Wheeler's "Delayed Choice" Experiment Done With Single Atoms

Looks like we now have the first "Delayed Choice" experiment done with single atoms, this one with single He atoms.

Indeed, the results of both Truscott and Aspect's experiments shows that a particle's wave or particle nature is most likely undefined until a measurement is made. The other less likely option would be that of backward causation – that the particle somehow has information from the future – but this involves sending a message faster than light, which is forbidden by the rules of relativity.

There are now many experiments that support QM's non-realism and quantum contextuality. This latest experiment adds to the body of evidence.

Zz.

Teleportation to a Solid-State Quantum Memory

The Gisin group has done it again! This time, they have managed to teleport a quantum state via photons and into a quantum memory in a form of a doped crystal.

Today, Felix Bussières at the University of Geneva in Switzerland and a few pals say they’ve taken an important step towards this. These guys have teleported quantum information to a crystal doped with rare-earth ions—a kind of quantum memory. But crucially they’ve done it for the first time over the kind of ordinary optical fiber that telecommunications that are in use all over the world.

This work has been published in Nature Photonics.

 Zz.

Light Stopped And Stored For More Than A Minute

Advances in this field of storing light have been astounding. It was only a few years ago that we had the amazing accomplishment of light being stopped and then "played back" out of Lene Hau's lab. And there have been more advances since then (read here). Now comes this latest paper (free access to the actual paper is available at that link).

There are two major accomplishment that are notable with this one:

1. They managed to store light and all of its coherent information for more than a minute, and

2. They are using a solid state medium, rather than atomic gasses, which will make this more viable for storing quantum information.

While solid-state devices would be preferable for applications, stopping light in solids is more challenging: stronger interactions between atoms and their environment severely limit the attainable coherence times. But the effect has been demonstrated in a special class of solids: crystals doped with rare-earth (RE) ions cooled at cryogenic temperatures. Since the atoms are naturally trapped in the crystal, the motion of RE atoms is limited and the transitions of interest take place between electronic levels (e.g., the 4f electrons of praseodymium) shielded from the crystal environment by outer full electronic shells (5s and 5p). This makes the coherence properties of these crystals exceptional.

Zz.

Catch And Release Of Photons

I'm always amazed by the nature of these experiments, and continue to be awed by what have been accomplished.

Two different groups published two different papers in this week's PRL, all reporting on the ability to store photons and then releasing them. What is even fascinating is that each group used different schemes to accomplish the same thing. I already reported on one of these results earlier. So check out the first link for a review of these two experiments, and you also get a link to free access to both papers.

Zz.

Store Photons. Then Make Them Intract With Each Other

It's amazing to see the development in this area. Just a few years ago, we were amazed at the accomplishments coming out of Lene Hau's lab in being able to not only store photons, but also to "replay" them back. Now comes this, which is another step in refining and improving the capability of storing photons.

What Charles Adams and colleagues at Durham University have now done is come up with a way of storing individual optical photons in highly excited states of an atomic gas. Once stored, the photons can be made to interact strongly, before being released again. An important feature of the technique is that it uses microwaves, which are also used to control some types of stationary qubit. 

 http://arxiv.org/pdf/1207.6007v3.pdf

Apparently, this is to be published in PRL.

Definitely a commendable accomplishment here in the evolution of our capability of using photons for quantum communications.

Zz.

Wineland and Haroche's PRL Papers

On the heels of AIP's free listing of the papers by this year's Physics Nobel Prize winners, the APS is making available for free the papers by these two authors that were published in PRL.

Zz.

Maxwell's Demon Exorcised?

Heh, sorry. Couldn't help with the pun.

It appears that there's now experimental evidence that erasing information requires the expenditure of energy. Consequently, this energy defeats the scenario presented in the Maxwell's Demon.

The results safeguard one of the most cherished principles of physical science: the second law of thermodynamics. This law states that heat will always move from hot to cold, or equivalently, that entropy — the amount of disorder in the Universe — always increases.

