Chinese Physicists Smash Distance Record For Teleportation

The ability to teleport photons through 100 kilometres of free space opens the way for satellite-based quantum communications, say researchers
 

Teleportation is the extraordinary ability to transfer objects from one location to another without travelling through the intervening space.

The idea is not that the physical object is teleported but the information that describes it. This can then be applied to a similar object in a new location which effectively takes on the new identity.

And it is by no means science fiction. Physicists have been teleporting photons since 1997 and the technique is now standard in optics laboratories all over the world.

The phenomenon that makes this possible is known as quantum entanglement,  the deep and mysterious link that occurs when two quantum objects share the same existence and yet are separated in space.

Teleportation turns out to be extremely useful. Because teleported information does not travel through the intervening space, it cannot be secretly accessed by an eavesdropper.

For that reason, teleportation is the enabling technology behind quantum cryptography, a way of sending information with close-to-perfect secrecy.

Unfortunately, entangled photons are fragile objects. They cannot travel further than a kilometre or so down optical fibres because the photons end up interacting with the glass breaking the entanglement. That severely limits quantum cryptography's usefulness.

However, physicists have had more success teleporting photons through the atmosphere. In 2010, a Chinese team announced that it had teleported single photons over a distance of 16 kilometres. Handy but not exactly Earth-shattering.

Now the same team says it has smashed this record. Juan Yin at the University of Science and Technology of China in Shanghai, and a bunch of mates say they have teleported entangled photons over a distance of 97 kilometres across a lake in China.

That's an impressive feat for several reasons. The trick these guys have perfected is to find a way to use a 1.3 Watt laser and some fancy optics to beam the light and receive it.

Inevitably photons get lost and entanglement is destroyed in such a process. Imperfections in the optics and air turbulence account for some of these losses but the biggest problem is beam widening (they did the experiment at an altitude of about 4000 metres). Since the beam spreads out as it travels, many of the photons simply miss the target altogether.

So the most important advance these guys have made is to develop a steering mechanism using a guide laser that keeps the beam precisely on target. As a result, they were able to teleport more than 1100 photons in 4 hours over a distance of 97 kilometres.

That's interesting because it's the same channel attenuation that you'd have to cope with when beaming photons to a satellite with, say, 20 centimetre optics orbiting at about 500 kilometres. "The successful quantum teleportation over such channel losses in combination with our high-frequency and high-accuracy [aiming] technique show the feasibility of satellite-based ultra-long-distance quantum teleportation," say Juan and co.

So these guys clearly have their eye on the possibility of satellite-based quantum cryptography which would provide ultra secure communications around the world. That's in stark contrast to the few kilometres that are possible with commercial quantum cryptography gear.

Of course, data rates are likely to be slow and the rapidly emerging technology of quantum repeaters will extend the reach of ground-based quantum cryptography so that it could reach around the world, in principle at least.

But a perfect, satellite-based security system might be a useful piece of kit to have on the roof of an embassy or distributed among the armed forces.

Something for western security experts to think about.

Ref: arxiv.org/abs/1205.2024: Teleporting Independent Qubits Through A 97 Km Free-Space Channel

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Quantum Rainbow Photon Gun Unveiled

  Published by MIT

A photon gun capable of reliably producing single photons of different colours could become an important building block of a quantum internet

We've heard much about the possibility of a quantum internet which uses single photons to encode and send information protected by the emerging technology of quantum cryptography.

The main advantage is of such a system is perfect security, the kind of thing that governments, the military, banks and assorted other groups would pay handsomely to achieve.

One of the enabling technologies for a quantum internet is a reliable photon gun that can fire single photons on demand. That's not easy.

One of the significant weaknesses of current quantum cryptographic systems is the finite possibility that today's lasers emit photons in bunches rather than one at a time. When this happens, an eavesdropper can use these extra photons to extract information about the data being transmitted.

So there's no shortage of interest in developing photon guns that emit single photons and indeed various groups have made significant progress towards this.

Against this background, Michael Fortsch at the Max Planck Institute for the Science of Light in Erlangen, Germany, and a few pals today say they've made a significant breakthrough. These guys reckon they've built a photon emitter with a range of properties that make it far more flexible, efficient and useful than any before--a kind of photon supergun.

The gun is a disc-shaped crystal of lithium niobate zapped with 582nm light from a neodymium-doped yttrium aluminium garnet (Nd:YAG) laser. Lithium niobate is a nonlinear material that causes single photons to spontaneously convert into photon pairs.

