
Is Quantum Entanglement Real? - dctoedt
http://www.nytimes.com/2014/11/16/opinion/sunday/is-quantum-entanglement-real.html
======
Xcelerate
_All_ components of a quantum system are always entangled. It's not some
mysterious property that arises under unusual circumstances. In fact, I would
argue that quantum entanglement is _the_ defining feature of quantum mechanics
that distinguishes it from classical mechanics.

However, the way that most articles (and even physicists) use the term
"entanglement" is in a loose way. Within a good approximation, a multi-
component quantum state can be represented as a tensor product of single
component quantum states. When this isn't a good approximation anymore, you
can say that the entanglement of the system is much more apparent.

To give an example of what I mean, there's a technique in quantum chemistry
known as density functional theory (DFT), which is used to compute ground
state energies of various molecules. For some molecules, it works pretty well.
The benefit of DFT is that it is a fast calculation technique (well, as far as
quantum chemistry goes), but it's speed comes at a price. Rather than using
the Coulombic interaction of every pair of electrons to compute the energy of
a molecule (or more technically, using every Coulombic interaction as terms in
the Hamiltonian), a probability cloud of electron positions is computed
instead, and this cloud is used as the energetic term.

This works really well for a lot of systems, but in some cases
(superconductors, metals, solid-state physics, van der Waals interactions),
this approximation falls apart because the effect of electron-electron
correlation significantly affects the energy of the system. In this case, the
full, non-separable wavefunction is required. The fact that this full
wavefunction cannot easily be broken into an approximation of simpler, single
electron wavefunctions means that the entanglement of the system is very
apparent.

~~~
gus_massa
I agree completely. Entanglement is everywhere.

To calculate the correct energy of the electrons in a molecule it's necessary
to use entanglement. But these molecular examples may be explained using some
weird classical models with local hidden variables, and with electrons that
interact with each other to conspire and provide the right result of the
measurement. The quantum mechanical explanation is actually more simple, but
not "intuitive".

Almost all the discussions about entanglement discuss the strange case of two
entangled particles that are far away. The distance between the particles is
only a trick to:

* be sure that one of them can't communicate to the other and tell the result of the measurement

* be sure that there is no a local hidden variable theory that explain the result

~~~
musername
why one hidden variable, when you could have one for each atom between the
entangled? :)

------
po
I have never been comfortable with the metaphors that tech reporters use for
describing quantum entanglement.

They always talk as if it's a spooky interaction but the metaphor falls down.
Here's an example: I take too halves of an Oreo cookie and send them in
opposite directions. Then I use a detector to measure how much filling is on
one half and instantly know how much was on the other half. Was that spooky?

I understand that at the physics level it's not like this, but the metaphors
are never satisfyingly 'spooky' to me as I can easily think up a classical
explanation.

~~~
kaoD
I feel I'll never get to understand entanglement (I hope it's like monads :P).
If it's not "spooky" but just half an Oreo... then what is so special about it
and why would physicist study it then?

~~~
tzs
Let's play an imaginary game. It will be you and a friend as a team against
the house, represented by me and my two assistants. Here's how the game works.

1\. You and your friend can consult with each other before the game starts and
agree on a strategy. You can build or obtain any equipment you think will help
you play the game.

2\. You and your friend, and any equipment you bring, meet me at the place
where the game starts. I have two spaceships waiting. You and one of my
assistants will get into spaceship A and go off into space. Your friend and my
other assistant will go into spaceship B and go off into space. You and your
friend can bring along whatever equipment you want.

3\. My assistants will take you to separate locations in space. The locations
will be at rest with respect to each other, and several light minutes apart.
My assistants will establish synchronized clocks between the two locations,
and then play starts. I'll only describe what goes on at your location. The
same thing is going on at your friend's location.

4\. There will be 1000 rounds, and each round lasts one minute. In each round
my assistant will use a random number generator to generate a single bit, 0 or
1, and will tell you the bit. You can assume that this random number is truly
random. There is no known way to predict it, and there is no correlation
between it and the random number generator at the other location. The rounds
start on a strict schedule so that you and your friend start round N on your
respective ships at the same time.

5\. After you are told the random number, you must generate a single bit, 0 or
1, by any means you wish. You can come up with it in your head. You can use
any equipment you brought. You can read it from a list of predetermined bits
you and your friend generated before the game started. You can try to
communicate with your friend (or with anyone else) if you brought
communications equipment, and use the results of that communication--but keep
in mind you are several light minutes from your friend.

6\. After the 1000th round, my assistant will fly you back to Earth where I
will be waiting. My assistants will each give me the list of random numbers
and responses. I will then compute your team score by adding up your scores
for each round. Scoring for each round is as follows:

• If you both received a 1 in the round from the random number generators,
then your team scores 1 point if you and your friend gave DIFFERENT bit values
for that round. I.e., if the random bits were both 1, then you score if you
gave 0 and your friend gave 1, or if you gave 1 and your friend gave 0.

