[Even with quantum teleportation and the existence of entangled
quantum states, faster-than-light communication still remains
impossible.]
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SUNDAY SCIENCE: EVEN WITH QUANTUM ENTANGLEMENT, THERE’S NO
FASTER-THAN-LIGHT COMMUNICATION  
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Ethan Siegel
March 9, 2023
Big Think
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*
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_ Even with quantum teleportation and the existence of entangled
quantum states, faster-than-light communication still remains
impossible. _

Ten yttrium atoms with entangled electron spins, as used to first
create a time crystal. Although these atoms have quantum properties
that are not wholly independent of one another, they are not in
identically cloned quantum states to one another, Chris Monroe,
University of Maryland

 

* For many, the notion of quantum entanglement, which can be
maintained even over very large distances, leads to the hope that it
could someday be used for faster-than-light communication. 
* But there are fundamental laws for both relativity and quantum
mechanics, and even though entangled quantum states do exist and obey
arcane rules, no information can ever be exchanged
faster-than-light. 
* As a result, faster-than-light communication doesn't occur,
irrespective of what your quantum mechanical setup is. Unless
something very exotic exists, faster-than-light communication isn't
possible.

One of the most fundamental rules of physics, undisputed since
Einstein first laid it out in 1905, is that no information-carrying
signal of any type can travel through the Universe faster than the
speed of light. Particles, either massive or massless, are required
for transmitting information from one location to another, and those
particles are mandated to travel either below (for massive) or at (for
massless) the speed of light, as governed by the rules of relativity.
You might be able to take advantage of curved space to allow those
information-carriers to take a short-cut, but they still must travel
through space at the speed of light or below.

Since the development of quantum mechanics, however, many have sought
to leverage the power of quantum entanglement to subvert this rule.
Many clever schemes have been devised in a variety of attempts to
transmit information that “cheats” relativity and allows
faster-than-light communication after all. Although it’s an
admirable attempt to work around the rules of our Universe, every
single scheme has not only failed, but it’s been proven that all
such schemes are doomed to failure. Even with quantum entanglement,
faster-than-light communication is still an impossibility within our
Universe. Here’s the science of why.

Flipping a coin should result in a 50/50 outcome of getting either
heads or tails. If two ‘quantum’ coins are entangled, however,
measuring the outcome of one of the coins (heads or tails) can provide
you with information to do better than random guessing when it comes
to the state of the other coin. However, that information can only be
transmitted, from one coin to the other, at light speed or
slower. frankieleon/flickr

Conceptually, quantum entanglement is a simple idea. You can start by
imagining the classical Universe and one of the simplest “random”
experiments you could perform: conducting a coin flip. If you and I
each have a fair coin and flip it, we’d each expect that there’s a
50/50 chance of each of us getting heads and a 50/50 chance that each
of us would get tails. Your results and my results should not only be
random, they should be independent and uncorrelated: whether I get
heads or tails should still have 50/50 odds irrespective of what you
get with your flip.

But if this isn’t a classical system after all, and a quantum one
instead, it’s possible that your coin and my coin will be entangled.
We might each still have a 50/50 chance of getting heads or tails, but
if you flip your coin and measure heads, you’ll instantly be able to
statistically predict to _better_ than 50/50 accuracy whether my
coin was likely to land on either heads or tails. This is the big idea
of quantum entanglement: that there are correlations between the two
entangled quanta that means if you actually measure the quantum state
of one of them, the other one’s state isn’t instantly determined,
but rather some probabilistic information can be gleaned about it.

By creating two entangled photons from a pre-existing system and
separating them by great distances, we can ‘teleport’ information
about the state of one by measuring the state of the other, even from
extraordinarily different locations. Interpretations of quantum
physics that demand both locality and realism cannot account for a
myriad of observations, but multiple interpretations all appear to be
equally good.  Melissa Meister/ThorLabs

How does this work, conceptually?

In quantum physics, there exists a phenomenon known as quantum
entanglement, which is where you create more than one quantum
particle — each with its own individual quantum state — where
something important about the sum of both states together is known.
It’s as though there’s an invisible thread connecting these two
quanta (or, if two coins were entangled according to the laws of
quantum mechanics, your coin and my coin), and when one of us makes a
measurement about the coin we have, we can instantly know something
about the state of the other coin that goes beyond our familiar
“classical randomness.”

Although this sounds like purely theoretical work, it’s been within
the realm of experiment for many decades. We’ve created pairs of
entangled quanta (photons, to be specific) that are then carried away
from one another until they’re separated by large distances, then we
have two independent measurement apparatuses that tell us what the
quantum state of each particle is. We make those measurements as close
to simultaneously as possible, and then get together to compare our
results. These experiments are so profound that research following
these lines was awarded a share of the 2022 Nobel Prize in physics
[[link removed]].

How does this work, conceptually?

