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Einstein reportedly bitter about not getting spooky action at a distance
May 30, 2014 9:26 AM   Subscribe

Researchers at the Kavli Institute of Nanoscience at the Delft University of Technology in the Netherlands have developed a technique for quantum data teleportation that uses deterministic methods to offer one hundred percent accuracy. Previous methods only worked reliably one in every 100 million attempts.

While these results still don't really get us to the promised land of Star Trek style teleportation, they might just mean new computer technology that can exploit these effects to communicate state information instantly and securely via entanglement, and more importantly, they just might definitively prove Einstein was wrong about something.
posted by saulgoodman (87 comments total) 30 users marked this as a favorite

 
they just might definitively prove Einstein was wrong about something

I don't get this. It's not new that there is such a thing as entanglement. Einstein's objection wasn't--so far as I understand it--to the fact of entanglement. His objection was that entanglement revealed that the theory of quantum physics was incomplete: because it could not account for "spooky action at a distance" other than by saying simply "well, that's just the way things are, baby."

I don't see that this latest development makes Einstein any more or less wrong about that than he already was.
posted by yoink at 9:36 AM on May 30 [7 favorites]


This is more like the ansible than a Star Trek teleporter. Still hugely cool and right out of science fiction
posted by T.D. Strange at 9:36 AM on May 30 [8 favorites]


Fascinating stuff. Current processors will probably be pushing up against the clockcycle "event horizon" problem in the next few generations (inability to propagate state from one edge of the die to the other, either via electrons or even optically, within a single clock cycle), and that's generally been viewed as a fundamental sort of performance limit. Jim Gray somewhat famously predicted that it would lead processors towards being "smoking, hairy golfballs" over time.

But of course that all goes out the window if you can propagate information instantaneously. And you only need to get it across a few inches, not feet.
posted by Kadin2048 at 9:36 AM on May 30 [4 favorites]


Wow, this is huge if true.
posted by BrotherCaine at 9:36 AM on May 30


Okay, somebody with more knowledge of this sort of thing correct me if I'm wrong here, but if this entanglement thing can be made to work on a commercial level, couldn't it be used for instantaneous, wireless communication over distances? I mean, forget really really fast Internet, I'm talking about high definition images being sent instantly from a camera on board a Voyager-type spacecraft halfway between Proxima and Earth.

'cause if that could be done, it's frigging beyond huge.
posted by Mooski at 9:48 AM on May 30 [4 favorites]


Einstein's objection wasn't--so far as I understand it--to the fact of entanglement.

Initially he was in the disbeliever camp. He dropped that fairly quickly though and moved on to the position that if it could be proved it would reveal an enormous hole in our understanding.
posted by Tell Me No Lies at 9:48 AM on May 30 [2 favorites]


Sigh. Everyone, repeat after me: you can't use entanglement to communicate faster-than-light. There is a sense in which quantum teleportation "happens" instantaneously, but it relies on classical communication.

This is an engineering breakthrough, not a change in our understanding of physics.
posted by teraflop at 9:49 AM on May 30 [22 favorites]


everything we know is wrong
but we knew that already
posted by philip-random at 9:58 AM on May 30 [3 favorites]


I don't get this. It's not new that there is such a thing as entanglement. Einstein's objection wasn't--so far as I understand it--to the fact of entanglement. His objection was that entanglement revealed that the theory of quantum physics was incomplete: because it could not account for "spooky action at a distance" other than by saying simply "well, that's just the way things are, baby."

The article kind of glosses over this but they're talking about Bell tests, which test for local hidden variable theories -- Einstein thought that such theories are possible. Bell tests are nothing new (they go back to the 70s), but proponents of local hidden variables claim that such tests are ineffective because they all have so-called loopholes. So there's a lot of research groups attempting to design tests without loopholes, and apparently this group at Delft achieved just that.
posted by maskd at 9:58 AM on May 30


I don't see that this latest development makes Einstein any more or less wrong about that than he already was.

It's just a hook the press is using because there's still a lingering perception Einstein was never wrong about anything. Your point is mostly true: We've known entanglement is real for some time now, though Einstein rejected it as leading to absurdities. We still don't know if entanglement can be exploited to transmit information at FTL speeds. They may have the tools to figure that out now and researchers are expected to begin working on that next, as I understand it. If so, then Einstein's wrong again, if only on a technicality, because information may be able to travel faster than light (unless that's a deceptive way to think about information that stretches the metaphor of "movement" too far).
posted by saulgoodman at 9:58 AM on May 30 [2 favorites]


Everyone, repeat after me: you can't use entanglement to communicate faster-than-light.

That page says it's impossible to send meaningful information through quantum entanglement, but the claim here is that a flipped bit – which is by definition meaningful information – has been transmitted via quantum entanglement. So I'm confused.
posted by Holy Zarquon's Singing Fish at 10:02 AM on May 30 [9 favorites]


I mean, forget really really fast Internet, I'm talking about high definition images being sent instantly from a camera on board a Voyager-type spacecraft halfway between Proxima and Earth.

Heh, that's funny. I have a fundamentally opposite viewpoint about the relative merits of those two things. I'd have phrased it "I mean, forget high definition images being sent instantly from a camera on board a Voyager-type spacecraft halfway between Proxima and Earth. I'm talking about really fast Internet!"
posted by gurple at 10:05 AM on May 30 [5 favorites]


If a science reporter mentions that something physically impossible by current understanding has happened, it hasn't happened.


Not 100% accurate but the sort of accurate that you'd darn well know if it wasn't.
posted by Zalzidrax at 10:05 AM on May 30 [1 favorite]


"So I'm confused."
Surely physicsmatt will stop by shortly to help us out.
posted by Blasdelb at 10:06 AM on May 30 [2 favorites]


Einstein reportedly bitter about not getting spooky action at a distance
The principle of generating small amounts of finite improbability by simply hooking the logic circuits of a Bambleweeny 57 Sub-Meson Brain to an atomic vector plotter suspended in a strong Brownian Motion producer (say a nice hot cup of tea) were of course well understood — and such generators were often used to break the ice at parties by making all the molecules in the hostess's undergarments leap simultaneously one foot to the left, in accordance to the theory of indeterminacy.

Many respectable physicists said that they weren't going to stand for this, partly because it was a debasement of science, but mostly because they didn't get invited to those sorts of parties.
--The Hitchhiker's Guide To The Galaxy
posted by zombieflanders at 10:08 AM on May 30 [6 favorites]


Sigh. Everyone, repeat after me: you can't use entanglement to communicate faster-than-light. There is a sense in which quantum

Oh, ok. That still makes sense.

We still don't know if entanglement can be exploited to transmit information at FTL speeds.

What the fuck, scientists? Can't you just tell me the friggin answer here.

Barring shit that happened right after the Big Bang, has anything gone FTL?
posted by hal_c_on at 10:15 AM on May 30


OMG, y'all, I was at that Dear Albert reading at NYU Skirball on Wednesday, and Alan Alda asked Brian Greene after the performance about the spooky business. And wanted to know what "spooky" is in German. Anyway, Greene thinks Einstein was wrong.
posted by droplet at 10:15 AM on May 30


That page says it's impossible to send meaningful information through quantum entanglement, but the claim here is that a flipped bit – which is by definition meaningful information – has been transmitted via quantum entanglement. So I'm confused.

But the "faster than light" part is what we're talking about, and that appears to be entirely invented by the media articles. I don't have access to the full Nature paper at the moment, but you'd think that would be the kind of thing that they'd put in the abstract if it were true.

