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Coherent Schrödinger's cat still confounds

Nov 23, 2011
14
comments

How to make a
cat from photons

The famous paradox of Schrödinger's cat starts from principles of quantum
physics and ends with the bizarre conclusion that a cat can be simultaneously in
two physical states – one in which the cat is alive and the other in which it is
dead. In real life, however, large objects such as cats clearly don't exist in a
superposition of two or more states and this paradox is usually resolved in
terms of quantum decoherence. But now physicists in Canada and Switzerland argue
that even if decoherence could be prevented, the difficulty of making perfect
measurements would stop us from confirming the cat's superposition.

Erwin Schrödinger, one of the fathers of quantum theory, formulated his
paradox in 1935 to highlight the apparent absurdity of the quantum principle of
superposition – that an unobserved quantum object is simultaneously in multiple
states. He envisaged a black box containing a radioactive nucleus, a Geiger
counter, a vial of poison gas and a cat. The Geiger counter is primed to release
the poison gas, killing the cat, if it detects any radiation from a nuclear
decay. The grisly game is played out according to the rules of quantum mechanics
because nuclear decay is a quantum process.

If the apparatus is left for a period of time and then observed, you may find
either that the nucleus has decayed or that it has not decayed, and therefore
that the poison has or has not been released, and that the cat has or has not
been killed. However, quantum mechanics tells us that, before the observation
has been made, the system is in a superposition of both states – the nucleus has
both decayed and not decayed, the poison has both been released and not been
released, and the cat is both alive and dead.

Mixing micro and macro

Schrödinger's cat is an example of "micro-macro entanglement", whereby
quantum mechanics allows (in principle) a microscopic object such as an atomic
nucleus and a macroscopic object such as a cat to have a much closer
relationship than permitted by classical physics. However, it is clear to any
observer that microscopic objects obey quantum physics, while macroscopic things
obey the classical physics rules that we experience in our everyday lives. But
if the two are entangled it is impossible that each can be governed by different
physical rules.

The most common way to avoid this problem is to appeal to quantum
decoherence, whereby multiple interactions between an object and its
surroundings destroy the coherence of superposition and entanglement. The result
is that the object appears to obey classical physics, even though it is actually
following the rules of quantum mechanics. It is impossible for a large system
such as a cat to remain completely isolated from its surroundings, and therefore
we do not perceive it as a quantum object.

While not disputing this explanation, Christoph Simon and a colleague at the
University of Calgary, and another at the University of Geneva, have asked what
would happen if decoherence did not affect the cat. In a thought experiment
backed up by computer simulations, the physicists consider pairs of photons (A
and B) generated from the same source with equal and opposite polarizations,
travelling in opposite directions. For each pair, photon A is sent directly to a
detector, but photon B is duplicated many times by an amplifier to make a
macroscopic light beam that stands in for the cat. The polarizations of the
photons in this light beam are then measured.

Two types of amplifier

They consider two different types of amplifier. The first measures the state
of photon B, which has the effect of destroying the entanglement with A, before
producing more photons with whatever polarization it measures photon B to have.
This is rather like the purely classical process of observing the Geiger counter
to see whether it has detected any radiation, and then using the information to
decide whether or not to kill the cat. The second amplifier copies photon B
without measuring its state, thus preserving the entanglement with A.

The researchers ask how the measured polarizations of the photons in the
light beam will differ depending on which amplifier is used. They find that, if
perfect resolution can be achieved, the results look quite different. However,
with currently available experimental techniques, the differences cannot be
seen. "If you have a big system and you want to see quantum features like
entanglement in it, you have to make sure that your precision is extremely
good," explains Simon. "You have to be able to distinguish a million photons
from a million plus one photons, and there is no current technology that would
allow you to do that."

Quantum-information theorist Renato Renner of ETH Zurich is impressed: "Even
if there was no decoherence, this paper would explain why we do not see quantum
effects and why the world appears classical to us, which is a very fundamental
question of course." But, he cautions, "The paper raises a very fundamental
question and gives us an answer in an interesting special case, but whether it
is general remains to be seen."

The research will be published in Physical Review Letters.

About the author

Tim Wogan is a science writer based in the UK

14 comments

Add your comments on this article

1

Jarek DudaNov
23, 2011 5:04 PMCracow, Poland

What is happening while oppening the box?