In quantum physics, there exists a phenomenon known as quantum
entanglement, which is where you create more than one quantum
particle — each with its own individual quantum state — where
something important about the sum of both states together is known.
It’s as though there’s an invisible thread connecting these two
quanta (or, if two coins were entangled according to the laws of
quantum mechanics, your coin and my coin), and when one of us makes a
measurement about the coin we have, we can instantly know something
about the state of the other coin that goes beyond our familiar
“classical randomness.”

Although this sounds like purely theoretical work, it’s been within
the realm of experiment for many decades. We’ve created pairs of
entangled quanta (photons, to be specific) that are then carried away
from one another until they’re separated by large distances, then we
have two independent measurement apparatuses that tell us what the
quantum state of each particle is. We make those measurements as close
to simultaneously as possible, and then get together to compare our
results. These experiments are so profound that research following
these lines was awarded a share of the 2022 Nobel Prize in physics
[[link removed]].

The best possible local realist imitation (red) for the quantum
correlation of two spins in the singlet state (blue), insisting on
perfect anti-correlation at zero degrees, perfect correlation at 180
degrees. Many other possibilities exist for the classical correlation
subject to these side conditions, but all are characterized by sharp
peaks (and valleys) at 0, 180, 360 degrees, and none has more extreme
values (+/-0.5) at 45, 135, 225, 315 degrees. These values are marked
by stars in the graph, and are the values measured in a standard
Bell-CHSH type experiment. The quantum and classical predictions can
be clearly discerned, and were identified at a variety of angles way
back in 1972 with Stuart Freedman’s PhD thesis. Richard Gill, 22
December 2013, drawn with R

What we find, perhaps surprisingly, is that the results for your coin
and my coin (or, if you prefer, your photon’s spin and my photon’s
spin) are correlated with one another! We’ve now separated two
photons by distances of hundreds of kilometers before making those
critical measurements and then measuring their quantum states within
nanoseconds of one another. If one of those photons has spin +1, the
other one’s state can be predicted to about a 75% accuracy, rather
than the standard 50% you would have classically expected from knowing
it’s either +1 or -1.

Moreover, that information about the other particle’s spin can be
known instantaneously, rather than waiting for the other measurement
apparatus to send us the results of that signal, which would take
about a millisecond. It seems, on the surface, that we can know some
information about what’s going on at the other end of the entangled
experiment not only faster than light, but at least tens of thousands
of times faster than the speed of light. Does this mean that
information is actually being transmitted at speeds faster than the
speed-of-light?

If two particles are entangled, they have complementary wavefunction
properties, and measuring one determines properties of the other. If
you create two entangled particles or systems, however, and measure
how one decays before the other decays, you should be able to test for
whether time-reversal symmetry is conserved or violated. David
Koryagin/Wikimedia Commons

On the surface, it might appear that information really is being
communicated at speeds faster-than-light. For example, you might
attempt to concoct an experiment that obeys the following setup:

* You prepare a large number of entangled quantum particles at one
(source) location.
* You transport one set of the entangled pairs a long distance away
(to the destination) while keeping the other set of entangled
particles at the source.
* You have an observer at the destination look for some sort of
signal, and force their entangled particles into either the +1 state
(for a positive signal) or a -1 state (for a negative signal).
* Then, you make your measurements of the entangled pairs at the
source, and determine with better than 50/50 likelihood
[[link removed]] what
state was chosen by the observer at the destination.

If this setup worked, you really would be able to know whether the
observer at the distant destination forced their entangled pairs into
either the +1 or the -1 state, simply by measuring your own particle
pairs after the entanglement was broken from afar.

The wave pattern for electrons passing through a double slit,
one-at-a-time. If you measure “which slit” the electron goes
through, you destroy the quantum interference pattern shown here.
Regardless of the interpretation, quantum experiments appear to care
whether we make certain observations and measurements (or force
certain interactions) or not. Dr. Tonomura; Belsazar/Wikimedia
Commons

This seems like a great setup for enabling faster-than-light
communication. All you need is a sufficiently prepared system of
entangled quantum particles, an agreed-upon system for what the
various signals will mean when you make your measurements, and a
pre-determined time at which you’ll make those critical
measurements. From even light-years away, you can instantly learn
about what was measured at a destination by observing the particles
you’ve had with you all along.

But is this right?

It’s an extremely clever scheme for an experiment, but one that
doesn’t actually pay off in any way. When you, at the original
source where the particle pairs were entangled and created, go to make
these critical measurements, you’ll discover something extremely
disappointing: your results simply show 50/50 odds of being in the +1
or -1 state. It’s as though the actions of the distant observer,
forcing their member of the entangled pairs to be in either the +1 or
the -1 state, had no effect on your experimental results at all. The
results are identical to what you’d expect had there never been any
entanglement at all.