Quantum computation relies on the ability to put particles into a superposition of multiple states, and entangle multiple particles ("qubits") so that their states are correlated. You can manipulate a qubit's quantum state using a variety of operations, but the catch is that you can't measure it without collapsing the superposition. In particular, you can't do simple things like copying a value from one quantum "memory location" to another.

You can use a specially-constructed sequence of entanglements and measurements to move a qubit's quantum state to a new location, at the cost of destroying the original one. But doing so requires sending classical information about what operations to perform at the other end, and this step still has to happen at light-speed or slower. That's what quantum teleportation is all about.

I'll qualify all this by saying that I'm by no means an expert in quantum physics, so I apologize if I got any details wrong. However, I'm pretty sure I'm substantially less wrong than the articles that are suggesting this can be used for FTL communication.
posted by teraflop at 10:20 AM on May 30 [4 favorites]


Communicating via entanglement requires sending additional information via conventional methods.

Measuring a quantum spin at point A allows you to "instantaneously" infer information about the state at its partner spin at point B, but that alone is not enough to send information between A and B. Say you (at point A) measure a spin's projection along the z-axis and get spin up. Now you instantly know that if someone at point B measures the partner spin along the z-axis, they'll get spin down. But that doesn't send any information. Aha, you say, I'll either measure along z (to send the bit 1) or not (to send the bit 0). Then they'll either get the "right" answer (if they measure along the same axis) or a random answer (if they measure along a different one). But they won't know if they got the right answer (neither will you) unless you send that information, using classical means.

Of course, this is only even an issue if you subscribe to the Copenhagen interpretation of quantum mechanics. The many worlds interpretation has no superluminal collapse whatsoever. It's just philosophically ... unpleasant for many people.
posted by Humanzee at 10:21 AM on May 30 [7 favorites]


MetaFilter: just philosophically ... unpleasant for many people.
posted by hippybear at 10:28 AM on May 30 [11 favorites]


I mean, forget really really fast Internet, I'm talking about high definition images being sent instantly from a camera on board a Voyager-type spacecraft halfway between Proxima and Earth.

You're conflating two different meanings of "speed" here...

1) Propagation Velocity (meters/sec) -- Terrestrial links already transmit data from point A to point B faster than you can perceive without instrumentation, because of the relatively short spans ... About 0.67x the speed of light for wire or fiber, 1.0x for radio (eg. cell). Increasing the links' speed from 0.67x to (say) 2.0x wouldn't improve your perceived Internet "speed" between client and server.* You would perceive faster response from a light-minutes-/light-hours-away spacecraft going from 1.0x speed of light (through vacuum) to 2x.

2) Bandwidth (bits/sec) -- Increasing the end-to-end bandwidth between client and server would improve your perceived Internet "speed," all other things being equal.


*NB: Sometimes you can detect a voice / video delay when multiple telephone or data switches get in the way, or for audio when you get a bad reflection (echo) from poor termination and/or duplexing on the far end of a longer-distance call. In the second case, you might perceive better call quality going from 0.67x to 2.0x.
posted by ZenMasterThis at 10:35 AM on May 30 [1 favorite]


Aha, you say, I'll either measure along z (to send the bit 1) or not (to send the bit 0). Then they'll either get the "right" answer (if they measure along the same axis) or a random answer (if they measure along a different one). But they won't know if they got the right answer (neither will you) unless you send that information, using classical means.

That makes sense; the nature of quantum mechanics means you can't just passively monitor for a state change, since "passive monitoring" doesn't exist. But you can't use a pre-arranged schedule to have both sides take their measurements at the same time without needing to signal each one?
posted by Holy Zarquon's Singing Fish at 10:53 AM on May 30


So the problem with practical application of this is that B can't tell if its result is due to A's measuring its half of the entangled pair (& therefore setting B's half) or not? Reminds me of trying to decipher a one-time pad encoded message without having access to the pad; all messages are equally likely & dependent on the pad the encoder chose to use.
posted by scalefree at 10:58 AM on May 30


You know, Roger Williams wrote about this in The Metamorphosis of Prime Intellect and bad (well weird anyway) things happened. Not sure we should be going there.
posted by elendil71 at 11:10 AM on May 30 [5 favorites]


Einstein might have avoided being bitter about not getting action if he had, like, showered and combed his hair.
posted by weapons-grade pandemonium at 11:11 AM on May 30


But you can't use a pre-arranged schedule to have both sides take their measurements at the same time without needing to signal each one?
The timing doesn't matter (you measure when the spin comes in), the orientation and parity in observed spins matters. You could pre-arrange orientation but that doesn't permit message passing. "I just measured spin down, so dude over at point B must have just measured spin up. Great!" You can't pre-arrange the spins because they're generated randomly by the entanglement process.

So the problem with practical application of this is that B can't tell if its result is due to A's measuring its half of the entangled pair (& therefore setting B's half) or not? Reminds me of trying to decipher a one-time pad encoded message without having access to the pad; all messages are equally likely & dependent on the pad the encoder chose to use.
Exactly right.

As far as I can tell, the only real use for quantum communication is an extremely secure communication channel.
posted by Humanzee at 11:13 AM on May 30 [1 favorite]


Say you (at point A) measure a spin's projection along the z-axis and get spin up. Now you instantly know that if someone at point B measures the partner spin along the z-axis, they'll get spin down.

So from a transmitting information point of view, this is basically like if I put a quarter in one envelope and a dime in another and give the envelopes to two people. If one opens her envelope and sees a quarter she "instantly" knows the other person will see a dime, no matter how far away the dime is.

From a physics point of view, the cool part is that the determination of which envelope has the quarter is actually not made until one of the envelopes is opened, but that doesn't make it any more useful for transmitting information.

It's about as disappointing as actually doing the Schrodinger's Cat experiment. All you see is a dead cat or a not-dead cat. You can't actually observe whether or not the cat's dead/not-dead state is indeterminate until it's observed.
posted by straight at 11:13 AM on May 30 [3 favorites]


It's just philosophically ... unpleasant for many people.

Nightmarish, I'd say. If proven true, I might kill myself immediately.*

In 50% of universes, maybe not this one.
posted by Thoughtcrime at 11:17 AM on May 30 [1 favorite]


As far as I can tell, the only real use for quantum communication is an extremely secure communication channel.

Basically it means that two people could pick a random number at the same time and get the same random number, then use that random number as a one-time pad for cryptography. But it seems like there would be a pretty limited set of situations where handing someone half of a set of entangled electrons would be better than just handing them one of two copies of a one-time pad.
posted by straight at 11:19 AM on May 30 [2 favorites]


I believe the more physics-literate folks when they say that quantum entanglement can't be used for FTL communication, but I don't understand why. I'm-a break the idea down as well as I can; somebody please tell me where I've gotten it wrong.

1. Certain quantum properties of two particles that are entangled are always mirror images of each other. If the Florpelometer shows a reading of 17.8 for particle A, it's gonna show a reading of -17.8 for particle B.

2. It is possible for people in white lab coats to perturb the quantum state of particle A.

2b. Not just to perturb it, but to deliberately "set" it to a desired value.

2c. And particle B will change its state to match particle A's new state.

2d. And this will happen instantaneously, not limited by the speed of light.

3. So we can continuously read the state of particle B to find out what states are being "set" on particle A.

Which part do I have wrong?
posted by escape from the potato planet at 11:22 AM on May 30


Straight got there first, but it's as simple as that - put a black ball in one bag and a white ball in another, then send one to Australia and one to the Moon. When the lunar bag is opened, the astronaut will know instantly what colour ball is in the Sydney bag, despite the speed of light delay between them.

The quantum bit is that there's no way to close the bag afterwards, so you'll always know if someone's intercepted it along the way and you know not to use that pair of balls for anything secure.