Doesn't no-cloning theorem forbid amplifiers from the proposed
experiment?

And generally in Schrodinger's cat experiment, while for
external observer the cat is in quantum superposition, if there would be
additional observer inside the box, he could finally get out and show video
recording what has really objectively happened inside.
Ok, one can use
magical interpretation that there was e.g. multiple universes and in each of
them the internal observer have seen something different and finally opening
the box has chosen one of these universes ... but there is also a bit
simpler explanation: quantum mechanics is not fundamental, but only
effective theory, among other representing information of subjective
observer.
For example if we would correct Brownian motion to make it in
agreement with thermodynamical principles (Maximal Entropy Random Walk and
extensions), it doesn't longer disagree with thermodynamical predictions of
quantum mechanics ( arxiv.org…1111.2253 ).

Edited by Jarek Duda on Nov 23, 2011 5:04 PM.

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2

M. AsgharNov
23, 2011 6:42 PM

Range of quantum entaglement

Interesting work but still a calculated guess game that has to
be tested. Question: How big an object has to, when it loses this property
of quantum coherence through the interaction with its environment, say
gravity, and becomes a classical entity? It has been shown experimentally a
few years back that a "piezoelectric vibrator" (PV) visible to the eye and
containing billions of atoms can be in an entangled state containing its
ground state and its first excited state. Can cat size PV do the same trick?
Why not?

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3

M. AsgharNov
23, 2011 7:17 PM

3-body state.

With another one in the cage along with the poison and the cat,
you have to have a 3-body entangled state.

Quote:

Originally posted by Jarek Duda View
comment
Doesn't no-cloning theorem forbid amplifiers from
the proposed experiment?

And generally in Schrodinger's cat
experiment, while for external observer the cat is in quantum
superposition, if there would be additional observer inside the box, he
could finally get out and show video recording what has really objectively
happened inside.
Ok, one can use magical interpretation that there was
e.g. multiple universes and in each of them the internal observer have
seen something different and finally opening the box has chosen one of
these universes ... but there is also a bit simpler explanation: quantum
mechanics is not fundamental, but only effective theory, among other
representing information of subjective observer.
For example if we
would correct Brownian motion to make it in agreement with thermodynamical
principles (Maximal Entropy Random Walk and extensions), it doesn't longer
disagree with thermodynamical predictions of quantum mechanics ( arxiv.org…1111.2253 ).

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4

Jarek DudaNov
23, 2011 7:45 PMCracow, Poland

Subjective knowledge-objective reality distinction

Quote:

Originally posted by M. Asghar View
comment
With another one in the cage along with the poison
and the cat, you have to have a 3-body entangled state.

Call
it as you want, but there is huge amount of wavefunction collapses inside
(like every deexcitation) - the internal observer can be aware of them,
while for the external one they would have to remain in superposition until
the box is opened.
Of course, from the external observer point of view,
this superposition is also of the internal observer state ... and while
opening the box, all possible hypothetical internal scenarios collapse into
the one which really happened there - e.g. recorded on video by the internal
observer.
So maybe in fact there was objectively happening inside only
this single final scenario and superposition was only representation of the
limited knowledge of the external observer?

Experiment is not
physical? So let the separation be spatial - they are e.g. 1 light year
away, so a year after cat's death, it would need to remain in superposition
in quantum mechanics of the far observer ...

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5

zrzzzNov 24, 2011 2:42 AM

Wouldn't you hear the meowing stop?

and perhaps a bad smell after some time? This Schrödinger fellow
should watch out for the ASPCA. They don't take kindly to animal testing.
Also, the FBI might want to know where he got that radioactive material
from...

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6

spintechNov 24, 2011 8:09 AM

The answer is...

Quantum Mechanics is a statistical theory. If we have 1000 cat
in the boxes, we can say that after a specific amount of time, for example
300 cats are dead. So what happens to an individual cat? Quantum Mechanics
cannot answer. It deals with probabilities, which by definition, means we
have an infinite number of experiments.

As for that PV experiment,
the quantum state is not "written" on the system as |1>+|2>. They make
measurements that result in similar quantities as if the system is in such a
state. The same results can be potentially predicted by a more exact
single-system theory.

Edited by spintech on Nov 24, 2011 8:10 AM.