Schematic of the third Aspect experiment testing quantum non-locality.
Entangled photons from the source are sent to two fast switches that
direct them to polarizing detectors. The switches change settings very
rapidly, effectively changing the detector settings for the experiment
while the photons are in flight. Different settings, puzzlingly
enough, result in different experimental outcomes. Chad Orzel

Where did our plan fall apart? It was at the step where we had the
observer at the destination make an observation and try to encode that
information into their quantum state, where we had previously stated,
“You have an observer at the destination look for some sort of
signal, and force their entangled particles into either the +1 state
(for a positive signal) or a -1 state (for a negative signal).”

When you take that step — forcing one member of an entangled pair
of particles into a particular quantum state — that action not only
breaks the entanglement between the two particles, but it doesn’t
break the entanglement and determine what that particle’s properties
were; it breaks the entanglement and places it into a new state that
doesn’t care about which state (+1 or -1) would have been
“determined” from making a fair measurement.

That is to say, the other member of the entangled pair is completely
unaffected by this “forcing” action, and its quantum state remains
random, as a superposition of +1 and -1 quantum states. What you’ve
done by “forcing” one member of the entangled particles into a
specific state is completely break the correlation between the
measurement results. The state you’ve “forced” the destination
particle into is now 100% unrelated to the quantum state of the source
particle.

A quantum eraser experiment setup, where two entangled particles are
separated and measured. No alterations of one particle at its
destination affect the outcome of the other. You can combine
principles like the quantum eraser with the double-slit experiment and
see what happens if you keep or destroy, or look at or don’t look
at, the information you create by measuring what occurs at the slits
themselves. Patrick Edwin Moran/Wikimedia Commons

The only way that this problem could be circumvented is if there
existed some way of making a quantum measurement that actually forced
a particular outcome. (Note: this is not something permitted within
the presently-known laws of physics.)

If you could do this, then someone at the destination could conduct
observations — for example, learning whether a planet they were
visiting were inhabited or not — and then use some unknown process
to:

* measure their quantum particle’s state,
* where the outcome will turn out to be +1 if the planet is
inhabited,
* or -1 if the planet is uninhabited,
* and thereby enable the source observer with the entangled pairs to
instantaneously figure out whether this distant planet is inhabited or
not.

Unfortunately, the results of a quantum measurement are unavoidably
random
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you cannot encode a preferred outcome into a quantum measurement.

 Even by taking advantage of quantum entanglement, it should be
impossible to do better than random guessing when it comes to knowing
what goes on at the other end of an entanglement experiment,
regardless if it’s about photon spins, coin flipping, or trying to
know what cards the dealer’s hand holds. Maksim and CSTAR/Wikimedia
Commons

As quantum physicist Chad Orzel has written
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there is a big difference between making a measurement (where the
entanglement between pairs is maintained) and forcing a particular
result — which itself is a change of state — followed by a
measurement (where the entanglement is not maintained). If you want to
control, rather than simply measure, the state of a quantum particle,
you’ll lose your knowledge of the full state of the combined system
as soon as you make that change-of-state operation happen.

Quantum entanglement can only be used to gain information about one
component of a quantum system by measuring the other component so long
as the entanglement remains intact. What you cannot do is create
information at one end of an entangled system and somehow send it over
to the other end. If you could somehow make identical copies of your
quantum state, faster-than-light communication would be possible after
all, but this, too, is forbidden by the laws of physics
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If you could somehow take a quantum state and make an identical copy
of it, it might be possible to concoct a faster-than-light
communication scheme. However, a valid no-cloning theorem was proven
back in the 1970s and 1980s by multiple independent parties, as the
act of attempting to even measure a quantum state (to know what it is)
fundamentally changes the outcome. minutephysics/YouTube

There’s an awful lot that you can do by leveraging the bizarre
physics of quantum entanglement, such as by creating a quantum
lock-and-key system
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virtually unbreakable with purely classical computations. But the fact
that you cannot copy or clone a quantum state
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merely reading the state fundamentally changes it — is the
nail-in-the-coffin of any workable scheme to achieve faster-than-light
communication with quantum entanglement. Many aspects of quantum
entanglement, which itself is a rich field of research, were
recognized in the 2022 Nobel Prize in physics
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There are a lot of subtleties associated with how quantum
entanglement actually works in practice
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but the key takeaway is this: there is no measurement procedure you
can undertake to force a particular outcome while maintaining the
entanglement between particles. The result of any quantum measurement
is unavoidably random, negating this possibility. As it turns
out, God really does play dice with the Universe
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and that’s a good thing. No information can be sent
faster-than-light, allowing causality to still be maintained for our
Universe.

_ETHAN SIEGAL has lived his life fascinated with one simple fact: we
have the capacity to figure out definitive, scientific answers to even
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he’s committed to not just figuring it out, but to sharing the
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