Archive for the ‘Weird 'n' spooky’ Category

Human eye could detect spooky action at a distance

Thursday, February 19th, 2009

mantanglement

It’s almost a year since Nicolas Gisin and colleagues at the University of Geneva announced that they had calculated that a human eye ought to be able to detect entangled photons. “Entanglement in principle could be seen,” they concluded.

That’s extraordinary because it would mean that the humans involved in such an experiment would become entangled themselves, if only for an instant.

Gisin is a world leader in quantum entanglement and his claims are by no means easy to dismiss.

Now he’s going a step further saying that the human eye could be used in a Bell type experiment to sense spooky-action-at-a-distance. “Quantum experiments with human
eyes as detectors appear possible, based on a realistic model of the eye as a photon detector,” they say.

One problem is that human eyes cannot se single photons–a handful are needed to trigger a nerve impulse to the brain.

That might have scuppered the possibility of  a Bell-type experiment were it not for some interesting work from Francesco De Martini and buddies at the Universityof Rome, pointing out how the quantum properties of a single particle can be transferred to an ensemble of particles.

That allows a single entangled photon, which a human eye cannot see, to be amplified into a number of entangled photons that can be seen. The eye can then be treated like any other detector.

This all looks like fun. The first person to experience entanglement –mantanglement–would surely be destined for some interesting press covereage.

But the work raises an obvious question: why is Gisin pursuing this line? The human eyeball could be put to use in plenty of optics experiments, so why the focus on mantanglement?

Could it be that Gisin thinks there is more to entanglement than meets the eye?

Ref: arxiv.org/abs/0902.2896: Quantum experiments with human eyes as detectors based on cloning via stimulated
emission

2D image created from a single pixel sensor

Wednesday, December 17th, 2008

ghost-imaging

Ghost imaging is a curious phenomenon that has had numerous physicists scratching their heads in recent years.

It works like this: take two beams of entangled photons and aim the first at an object. The transmitted photons from the object are then collected by a single pixel detector.

The second beam is aimed at a CCD array without ever having hit the object.

It turns out it is possible to reconstruct an image of the object–a so-called ghost image–by matching the data from the two detectors, even though the single pixel detector has no spatial resolution.

When this was first demonstrated in 1995, everybody was amazed by the strange power of quantum entanglement.

But later, various groups showed that entangled beams weren’t necessary at all and that ordinary light from a pseudothermal source would do the job just as well.

While interesting, that doesn’t actually rule out the possibility that the two beams may be correlated in some entangled-like quantum way, however.

So the question of whether quantum entanglement is responsible or not has remained open. Until now.

Yaron Silberberg and pals from the Weizmann Institute of Science in Israel, have carried out an ingenious experiment that settles the matter.

They use only one beam, which they use to illuminate the object, and collect the transmitted photons using a single pixel detector. They then calculate theoretically what the second beam should look like and combine the single pixel data with this “virtual beam”.

And get this: they still see a ghost image. That’s a 2D image from a single pixel detector! And a pretty convincing demonstration that quantum entanglement cannot be responsible.

The question now is: what kind of classical information processing allows the reconstruction of a 2d image from a single pixel sensor? That’s a real puzzle.

Ref: arxiv.org/abs/0812.2633: Ghost Imaging with a Single Detector

Quantum direct communication: secrecy without key distribution

Friday, December 5th, 2008

quantum-direct-communication

An interesting development in the world of quantum encryption.

In the last couple of years, we’ve seen a number of quantum key distribution systems being set up that boast close-to-perfect security (although they’re not as secure as the marketing might imply).

These systems rely on two-part security. The first is the quantum part which reveals whether a message has been intercepted or not. Obviously this is no use when it comes to sending secret message because it can only uncover eavesdroppers after the fact.

So Alice sends a one time pad over this quantum channel that she and Bob can later use to encrypt and send a message classically. If this key is compromised, Alice sends another.

What guarantees the security is not quantum mechanics but the second part of the system: the one time pad.

Today, Seth Lloyd and colleagues at the Massachusetts Institute of Technology in Cambridge, publish a way of guaranteeing security over a quantum channel without having to fall back on a one time pad.

Their idea is to send a message over a standard quantum channel without bothering with a one time pad. The security, they say, can be monitored by randomly checking the channel to see whether any of the qubits are being lost (potentially to Eve).

The security of the channel then depends on how much loss of information Alice and Bob are willing to accept, but can always be improved by checking more often for eavesdroppers.

Quantum direct communication, as the team call it, looks interesting. But it will be demanding to implement, not least because any noise in the channel will look like an eavesdropper. So it looks as if this idea will have to be limited to short range applications where noise can be kept to a minimum.

