26 ms·

I’ve been working on quantum computing and quantum communication for 15 years now and what I really want people to know is that entanglement is “beyond classica

by gaze 3y ago

I’ve been working on quantum computing and quantum communication for 15 years now and what I really want people to know is that entanglement is “beyond classical correlation.” Correlation which is not beyond classical is shaking up a shoebox with a pair of gloves in it, and having two people take the gloves far away from each other to then observe what hand they got. They then understand the hand the other has. There’s one bit of information here corresponding to which hand went to person A. Entanglement is just this but “more.” You can’t communicate with a pair of gloves. Person A does not know when person B has realized person A knows what glove they have. Just the same, it does not matter who matters which of the two particles in a Bell pair first! Many quantum information theorists don’t even believe the wave function is “real” but just a mathematical tool for making predictions about measurement outcomes. You should come at entanglement from this angle, because the main difference between the bell test and the pair of gloves is the state of the particles is undetermined before measurement, and the chosen measurements will result in different correlations between the measurement outcomes.

petermcneeley 3y ago
"it does not matter who measures"
MPSimmons 3y ago
In my mind, I see it like you've got a line bisecting another line. You don't know what the angles are until you measure one, and once you measure it, you know for certain what the other angle is, whether that person does or not. But you can't change the angle and you can't send information using them.
- Jensson 3y ago
  Except the line is spinning around faster than you can see and you measuring it makes it stop. Your example makes it sound like the angle was decided before you measured.
d0mine 3y ago
Feynman explains quantum correlation using boxes with 3 buttons https://youtube.com/watch?v=ZcpwnozMh2U&t=17m55s https://youtube.com/watch?v=ZcpwnozMh2U&t=17m55s
bilsbie 3y ago
Thanks. So how is it not analogous to the glove example?
- trimethylpurine 3y ago
  It's a trick question. Gloves go in a glove box.
- notfed 3y ago
  Yeah really, they didn't give any example of the "more"...
- Jensson 3y ago
  The gloves doesn't change their state when you check on them. Quantum particles do. Example: Lets say you check if the glove is white or black, you see it is white. Then you check if the glove is half white/half black, or half black/half white, you see it is half white/half black. Then you check again if it is white or black, now it is black. That is how quantum gloves would work, but real gloves doesn't work that way.
  - trimethylpurine 3y ago
    >Then you check again You only get to check once, I think. The rest doesn't matter, supposedly.
    - Jensson 3y ago
      That is false, it changes every time you check along another axis. If you make the same check it doesn't change, but if you change what you check then you can update the particle as my example shows. Changing a particle like this by making repeated measurements is a standard example in undergrad quantum mechanics. Edit: There is always a part of the particles state that you can't know, you know Heisenberg's indeterminacy principle, so if you measure position you now makes velocity undetermined, and then if you measure velocity now you make position undetermined, and then if you go back and try to measure position the particle will be in a new random spot.
ynniv 3y ago
My understanding is that it's like two of the same scratch-off lottery ticket. There are two spots you can scratch, only one has a prize, we don't know which one that is but it's same for both tickets. The tickets are taken to separate rooms. The people in each room then pick a spot and scratch it. How likely is it that at least one of them will win? The odds of one scratch winning are of course 50%, and the odds of the other person winning are of course also 50%. The odds of both people winning are 25%, and both people losing also 25%. The CHSH experiment suggests otherwise: that when one person loses the other person is less likely to also lose, such that the likelihood of both losing is only 15%. How can this happen? I haven't a clue. I'm not a theoretical physicist, and I haven't personally conducted this (~ $5,000) experiment. But it's a result that deeply bothers me. If you build this system you can then scratch a whole lot of tickets. Each ticket has a 50% chance of a side having the prize, but as long as both parties are consistently scratching the same side of entangled pairs they will win more often than they lose, revealing whether or not they are in agreement. I have no idea how this could be true, but people keep running the experiment and keep getting similar results. And, it seems like this could result in faster than light communication, so there's probably a better ELI-5 out there.
- amluto 3y ago
  > The CHSH experiment suggests otherwise: that when one person loses the other person is less likely to also lose, such that the likelihood of both losing is only 15%. How can this happen? I haven't a clue. That’s easy, even without quantum mechanics. All you need is to have a different distribution of lottery tickets. Print tickets such that, if one wins, the other is more likely to win, which you can do by having the number of winning spots per ticket be random and correlated. For example, there could be a 50% chance that neither ticket has a winning spot and a 50% chance that each ticket has one winning spot. But you could imagine a pair of tickets with internal radios, such that, when you scratch one, it tells the other one, and together they simulate a general function where the joint probability of (win on ticket 1, win on ticket 2). If you set up the probabilities appropriately, then you can’t use the tickets to send a signal, and the result is referred to in the literature as “non-signaling boxes” (the magic tickets are the boxes). Quantum mechanical entanglement can do something like this, except that the probability distributions you can generate with entanglement are a subset of the more general non-signaling boxes.