In the nineteenth century, the Scottish scientist James Clerk Maxwell proposed a scenario that seemed to violate this law. In a gas, hot molecules move faster than cold ones. Maxwell imagined a microscopic intelligent being, later dubbed a 'demon', that would open and shut a trapdoor between two compartments to selectively trap hot molecules in one of them and cool ones in the other, defying the tendency for heat to spread out and entropy to increase.

Landauer’s theory offered the first compelling reason why Maxwell’s demon couldn’t do its job. The demon would need to erase (‘forget’) the information it used to select the molecules after each operation, and this would release heat and increase entropy, more than counterbalancing the entropy lost by the demon.

It is very difficult to go against the 3rd Law, I tell ya!

Zz.

Can Quantum Theory Be Derived From More Fundamental Principles?

It is an intriguing question and certainly something that a number of theorists have been working on. The latest in such a development is the latest paper published in PRA this week, and covered this this week's APS Physics (you also get a free download of the actual paper)[1].

Abstract: We derive quantum theory from purely informational principles. Five elementary axioms—causality, perfect distinguishability, ideal compression, local distinguishability, and pure conditioning—define a broad class of theories of information processing that can be regarded as standard. One postulate—purification—singles out quantum theory within this class.

Quantum mechanics continues to be one of the most puzzling and amazing theory of our time.

Zz.

[1] G. Chiribella et al., Phys. Rev. A v.84, p.012311 (2011).

Bell-Type Experiments With No Loopholes?

In case you missed it, there's a very fascinating article in the March 18, 2011 issue of Science (p.1380). It describes the physics of quantum entanglement, which is essentially 2 separate phenomena - superposition and non-locality - and the Bell-type experiments that have demonstrated it (i.e. violation of Bell inequality), but up to a certain point. The article deals with the 2 types of loopholes - detection and locality loopholes - that still plague the experiments, and the efforts to design experiments that are devoid of these loopholes.

With their eyes on the prize, a group led by Paul Kwiat of the University of Illinois, Urbana-Champaign, has been collaborating with engineers at the U.S. National Institute of Standards and Technology (NIST) in Boulder, Colorado, to develop photon detectors with near 100% efficiency. “Those are good enough to perform a loophole-free test,” says team member Joseph Altepeter of Northwestern University in Evanston, Illinois. The struggle now is to chain these components together with optical fibers across a large enough distance to keep the communication loophole shut. “Essentially the pieces are all in place, but the devil is in the detail,” Altepeter says.

Meanwhile, Weinfurter and his colleagues are tackling the problem from an entirely different angle. They were inspired by an experiment, carried out in 2001 by David Wineland's team at NIST, that successfully closed the detection loophole using atoms rather than photons. Because atoms are far more hefty than flighty photons, Wineland realized, they are less likely to escape the apparatus, so they provide a potentially perfect detection rate. The team performed a Bell test that compared how often the energy levels—high or low—of electrons in entangled pairs of atoms matched up. Once again, quantum mechanics was hailed victorious, as the level of correlations exceeded Bell's inequalities. But it was not a resounding win because the atoms were close enough together to have influenced each other. In other words, the researchers had closed the detection loophole but in the process were forced to leave the communication loophole open.

Building on Wineland's experiment, Weinfurter's group is attempting to tie up both loopholes at once, by weaving photons together with atoms to reap the benefits of both. The idea is to start with two initially unentangled atoms in separate laboratories—ideally more than 100 meters apart, so that the atoms cannot influence each other over the course of the test. Each atom emits a photon; the two photons are captured and transmitted along optical fibers to a third location, where they are entangled. “The magic is that as soon as the photons are entangled, their parent atoms automatically become entangled, too,” explains Weinfurter's collaborator Marek Zukowski at the University of Gdansk in Poland.

