Quantum copies do new tricks

Mar 22, 2012

One of the strange features of quantum information is that, unlike almost every other type of information, it cannot be perfectly copied. For example, it is impossible to take a single photon and make a number of photons that are in the exact same quantum state. This may seem minor, but it's not. If perfect copying was possible, it would, among other things, be possible to send signals faster than the speed of light. This is forbidden by Einstein's theory of relativity.

For years, scientists have been experimenting with the idea of approximate quantum copying. A recent paper published in Physical Review Letters (PRL), by Sadegh Raeisi, Dr. Wolfgang Tittel and Dr. Christoph Simon of the Institute for at the University of Calgary takes another step in that research.

They showed that it is possible to perfectly recover the original from the imperfect quantum copies. They also proposed a way that his could be done in practice.

"Copying classical information is very important in our daily lives," says paper co-author Simon. "Think of the prevalence of photocopiers, faxes, scanners. It was quite surprising for physicists when they realized that the same thing is not possible for , at least not perfectly. It is then important to study what exactly is possible and what isn't."

The research can be used in a variety of ways. First, it shows clearly that quantum information is preserved when copied. Even though the copies may be imperfect, the original can be recovered. In practical terms, it might lead to a precision measurement technique based on for samples that have very low contrast, such as living cells.

The Institute for Quantum Information Science is a multidisciplinary group of researchers from the areas of computer science, mathematics and physics.

"At the fundamental level our world is governed not by classical physics, but by quantum physics," says Simon. "We are trying to understand the consequences of that for fundamental concepts such as information and trying to use this understanding to develop new kinds of quantum technology."

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SpaceColonizer
1 / 5 (2) Mar 22, 2012
Um... my understanding was that quantum entanglement DOES NOT conflict with relativity because relativity says than "a particle" cannot be accelerated faster than light because it would require absolute mass/infinite energy. But with quantum entanglement there is no particle traveling between point a and b... so the fact that information can be passed faster than it would have taken a particle to traverse that distance does not violate relativity.
Silverhill
5 / 5 (2) Mar 22, 2012
Unfortunately, even information cannot be sent FTL.
If the state of one member of an entangled pair of photons is measured, the state of the other is instantly determined -- but the information about which state was found at which location will still be unknowable to both parties until enough time has elapsed that a photon can be sent between them.
StarGazer2011
2.6 / 5 (5) Mar 23, 2012
re entanglement, i dont see how any information is travelling at all, intantly or otherwise. The information is shared at the point of entanglement, everything after that is just an argument from the residual.
For example, if I take a blue ball and a red ball, blindfold myself, and put each in a separate box and seal them; then send one to Timbuktu; then open my box, see that it has a red ball and hence infer that the other one in timbuktu must be red, no actual information has travelled between Timbuktu and myself.

What am I missing; or is entanglement over hyped?
Skepticus
3 / 5 (4) Mar 23, 2012
The research can be used in a variety of ways. First, it shows clearly that quantum information is preserved when copied. Even though the copies may be imperfect, the original quantum state can be recovered

Looks like Star Trek-style teleporters may not need Heisenberg Compensator after all!
chasehusky
5 / 5 (3) Mar 23, 2012
It's incredibly frustrating that the Physorg editors cannot be bothered to link to the appropriate paper a good portion of the time:

Sadegh Raeisi, Wolfgang Tittel, and Christoph Simon, "Proposal for Inverting the Quantum Cloning of Photons", Phys. Rev. Lett., 108(12), 2012. (URL: link.aps.org/doi/10.1103/PhysRevLett.108.120404)
DamienS
5 / 5 (1) Mar 23, 2012
re entanglement, i dont see how any information is travelling at all

Correct! No information can be sent by this means. Your blue and red ball example is a classical one which can in no way be compared to quantum entanglement. It's not that you simply don't know the initial states (classical), but that all states exist at the same time (quantum superposition) until a measurement is made.
Foolish1
3 / 5 (2) Mar 23, 2012
if I take a blue ball and a red ball, blindfold myself, and put each in a separate box and seal them; then send one to Timbuktu; then open my box, see that it has a red ball and hence infer that the other one in timbuktu must be red, no actual information has travelled between Timbuktu and myself.