superposeur 3y ago
> Many quantum information theorists don’t even believe the wave function is “real” but just a mathematical tool for making predictions about measurement outcomes You are correct that many say this, but this is a constant source of frustration for me (a physicist who does believe the wave function is the only real thing). These physicists never seem to articulate what then is supposed to actually be “real” (under their definition) or what laws govern the “real” things as opposed to the wave function. Implicit seems to be that there is some kind of separate classical realm that obeys non quantum mechanical laws. Is this separate classical realm to be understood as a macroscopic limit of the quantum realm? But if so then the whole picture is circular since wavefunctions are supposed to be mere bookkeeping devices for classical things. Or to put this more succinctly, if the wave function is a mere bookkeeping device, then bookkeeping for __what__? (I should mention that yes I know about QBism and all that and my confusion is not for lack of talking to QBists about such things — I just can’t make heads or tails of what they tell me!)
- ranguna 3y ago
  > bookkeeping for __what__? Bookiping for things that happen so fast and at such a small scale that our current technological tools cannot fully capture data with enough precision and accuracy for us to make an good model of the underlying behaviour. Imagine you put a spinning ball on top of a frozen lake on a windy day. You are going to observe the ball spin continously towards the same orientation until several gusts of wind make it spin the other way around, and this goes on and on for hours. From the measurements of the wind and current spin of the ball, you can make a model that accurately and precisely predicts the spin of the ball on the almost frictionless frozen lake. Now imagine your spinning ball is extremely small you can't even see it or any "wind" with any tools that you currently have, but you can still measure its spin to a certain precision. Now imagine this "wind" is so strong and volatile that the tools you have sometimes takes a somewhat accurate physical snapshot of it and sometimes it just misses the gust. You take a look at the somewhat accurate measured spin and it seems to have changed without any reason, but it was just because your tooling is not accurate and precise enough to capture all the quantum gusts of wind that influence the quantum ball, so it looks like the ball is changing its spin randomly, whist in reality, we just can't precisely measure whatever is influencing the ball's spin with our current tooling. The influences are there, it's just that our tooling can't capture them precisely and accurately enough. To combat that, we continously measure the ball's spin and we are able to figure out a pattern, not a precise one (because again, our tooling is not precise and accurate enough), but a pattern based on probability of the ball being in a specific spin state, we can even combine this with our inaccurate measurements of the quantum wind and further improve the accuracy of the probability. But never to a precise pattern, because our tooling sometimes misses certain wind states that it looks like the ball chagend spin randomly. If we had tools that precisely and accurately measured the ball's spin and the quantum wind, we would be able to build a precise and accurate model of the spin based on those measurements. But we can't, although, we still want to make science around these inaccurate measurements, and probability based patterns are just enough for the science we want to make. The wave function is just the result of our lack of precise and accurate measuring tools and measuring methods at this quantum scale. And for now, it's good enough for the science we want to do.
Strilanc 3y ago
I would describe classical correlations as downgraded entanglement. Correlation is what's left when entanglement decoheres / undergoes uncontrolled phase noise. Things you should be able to do, like win the Mermin-Peres magic square game 100% of the time, aren't possible when the entangled qubits you'd use to do it aren't protected from phase noise. Decoherence is sort of analogous to air. It's so ubiquitous in your life that you don't really think about it, but you'd notice immediately if it was removed. Being steeped in air your whole life has twisted your physical intuitions. That's why Aristotle thought "objects come to rest" when actually objects move at constant speed unless acted upon by a force. Similarly, being steeped in decoherence has twisted your physical intuitions. Like thinking "adding more ways for something to happen must make it more likely" or "a particle's position is independent of its momentum" or "I can measure an object without affecting it". But actually different paths can interfere, and momentum is the Fourier transform of position, and measurements apply phase noise. Decoherence is so ubiquitous that it's a huge challenge to engineer systems that suppress it. This is why quantum computers are so hard to make, and why quantum error correction has so much more overhead compared to classical error correction. Classical error correction only has to fix bit flips, and it can do so by making phase flips worse (which it does). Quantum error correction has to simultaneously fix bit flips and phase flips. When two particles are entangled, rotating one around the X axis by an angle A and the other by an angle B and then measuring produces measurement results that agree with probability cos^2(A-B). Same as the probability of a photon with polarization angle A passing through a polarizing filter with angle B. Decohere the entanglement before the rotations, and the measurements will instead agree with probability cos^2(A)cos^2(B) + sin^2(A)sin^2(B). Note that cos^2(A-B) = cos^2(A)cos^2(B) + sin^2(A) sin^2(B) + 2 cos(A) cos(B) sin(A) sin(B), meaning decoherence is taking away the 2 cos(A) cos(B) sin(A) sin(B) interference term. That's the downgrade. That's what makes your best possible CHSH win rate drops from 85% to 75%. If entanglement allowed sending messages, the initial CHSH win rate would be 100% instead of 85%.