These newly entangled atoms can then take the Bell test, with a perfect detection rate, while sitting far enough apart to keep the communication loophole closed. “The setup is being tried in two neighboring labs right now,” Zukowski says. “When we are happy that everything is working, we will try it in two distant labs.

Of course, the article then threw another wrench in the possible closure of these loopholes by pointing out the possibility of a "freedom-of-choice" loophole that can go back "... far back as the big bang..." Oy vey!

I think such superdeterminism needs to be shown to be influential for me to start putting any degree of validity on it.

It is a good article if you have access to it.

Zz.

"Switching Off" the Heisenberg Uncertainty Principle

A new scheme in quantum information measurement has been proposed that might, under certain situation, defeat the limit imposed by the Heisenberg Uncertainty principle (HUP). Instead of gathering information about the state from a "classical memory", the "... particle is prepared entangled with a quantum memory.. ", which is still a device that we do not have yet.

Heisenberg’s uncertainty principle applies wherever predictions about measured quantum mechanical variables are made on the basis of classical data. It arises due to the fact that, in quantum mechanics, there are no clearly defined 0 and 1 states like those of a bit in a classical computer, and instead several alternative possibilities can exist simultaneously. “If we collect the available information about a particle in a quantum memory, this makes this information more valuable than information gathered in a classical way,” says Renato Renner, Assistant Professor at the Institute for Theoretical Physics of ETH Zurich and co-author of the paper. These quantum data then theoretically allow measured variables to be predicted with any desired precision, and the Heisenberg uncertainty becomes arbitrarily small.

We will just have to see if such a quantum memory can be produced.

Zz.

Random Numbers Certified by Bell’s Theorem

Haven't had time to closely read this yet, but it is a good one.

Abstract: Randomness is a fundamental feature of nature and a valuable resource for applications ranging from cryptography and gambling to numerical simulation of physical and biological systems. Random numbers, however, are difficult to characterize mathematically, and their generation must rely on an unpredictable physical process. Inaccuracies in the theoretical modelling of such processes or failures of the devices, possibly due to adversarial attacks, limit the reliability of random number generators in ways that are difficult to control and detect. Here, inspired by earlier work on non-locality-based and device-independent quantum information processing, we show that the non-local correlations of entangled quantum particles can be used to certify the presence of genuine randomness. It is thereby possible to design a cryptographically secure random number generator that does not require any assumption about the internal working of the device. Such a strong form of randomness generation is impossible classically and possible in quantum systems only if certified by a Bell inequality violation15. We carry out a proof-of-concept demonstration of this proposal in a system of two entangled atoms separated by approximately one metre. The observed Bell inequality violation, featuring near perfect detection efficiency, guarantees that 42 new random numbers are generated with 99 per cent confidence. Our results lay the groundwork for future device-independent quantum information experiments and for addressing fundamental issues raised by the intrinsic randomness of quantum theory.

S. Pironio et al., Nature v.464, p.1021 (2010).

A news report on this paper can be found here.

Zz.

The Inadequacy of Everettian Accounts of Evolution, Probability, and Scientific Confirmation

This is certainly a provocative title. The manuscript, which is a chapter in an upcoming book, I would guess, directly questions whether Everette's "Many-World" model for quantum theory is in fact adequate, or whether it has actually failed.

Abstract: There is a compelling intellectual case for exploring whether purely unitary quantum theory defines a sensible and scientifically adequate theory, as Everett originally proposed. Many different and incompatible attempts to define a coherent Everettian quantum theory have been made over the past fifty years. However, no known version of the theory (unadorned by extra ad hoc postulates) can account for the appearance of probabilities and explain why the theory it was meant to replace, Copenhagen quantum theory, appears to be confirmed, or more generally why our evolutionary history appears to be Born-rule typical. This article reviews some ingenious and interesting recent attempts in this direction by Wallace, Greaves, Myrvold and others, and explains why they don't work. An account of one-world randomness, which appears scientifically satisfactory, and has no many-worlds analogue, is proposed. A fundamental obstacle to confirming many-worlds theories is illustrated by considering some toy many-worlds models. These models show that branch weights can exist without having any role in either rational decision-making or theory confirmation, and also that the latter two roles are logically separate. Wallace's proposed decision theoretic axioms for rational agents in a multiverse and claimed derivation of the Born rule are examined. It is argued that Wallace's strategy of axiomatizing a mathematically precise decision theory within a fuzzy Everettian quasiclassical ontology is incoherent. Moreover, Wallace's axioms are not constitutive of rationality either in Everettian quantum theory or in theories in which branchings and branch weights are precisely defined. In both cases, there exist coherent rational strategies that violate some of the axioms.