What am I missing; or is entanglement over hyped?


The hype deserving part using your analogy is Colors of both balls are determined *AFTER* the box is opened and color of either ball is measured.

The idea a predetermined answer actually exists and we just can't access it due to..ah..blindfolds is called a "hidden variable" theory. No verifiable version of any such theory has yet to survive experimental investigation.
Foolish1
1.7 / 5 (3) Mar 23, 2012
Um... my understanding was that quantum entanglement DOES NOT conflict with relativity because relativity says than "a particle" cannot be accelerated faster than light because it would require absolute mass/infinite energy. But with quantum entanglement there is no particle traveling between point a and b... so the fact that information can be passed faster than it would have taken a particle to traverse that distance does not violate relativity.


Even objects which propogate FTL do not "violate" realitivity. They simply violate human sense of realism which as realitivity and the quantum world has thought us was never worth much to begin with.
GoodElf
5 / 5 (1) Mar 23, 2012
Entanglement is instantaneous. The information sent by this means does not "travel" anywhere. In a recent paper "Impossibility of covariant deterministic nonlocal hidden-variable extensions of quantum theory" Nicolas Gisin PHYSICAL REVIEW A 83, 020102(R) (2011) (please download from NG's Homepage)... a special frame is necessary to enable this phenomenon. A kind of universal "now". Einstein had such a frame when he introduced a special way to synchronize clocks in different inertial frames of reference in Special Relativity. Alas there is no easy way to extend this concept beyond inertial frames. Properly synchronized, despite separation, the times represent the same arbitrary "0" clock time.
Gisin is noted for his experiments which demonstrated this anomaly. See "Physicists Seek Answers to Quantum Correlations"
http://www.physor...526.html
Though plausible the caution here is this "proposed" experiment has not yet been done
TheQuietMan
2 / 5 (1) Mar 24, 2012
I suspect the author of this article has no clue. The whole point of whole catalogs of quantum phenomena is that particles are at identical states, from LASERs to Bose Einstein condensates. We are not talking Xerox copies.
GoodElf
not rated yet Mar 24, 2012
The whole point of whole catalogs of quantum phenomena is that particles are at identical states... We are not talking Xerox copies
Agreed. However entangled quantum particles exhibit non-local orthogonality. Pairs of BBO down-converted photons are in orthogonal states which are "clones". This property is not exhibited by LASERS because each boson in a single transmitted "wave" come from separate sources... unless they are very special sources which emit single source-entangled photons a single pair at a time. This pair is then duplicated several times ("Xeroxed") to make up to 8 simultaneously entangled photons.
"First Observation Of 8 Entangled Photons Smashes Entanglement Record"
http://www.techno...v/26837/
Exactly reversing this process is what the experiment is "supposed" to be capable of doing. Conjecture is that "reversing" all these 8 photons back into 1 will not restore the original phase state and change the entanglement regime. Paper says "no".
Tausch
1 / 5 (2) Mar 25, 2012
...
it is impossible to take a single photon and make a number of photons that are in the exact same quantum state...


Alternative wording is... within measurable limits, physically indistinguishable states have different quantum states.

We have quantum information theory...asserting three theoretical foundational pillars:

Here the review;
http://www.physor...lly.html

Summarized -
1.)The no-cloning theorem
2.)The no-deleting theorem
3.)The no-hiding theorem

Quiet Man -
...particles are at identical states...


No human contortionist of the physical or mental universe can offer you a line of reasoning to uphold your assertion quoted above.