• Otherwise, your team scores 1 point if you and your friend gave the SAME bit
value for that round.

Your win prizes if your score is above 800.

Question: what is your plan?

You can win 75% of the rounds with a very simple strategy: always pick 0.
You'll win the rounds where the random bits were 00, 01, or 10, and lose when
they were 11. That turns out to be the best you can with non-quantum methods.

If you use entanglement you can do better. You prepare 1000 pairs of qubits,
numbered 1 to 1000. The two qubits in each pair are entangled in what is
called a Bell state. It's an equal superposition of the two states "both
qubits are 0" and "both qubits are 1". What this means is that if you take
these two qubits and you measure one and your friend measures the other, AND
YOU MEASURE IN THE SAME BASIS (I'll explain that in a moment), you will either
both get 0 or both get 1.

What I mean by measure in the same basis is this. Suppose your qubits are
implemented by, say, polarized photons. A photon can be polarized at any angle
from 0 to 360 (I'm going to use degrees instead of radians because I think
that will be more comfortable for most). To make qubits out of this, you might
decide that 0 is represented by polarization at 0 degrees, and 1 by
polarization at 90 degrees. To see if a qubit is a 0 or a 1, you could try to
pass it through a polarizing filter set to 90 degrees. If it comes through, it
was a 1, and if it is blocked it was a 0. If we are using 0/90 degrees for our
qubits, we say we are using the 0 degree basis.

What happens if you pass that qubit through a polarizing filter at 45 degrees?
Half the time it will pass, and half the time it will be blocked. So a qubit
that has a definite value in the 0 basis can be any value with equal
probability in the 45 degree basis. How about if we measure using a 22.5
degree basis? It can still be any value, but now there is about an 85% chance
it will have the same value as in the 0 degree basis. In other words, if
someone generates a qubit with a particular binary value in the 0 degree
basis, and you measure it in the 22.5 degree basis, 85% of the time you'll get
the same value that the person who made it set it to in their basis.

In general, if we have two qubits in the same state, and you measure one of
them in one basis, and I measure the other in a basis that is T degrees
different from yours, we will get the same measurement with probability
cos(T)^2. E.g., if we use the same basis (T=0), then we get the same
measurement all the time. If my basis is 90 degrees from yours, we'll get
opposite measurements. If my basis was 45 degrees from yours, half the time my
measurement would agree with yours and half the time it would not.

Here is how you use your prepared qubits to improve your game outcome. As I
said, you take pairs of qubits and put that system of two qubits into an equal
mix of the states "both qubits are 0" an "both qubits are 1". When you head
off to space, you take one of the qubits from each pair, and your friend takes
the other.

For round N, after you get the random bit from my assistant, you take your
qubit for round N, and you measure it to see if it is a 0 or a 1. If my
assistant gave you a 0 bit, you measure in the 0 degree basis. If my assistant
gave you a 1 bit, you measure in the 45 degree basis. You report the result of
your measurement as your bit.

Your friend measures in the 22.5 degree basis if he gets a 0 random bit, and
he measures in the -22.5 degree basis if he gets a 1 random bit. He reports
the result of his measurement as his bit. Here's a diagram showing the angles
you each use depending on the random bit you receive:

    
    
          1---- friend -----0
          |                 |
       -22.5      0       22.5      45
                  |                  |
                  0----- you --------1
    

Note that if the random bits are both 1, then you two do your basis and his
are 67.5 degrees apart. On the other three cases, they will only be 22.5
degrees apart.

So, in all cases except both random numbers being 1, which means that you want
to pick the same bit as your friend, your measurements will agree with
probability cos(22.5)^2 = 0.854.

In the case where both random numbers are 1, which means you and your friend
want to NOT agree, you will agree with probability cos(67.5)^2 = 0.146, which
means you will disagree with probability 0.854.

Net result: your probability of winning a given round is 0.854, which is quite
a bit better than the 0.75 that is the best you can get with a non-quantum
approach.