by gaze 3y ago

petermcneeley 3y ago

"it does not matter who measures"

MPSimmons 3y ago

In my mind, I see it like you've got a line bisecting another line. You don't know what the angles are until you measure one, and once you measure it, you know for certain what the other angle is, whether that person does or not. But you can't change the angle and you can't send information using them.

Jensson 3y ago
Except the line is spinning around faster than you can see and you measuring it makes it stop. Your example makes it sound like the angle was decided before you measured.

d0mine 3y ago

Feynman explains quantum correlation using boxes with 3 buttons https://youtube.com/watch?v=ZcpwnozMh2U&t=17m55s https://youtube.com/watch?v=ZcpwnozMh2U&t=17m55s

bilsbie 3y ago

Thanks. So how is it not analogous to the glove example?

trimethylpurine 3y ago
It's a trick question. Gloves go in a glove box.
notfed 3y ago
Yeah really, they didn't give any example of the "more"...
Jensson 3y ago
The gloves doesn't change their state when you check on them. Quantum particles do. Example: Lets say you check if the glove is white or black, you see it is white. Then you check if the glove is half white/half black, or half black/half white, you see it is half white/half black. Then you check again if it is white or black, now it is black. That is how quantum gloves would work, but real gloves doesn't work that way.
- trimethylpurine 3y ago
  >Then you check again You only get to check once, I think. The rest doesn't matter, supposedly.
  - Jensson 3y ago
    That is false, it changes every time you check along another axis. If you make the same check it doesn't change, but if you change what you check then you can update the particle as my example shows. Changing a particle like this by making repeated measurements is a standard example in undergrad quantum mechanics. Edit: There is always a part of the particles state that you can't know, you know Heisenberg's indeterminacy principle, so if you measure position you now makes velocity undetermined, and then if you measure velocity now you make position undetermined, and then if you go back and try to measure position the particle will be in a new random spot.

ynniv 3y ago

My understanding is that it's like two of the same scratch-off lottery ticket. There are two spots you can scratch, only one has a prize, we don't know which one that is but it's same for both tickets. The tickets are taken to separate rooms. The people in each room then pick a spot and scratch it. How likely is it that at least one of them will win? The odds of one scratch winning are of course 50%, and the odds of the other person winning are of course also 50%. The odds of both people winning are 25%, and both people losing also 25%. The CHSH experiment suggests otherwise: that when one person loses the other person is less likely to also lose, such that the likelihood of both losing is only 15%. How can this happen? I haven't a clue. I'm not a theoretical physicist, and I haven't personally conducted this (~ $5,000) experiment. But it's a result that deeply bothers me. If you build this system you can then scratch a whole lot of tickets. Each ticket has a 50% chance of a side having the prize, but as long as both parties are consistently scratching the same side of entangled pairs they will win more often than they lose, revealing whether or not they are in agreement. I have no idea how this could be true, but people keep running the experiment and keep getting similar results. And, it seems like this could result in faster than light communication, so there's probably a better ELI-5 out there.

amluto 3y ago
> The CHSH experiment suggests otherwise: that when one person loses the other person is less likely to also lose, such that the likelihood of both losing is only 15%. How can this happen? I haven't a clue. That’s easy, even without quantum mechanics. All you need is to have a different distribution of lottery tickets. Print tickets such that, if one wins, the other is more likely to win, which you can do by having the number of winning spots per ticket be random and correlated. For example, there could be a 50% chance that neither ticket has a winning spot and a 50% chance that each ticket has one winning spot. But you could imagine a pair of tickets with internal radios, such that, when you scratch one, it tells the other one, and together they simulate a general function where the joint probability of (win on ticket 1, win on ticket 2). If you set up the probabilities appropriately, then you can’t use the tickets to send a signal, and the result is referred to in the literature as “non-signaling boxes” (the magic tickets are the boxes). Quantum mechanical entanglement can do something like this, except that the probability distributions you can generate with entanglement are a subset of the more general non-signaling boxes.