Which is damned useful, but does not give us the ansible.

Which is a real shame, but that's how the universe is built.

(I don't understand how teleportation - or electron tunnelling - doesn't violate c, but I accept it doesn't.)
posted by Devonian at 11:24 AM on May 30


There are two nice properties of quantum communication that make it better than a one-time pad.
1) it's really, truly random. The NSA can't trick electrons into using a bad random number generator, there are no back doors.
2) It can't be copied. Anyone trying a man-in-the-middle attack can be detected and thwarted.

escape from the potato planet, you're wrong about 2b. The state can be collapsed to one of two distinct values, but it's random and not under the control of the people in white lab coats.
posted by Humanzee at 11:28 AM on May 30 [3 favorites]


Gotcha! Thanks.
posted by escape from the potato planet at 11:29 AM on May 30


1) it's really, truly random. The NSA can't trick electrons into using a bad random number generator, there are no back doors.

I suspect that any implementation of this which generates enough random numbers to be useful for communication would require software that could have a back door in it.
posted by straight at 11:32 AM on May 30 [2 favorites]


Well you can set a spin but setting it breaks entanglement before the quantum state can propagate to the other entangled particle.
posted by Talez at 11:34 AM on May 30 [2 favorites]


I've not got time to look at this in any detail right now (England vs Peru in half an hour innit), but the two observations I can glean from that article are:

(1) It sounds a like they've managed to create a quantum system that entangles and decoheres nicely (i.e they can make entanglement reliably work in real life, rather than it being a breakthrough in the theory)

(2) The stuff about Einstein being wrong is really annoying (because that's how science works, he isn't even really wrong in this case, and he called something 'my greatest blunder' from which it should be possible to infer that he was aware of being wrong at least twice).

sorry if that got a bit ranty, I've headed over here because arguing that exercise doesn't cure cancer on facebook is doing my head in.
posted by Ned G at 11:39 AM on May 30 [3 favorites]


“The only thing known to go faster than ordinary light is monarchy, according to the philosopher Ly Tin Weedle. He reasoned like this: you can't have more than one king, and tradition demands that there is no gap between kings, so when a king dies the succession must therefore pass to the heir instantaneously. Presumably, he said, there must be some elementary particles -- kingons, or possibly queons -- that do this job, but of course succession sometimes fails if, in mid-flight, they strike an anti-particle, or republicon. His ambitious plans to use his discovery to send messages, involving the careful torturing of a small king in order to modulate the signal, were never fully expanded because, at that point, the bar closed.”


― Terry Pratchett
posted by poe at 11:40 AM on May 30 [20 favorites]


Well you can set a spin but setting it breaks entanglement before the quantum state can propagate to the other entangled particle.

Oh! That's where I was getting hung up too. Now sense is made.
posted by Holy Zarquon's Singing Fish at 11:41 AM on May 30 [2 favorites]


METAFILTER: So I'm confused
posted by philip-random at 12:10 PM on May 30 [2 favorites]


The stuff about Einstein being wrong is really annoying (because that's how science works, he isn't even really wrong in this case, and he called something 'my greatest blunder' from which it should be possible to infer that he was aware of being wrong at least twice).

No, that was about General Relativity and the cosmological constant -- Einstein introduced it because it was the only way to get a static universe, which he believed in and was disproved by Hubble's observations in 1929. We'll never know Einstein's thoughts on Bell's theorem, because he died before it was published in 1964.
posted by maskd at 12:35 PM on May 30


The state can be collapsed to one of two distinct values, but it's random and not under the control of the people in white lab coats.

Stupid question - couldn't the simple fact of measurement or not by one side be used as a communication channel?

I'm referring to Prof. John Cramer's work over at UW.
posted by Ryvar at 12:50 PM on May 30


Damn...those HFT guys are going to start buying before they short!
posted by malocchio at 12:53 PM on May 30 [3 favorites]


That would actually be the literal truth driving development of superluminal communication, were such a thing possible, yes.
posted by Ryvar at 1:01 PM on May 30


OK, this cannot be used for "instantaneous" communication, in the sense of the previously-mentioned sci-fi "ansible." An ansible (coined by Le Guin, most people know the word from Ender's Game) is a fictional device that acts as a faster-than-light radio, communicating between points with zero time-delay.

This is impossible, and its impossible because the Universe obeys general relativity. What GR tells us is that information cannot propagate faster than the speed of light in a vacuum (called c, equal to about 300,000 km/s). This is a consequence of living in a causal (effects follow causes) Universe where any observer sees the same laws of physics (this is what we mean by relativity). Under these assumptions, which are borne out to a high degree of accuracy by experiments and provide the only consistent theoretical framework for Nature which I am aware of, faster-than-light communication is equivalent to time-travel. That is to say, if you gave me an ansible, then I can use it to place a call back in time. Though doing so might require using relativistic satellites and other very expensive pieces of equipment (I'm a theorist, such magitek is free for me), and no guarantee someone at the other end can pick up the call.

I say this just to point out that when you say "oh, FTL radio is awesome, I can communicate with far-away space probes" or whatever (and really, the obvious killer app is breaking the shit out of the stock market on micro-second trading), you are ignoring the far bigger use, which is becoming the complete master of all Space and Time. For some reason, most sci-fi with ansibles sort of misses that point.

Quantum entanglement allows for "instantaneous" transmission of quantum states; Einstein's spooky action at a distance. However, this is not FTL communication. No information can be conveyed this way.

Some people have explained why not previously, but I'll spell it out again. Entanglement forces two wavefunctions to become intermeshed in such a way that two wavefunctions are correlated. That is, when a measurement is performed on one particle, which collapses the wavefunction into a definite state, you immediately and unambiguously know what the result of the same measurement on the other particle will be. Let's go through an example.

The canonical version of this is the spin of a particle, measured relative to a particular magnetic field (which you have presumably set up in your lab). Particles can only have a limited number of orientations relative to the magnetic field: spin-1/2 fermions for example can only have 2 orientations: spin oriented with the field (let's call that "up") and spin oriented against the field (call it "down"). If you don't do anything special, before you measure a particle's spin it can be in either orientation. The wavefunction is an even mixture of up and down. The wavefunction notation for this would be something like:
1/\sqrt{2} (|up>+|down>) or
1/\sqrt{2} (|up>-|down>) or most generally
1/\sqrt{2}(e^{i\phi_1}|up>+e^{i\phi_2)|down>).
Once I measure the spin, it collapses into up or down, and I have no a priori knowledge of which it will be (no "hidden variables").

So now let's take two particles in my magnetic field set up, and let's arrange things so that their wavefunctions are completely entangled. Maybe I arrange things so that the total spin is zero, so if particle A is up, than I know particle B is down, and vice versa. However, before I make the measurement, I don't know what A's spin is or B's spin.

Before I measure, I separate the two particles. I'm a theorist, so I imagine separating them by some incredible distance. Put one on Jupiter say (30 light-minutes away). I have to keep the magnetic field intact, and I can't allow anything to interfere with either particle in such a way that it forces a particle to have a definite spin, for reasons that will be clear in a moment. Now I have two correlated particles separated by huge distances. I then measure the spin of particle A. Let's say I find A's spin is "down." Now I immediately know B's spin is "up" without having to wait 30 minutes to get a message from Jupiter saying what the spin is. Whoo hoo! I have FTL transmission of information.

Except not. The two particles have to start together; so the information about the entangled states propagated out to the end location of particle B at speeds less than light; only as fast as the rocket carrying the particle. It is in some way analogous to having two rocks: one white, one black. I grab one rock and don't look at the color. You grab the other and fly to Jupiter. I now reveal that I am holding the black rock; I know instantly you have the white one, but nothing propagated faster than light.