Try to duplicate this with a "half an Oreo" model of entanglement, and it
won't work.

~~~
po
Thanks! This is a pretty good description of a scenario where quantum effects
can do something that a classical model can't.

------
softdev12
Even with experiments still ongoing, quantum entanglement is most likely real.
I think David Kaiser wouldn't suggest otherwise.

There's a real nice section in wikipedia re: the testing of quantum
entanglement (see
[http://en.wikipedia.org/wiki/Quantum_entanglement](http://en.wikipedia.org/wiki/Quantum_entanglement))

"Systems which contain no entanglement are said to be separable. For 2-Qubit
and Qubit-Qutrit systems (2 x 2 and 2 x 3 respectively) the simple Peres-
Horodecki criterion provides both a necessary and a sufficient criterion for
separability, and thus for detecting entanglement. However, for the general
case, the criterion is merely a sufficient one for separability, as the
problem becomes NP-hard.[59][60] A numerical approach to the problem is
suggested by Jon Magne Leinaas, Jan Myrheim and Eirik Ovrum in their paper
"Geometrical aspects of entanglement".[61] Leinaas et al. offer a numerical
approach, iteratively refining an estimated separable state towards the target
state to be tested, and checking if the target state can indeed be reached. An
implementation of the algorithm (including a built in Peres-Horodecki
criterion testing) is brought in the "StateSeparator" web-app."

------
danbruc
Good opportunity to again promote »The Theoretical Minimum« [1] by Leonard
Susskind (born June 1940, director of the Stanford Institute for Theoretical
Physics with research interests in string theory, quantum field theory,
quantum statistical mechanics and quantum cosmology). It is pretty accessible
and a key takeaway is that there is a lot of misinformation in the wild and
especially on the internet when it comes to (modern) physics as he tell his
class not only once when someone asks about something he read somewhere.

[1] [http://theoreticalminimum.com/](http://theoreticalminimum.com/)

~~~
gus_massa
The site looks interesting, but it's difficult to find the information that is
related to the discussion. Does this site have a page that discusses
specifically entanglement? EPR paradox? Bell inecuality? I'd like to see a
link to the specific relevant page and not a link to the general site.

~~~
danbruc
Here is the course on quantum entanglement [1] but depending on prior
knowledge it may be wise to first go through the courses on classical [2] and
quantum [3] mechanics. He tries to keep everything as self-contained as
possible but does not go through the very basics things like Lagrangians and
Hamiltonians every time.

[1] [http://theoreticalminimum.com/courses/quantum-
entanglement/2...](http://theoreticalminimum.com/courses/quantum-
entanglement/2006/fall)

[2] [http://theoreticalminimum.com/courses/classical-
mechanics/20...](http://theoreticalminimum.com/courses/classical-
mechanics/2011/fall)

[3] [http://theoreticalminimum.com/courses/quantum-
mechanics/2012...](http://theoreticalminimum.com/courses/quantum-
mechanics/2012/winter)

------
bhouston
Some questions:

\- Can we use this to send large amounts of data around the world quickly? Can
it replace intercontinential data links?

\- Can we use this for interstellar communication channels of limited amounts
of data?

\- Can we use this to pass messages between two locations in a way that is
immune to easedropping?

\- What are other real-world applications of this?

~~~
beloch
It's a very common misconception that entanglement allows faster than light
communication. It doesn't.

Describing quantum phenomena with real world metaphors always breaks down if
we carry things far enough. I'm going to use coin flipping as a metaphor to
make a specific point, but don't assume this metaphor will still work if you
try to extend it. To even begin to understand entanglement you really need to
look at the math. That being said, let's proceed.

 _The Setup:_ (this explains how the metaphor is being made, but you can skip
to the payoff if you just want the what without any of the why).

We can think of a pair of entangled particles as a pair of coins. If we flip
each coin, we have a fifty/fifty odds of gettings heads or tails. This is what
we normally think of, in the macroscopic world, as a random outcome (It's not
really, but let's not go there). An entangled pair of coins behaves very
unusually. The outcome of flipping one coin is related to the outcome of the
other coin. For example, if one coin comes up as heads, the other will
_always_ come up as tails, or vice-versa. Let's say we separate the two coins
by several light years and then flip them. The strange correlation will hold.
For a non-Quantum particle there are several possible explanations for this
behavior. We might think that, somehow, the measurement of one particle causes
the outcome of the other article to change. However, time depends on reference
frame. If the two coins are in relative motion to each other in the right
manner, which particle was measured first is relative (There is no universally
preferred reference frame). i.e. One observer might correctly say that the
first coin was measured before the second, but another observer might see
things reversed. So, which measurement is the cause and which is the effect?
Another notion is that the outcome was preordained when the coins were made.
I'd have to go into the math a bit to adequately explain this, but with
entangled quantum particles you can create experiments that demonstrate this
is impossible. Tests of Bell inequalities are especially notable for this.
This is Einstein's "spooky action at a distance", a behavior exhibited by
entangled particles.

 _The Payoff:_

So, if a measurement performed on two entangled particles is like flipping a
pair of coins, how can we use that to communicate. Well, we can't. Not
directly. You see, we can't force a coin to come up heads or tails. If we look
at the outcomes of just one coin they will always appear random and
uncontrollable. It's when we talk to the guy with the other coin that we'll
say, "Hey, I knew what you were going to get because of what I got!". So,
classical communication, limited by the speed of light, is necessary to figure
out what these correlations are. So forget about FTL communication. However,
what about communicating secrets? Well, that's a whole other kettle of fish!
It turns out quantum correlations are _fantastic_ for communicating secret
keys. Google quantum cryptography.

