

Researchers Advance 'Quantum Teleportation' - todayiscrown
http://www.jpl.nasa.gov/news/news.php?feature=4384

======
lisper
> When Alice measures the state of her photon, Bob's photon changes state as
> well.

NO NO NO! Ten thousand times no! Bob's photon does NOT change state as a
result of Alice's measurement. It is in exactly the same state that it was in
before: entangled, which is to say, if you consider it in isolation, it is in
a quantum mixed state. NOTHING changes on Bob's side as a result of Alice's
measurement.

It's bad enough that the popular press chronically gets this wrong, but NASA
really ought to know better.

~~~
jessriedel
> Bob's photon does NOT change state as a result of Alice's measurement. It is
> in exactly the same state that it was in before: entangled, which is to say,
> if you consider it in isolation, it is in a quantum mixed state. NOTHING
> changes on Bob's side as a result of Alice's measurement.

As usual in these situations, it depends on being precise about what one is
referring to with words like "state". But with the common definition -- a
density matrix updated to incorporate all macroscopic data -- the state of A
certainly does change following the measurement of B. It goes from fully mixed
to pure.

Of course, Bob's personal epistemic state (which doesn't include data that he
hasn't yet received from Alive) doesn't change.

You can try to say that Alice and Bob's photons have merely become entangled
with the measuring apparatus, or some similar many-worlds-type statement, but
then you'd be using "state" in a very different way than the vast majority of
experimental physicists. You might think that definition better fitting with
reality, but it certainly wouldn't make the NASA press release wrong.

~~~
lisper
It's more than Bob's personal epistemic state that doesn't change. The
physical state of his photon doesn't change either. The change from "fully
mixed" to "pure" is a change in the mathematical representation of the
situation, not a change in Bob's local physical reality. There is no
experiment Bob can perform on his photon that will reveal to him whether or
not Alice has measured her photon or not (note that this remains true EVEN
AFTER he has received Alice's bits).

The difference between a mixed and a pure state is simply a difference in
mathematical perspective, not a difference in the underlying physical reality.

~~~
jessriedel
I'm well aware of the physics of the situation. It's my day job. I'm just
telling you that NASA hasn't said anything false. The common meaning of "the
state of the photon" is exactly as I described above, and it certainly does
change. Your discussion of "underlying reality" is besides my point, since
those words don't appear in the original article.

> There is no experiment Bob can perform on his photon that will reveal to him
> whether or not Alice has measured her photon or not (note that this remains
> true EVEN AFTER he has received Alice's bits).

I'm not sure what you mean here, but a reasonable interpretation of your words
would be false. You consider a situation where Bob has received bits from
Alice even when she might not have actually made a measurement. If so then I
guess you're saying that Alice just makes these up? If so, then Bob can
certainly check to see if Alice has actually made the measurement she claims
to have by making a local measurement on his photon, and this can result in an
outcome that lets Bob definitively determine that Alice is lying to him. [Of
course, sometimes Bob's result agrees with Alice's claimed bits; in this case,
Alice has simply guessed correctly for what she will end up receiving in her
(now fully determined) experiment.]

~~~
gus_massa
Can I agree with you both?

After a few curses of quantum mechanics (with the Sakurai book) and working in
the field I still not sure if the press release is misleading or just wrong.

One of the important details is that Bob can't do any experiment to
distinguish the photon before Alice measurement and the state of the photon
after Alice measurement. If it's not possible for him to distinguish the
states, has it really changed?

On the other hand, if Bobs makes a measurement after Alice measurement, the
result should "agree" with the result Alice measurements. So, has something
changed?