The only difference here is the "collapsing wavefunction." Which appears to occur instantly, and due to the lack of hidden variables, really means that the two particle's spins were not determined until I make a measurement on one. At which point, yes, the wavefunction collapse propagates instantly to the far away particle. Aha, you say: if I can force particle A to have a definite spin, than I can communicate with particle B instantly.

Except you can't. To force particle A's spin to be up or down, I will have to perturb the system; change the field, scatter a photon, something. When I do that, I will necessarily dis-entangle the relationship of particle B's spin with particle A's. This is why when I imagine transfer particle B to Jupiter or wherever, I can't just take the particle out of the magnetic field and throw it over to Jupiter, I have to keep it carefully isolated from outside interactions which alter it's spin wavefunction. Being entangled is not something that happens between two particles and remains true for all time; after the experiment is done I can change particle A's spin all I like and particle B isn't going to change along with it. This is not sympathetic magic.

So by its very nature, entanglement precludes FTL communication of information. The moment one party holding half the entangled states futzes with their particles, they will either collapse the wavefunction (which gives a random result, not useful for communication) or decorrelate their particles' with the other party's states. The useful application (and why two entangled states are more useful than my black/white rock example) is that these can be use for un-interceptable, un-tappable 1-time crypographic pads. With rocks or regular bits of information or whatever, while you're flying off to Jupiter, someone can steal my half of the pad, copy it, and return it unnoticed. Then, when I open the pad (which will typically be a list of bits "11100111100101010....") and use it to encrypt my message (say, by converting my message into binary and adding the pad's bits to the binary code of the message), someone can intercept and decrypt my message without either me or the intended recipient knowing.

With quantum entanglement, this is impossible. This is because quantum states are "unclonable." If I have a state in some particular wavefunction, I can collapse the wavefunction (via measurement) and copy the classical states (I measure particle A's spin is up, and now I can make another particle with spin up to copy the first). But before I measure the state, I can't make another quantum system with the same (unmeasured) wavefunction. That means I cannot steal your box of entangled bits, clone the entanglement state, and so steal your 1-time pad once you make your measurement.

So if I was building a 1-time pad using entangled states (say you're flying to Europa looking for life in the Europan Ocean, and you need to radio back to me and tell me what you found without the rest of the world knowing, because humanity is a bunch of panicky monkeys), I'd take a huge number of entangle particle pairs (say, 1 gb worth). I call spin up "1" and spin down "0," and you take half and I take half. You fly to Jupiter, land on Europa, and start your mission as Speaker-to-Shellfish. You need to report back, so you measure the spins, get a random string of digits (say "101011001.....") and immediately know I have the compliment of that code (in this case "010100110..."). You take your message, encode it, beam it to me, and sleep soundly knowing that only I have the compliment of your encoding pad and so only I know that King Graz'k of the Cuttlefish People has declared war on Belgium unless waffle deliveries proceed immediately.

Now let's say that the Belgian intelligence service, hearing pre-flight rumors of the Cuttlefish People's antipathy and fearsome electrotridents, have snuck into my house and stole the box containing my entangled bits (before you sent your message) with the plan of copying it and replacing the box all unnoticed by me. If they could clone the wavefunctions, entanglements and all, then when I decode your message, so can they (assuming they pick up the radio transmission), because their clones bits would collapse to the same set of spins as mine once you made your measurements. However, since cloning is impossible, they cannot do that. They could measure the spins of my particles, "prematurely" collapsing your spins and giving them access to the 1-time pad you have (since when they measure the spins they collapse your spins), but then when I measure I'll get a different 1-time pad (unless they take special steps to keep the spins from changing over time post measurement). So when I get your message, my 1-time pad differs from yours, I decode only random noise, and I know the Belgians are on to us. And presumably we could have set up communication protocols requiring some sort of handshake from both parties acknowledging that the messages were readable before the important information was sent, but that's way too much effort for me to figure out now and is the domain of non-quantum crypto I'd think.

What about quantum teleportation? Can that be used to send information? No, for the same reasons as entanglement. These scientists can now "teleport" a quantum state instantaneously. That's very important and useful, but not for FTL communication. To teleport a quantum state, I start with two systems that are generally the same (two identical set of particles with spins, to use the same example I had used before) and overwrite the 2nd set of particles' wavefunction with the 1st set's. However, remember that cloning quantum states is impossible. So in doing so, the "teleported" state is destroyed. I might start with 2 boxes of 500 particles, one of which has all 500 spins carefully entangled with each other, but once I teleport the state of box A to box B, the wavefunctions of box A are completely changed. So I can't teleport you the 1-time pad and keep the original to use, nor can I set up the spins with a particular message and teleport it to you in the blink of an eye. You can try to come up with ways around this, but in all cases one of two things (usually both) have to have occurred:

- Either you started both systems next to each other than moved them away slower than light before pulling some quantum teleportation scheme and/or
- you have to communicate some classical information through slower-than-light channels in order for the "FTL" quantum information to be useful.

In the box o' bits example above, the boxes started next to each other, then moved apart at sub-light speeds, then I had to send a coded message, otherwise the boxes are a bunch of random bits and not useful information.

Ryvar: I am very very very very very skeptical that the outlined scheme would work as advertised and allow FTL communication. I am happy to be proved wrong, but my prior on this being true is amazingly low. I think I see the set-up they're envisioning, but I can't come up with a configuration in which all the relevant parties are not within each other's light-cones (which is just a fancy way to say that I feel like they can do all sorts of clever quantum tricks to communicate, but they are close enough together that it would be equally effective just to pick up the radio and call each other using boring old photons). But hey, that's just me, and I've been thinking about it for all of 10 minutes.
posted by physicsmatt at 1:16 PM on May 30 [60 favorites]


physicsmatt! You came!
posted by Tell Me No Lies at 1:17 PM on May 30 [1 favorite]


I think it was Stanislaw Lem's Cyberiad that he had two robotic Constructors, both of whom wanted to play a trick on opposing kings. In order to ensure they were coordinated, they used a quantum entangled widget that would turn red when measured. Sure enough, when either constructor looked at their widget, it was red ....
posted by Blackanvil at 1:20 PM on May 30


physicsmatt! You came!

I was scrolling down the page and halfway through I thought: Th.. this... this satisfactorily technical yet still highly readable wall of text... it... it must be him!
posted by procrastinator at 1:45 PM on May 30 [8 favorites]


In the box o' bits example above, the boxes started next to each other, then moved apart at sub-light speeds, then I had to send a coded message, otherwise the boxes are a bunch of random bits and not useful information.

Yeah, this is where I'm confused. It seems like for the teleportation case there comes a point where you can detect that the state of the receiving end changed. Not what the change means, etc, just that it was in one state and suddenly it is in another.

Is that not the case?
posted by Tell Me No Lies at 1:46 PM on May 30


Thanks for the great explanation physicsmatt!

I remember reading an article that, at least on the surface, seems to suggest the possible existence of what might be a loophole where some types of "gentle" measurements can be made without collapsing the wave function. If this is indeed the case then wouldn't that imply that this could potentially be done to entangled particles as well and those measurements could be used to transmit information through the system continuously and indefinitely?

I do understand that this can't really be possible for the very reasons you stated above so I'm wondering how this loophole is closed. Is it because the feedback mechanism they use to stabilize the system would have to be applied to both particles and that can't be done because of their spatial separation?
posted by Hairy Lobster at 1:47 PM on May 30 [1 favorite]


Ah, forgot to link in the article.