We are talking about energy distribution - nothing is copied - as you rightly noted - all energy distribution occurs by way of energy transfer resulting in quantum eigenstates - the word 'copy' makes no sense in quantum information theory.
Tausch
1 / 5 (2) Mar 25, 2012
First, it shows clearly that quantum information is preserved when copied


Let's help the author of the article out:

First, it shows clearly that quantum information is preserved when TRANSFERRED


There now. That wasn't so hard now, was it? And almost one 100% correct. For purists, still not 100% correct.
Why?
Callippo
1 / 5 (3) Mar 25, 2012
Unfortunately, even information cannot be sent FTL.
In single experiment - yes. But in parallel experiments this limit can be increased significantly. Another increasing of information speed is enabled, if we resign to the determinism of this information. For example in dense aether wave model the gravitational waves do propagate superluminaly and they do manifest with CMBR noise. The shielding of remote massive object should manifest with significant increasing of CMBR noise. This noise comes from all directions, so we cannot realize immediately, where the sender of information is, but such information will be transferred superluminaly. The experiments of Gunter Nimtz with superluminal information transfer via evancescent waves between two prism have similar character.
Callippo
1 / 5 (3) Mar 25, 2012
Another way of superluminal speed of information transfer the so-called metamaterial provide. It's all based on the same phenomena: the light waves are transverse, so they're do propagate trough vacuum with limited speed. At the moment, when we force them to propagate like the gravitational i.e. longitudinal waves, the speed of their spreading increases significantly. The spreading of such light will not be deterministic at the price. This aspect of metamaterials is ignored in quiet because it violates the relativity, although this way of superluminal light propagation has been already observed at the case of spreading of light trough clouds of dense interstellar gas, where strong dispersion occurs. Such dispersion must be connected with increasing of wavevelength of light, i.e. with negative refraction of environment for being able to work
Callippo
1 / 5 (2) Mar 25, 2012
They showed that it is possible to perfectly recover the original from the imperfect quantum copie
This article is incorrect in this regard, as the perfect quantum copying is known both experimentally, both theoretically as the solution of ElitzurVaidman's bomb-testing problem. Without it we could never separate the atoms by their spin in Stern-Gerlach-like experiment, for example, because the uncertainty principle of QM clearly says, you cannot know the non-commutative quantity (like the spin) and the commutative one (like the position) with unlimited precision. The principle of weak measurement in consecutive series of experiment demonstrates though, under certain condition it's still possible and the atoms can be separated by their spin with repetitive diffraction completely.
Callippo
1 / 5 (2) Mar 25, 2012
In this sense the quantum copping of photons occurs during every experiment with entangled photons, like this iconic experiment. http://www.davidj...gler.png This experiment provides two streams of entagled photons via passing of UV light trough nonlinear BBO crystal. We can amplify these photons via lasing, which would preserve their spin, i.e. polarization and we can use them in additional experiment, providing we will find another nonlinear crystal, which will behave in similar way at longer wavelengths. In this way we could get four entangled photons from two and so on. Note that the wavelength of photons is always changing toward lower values at the price: the quantum copying is always dissipative.
Tausch
2.3 / 5 (3) Mar 25, 2012
...the quantum copying...is always dissipative - C


There are no 'copies' in quantum information theory.
Just a transfer of information.

The rest of your research you must forge alone.
Until someone...anyone understands you.
kochevnik
1 / 5 (1) Mar 25, 2012
"it is impossible to take a single"That goes against the basic operation of a laser. Under inverted energy populations you get stimulated emission which produces, AFAIK, perfect copies.
Tausch
1 / 5 (2) Mar 25, 2012
You have photons, physically and measurably, indistinguishable from each other - in that sense you lasers exhibiting what you insist on labeling as 'cascading''copies' of photons. The quantum states of each photon giving rise to measurably indistinguishable physical states of the photons are different - not identical.

Quantum states are never physical states.
So the word 'copy' always refers to a physical state.
marraco
1 / 5 (2) Mar 25, 2012
"If the state of one member of an entangled pair of photons is measured, the state of the other is instantly determined"

Non compatible with Relativity definition of simultaneity. It cannot be "instantly" for all observers.
Foolish1
1 / 5 (1) Mar 26, 2012
Non compatible with Relativity definition of simultaneity. It cannot be "instantly" for all observers.


Its not like either party can know when the event occured so what does it matter? Why not?