superposeur 3y ago

> Many quantum information theorists don’t even believe the wave function is “real” but just a mathematical tool for making predictions about measurement outcomes You are correct that many say this, but this is a constant source of frustration for me (a physicist who does believe the wave function is the only real thing). These physicists never seem to articulate what then is supposed to actually be “real” (under their definition) or what laws govern the “real” things as opposed to the wave function. Implicit seems to be that there is some kind of separate classical realm that obeys non quantum mechanical laws. Is this separate classical realm to be understood as a macroscopic limit of the quantum realm? But if so then the whole picture is circular since wavefunctions are supposed to be mere bookkeeping devices for classical things. Or to put this more succinctly, if the wave function is a mere bookkeeping device, then bookkeeping for __what__? (I should mention that yes I know about QBism and all that and my confusion is not for lack of talking to QBists about such things — I just can’t make heads or tails of what they tell me!)

ranguna 3y ago
> bookkeeping for __what__? Bookiping for things that happen so fast and at such a small scale that our current technological tools cannot fully capture data with enough precision and accuracy for us to make an good model of the underlying behaviour. Imagine you put a spinning ball on top of a frozen lake on a windy day. You are going to observe the ball spin continously towards the same orientation until several gusts of wind make it spin the other way around, and this goes on and on for hours. From the measurements of the wind and current spin of the ball, you can make a model that accurately and precisely predicts the spin of the ball on the almost frictionless frozen lake. Now imagine your spinning ball is extremely small you can't even see it or any "wind" with any tools that you currently have, but you can still measure its spin to a certain precision. Now imagine this "wind" is so strong and volatile that the tools you have sometimes takes a somewhat accurate physical snapshot of it and sometimes it just misses the gust. You take a look at the somewhat accurate measured spin and it seems to have changed without any reason, but it was just because your tooling is not accurate and precise enough to capture all the quantum gusts of wind that influence the quantum ball, so it looks like the ball is changing its spin randomly, whist in reality, we just can't precisely measure whatever is influencing the ball's spin with our current tooling. The influences are there, it's just that our tooling can't capture them precisely and accurately enough. To combat that, we continously measure the ball's spin and we are able to figure out a pattern, not a precise one (because again, our tooling is not precise and accurate enough), but a pattern based on probability of the ball being in a specific spin state, we can even combine this with our inaccurate measurements of the quantum wind and further improve the accuracy of the probability. But never to a precise pattern, because our tooling sometimes misses certain wind states that it looks like the ball chagend spin randomly. If we had tools that precisely and accurately measured the ball's spin and the quantum wind, we would be able to build a precise and accurate model of the spin based on those measurements. But we can't, although, we still want to make science around these inaccurate measurements, and probability based patterns are just enough for the science we want to make. The wave function is just the result of our lack of precise and accurate measuring tools and measuring methods at this quantum scale. And for now, it's good enough for the science we want to do.

Strilanc 3y ago

I would describe classical correlations as downgraded entanglement. Correlation is what's left when entanglement decoheres / undergoes uncontrolled phase noise. Things you should be able to do, like win the Mermin-Peres magic square game 100% of the time, aren't possible when the entangled qubits you'd use to do it aren't protected from phase noise. Decoherence is sort of analogous to air. It's so ubiquitous in your life that you don't really think about it, but you'd notice immediately if it was removed. Being steeped in air your whole life has twisted your physical intuitions. That's why Aristotle thought "objects come to rest" when actually objects move at constant speed unless acted upon by a force. Similarly, being steeped in decoherence has twisted your physical intuitions. Like thinking "adding more ways for something to happen must make it more likely" or "a particle's position is independent of its momentum" or "I can measure an object without affecting it". But actually different paths can interfere, and momentum is the Fourier transform of position, and measurements apply phase noise. Decoherence is so ubiquitous that it's a huge challenge to engineer systems that suppress it. This is why quantum computers are so hard to make, and why quantum error correction has so much more overhead compared to classical error correction. Classical error correction only has to fix bit flips, and it can do so by making phase flips worse (which it does). Quantum error correction has to simultaneously fix bit flips and phase flips. When two particles are entangled, rotating one around the X axis by an angle A and the other by an angle B and then measuring produces measurement results that agree with probability cos^2(A-B). Same as the probability of a photon with polarization angle A passing through a polarizing filter with angle B. Decohere the entanglement before the rotations, and the measurements will instead agree with probability cos^2(A)cos^2(B) + sin^2(A)sin^2(B). Note that cos^2(A-B) = cos^2(A)cos^2(B) + sin^2(A) sin^2(B) + 2 cos(A) cos(B) sin(A) sin(B), meaning decoherence is taking away the 2 cos(A) cos(B) sin(A) sin(B) interference term. That's the downgrade. That's what makes your best possible CHSH win rate drops from 85% to 75%. If entanglement allowed sending messages, the initial CHSH win rate would be 100% instead of 85%.