Nevertheless, a cool idea.

Ref: arxiv.org/abs/0802.0656: Quantum Direct Communication with Continuous Variables

Loop quantum cosmology: a brief overview

Wednesday, December 3rd, 2008

quantum-loop-cosmology

Abhay Ashtekar, a physicist at the Pennsylvania State University is one of the founders of loop quantum cosmology and also a part-time populariser of science.

Today, he uses both of these attributes to produce a fascinating overview of loop quantum cosmology that non-specialists will find enlightening.

A recommended read.

Ref: arxiv.org/abs/0812.0177: Loop Quantum Cosmology: An Overview

Quantum test found for mathematical undecidability

Tuesday, December 2nd, 2008

mathematical-undecidability

It was the physicist Eugene Wigner who discussed the “unreasonable effectiveness of mathematics” in a now famous paper that examined the profound link between mathematics and physics.

Today, Anton Zeilinger and pals at the University of Vienna in Austria reveal this link at its deepest. Their experiment involves the issue of mathematical decidability.

First, some background about axioms and propositions. The group explains that any formal logical system must be based on axioms, which are propositions that are defined to be true. A proposition is logically independent from a given set of axioms if it can neither be proved nor disproved from the axioms.

They then move on to the notion of undecidability. Mathematically undecidable propositions contain entirely new information which cannot be reduced to the information in the axioms. And given a set of axioms that contains a certain amount of information, it is impossible to deduce the truth value of a proposition which, together with the axioms, contains more information than the set of axioms itself.

These notions gave Zeilinger and co an idea. Why not encode a set of axioms as quantum states. A particular measurement on this system can then be thought of as a proposition. The researchers say that whenever a proposition is undecidable, the measurement should give a random result.

They’ve even tested the idea and say they’ve shown the undecidability of certain propositions because they generate random results.

Good stuff and it raises some interesting issues.

Let’s leave aside the problem of determining whether the result of particular measurement is truly random or not and take at face value the groups claim that “this sheds new light on the (mathematical) origin of quantum randomness in these measurements”.

There’s no question that what Zeilinger and co have done is fascinating and important. But isn’t the fact that a quantum system behaves in a logically consistent way exactly what you’d expect?

And if so, is it reasonable to decide that, far from being fantastically profound, Zeilinger’s experiment is actually utterly trivial?

Ref: http://arxiv.org/abs/0811.4542: Mathematical Undecidability and Quantum Randomness

Quantum cloaking makes molecules invisible

Friday, November 14th, 2008

Cloaking is surely the zeitgeist topic of the moment and for proof, you need look no further than the work of Jessica Fransson from the University of Upssala in Sweden and colleagues. This is a group who have who have applied the ideas of cloaking to the quantum world and come up trumps. the result is a design for a molecular cloak that could turn out to be extremely useful.

First what does it mean to see or not see a quantum object? Fransson and co say that seeing is equivalent to detecting quantum objects and in the case of molecules that means looking for the terahertz radiation they produced when they vibrate.

“We propose a method for detecting and manipulating quantum invisibility based on THz cloaking of molecular identity in coherent nanostructures,” says Fransson and buddies.

In practice, this means designing quantum corals, elliptical nanostructures, that absorb terahertz waves of specific frequencies. When a molecule that emits this frequency is placed at the focus, it cannot be spotted. It is essentially invisible.

Useful? You bet. Such a quantum coral would be ideally suited to detecting molecules of specific species while ignoring others. For example, if you have a particular molecular species that poisons your measurements, then what you need is a cloak that will make it invisible to your detectors

It’s ideas like this that are going to make cloaking mighty useful one of these days.

Ref: arxiv.org/abs/0811.1782: Quantum Detection and Invisibility in Coherent Nanostructures

Solving a quantum conundrum

Monday, November 3rd, 2008

“Can one be convinced of the correctness of the computation of every quantum circuit, namely, every quantum experiment that can be conducted in the laboratory?” ask Dorit Aharonov and colleagues from the Hebrew University of Jerusalem in Israel.

That’s an interesting question of quantum computer science. If you can’t simulate the answer to calculation on a classical computer, how do you know the answer that a quantum computer arrives at is correct.

There’s a clue, says Aharanov, in Shor’s algorithm, which factors large numbers on a quantum computer. There is no way of knowing in advance whether the factors Shor’s algorithm finds are correct because that would be tantamount to the seemingly impossible task of  factoring numbers in polynomial time. But you can determine the veracity of the answer in retrospect simply by multiplying the factors together, which can be done easily on a classica lcomputer.