So the 582nm photons ricochet around inside the disc and eventually emerge either as unchanged 582nm photons or as a pair of entangled photons with about twice the wavelength (about 1060nm). This entangled pair don't have quite the same wavelength and so all three types of photon can be easily separated.

The 582 nm photons are ignored. Of the other pair, one is used to transmit information and the other is picked up by a detector to confirm that the other photon is ready form transmission.

So what's so special about this photon gun? First and most important is that the gun emits photons in pairs. That's significant because the detection of one photon is an unambiguous sign that another has also been emitted. It's like a time stamp that says a photon is on its way.

This so-called photon herald means that there can be no confusion over whether the gun is secretly leaking information to a potential eavesdropper.

This gun is also fast, emitting some 10 million pairs of photons per second per mW and also two orders of magnitude more efficient than other photon guns.

These guys can also change the wavelength of the photons the gun emits by heating or cooling the crystal and thereby changing its size. This rainbow of colours stretches over 100nm (OK, not quite a rainbow but you get the picture).

That's important because it means the gun can be tuned to various different atomic transitions allowing physicists and engineers to play with a variety of different atoms for quantum information storage.

All in all, an impressive feat and clearly an enabling step along the way to more powerful quantum information processing tools.

Ref: arxiv.org/abs/1204.3056: A Versatile Source of Single Photons for Quantum Information Processing

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Can The Human Brain See Quantum Images?

Nobody knows whether humans can access exotic images based on quantum entanglement. Now one physicist has designed an experiment to find out

The strange rules of the quantum world lead to many weird phenomena. One of these is the puzzling process of quantum imaging, which allows images to form in hitherto unimagined ways.

Researchers begin by creating entangled pairs by sending a single laser  beam into a non-linear crystal, which converts single photons into entangled pairs of lower frequency photons, a process known as parametric down conversion. A continuous beam generates a series of pairs of entangled photons.

Next, they send the entangled photons towards a pair of detectors. Each member of an entangled pair by itself fluctuates in random ways that make its time and position of arrival uncertain.

Use one of the detectors to receive just one half of the entangled photons and the result is a blur, smeared by the process of randomness.

But use two detectors to receive both sets of photons and the uncertainties disappear, or at least are dramatically reduced. In this case, the 'image' is pinsharp. The uncertainty disappears because of the quantum correlation between the entangled pairs.

Researchers have extended this technique by superimposing a pattern on the wavefront of the initial laser beam, creating shapes such as a donut. They've shown that a single detector alone cannot 'see' a such a donut image even though it appears clean and sharp when two detectors pick up both sets of the entangled pairs.

These strange pictures are called quantum images or higher order images and quantum physicists think they can use them to carry out exotic processes such as sending information secretly and performing quantum lithography.

Today, Geraldo Barbosa at Northwestern University in Evanston, Illinois, raises another interesting possibility. He asks whether it is possible for humans to see higher order images and suggests that a relatively simple experiment could settle the question.

This experiment consists of a laser beam shaped into an image, such as the letter A. This laser then hits a non-linear crystal, generating entangled pairs of photons that retain this image shape. The set up is such that these photons are then detected, not by conventional detectors, but by human eyeballs.

The question is whether the human retina/brain combination can access the correlation that exists between the entangled pairs. If so, the human would see the letter A. If not, he or she would see only a blur.

Of course, there are some significant experimental challenges. One is to design the experiment in a way  that ensures the subject can only receive the image through this quantum process and not through some other channel, such as talking to the experimenter. However, that should be straightforward for any psychologist to design.

Another problem, however, is that the retina can only detect photons in groups of 7 or more and these have to arrive within a specific time window. Only then can a human subject 'see' the result. Generating the required intensity of entangled photons is one challenge.

The key question is whether the entanglement survives this group process. If the brain can access the quantum correlations, the image will be visible. If not, the result will be a blur.

That's a fascinating experiment not least because a positive result would be astounding. It would show that we humans can essentially 'see' entanglement.

Barbosa points out that new forms of imaging are not unknown in the animal world. Various animals and insects see in the infrared and ultraviolet, giving them an entirely different perspective on the world.

There is also some evidence that birds can 'see' the earth's magnetic field thanks to the quantum interaction between the field and light sensitive molecules in their retinas.

So the possibility that new ways of seeing the world can emerge is not unprecedented. However, the idea that humans can access higher order images thanks to quantum entanglement is clearly an idea of a different ilk.

Perhaps the most exciting aspect of Barbosa's idea is that it appears feasible now. There's no reason why this experiment couldn't be done in any quantum optics lab in the near future.

We'll look forward to seeing the results.

Ref: arxiv.org/abs/1202.5434: Can humans see beyond intensity images?

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