It's 26 pages long, and at this point, I haven't had the chance to read it yet. But you might have time, so I'm not going to deprive you of the pleasure (torture?) of reading it. :)

Zz.

How Fundamental Particles Lose Track Of Quantum Mechanical Properties

We have another report on the study on the mechanism of the differences between the quantum world and our classical world. This is another study on the effect of decoherence on a quantum system that couples to an external "bath".

It would be interesting to compare this to an earlier report on the emergence of a classical system from a single-particle state after just one interaction. It is also interesting to see how Roger Penrose would handle this. He seems to think that our classical world emerges due to some coupling or interaction between the quantum systems and gravity, and that eventually destroys the quantum system and out comes the classical system. These two papers above seems to indicate that the mere act of decoherence might be sufficient to produce the classical world.

Zz.

Physicists Successfully Store and Retrieve Nothing

This could easily fit in as an episode of the Jerry Seinfeld series.

It appears that there is such a thing as a "squeezed vacuum", and it takes some effort to store and retrieve this "nothingness".

To see what this is, begin with a normal light wave. Classically, this is a smooth wave of electromagnetic fields with equally spaced peaks and dips. But throw in quantum mechanics and things get more complicated. The precise height of the wave becomes uncertain, so the wave gets fuzzy (see figure). Physicists have learned how to manipulate that inevitable uncertainty--for example, making it smaller at the peaks and larger in between. That makes "phase-squeezed light." Now imagine turning down the intensity of the phase-squeezed light to zero. The wave itself goes away, but the waxing and waning uncertainty remains, creating a squeezed vacuum.

It's interesting that two separate groups produced work on this at almost the same time. This, of course, is not unusual, and PRL, Nature, and Science have been known to put such things in the same issue. It serves to reinforce the discovery.

Zz.

Atoms Swap Spins

This is another rather cool experiment (pun intended) published in this week's Nature[1]. They have managed to "swap" spin states between cold Rb atoms.

Anderlini et al. find a way to make use of a similar symmetry-based constraint. They work with bosonic rubidium atoms, 87Rb, that have a symmetric total wavefunction. This wavefunction has two components: a spin component describing the internal state of the atoms, and a spatial component describing their locations. Because of the fixed exchange symmetry of the total wavefunction, the symmetries of the spin and spatial wavefunctions are precisely related: if the spin wavefunction for 87Rb atoms is symmetric, then the spatial wavefunction is also symmetric, and vice versa. Crucially, antisymmetric spatial wavefunctions hinder particles from getting close to each other, whereas symmetric spatial wavefunctions favour it. Because the atoms interact effectively only when they come into contact, particles in symmetric spatial states interact with each other, whereas particles in antisymmetric spatial states do not.

Anderlini et al. stored quantum information in the atoms' spin wavefunction, such that the stored bits determined its symmetry character — symmetric, antisymmetric or a superposition of both. The spin wavefunction also controlled the spatial wavefunction through the direct link between their symmetries, and so determined the collisional properties. Thus, the state of the quantum bits controlled the atoms' interactions.

Also read the News and Views review in the same issue of Nature, and also a report on this work on PhysicsWeb.

Zz.

[1] M. Anderlini et al. Nature v.448, p.452 (2007).