Here it is.
posted by Hairy Lobster at 1:47 PM on May 30


So it sounds like—and correct me if I'm wrong—entanglement isn't some mystical connection between two particles, where each is "responding" to the other's state. Creating a pair of entangled particles is more like running two identical computer programs at the same instant, such that each one's state at any given moment will be identical to the other's. (For the purposes of this analogy, we're talking about a program that doesn't accept any kind of input during execution.)

Is that right?

If so, why does every news article ever completely misrepresent it?
posted by escape from the potato planet at 1:58 PM on May 30


I suppose, then, that my somewhat-less-than-technical response must be: Dang.
posted by Mooski at 1:59 PM on May 30


So if this tech was brought to market and was being used for consumer devices ( assuming its affordable for typical consumers) for users with limited to average tech know-how, what would the advertising say? ( without lying/misleading)
posted by Bwithh at 1:59 PM on May 30


So it sounds like—and correct me if I'm wrong—entanglement isn't some mystical connection between two particles, where each is "responding" to the other's state. Creating a pair of entangled particles is more like running two identical computer programs at the same instant, such that each one's state at any given moment will be identical to the other's. (For the purposes of this analogy, we're talking about a program that doesn't accept any kind of input during execution.)

Close. They'll be opposite of each other not identical. But you can't force which one is in state X & which is in state Y at any given moment. All you can say is, if one is X then the other one must be Y.

I think.
posted by scalefree at 2:11 PM on May 30


escape from the potato planet: "Creating a pair of entangled particles is more like running two identical computer programs at the same instant, such that each one's state at any given moment will be identical to the other's."

I don't think that's quite accurate. To quote physicsmatt:

"The only difference here is the "collapsing wavefunction." Which appears to occur instantly, and due to the lack of hidden variables, really means that the two particle's spins were not determined until I make a measurement on one. At which point, yes, the wavefunction collapse propagates instantly to the far away particle."

My reading of this is that there is a "live" and instant connection between entangled particles. If the wavefunction collapses for one particle it also instantly collapses for the other. But the result of the collapse is not predetermined/set at the time of entanglement. I think your analogy corresponds more to the notion of hidden variables which I believe has been dismissed. But even though this connection appears to be real it's useless in terms of establishing communication/transmitting information FTL because it'll only work exactly once because the entanglement ends with the collapse.
posted by Hairy Lobster at 2:19 PM on May 30 [2 favorites]


All hail King Graz'k of the Cuttlefish People
posted by djseafood at 2:24 PM on May 30 [2 favorites]


So, can I get a little more clarification on the "uncloneable" thing? Is it not possible to manually set the spin of a particle? Explain how this scenario doesn't subvert the secure pad thing:

I create a pair of tangleboxes A and B for communicating with Jupiter. I send B off to Jupiter, and keep A. The Belgians sneak in, steal A, and read it, locking the spins for both A and B, and recording the bits now stored in A (and thus also knowing what B will read).

Why can't they create a new fake A' to give me, with the spins not entangled but already set to match the bits ripped from A? Is it not possible to set spins directly, or sort particles by spin in order to arrange them as they'd been read from A? Is there something in a measurement that distinguishes a set-spin particle from a freshly-measured one?
posted by NMcCoy at 2:25 PM on May 30


Tell Me No Lies: I'm not an experimentalist, but my strong suspicion is that you cannot tell that the state has changed without looking at the wavefunction in such a way that the teleportation could not occur.

This is the basic problem with trying to use wavefunctions for communication. Wavefunctions are the "real" thing that particles are "made" out of. We are all quantum, and the real question is why we don't notice it (it's because hbar is small, by the way). However, we don't get to directly see wavefunctions, we get measurements which are proportional to wavefunctions squared, and the measurements themselves collapse wavefunctions, changing them. Do if you want to check to see if your box o' bits was perturbed, you are going to have to measure it, which collapses the wavefunction, rendering it useless as a teleportation target.

Lobster: As above, my strong suspicion is that you'd have to nudge both wavefunctions to keep them entangled. Though I could be underthinking this and there might be something far more subtle going on (confession time guys, I'm answering these questions in lieu of working, so you have to excuse a certain amount of glib laziness).

escape: because most science journalists aren't scientists and scientists are not great at explaining what we do as a general rule. The difficultly is that wavefunctions, and thus entanglement, have unique properties that make them fundamentally different from your example - for example as Lobster points out, the state is not determined prior to collapse, unlike two programs running deterministically in parallel. So in this case, sure there's a useful analogy to be made with some classical system to make a particular point, but in general a wavefunction or pair of entangled wavefunctions can do things that no classical system can do, and so your average quantum computing expert doesn't want to make an analogy that fails to capture the nuance. This is especially true since the nuance is the reason that quantum computing is actually interesting.

So we describe wavefunctions in a way that sounds like magic, and then we talk about teleporting the quantum state, or spooky-action-at-a-distance, which are all perfectly fine terms and correct in context, but due to the communication barrier and the previous emphasis on the crazy mad properties of wavefunctions, it gets garbled into things that sound like the sci-fi we know and love.

It's a problem, and a big one, but I don't know if there's a real answer to it. Things like quantum computing and quantum interference and wavefunction collapse are really fucking interesting and really fucking cool, and in fact in some ways more interesting and cool because they can't be used to FTL.

If there's one thing to get across from this is that often in physics it would look like there's an "easier" way to build a Universe, a way that would allow us FTL or 2nd law violations or something awesome like that. However, the laws of physics are far more subtle than the surface reading, and they work in such a way that prevents these things from happening. Yes it sucks (I want a starship as much or more than you, trust me), but then you dig deeper and you discover that if you'd gone with the "easy" way to build the laws of physics, you'd have run into some much more dangerous and crazy issue that would be Bad for a consistent Universe. For example, the more I learn about it, the more it becomes clear I don't know how you could build a Universe that makes sense without relativity or something very similar to it; yes it's counterintuitive when you first learn it, but then you realize that not having it means you can't actually be consistent in how things like space and time and events actually interrelate.
posted by physicsmatt at 2:28 PM on May 30 [7 favorites]


Creating a pair of entangled particles is more like running two identical computer programs at the same instant, such that each one's state at any given moment will be identical to the other's.

I like to imagine it more like those friendship necklaces where you have a heart divided down the middle and one person takes the "best" partand the other takes the "friends" part.

Or there's that ancient debt stick thing where a piece of stone or bone would be etched then split down the middle, joining them later would be evidence of their relationships.
posted by odinsdream at 2:40 PM on May 30


The stuff about Einstein being wrong is really annoying (because that's how science works, he isn't even really wrong in this case, and he called something 'my greatest blunder' from which it should be possible to infer that he was aware of being wrong at least twice).

No, that was about General Relativity and the cosmological constant -- Einstein introduced it because it was the only way to get a static universe, which he believed in and was disproved by Hubble's observations in 1929. We'll never know Einstein's thoughts on Bell's theorem, because he died before it was published in 1964.
Yeah, I know, I was just using it as an example of how Einstein isn't / wasn't actually infallible, which seems to be a weird trope that loads of people have swallowed. I was secretly hoping that someone would object to my statement on the grounds that the cosmological constant may not have actually been a blunder (depending on your opinions on dark energy). In that case I was ready to point out that the statement 'my greatest blunder' was in itself a blunder, and then I was ready to feel smug.
posted by Ned G at 2:56 PM on May 30


Whatever collapses the wavefunctions of the two particles does travel FTL, though, doesn't it?
posted by wemayfreeze at 2:57 PM on May 30


wemayfreeze: "Whatever collapses the wavefunctions of the two particles does travel FTL, though, doesn't it?"