I think that the press release it's completely misleading and perhaps wrong.

~~~
jessriedel
> I still not sure if the press release is misleading or just wrong

There is absolutely no doubt that if the reader applies an intuitive classical
interpretation of the term "state" to what physicists mean by a quantum state,
then the reader will end up with some wrong impressions (and some right ones).
The fact is that there are certain things that are going on that just don't
map well on to normal, classical language. And this is not the readers fault;
he's a normal, lay human being, and _really_ smart people have been fighting
over how to interpret the (unambiguous) experimental data for almost a century
now without reaching a consensus.

Now, it would probably be better if physicists used the term "qustate" to
refer to the state, and that they used this press releases, and that they said
stuff like "when Alice makes her measurement, the qustate of Bob changes
instantaneously". That way no one would go applying their classical intuition
about what a "state" ought to be to the density matrix. Instead, they would
ask "what's a 'qustate'?", and someone would say "well, it's sort of like a
classical state...but it's not the same...and it's sorta like an epistemic
probability distribution...but also not really...and the difference are subtle
and very profound and you have to be careful and it takes a good amount of
studying before you can understand." And that would be great and maybe a few
more people would go actually read about it. But as it stands physicists are
more than happy to allow laymen to misinterpret their terminology when it
leads to their work sounding more exciting and getting more funding. There is
definitely something of a Motte-and-Bailey going on. (
[http://slatestarcodex.com/2014/11/03/all-in-all-another-
bric...](http://slatestarcodex.com/2014/11/03/all-in-all-another-brick-in-the-
motte/) )

> On the other hand, if Bobs makes a measurement after Alice measurement, the
> result should "agree" with the result Alice measurements. So, has something
> changed?

Actually, the mere agreement of Alice and Bob's measurement doesn't really
give evidence that anything on Bob's side has changed. If I jumble a blue
marble and a red marble in an urn and then put them in separate brown bags and
give you one at random, then when you look in your bag that will instantly
determine what I'll get when I look in mine, but it didn't change anything
about my bag in the normal, physical sense.

The way in which quantum mechanics differs from this situation is subtle but
significant.

~~~
lisper
> physicists are more than happy to allow laymen to misinterpret their
> terminology when it leads to their work sounding more exciting and getting
> more funding

Yes, I think you hit the nail on the head here.

------
chm
I think this is the paper:
[http://arxiv.org/abs/1401.6958](http://arxiv.org/abs/1401.6958) .

------
EGreg
I've had this conversation on HN and I came to the conclusion there isn't much
semantic difference between two entangled photos and, say, two sides of a coin
split down the middle. Alice and Bob could have just as easily had two sides
of a coin they didn't look at, and the situation would be the same. As soon as
you look at what you have, you know what the other person has. If you don't
look, but a machine interacts with it, than the same situation arises.

The two entangled photons were once close to each other, just like the two
sides of the coin. So what's the difference here?

Looking at a photon you can't tell whether someone else has looked at the
entangled photon. Same with the sides of the coin.

[http://www.scientificamerican.com/article/quantum-
entangleme...](http://www.scientificamerican.com/article/quantum-entanglement-
creates-new-state-of-matter1/)

~~~
inclemnet
There's a big, very important difference - the quantum state can display non-
classical correlations beyond what the classical coin model can describe.

We can see this by considering two electrons in an entangled spin state - say,
one is spin up, and the other is spin down, but the entangled state means you
don't know which is which, only that Alice and Bob will get consistent answers
when they measure the spin in the vertical direction. At this point,
everything maps fine to the two-half-coins idea, all we know is that they have
opposite spins.

What's different in the quantum case is that Alice and Bob could instead
decide to measure the spin in the left/right direction. Following the rules of
quantum mechanics, a given spin up or down state has an undetermined spin
left/right state, so when you measure the spin in the left/right direction it
has exactly 50% chance of being each one.

If the original states were really just like the coin halves, this new
measurement would be simply uncorrelated between Alice and Bob - they'd start
with different states (up or down), but the left/right measurement would
destroy that information and they'd both get a random answer because the
individual elecrons end up with an individually random left/right spin
direction.

The reality is actually different; if we do the measurement maths on the
quantum state rather than assuming it's predetermined like the coins, it turns
out the left/right spin is _still entangled_. That means that when Alice and
Bob measure the spin in the left/right direction, they'll always find that one
of them gets left and the other gets right. This would not be possible if the
quantum states were predetermined like the coins.

So, the coins analogy is not a bad way to understand some of the basics of
what you expect, but it absolutely is not a fully accurate description of
what's going on. Maybe you already knew that, but I wanted to be clear that
there's very much more to entanglement, because this is the source of several
common misconceptions.

(Of course quantum teleportation is a further thing again, but the extra non-
classical mathematics of entanglement are still important, it's not just coin
halves.)

~~~
EGreg
Thanks for giving me more information! I am just learning about this stuff.
However, it seems to me that a classical explanation is enough here, as well.
Why would the measurement of Up/Down spin destroy information about Left-Right
spin in the classical case? It gives no information about left-right spin. In
a way, that's what happened in the link I shared.

