
Physicists Create Quantum Link Between Photons That Don't Exist at the Same Time - rosser
http://www.wired.com/wiredscience/2013/05/quantum-linked-photons/
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iandanforth
Does this work only in one direction? Is there any reason to see the first
measurement as causal? Could the second measurement be interpreted as causal
that then propagates backward in time?

I'm really trying to get at if the linear description of events in the article
is just a convenient simplification, or if physicists thinks that there is
time order here.

~~~
Steuard
My best understanding is that in general, issues related purely to quantum
entanglement are entirely unrelated to causality. As noted in the article,
even the "spooky action at a distance" that made Einstein so uncomfortable
cannot be used to transmit information faster than light. I haven't checked
the math, but I assume that the same sort of properties mean that this system
couldn't be used to transmit information into the past.

So issues of causality don't really arise: this is just a neat demonstration
that the weird correlations implied by quantum mechanics can be separated in a
"direction" that hadn't been tested before. (That is, there's definitely time
passing in this system and there is definitely entanglement between objects
existing at different times, but there's also no real tension between those
facts.)

~~~
drostie
Let me be a little more precise here. What you basically need to know here is
that entanglement describes a _correlation between measurements_ , and you
cannot know whether that correlation exists until you collect those
measurements together. This is the fundamental reason why it does not
"transmit information" faster than light in a certain sense. However, that
phrasing is misleading because you can use it to do things which would
otherwise require transmitting information faster than light.

I like to illustrate this by a game based on "GHZ states" which I call
"Betrayal." The idea is that there are 3 people working together, but I'm
going to make one of them work at cross-purposes to the other two. If the team
can recover gracefully from my mischief with high probability, then they all
win a big cash prize.

The game is simple: the three people can prepare however they want in advance,
but then they must go into different (relativistically-separated) rooms, look
at a screen with words on it, and hit either a button labelled 0 or a button
labeled 1. Then I collect these three numbers and sum them up, to get The Sum.
So if Alice hits 1, and Bob hits 0, and Carol hits 1, then the Sum is 2.

On the screens in the rooms, I give them a task. Sometimes I do a "control"
experiment: I tell all three of them "Make the sum even," and the team wins if
it's even. Sometimes I create a _traitor_ : I tell two of them, "make the sum
odd", and one of them "make the sum even", and the team wins if it's odd.

Three classical players cannot beat this game 100% of the time, no matter how
they prepare in advance. Three quantum players (i.e., three players sharing an
entangled Greenberger-Horne–Zeilinger state) can. So if we repeat the game
enough times, they can convince me that they can beat the game with higher
probability than the classical limit, and thus win the big cash prize.

~~~
tmzt
What makes the quantum players able to beat the limitation of the classical
players with multiple trials?

Every time I read an article discussing quantum mechanics, particularly new
results in the field, I get more and more of the feeling that we are just
missing something. According to the article entanglement can occur on the
scale of lightyears but those entanglements cannot be used to transmit
information faster than the speed of light.

The linked article on Schrodinger's Hat seems to be violate another rule about
observation, but there's always this caveat that prevents it from violating
some quantum principal.

~~~
drostie
It's just that they have access to an operation which classically doesn't
exist, because their probabilities are complex numbers rather than real
numbers. (Just as importantly, there are known limits to how great their
correlation can be; the nice thing about Betrayal is that you can quickly
prove that six classical random variables don't work no matter how they're
jointly distributed.)

So what is this strange operation? There exist two nice "superposition over
all states" quantum states for the three bits held by the three players:

    
    
        +++ = 000 + 001 + 010 + 011 + ... + 111
        −−− = 000 − 001 − 010 + 011 + ... − 111
    

Separately those states are not entangled: that is, +++ is made from the
separable (0 + 1)(0 + 1)(0 + 1) while −−− is made from the separable (0 − 1)(0
− 1)(0 − 1). In both "pure" states any bit pattern from 000 to 111 has equal
probability. Quantum mechanics now lets these observers have the superposition
state:

    
    
        (+++) + (−−−) = 000 + 011 + 101 + 110
    

This is an entangled state. In this state you cannot be sure which of these
four will occur, but they will each occur with even probability and the sum
will be even. So that's the "control" experiment covered. But we could solve
the "control" experiment with the 000 state too. What about the "traitor"
experiment?

Here's where you need the complex numbers. Each of the "make the sum odd"
people maps (+),(−) → (+),i(−). This is called a phase rotation, and you might
know i² = -1 in the complex plane. These separate acts shift the global state
to:

    
    
        (+++) + (−−−) → (+++) + i²(−−−) = (+++) − (−−−)
    

If you work it out you will find:

    
    
        (+++) − (−−−) = 001 + 010 + 100 + 111
    

So even though _locally_ nobody can tell what's happened (every single person
still has a 50/50 chance of seeing 0 or 1 by themselves), the _global_ sum
changes due to this phase rotation. That is what entanglement can get you,
large-scale correlations.