As for other applications, google quantum computing. Entanglement is
absolutely intrinsic to this field. There are also applications in medical
imaging and metrology (i.e. standards of measurements).

~~~
VieElm
What about using those entangled states to coordinate actions? You could plan
ahead and say if you use heads or tails as binary, flip enough to get digits
to use as (hopefully hospitable) coordinates and a time to meet. When the two
parties meet successfully that location was coordinated using communication
faster than light, no?

Maybe you could that same mechanism for superfast decision making
communication in automated equipment. Drone dispersal, late stage missile
targeting. Think shooting a bunch of missiles into the sky with a map of
possible locations and then entanglement used to have each missile pick its
target, no missile would hit the same target and no one would know which
missile was going where at launch. Stuff like that works?

You could use it for frequency hopping on radios making tracing communication
pretty difficult.

~~~
MichaelGG
You could just stick a random seed and tell them to meet at a place determined
by that seed. What's the difference?

The closest use is probably quantum cryptography, somehow, I'd guess. But
that's still not FTL communication.

~~~
VieElm
I'm not sure how you use a random number generator to randomly generate values
that generate knowledge about previous uses. Those missiles aren't picking
their targets randomly, they know where the other missiles are going and where
they have to go. No missile is going to hit the same target, with a random
number generator they would.

------
jmcminis
It seems the assertion is that there are 2 options, 1\. quantum entanglement
exists 2\. "Some unnoticed causal mechanism in the past may have fixed the
detectors’ settings in advance, or nudged the likelihood that one setting
would be chosen over another."

The human brain may or not be deterministic, but it is certainly a complex
system. For the results to be so consistent for the multitude of experiments
that have been performed, it seems highly unlikely that #2 is the case. I
think in this case it's pretty fair to apply Occam's razor.

On a side note, I hate titles for popular physics articles. The next article
titles: "5 super simple steps to splitting the atom" "20 crazy tips for
growing high temperature superconductors" "Make free cash farming Higgs
bosons"

~~~
snowwrestler
Option 2 is similar to the Novikov self-consistency hypothesis of time travel,
which states that if an action would cause a time travel paradox, its
probability is zero.

Under this hypothesis, even if entanglement does allow changes to move faster
than light--essentially travelling backward in time--any action that would
expose this time travel would be forbidden. The scientist is physically
prevented from making the "wrong" choice when setting the detector.

The problem with this hypothesis is that it seems essentially untestable.

Another way to think of option 2 is that the universe is 100% deterministic.
Under this hypothesis, the speed of entanglement is only apparent, not real--
like planning with a friend to both turn on your flashlights on opposite
hilltops 2 hours from now. Without knowing that it was planned in advance, an
observer would wonder how you reacted to one another so fast.

This is also impossible to fully test; at best you could prove that the
consistency extends as far back as we can see into the past. And that is
exactly what these scientists will be testing.

~~~
jmcminis
The Novikov self-consistency hypothesis is pretty neat. I like the thought of
the researcher travelling back in time to create the apparatus such that the
result always confirms quantum entanglement. It's classical and not quantum so
it's not clear to me how it relates. I'm not sure anyone (besides
philosophers) takes the loopholes seriously.

------
jaekwon
I assume that photons act as waves and not particles. Though they have
quantized energy (frequency), there's no reason for me to think that they have
locality. If so...

The most plausible explanation I can think of is a Monty Hall paradox at the
source of the emitter. Thousands of photon pairs? How are they generated? Is
their emission dependent on the destination via say, pilot wave resonance?
Either resonance or waves that travel backwards in time. Experiments of
entanglement speed should point towards one or the other.

------
karmakaze
I see nothing counter-intuitive about quantum entanglement. Photons travel at
the speed of light--time does not exist for them. Alter one, the entangled one
is instantaneously connected.

~~~
krapp
"If you think you understand quantum mechanics, you don't understand quantum
mechanics." \- Richard Feynman

~~~
readerrrr
I wish you comment were downvoted instead of karmakaze's one.

He is expressing an opinion and even if he is wrong, it is still an
interesting idea, and shows some effort. A decent short explanation could show
the mistake, and make a good comment->answer combo.

Instead you quoted a comment which is out of context ( Feynman's comment is
intended for physicists of his level ), contributing nothing.

( Please don't take this as a personal attack ( it isn't), but more a thought
what's wrong with HN in light of recent debate )

~~~
krapp
No, you're right - it was snide and I should have reconsidered it.