My understanding is that the measurement that results in the collapse of the wave function is made in one place only. But the wave function collapses instantaneously for both particles as a result. I'm not sure there is actually anything involved here that can be described as "travelling" in any way such as the bosons exchanged between particles as force carriers. Entanglement appears to be very different from force based interactions which cannot be FTL. To me this is a big part of why this is so hard to grasp intuitively because all analogies we can make are ultimately doomed because they're based on our daily experiences of interactions in space-time and we know nothing else.
posted by Hairy Lobster at 3:19 PM on May 30


if you want to check to see if your box o' bits was perturbed, you are going to have to measure it, which collapses the wavefunction, rendering it useless as a teleportation target.

Suppose that, prior to setting up a quantum telegraph, Alice and Bob agree on exactly when Alice will send a message to Bob, and how long the message will be.

Instead of using the spin of an electron per-se as the encoding medium, the message is encoded in the perturbedness of the particles--to set a particle to 1, Alice will observe that particle; to set it to 0, she will not.

Bob waits until after the agreed-upon time, checks all of his particles for their perturbedness, and knows which ones Alice flipped. If she flipped them soon enough before the agreed time, this would require the perturbedness to travel faster than light.

Why wouldn't this work?
posted by LogicalDash at 3:56 PM on May 30


deleted; I don't know what I'm talking about
posted by escape from the potato planet at 4:18 PM on May 30 [3 favorites]


"star trek" always bugged me because it was inconsistent with physics. when an object approaches the speed of light from the perspective of a (relatively) stationary observer, it undergoes the three lorentz transformations, mass, time, and length along the axis of its motion, which shrinks. consequently, whether it's an elephant or a starship blowing past you, it will look like an edge-on tortilla. if i had been gene roddenberry, the in-hull shots would be the same, but the out-hull shots would be rapidly moving tortillas.
posted by bruce at 4:26 PM on May 30 [1 favorite]


deleted; I don't know what I'm talking about

escape from the potato planet is an example to us all.
posted by justsomebodythatyouusedtoknow at 4:35 PM on May 30 [5 favorites]


LogicalDash, because the particles Bob observes don't come with a flag that says "observed by Alice." Their wavefunctions are collapsed or not, depending on whether they were observed, but that's not something Bob can know.
posted by physicsmatt at 4:53 PM on May 30 [2 favorites]


LogicalDash: "Why wouldn't this work?"

I have several ideas but, of course, they might all be wrong

1) can you check whether the waveform has collapsed on the receiver's end without collapsing it yourself or without disentangling the particles

2) if (1) is not a problem then timing is. The universe, as physicsmatt pointed out, appears to obey general relativity. Because relative motion and gravitation affect how clocks run the receiver must know everything that happened and continues to happen to the sender in order to determine the status of their clock relative to their own in order to decide when to check. This information however is only available through observations and communications subject to the speed of light limit. So, while it may be possible to encode information in a multi-entangled-pair setup this way (provided (1) isn't a problem) you're not able to reliably read it out before the same information could have been communicated to you at light speed.
posted by Hairy Lobster at 4:57 PM on May 30


Nightmarish, I'd say. If proven true, I might kill myself immediately.

Are you a friend of Wigner?
posted by sebastienbailard at 5:52 PM on May 30


LogicalDash: as I understand it (which is admittedly not all that well) what you propose is essentially what Prof. Cramer is working on - at a slightly larger scale where the collapse is observed probabilistically and No Cloning is not violated.

I strongly suspect that even if he does get it working, it'll turn out Novikov was correct - the probability of an event that would give rise to a paradox is zero.
posted by Ryvar at 6:57 PM on May 30


The wife says:

1) How to check whether Belgium swiped Bob's box o' bits: When measuring their bits, Alice and Bob each randomly and independently decide whether to measure each bit's spin along the z direction or along the (say) x direction. After measuring, Alice sends her sequence of choices of measurement axis to Bob (using an ordinary radio). Each bit which they happened to measure along the same axis goes into the one-time pad. Each bit which they happened to measure along different axes gets thrown in the trash (because the results of these measurements will not be correlated between Alice and Bob). Belgium's own random choices of measurement axis won't match the ones Alice and Bob are keeping, so there will be gaps in Belgium's one-time pad. Then Alice and Bob can openly compare (again by ordinary radio) the first several bits in their one-time pads to check whether they match. Belgium's interception will make the first-several-bit comparison reveal mismatches in the bits for which Belgium measured along the "wrong" axis compared to Alice (thus breaking the entanglement and causing Bob's subsequent measurement along the other axis to be randomized).

2) There is no way to determine whether an electron you have lying around is in a superposition of states just by making a measurement on that electron. You measure its spin along some chosen axis; you get either "up" or "down".

3) As far as I can tell, the weak-measurement stuff buys you nothing compared to any other quantum measurements, and therefore there is no point to it. But this probably means I am missing something.
posted by sebastienbailard at 7:06 PM on May 30 [1 favorite]


physicsmatt: We are all quantum.
posted by SPrintF at 9:08 PM on May 30


As I understand it, entanglement lets you do two things: send a quantum state from one place to another without having to physically move the quantum state around, or send two classical bits using only one classical bit of signal.

However, in both cases you have to generate an entangled state, send the two halves of that off to the two places at normal sub-light speeds ahead of time, and then after doing quantum jiggery pokery (measurements, applying unitary operations, blah blah), you have to send a classical bit at normal sub-light speed to enable the other person to do some corresponding jiggery pokery which reveals the effect.

Another case of the laws of physics being rigged to prevent cool stuff which I've complained about before.
posted by larkery at 3:02 AM on May 31


LogicalDash, because the particles Bob observes don't come with a flag that says "observed by Alice." Their wavefunctions are collapsed or not, depending on whether they were observed, but that's not something Bob can know.

I'm sorry, I'm still not getting it then. I'm really trying.

How does this differ from the Belgians having cloned your bits? Because the guy on Jupiter knows when that's happened (right?)
posted by newdaddy at 6:29 AM on May 31


newdaddy, no worries, I probably wasn't clear, as I was trying to cover several hypotheticals at once.

Here's the physics in the real world, and what it means for the toy quantum encryption story I used as an example:
- cloning quantum states is impossible. This means that you cannot take an unmeasured wavefunction and "xerox" that wavefunction to a 2nd system while leaving the original wavefunction intact and unmeasured. That means, in my example, the Belgians can't steal the box of unmeasured wavefunctions*, make a quantum copy of the wavefunctions and replace the box while leaving the entangled wavefunctions otherwise intact.

- it is of course completely possible for someone to steal the box o' bits, measure the wavefunctions and collapse the probabilities into a particular set of spin states. If no other non-quantum security measures (some sort of identity validation procedure) were taken, it is in this case possible for them to steal, measure, and then replace the now-collapsed box o' bits and have me be none the wiser. They could then listen in to my conversation by having a copy of the 1-time pad (in a certain sense, they have the original 1-time pad, as they measured the box first. I have the copy, since I just got the spins after their wavefunctions had collapsed).

- this is not the most secure way to do quantum cryptography, as you have identified. However, as sebastianbailard's wife correctly identified, there are better, slightly more complicated ways to construct the 1-time pad that can completely avoid the problem. I'm going to pretend the reason I didn't bring it up before was that I was magnanimously trying to keep it simple, rather than the other possibility which is that I was being kind of lazy and not thinking things through in the detail required. Anyway, thanks to the unnamed Mrs. Bailard for reminding us.

This more complicated scheme uses the fact that you can often build entangled systems where you can make measurements of a property in one of several ways, each of which is mutually exclusive. The canonical system is two entangled photons, where their polarizations are entangled in such a way that whatever the polarization of one photon is, the other photon has the opposite. Great, this is just like the box of spin states I was using before for the purposes of the general idea. I'll come back the spins in a moment, but work first with photons for clarity, as you can actual check some of the strange properties required directly.