As for proving that classical probabilities cannot do this, take six random
variables no matter their joint distribution, call them Ao, Ae, Bo, Be, and
Co, Ce -- what Alice, Bob, and Carol do when they're told to make the sum odd
or even, respectively. The problem asks to make Ao + Bo + Ce ≡ Ao + Be + Co ≡
Ae + Bo + Co ≡ 1 (mod 2) while Ae + Be + Ce ≡ 0 (mod 2). Adding those four
equations together gives 2 * (Ao + Bo + Co + Ae + Be + Ce) ≡ 3 (mod 2), but 3
isn't even. So it's not possible to satisfy all four equations all of the time
with classical probabilities.

------
Schiphol
If one abandons the idea that there is a privileged 'now' (what philosophers
call the A-theory of time, which is already difficult to make compatible with
special relativity) this effect is not weirder than simultaneous entanglement
-- admittedly, this is already sufficiently weird.

(Most physicists, and most philosophers, believe that the right theory of time
is a B-theory of time: once every event is lain down in a four-dimensional
spacetime diagram, there is nothing else to describe; no facts, in particular,
about which of these different times is really _now_ , the time at which the
whole universe is.)

------
dxhdr
At the risk of being "that guy," a few thoughts popped into my head after
reading this article. I couldn't help but reflect on the possibility of us
just completely misunderstanding how the universe works. It wouldn't be the
first time in history. What if:

Our concept of time and matter is wrong, or our idea of quantum physics is
interesting and useful, but ultimately wrong.

Perhaps I'm just a simpleton but part of me really wants to believe we're
missing something fundamental that would make all of this so much easier to
accept and understand.

~~~
Xcelerate
At a mathematical level, it's pretty elegant how quantum mechanics works. At
the level of the article, not so much.

Once you _get_ the math (and it really kind of clicks all at once), it's not
too hard. The problem is that the math has a lot of historical baggage and the
ways of translating equations into prose are not the most effective I think.

The whole "uncertainty" thing bothers me. I think there's better ways of
expressing this concept to the public.

I am, however, bothered by the fact that QM is not deterministic. Most
experiments have ruled out loopholes for a deterministic universe, but it
still just weirds me out that randomness is inherent to the measurement of
observables.

~~~
tedsanders
There are certainly interpretations of QM that are deterministic. See many
worlds, for example.

~~~
andrewflnr
Personally, I've never found that a comforting alternative.

~~~
scotty79
It just because people tell about it as if all the possible divergent
trajectories through phase space happen not just ours. What they actually mean
is that the possibilities are vast and we have no idea why reality chose
exactly this path because it's not in any way priviledged.

It's like a ball a top of the totally symmetric hill. If you consider phase
space it contains ball rolling down in all the possible directions but noone
claims that ball rolls in all of them in alternate universes. It rolls down in
exactly one way though we might never have any idea why it chose this one.

~~~
andrewflnr
That sounds exactly like non-determinism, which is what we were trying to
avoid.

------
TheEzEzz
I'm a little baffled when articles come out claiming some weird relativity-
breaking quantum effect and fail to mention that the no-collapse
interpretation of quantum mechanics explains the effect easily without
breaking relativity.

~~~
hiker
What's the easy no-collapse interpretation explanation?

~~~
oofabz
There are a few, the most realistic is the Bohm interpretation:
<http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory>

The basic idea is that observed decoherence is not caused by wave collapse,
but is caused by interaction with noise. For example, consider the double-slit
experiment. If you allow light to pass through two slits, it forms an
interference pattern. If you put a detector in one of the slits, the pattern
collapses.

The Copenhagen interpretation says this is because the observation collapses
the waveform of the photon. The Bohm interpretation says that the photon
interacted with your detector and its phase was knocked out of sync.

------
ivan_ah
original article is here: <http://arxiv.org/abs/1209.4191>

------
krcz
Could they possibly create 1-2 and 3-4 entangled pairs at the same time,
measure 1 & 4 and then use this "projective measurement" trick to entangle 3
and 4? If so, could this affect the measurements of 1 and 4 in the past?

~~~
pep-ok
yes, i think, if you mean 2 and 3. you have 1-2 and 3-4, then you entangle 1-4
which I think should lead to 2-3 entanglement without the 2-3 couple ever
interacting at all.

I think it is called entanglement teleportation.

~~~
krcz
Yeah, I've meant entangling 2 & 3 pair and measuring 1 & 4\. But my concern is
if we can still use this trick _after_ measuring 1 & 4?

~~~
pep-ok
I am not sure I understand you.

~~~
krcz
I hope that can make things clear: <http://imgur.com/5mfB4m0> .

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jmanamj
The question now is, how to use this to mine Bitcoins?

------
calhoun137
I love how at the end of all these physics articles there is always some guy
who is like "O HELL YEA WE CAN USE THIS FOR QUANTUM COMPUTERS!"

~~~
femto
Those are probably the magic words that make the state security apparatus
throw wads of cash at you.

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ywang0414
Let's use this to keep a client in sync with the server. And also keep
multiple instances of server in synch with each other.