Suppose Alice and Bob had unlimited pairs of marbles labeled "RU" "RD", "LU"
and "LD" paired up with their opposites respectively, e.g. RU with LD. Alice
would randomly select four marbles, one from each pair, and Bob would select
the corresponding opposites. Now they separate and go to their homes, not
knowing what the marbles are.

Alice checks the first letter on the marbles only, and sees that she has R*
and R*. She concludes that her spin is "LEFT" and that bob has the "RIGHT"
marbles. Now, she decides to check all the letters on her marbles and realizes
she has, say, two RT marbles. She knows that Bob has two LD marbles.

Can a spin be not 1 or -1? Could it be in between? That would be the case here
if Alice had RU and RD, and once she checks both letters, she knows what Bob
has as a result.

But in the classical case discovering first letter doesn't give you any
information about the other. Doesn't that capture everything?

~~~
inclemnet
Just to be initially clear, 'spin' is a bit more subtle in quantum mechanics
than just 'it's actually spinning', but it's an okay conceptual start.

> Why would the measurement of Up/Down spin destroy information about Left-
> Right spin in the classical case?

It wouldn't, if you like, which would make it another thing that would be
different in classical mechanics, though it's a little strained because it's
assuming a lot about quantum states being like classical states.

In quantum mechanics the different spin operators do not commute, which means
if you measure it in the vertical direction (and get e.g. up) then in the
left-right direction, you get each of left or right with 50% probability. But
then if you measure it in the up/down direction you don't get up again, you
get up/down each with 50% probability, the original state is irrelevant. This
is obviously different to your marbles, which just have letters that don't
change.

> Can a spin be not 1 or -1? Could it be in between?

No. When you measure the quantum state you get an eigenfunction, though the
state before collapse could be a superposition. Indeed, this is the case for
different directions - spin up is a superposition of equal parts spin left and
spin right in the horizontal spin basis, which is why you get each direction
equally if you measure it in that direction.

> Doesn't that capture everything?

You're still trying to encode everything in a hidden variables theory, where
the quantum states secretly know everything beforehand about what it will do,
but the observer can't access the information without performing specific
measurements. This immediately fails as above, because the quantum states
inherently don't have fixed up/down and left/right components and each
measurement in a different basis gives a random result.

You can try to fix things by adding more complex rules about the letters on
the marbles changing when you read them, but it turns out there are
fundamental limitations on what physical results any such theory can predict.
This is famously addressed by Bell's theorem (as in, seriously famously, this
is really important), which demonstrates specific limitations on what physical
results are allowed by such models, and experiments have shown that quanum
mechanics does breach these limitations. I'm not sure how to explain more
simply what's going on though.

(Strictly, we can allow hidden variables (though more complex than a
predetermined left/right and up/down as above) by breaking the speed of light
to let the separated states communicate instantly, but this brings up its own
problems.)

~~~
EGreg
I thought something like pilot wave theory gets around Bell's theorem. Also
since Bell's theorem makes an assumption of local realism and nothing-goes-
faster-than-light, how does the description of entanglement as "spooky action
at a distance" not violate those?

Sheesh, these are awfully handwavy questions for someone who was in a math ph
D program. I should really sit sown and learn quantum mechanics for a while
with the math. How long do you think it would take to reason intelligently
about it? Going by what Feynman said, maybe I can't.

~~~
inclemnet
> I thought something like pilot wave theory gets around Bell's theorem

Bell's theorem remains true, but pilot wave theory achieves the quantum
results by sacrificing locality instead of hidden variables.

> how does the description of entanglement as "spooky action at a distance"
> not violate those?

It's just a name, and an old one at that. I suppose if you ascribe it to non-
locality then it really does involve something FTL on some level, but even
with a standard locality-preserving, no hidden variables theory, no
_information_ travels faster than light so you don't actually hit a FTL
problem. The strange thing is that the quantum state appears to behave
consistently regardless of its spatial extent, which is weird.

> How long do you think it would take to reason intelligently about it?

I would have thought that someone in a PhD mathematics program would be well
equipped to understand the mathematics - maybe the hard part is finding a good
resource that takes things in a good order. I'm afraid I can't really
recommend anything, though Feynman's stuff is probably good.