Photon polarization can be measured in one of two ways: linear or circular. Linear polarization is either parallel -- or perpendicular | relative to some axis. Your polarized sunglasses are linearly polarized: this is because light reflecting off of water comes off polarized parallel to the water surface, so -- relative to the surface of the Earth. Thus sunglasses are built so that only light perpendicular | to the ground can get through. (Test this by rotating polarized sunglasses while looking at the light reflecting off a lake, you'll see the reflected light get brighter as you rotate the sunglasses 90 degrees from "normal"). Scattered sunlight is also linearly polarized, and since the only reason the sky isn't space-black is due to scattering of light, the sky itself is polarized, with the amount of polarization increasing away from the Sun (as the angle of scattering increases, so does the polarization). Vikings may have used polarizing crystals called "sunstone" to find the location of the Sun despite clouds as a navigational aid.

Light can also be circularly polarized, either right-handed R or left-handed L relative to the motion of the light itself. Some crystals have circular polarizing effects (taking unpolarized light and letting through only the R or L components, cutting the intensity in half). Let me stick with the linear polarization, because it's a bit easier.

So I have photons that are entangled in such a way that whatever polarization I measure for one, the other has "the opposite." But polarization relative to what? Let's say both I and the person which whom I want to share the photons with can define the same frame of reference easily (so maybe not too practical for the Jupiter example, but easy enough if I'm sending the photons down a fiber optic cable). For example, we can both agree to measure linear polarization vertically or horizontally relative to the Earth's surface (ignore the curvature here, pedants), using a cross shaped filter +. We could also measure the polarizations using to the same filter, but tilted at 45 degrees X.

I generate a huge number of entangled photon pairs, entangled in such a way that their net polarization is zero (equal -- or | relative to any filter). If I measure one photon of a pair using the + filter, I get either -- or |, and the person on the other end will get the opposite: | or --, assuming they too use the + filter. And if I measure with the X filter, I get either / or \, and the other person gets the compliment: \ or /. (I'm feeling pretty clever about the ASCII art here folks).

Now, what happens if I measure using the + filter and the other person uses X? Well, when I use the + filter, I get -- or |, but both of those polarizations turn out to be a linear combination of / and \. You can actually prove this with three linearly polarized filters: take two filters (say the lenses of polarized sunglasses), and put them so that their polarization directions are at right angles. Then no light gets through, since anything going past the first filter must be |, and so has no -- component to get past the 2nd filter. Now take a 3rd filter, and put in between the first 2. Rotate this new filter so that its polarization is 45 degrees from the other two (so if the first is vertical, and the 2nd horizontal, this new filter is now pointing at a 45 degree angle from the floor). Now light gets through all three even though it couldn't get past two. This is because the | light coming through the 1st filter has equal components of / and \ (45 degree polarized light) and so half of the light going through the 1st filter can get through this new angled one. But \ and / light are half | and half --, so half the light coming through the angled filter can now get through the final horizontal polarization. Isn't quantum mechanics fun?

Going back to our entangled photon pair, if I use the + filter, I get -- (say), and I collapsed your photon into a | state, but since that | state is still a superposition of \ and /, when you use the X filter, I have no idea whether your measurement of your half of the entangled pair yielded a / or \ result.

So how do we weaponize this for encryption purposes? I make a bunch of entangled photon pairs. I send you one photon from each pair. I measure the polarizations, randomly picking the + or X filter each time to make the measurement. You also randomly decide which filter to use. We then call each other up, in the clear without encryption. I give you the filters I used BUT NOT the polarization results.

So I might say "++X+XX+X+X++XXXXX..."
You used filters "X++X++X++XX+X+X+..."

We compare our filter choices, and throw out the measurements done where your choice and my choice don't match. So in my random string above, we ignore the first measurement (I picked + but you used X), use the 2nd (both +), ignore the 3rd (I used X, you +), and so on. This is because we now know that whatever result I got from the first measurement, it was with a filter that you didn't pick, and so my measured polarization tells me nothing about yours. So it's not helpful for the 1-time pad. We then build the pad out of the measurements used with identical filter choices (so on average 1/2 of the photons are not useful for the 1-time pad), with some prearranged scheme where -- and / (say) are 1 bits, and | and \ (say) are 0 bits.

This more complicated scheme means we have a way to ensure that no man-in-the-middle attack can work. Here's why. Say that, in transit, the evil Belgian security forces intercept the photons and measure their polarization, collapsing their (and hence my entangled photons') polarizations. The Belgians need to pick filters too, and if I am sufficiently random in my selection of filters, there is no way for them to pick the filter sequence either of us chose. So they pick a random set of filters, measure the photons, collapse their wavefunctions, and send them on to you, all undetected. These photons are now all in |, --, / or \ states (depending on filter choice and how the wavefunction collapsed). You now pick a filter sequence. If you pick + when the Belgians picked +, than yay, the Belgians know which polarization you and I have, and if I picked the + filter too, they have that element of our 1 time pad (if I pick X, they know what polarization you have, but we're throwing that photon out since I don't know due to my inconsistent choice of filter). If you pick + and they pick X, then your measurement of polarization has nothing to do with the result they get. They would know my result, but you and I disagree.

So, to make sure no one is intercepting our messages, in addition to sharing the string of filter choices, we can also share the first several results from the measurements in which we both picked the same filter. If you and I both pick the + filter, but the Belgians chose X instead, then 50% of the time, your measurement of -- or | will not be the compliment of my measurement of | or --. This will mean sometimes your result will not match my expectations, indicating the presence of a man in the middle. We can share however many bits we want (which will then not be used in the construction of the 1-time pad), and the more we share, the higher the statistical confidence will be that we are or are not being overheard. I think every bit in agreement between us could have happened 3/4 of the time if there was someone overhearing us: 50% chance of them using the right filter, if they use the wrong filter, then a 50% chance that your now-uncorrelated measurement matches my expectation by pure chance. So the probability that we are being overheard and didn't know it goes like (3/4)^n for n shared bits. So after 16 shared bits there's only a 1% chance we're being overheard without realizing it.

How does this work for the spin example? Well, we do exactly what Mrs Bailard suggested: it turns out that spins along one axis (say the z) axis, can be written as an equal combination of spins along a different axis (either the x or y), so we can play exactly the same game as with the photons. You and I split pairs of entangled spins, each pick random directions to measure their spins, then share through open channels the axes we chose to measure the spins along, then throw out the bits where we didn't chose the same direction. Now, if someone swiped my box o bits, made their measurements, we can determine that this happened by sharing the first n spin measurements after we throw out those spins where we made incompatible measurements.

Hope that helps, but I'm going to guess this just raises more questions.

*the technical term is quantum bits, or qubits, which I am pleased to say is a word invented by my undergraduate mentor.
posted by physicsmatt at 12:49 PM on May 31 [1 favorite]


So...I was looking at my "Recent Activity" page and saw physicsmatt's comment above which, because of its typically generous length was cut off half way through and ended on this line:
So I have photons that are entangled in such a way...
which I could not help but read in the voice of a Jewish grandmother "Oy! I have photons that are entangle in SUCH a way! You wouldn't beLIEVE! But I mustn't complain..."
posted by yoink at 1:27 PM on May 31 [2 favorites]


So what I'm hearing from people here is that I need to work on my laconicism.
posted by physicsmatt at 1:34 PM on May 31


So what I'm hearing from people here is that I need to work on my laconicism.