Aharanov and co use this as the model  for a new type of quantum proof that is analogous to the interactive proof systems used in classical computer science.

The group says the approach could be used to tackle a number of interesting problems in quantum computer science such as proving the performance of a quantum computer for a customer wishing to buy one for their own tasks.

That’ll be handy since clock cycles aren’t going to make the grade in the quantum computing age.

Ref: arxiv.org/abs/0810.5375: Interactive Proofs For Quantum Computations

Entangled photons to produce better quantum images

Thursday, October 16th, 2008

A while back, we saw how quantum imaging had been put on a firmer theoretical footing, thanks to some new thinking by Seth Lloyd at MIT.

Quantum imaging involves sending one of a pair of entangled photons towards an object while holding on to the other.

For a long while nobody was quite sure what benefit you might get from this entanglement. Some physicists speculated that it could be possible to produce reflection-free images by measuring the entangled twin that you hang on to, even if the other photon never returns.

What Lloyd calculated was that illuminating an object with entangled photons can increase the signal to noise ratio of the reflected signal by a factor of 2^e, where e is the number of bits of entanglement. That’s an exponential improvement.

Now he and a few pals have filled in a few details in the scheme that make it more realistic. done the experiment and shown that Lloyd was right on the money. They sent photons towards an object and used the reflection to determine whether the object was present or absent.

When they used entangled photons, this process was much more efficient.

The result is effectively the first quantum image taken with entangled photons .

Now all we’re waiting for is experimental proof of the scheme which, if I’m not mistaken, won’t be long in coming. The work was part funded by DARPA’s Quantum Sensor Program so it’ll be interesting to see what plans the organisation has for this technique.

Ref: arxiv.org/abs/0810.0534: Quantum illumination with Gaussian states

Loophole found in quantum cryptography photon detectors

Tuesday, September 23rd, 2008

quantum-cryptoflaw.jpg

If you’re hoping to secure your data using quantum cryptography, you might want to find a shoulder to cry on.

Quantum cryptography ought to be 100 percent secure. In theory , it provides perfect security against eavesdroppers. But in practice, a number of loopholes have emerged (see here and here). And today, Vadim Makarov and pals at the Norwegian University of Science and Technology reveal another.

One crucial piece of kit that every quantum cryptographer needs is a detector capable of spotting single photons. And the detector of choice in about half of quantum cryptography experiments is the Perkin Elmer SPCM-AQR detector module. “Until recently, this has been the only commercially available Si single photon detector model,” say Makarov and buddies.

Sadly, it turns out to have a significant flaw. The Norwegian team says that bombarding the machine with bright optical pulses can override the control circuitry in a way that allows an eavesdropper to control its output. That gives Eve a way of staging a successful intercept attack.

I know what you’re thinking: why not switch to the gear used in the other half of quantum crypto experiments? The answer is that Makarov and pals have already shown that these devices have a vulnerability.

All is not lost, however. Now that the vulnerability has been revealed it should be straightforward to implement extra security to foil such an attack.

But the implications are clear. The eternal cat and mouse game between eavesdroppers and their victims looks set to continue. Which means that quantum cryptography may never be perfect.

Ref: arxiv.org/abs/0809.3408: Can Eve control PerkinElmer actively-quenched single-photon detector?

How to measure macroscopic entanglement

Monday, August 18th, 2008

Macroscopic entanglement

If macroscopic objects become entangled, how can we tell? The usual way to measure entanglement on the microscopic level is to carry out a Bell experiment, in which the quantum states of two particles are measured.  If the results of these measurements fall within certain bounds, the particles are considered to be entangled.

These kinds of quantum measurements are not possible with macroscopic bodies but recent work suggests there may be other ways to spot entanglement. Vlatko Vedral  at the University of Leeds and pals outline one of these on the arXiv.

Their idea is based on the third law of thermodynamics which states that the entropy at absolute zero is dependent only on the degeneracy of the ground state. This in turn implies that the specific heat capacity of a material must asymptotically approach zero as the temperature gets closer to absolute zero. But if particles within the material were entangled, Vedral and pals say this would not be the case.

That kind of thinking suggests a straightforward experiment: simply measure the heat capacity of a material as its temperature drops to zero. If it doesn’t asymptotically approach zero, then you’ve got some entanglement on your hands.

Best of all, measuring heat capacity is standard technique so there’s no reason this can’t be done pronto.

Ref: arxiv.org/abs/quant-ph/0508193: Heat Capacity as A Witness of Entanglement