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7

leamphilNov 24, 2011 12:30 PM

I never really understood the Schrodinger's Cat experiment -
there is an essential difrerence between a twin slit experiment where trying
to observe which slit a photon went through collapses the wave function, and
observing a cat in a box - if one looked into the sealed box then that would
not affect any radioactive decay that may or may not have killed the cat -
so unless one can find a trigger for the death (or otherwise) of the cat
that prevents all kinds of observation to check what happens, I don't think
you have an equivalent system. All you have is a cat that may be dead or
alive but you don't know which - which is not the same as it being both at
the same time.

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8

M. AsgharNov
24, 2011 12:38 PM

Entangled state

PLease do not forget that we are dealing with an entagled
state.
If you have 100 cats and a poison, you have to write an entangled
state for all of them, the so-called Bell
state.

Quote:

Originally posted by spintech View
comment
Quantum Mechanics is a statistical theory. If we
have 1000 cat in the boxes, we can say that after a specific amount of
time, for example 300 cats are dead. So what happens to an individual cat?
Quantum Mechanics cannot answer. It deals with probabilities, which by
definition, means we have an infinite number of experiments.

As for
that PV experiment, the quantum state is not "written" on the system as
|1>+|2>. They make measurements that result in similar quantities as
if the system is in such a state. The same results can be potentially
predicted by a more exact single-system
theory.

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9

Jarek DudaNov
24, 2011 6:29 PMCracow, Poland

QM = information/statistics + wave nature

There is more in quantum mechanics than just statistical physics
- it also "have in mind" that particles have wave nature required e.g. in
interference.
That the duality has two sides - particle is corpuscle with
some kind internal periodic process (Zitterbewegung/de Broglie's clock/is breather),
which creates periodic wave-like perturbations of the surrounding
field.
This clock was recently observed experimentally (nice Hestenes paper about it) and in this interpretation
there was observed interference or orbit quantization even for macroscopic objects with
wave-particle duality.

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10

martin07Nov 25, 2011 11:11 PMBracknell,
United Kingdom

Validity of the Proposed Experiment

The author states that when the polarisation of either photon A
or B is measured the entanglement between them is broken. The question I
have is that when the 'amplifier' produces a new photon/s with the same
polarisation as the original it must surely have to establish the
polarisation of this photon otherwise it can only produce randomly polarised
photons.

Therefore if this amplifier measures the polarisation,
needed to produce replicas of the original, the entanglement will, by the
author's assertion, be broken and the experiment will fail or produce false
results.

Just a thought.

Martin

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11

gzpablo50Nov
28, 2011 10:22 PM

Can a photon in a superposition state generate another photon in
a superposition state such those photons will have the same polarization
when measured?

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12

emaaloufNov 28, 2011 10:55 PM

A purely phsical perspective

This argument was valid in 1939.

Now days we can aim a
"thermal" camera at the box and watch the cat's thermal signature.
Superposition = voided.

It's time for this argument to be removed
from physics and for all physicists wasting their time on it to move to
bigger and more complicated things.

Dark matter and energy, the higgs
boson, particle/wave duality in specific states, black holes and
superluminal particles as few examples.

Cheers.

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13

pmartelNov 29, 2011 1:44 AMStoneham,
United States

The box was assumed to be opaque. That would include thermal
wavelengths.
Quote:

Originally posted by emaalouf View
comment
This argument was valid in 1939.

Now days
we can aim a "thermal" camera at the box and watch the cat's thermal
signature. Superposition = voided.

It's time for this argument to
be removed from physics and for all physicists wasting their time on it to
move to bigger and more complicated things.

Dark matter and energy,
the higgs boson, particle/wave duality in specific states, black holes and
superluminal particles as few
examples.

Cheers.

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14

JPDemersNov 29, 2011 5:21 AM

Missing the point

Quote:

Originally posted by emaalouf View
comment

Now days we can aim a "thermal" camera at
the box and watch the cat's thermal signature. Superposition = voided.

It's a common mistake to focus on the cat, but in fact
there's no need for the cat at all. Schroedinger stuffed the poor animal
into the box merely to add force to his argument. The paradox remains, even
if you're merely discussing the (macroscopic) poison being simultaneously
released and not released, or the (macroscopic) Geiger counter
simultaneously recording and not recording an event. The latter are
physically just as mysterious as a live/dead cat, but they don't grip the
imagination in quite the same way.

Edited by JPDemers on Nov 29, 2011 5:59 AM.

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