More physicsmatt is better physicsmatt.
posted by yoink at 2:23 PM on May 31 [4 favorites]


A question about relativity. Was Einstein's insight that there must be a maximum possible speed and he guessed it was probably the speed of light, or was there something about light in particular that made him realize nothing could go faster than light?
posted by straight at 6:20 PM on May 31


If I remember correctly then it was the experimental search for the "luminiferous aether", the assumed stationary medium through which light was thought to propagate, which yielded data suggesting that speed of light must be invariant and independent of the relative motion of the observer. This was dismissed as impossible by most but Einstein decided to accept the results at face value and proceeded from there.
posted by Hairy Lobster at 9:31 PM on May 31


The aberration of light as studied by James Bradley in 1729, the Fizeau experiment, and the Michelson-Morley experiments all seemed to show that light did not depend on the medium it was traveling through.

I think Lorentz and Poincare also contemplated the constancy of the speed of light and simultaneity. The maximum possible speed idea may come from Poincare's observation that x^2+ y^2+ z^2- c^2t^2 is invariant as discussed here.
posted by Golden Eternity at 12:17 AM on June 1


It was known from the Michelson-Morley experiments that the speed of light was measured to be the same in all reference frames, so as pretty much always, experiment was the driver of theory. Many physicists had been working on how to resolve this paradox within their understanding of physical laws. It was apparent that the solution required some sort of transformation in how lengths and times were measured in different frames; the relevant transformations had already been worked out before Einstein, which is why they are called Lorentz transformations and not Einstein transformations. Einstein's huge leap forward was to take these transformations as a statement about real, physical changes in lengths and time intervals, not some sort of mathematical trick, and finding a consistent theoretical framework in which to embed them (which all comes from the realization that everyone moving with constant speed must observe the same laws of physics, since they all are in inertial reference frames. This is actually just Galileo's observation updated to include the experimental fact that "the speed of light" is apparently a law of physics that everyone has to measure to be the same).

Also, if the particle of light - the photon - has some small, nonzero mass, then light would not travel at the maximum possible speed in the Universe: c ~ 3 x 10^8 m/s would not be "the speed of light." However, c would still be the cosmic speed limit, and relativity would remain unchanged. This would have consequences for electromagnetism, but not for the deeper structure of how spacetime works.

It turns out any massless particle always travels at c, in the Universe we live in, this includes gravitatons, photons, and gluons. In a universe with slightly different laws of physics, one or all of these particles could have non-zero mass, and so could not travel at c in any reference frame. So you could imagine a Universe where nothing was capable of moving at the cosmic speed limit (though I think you'd run into problems with a massive graviton, not sure though), though the existence of the speed limit would still have important effects.

The experimental limit on the photon mass is that it is < 10^-18 eV, or < 2 x 10^-54 kg. If you imagine that photons had mass near the maximum allowed by this limit, the Michelson-Morley experiments would not have been sensitive enough to detect a variation in the measured speed of light in different reference frames. The mass limit actually comes not from direct measurements of photon speeds, but by careful measurements of long range electric and magnetic fields. Adding a photon mass makes electric fields drop not as 1/r^2, but as 1/r^2 * exp(-m*r), where m is the mass of the photon, which would modify Coulomb's law in electrical conductors. The 10^-18 eV limit comes from the existence of Milky Way-sized magnetic fields, which would be impossible if the photon had a non-zero mass larger than the quoted limit.

For comparison, neutrinos, which were originally thought to be massless (and thus traveling at c) until this was conclusively demonstrated not to be the case in 1998 (though theorists had postulated that they had mass in the 1960's), have masses of less than about an eV (technically, it is still possible for the lightest neutrino to still be massless. There are three, and at least two of them have non-zero masses).

I intended this comment to be short. I swear.
posted by physicsmatt at 7:40 AM on June 1 [3 favorites]


Thanks, Hairy Lobster, Golden Eternity, and physicsmatt.
posted by straight at 3:41 PM on June 2


For the record, the FTL comms I described in The Metamorphosis of Prime Intellect are not based on entanglement. In that story the Universe is a computer and the simulation has a bug, which results in both the Correlation Effect as observed by Dr. Stebbins and his team and the larger hack which Prime Intellect uses to gain full control.

I did claim an entanglement-based comm system for Revelation Passage but it was in the last of five stories, and I made them wait over a hundred million years before letting them have it so we can assume there were difficulties.

The basic philosophical problem with these experiments is that the Universe does indeed "protect" us from FTL communication but is very obviously more than willing to use some kind of FTL communication for its own purposes, assuming the state vector of the entangled particles truly remains potentially random until it is read. While the "communication" may not be accomplished by the usual vectors like bosons there has to be some kind of instantaneous connection between the two widely separated particles in order for the results to align.

This is far from the only thing like that in QM. The single-photon double-slit interference experiment is just as freaky, and much simpler.

My problem with these things is that they strongly resemble the behavior of a system which has been designed to give a certain result but whose workings are being observed at a scale which was not intended by design, and at which they don't make a damn bit of sense. We have come so far from the comfortable analogy of particles as simply interacting billiard balls that it is almost impossible for professionals to talk to lay people about how the Universe works. The system gives a result, we are told, and it does so consistently; the math does not lie. But first you tell us there's no such thing as magic, and then you tell us how these quantum results arise, and being relative simpletons we are all like that's magic, dudes. The particles have a magic FTL phone to coordinate their result, why can't we have one?

If the Universe is a simulation, it obviously wants very strongly to represent itself otherwise, even though the simulation might not have been intended to be observed in such detail; it's as if you somehow brought calipers and a microscope into the halls of a video game like Doom and started asking embarrassing questions about the wall textures, and making up elaborate mathematical theories about why they repeat in certain ways, when the real answer is that the whole thing is something very different than it appears.
posted by localroger at 4:55 PM on June 2 [2 favorites]


I think what you're talking about, localroger, is just the fact that the universe isn't fractal. The stuff at small scales like photons and electrons is mostly unlike anything we have any direct experience of. And yet, try as we might, we can't help but think they must be something like little balls and get freaked out when they do stuff no ball could do.

But they're not balls. They are something else that we've never seen before, and it's not surprising that our intuitions about what is and isn't possible (even what is and isn't weird) are unreliable when applied to something so far outside our experience.
posted by straight at 5:16 PM on June 2


Well straight the thing is I have seen things that act like the universe's not-ball particles before. I've been working with computers for nearly 40 years and they look to me exactly like the kind of thing you get when you're trying to make a digital approximation of an information-dense analog phenomenon.

The impressive thing about theoretical improvements through the early 20th century is that each advance gave us a simpler and more elegant way to describe what had previously been unknown or arbitrary. The planets follow simple ellipses, the ellipses are the result of a simple gravitational formula, a few equations give rise to all the splendors of the electramognetic spectrum.

And then with the observation of quantum effects it all falls apart. Every theory is more complicated than the last in order to cover all the known observations. We have these effects that seem to require instantaneous connection across large distances in order for the math to work, even though the same math denies us such connections in any useful mode. The amount of information represented by a particle goes up instead of down, as all kinds of weird states need to be provided for.

Here's a kick in the pants: What causes the state vector to collapse? In other words, what is an "observation?" Nobody knows, right? Well suppose decoherence is forced when the information processing load of maintaining coherence passes a certain threshold?

That would be a bit of a kick in the balls to the folks who want to build quantum computers. It would also suggest that the universe is made up of more than just passive equations, whether those equations are describing little balls or something more exotic.
posted by localroger at 5:55 PM on June 2 [1 favorite]


Quantum teleportation is just the latest mind-blowing innovation made possible by breakthroughs in subatomic physics
posted by homunculus at 7:29 PM on June 7


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