[Quantum Communication] More reliable way to produce single photons

[Hamartia Antidote]

http://phys.org/news/2016-11-reliable-photons.html

More reliable way to produce single photons


Credit: University of Bath
Physicists at the University of Bath have developed a technique to more reliably produce single photons that can be imprinted with quantum information.

The invention will benefit a variety of processes which rely on photons to carry quantum information, such as quantum computing, secure quantum communication and precision measurements at low light levels.

Photons, particles of light, can be imprinted with information to be used for things like carrying out calculations and transmitting messages. To do this you need to create individual photons, which is a complicated and difficult process.

However researchers from the Centre for Photonics and Photonic Materials have implemented a new way to improve the performance of single-photon sources using fibre-optics and fast optical switches.

They combined several individual sources of photons using optical switches, a technique called multiplexing, using fibre optics fabricated at the University. The resulting device not only makes generating single photons more reliable but also allows control of properties of the photons created, including their colour.

Dr Robert Francis-Jones, from the Centre for Photonics and Photonic Materials, said: "Developing improved sources of single photons is one of the most pressing issues in quantum information processing. Through this research we hope to accelerate the transition of quantum-enhanced technologies from the lab to applications such as drug discovery."

The study is published in the journal Optica.

 

[Hamartia Antidote]

http://phys.org/news/2016-11-single-photon-convertera-key-component-quantum.html
Researchers develop single-photon converter—a key component of quantum internet


A single photon converter (a yellow-orange box) installed on an optical fiber of the laboratory setup. Credit: UW, Grzegorz Krzysewski
A Polish-British team of physicists has constructed and tested a compact, efficient converter capable of modifying the quantum properties of individual photons. The new device should facilitate the construction of complex quantum computers, and in the future may become an important element in global quantum networks, the successors of today's Internet.

Quantum internet and hybrid quantum computers, built out of subsystems that operate by means of physical phenomena, are now more than just the stuff of imagination. In an article published in Nature Photonics, physicists from the University of Warsaw's Faculty of Physics (FUW) and the University of Oxford report the development of a key element of such systems: an electro-optical device that enables the properties of individual photons to be modified. Unlike existing laboratory constructions, this new device works with previously unattainable efficiency and is at the same time stable, reliable and compact.

Building an efficient device for modifying the quantum state of individual photons was an exceptionally challenging task, given the fundamental differences between classical and quantum computing.

Contemporary computing systems are based on the processing of groups of bits, each of which is in a specific state: either 0 or 1. Groups of such bits are continually transferred between different subcomponents within a single computer, and between different computers on the network. We can illustrate this figuratively by imagining a situation in which trays of coins are being moved from place to place, with each coin showing either heads or tails.


A single photon -- a carrier of quantum information -- travels like a spinning coin, in a superposition of states. Modyfing its properties is extremely hard and should be done carefully, without destroying the superposition. Credit: FUW, Grzegorz Krzyzewski
Things are more complicated in quantum computing, which relies on the phenomenon of superposition of states. A quantum bit, known as a qubit, can be both in the 1 state and the 0 state at the same time. To extend the metaphor of the coins, this is analogous to a situation in which each coin is spinning on its edge. Information processing can be described as "quantum" processing as long as this superposition of states is retained during all operations—in other words, as long as none of the coins gets tipped out of the spinning state while the tray is being moved.

"In recent years, physicists have figured out how to generate light pulses with a specific wavelength or polarization, consisting of a single quantum—or excitation—of the electromagnetic field. And so today, we know how to generate precisely whatever kind of quantum 'spinning coins' we want," says Dr. Michal Karpinski from the Institute of Experimental Physics (FUW), one of the authors of the publication. "But achieving one thing always leaves you wanting more. If we now have individual light quanta with specific properties, it would be useful to modify those properties. The task is therefore to take a spinning silver coin and move it from one place to another, while quickly and precisely turning it into a gold coin, naturally without tipping it over. You can easily see that the problem is nontrivial."


Existing methods of modifying individual photons have utilized nonlinear optical techniques—in practice, attempting to force an individual photon to interact with a very strong optical pump beam. Whether the photon actually gets modified is a matter of pure chance. Moreover, the scattering of the pump beam may contaminate the stream of individual photons. In constructing the new device, the group from the University of Warsaw and the University of Oxford decided to use a different physical phenomenon: the electro-optic effect occurring in certain crystals. It provides a way to alter the index of refraction for light in the crystal by varying the intensity of an external magnetic force that is applied to it (in other words, without introducing any additional photons).

"It is quite astounding that in order to modify the quantum properties of individual photons, we can successfully apply techniques very similar to those used in standard fiber-optic telecommunications," Dr. Karpinski says.


Usually, due to the properties mismatch, the majority of single photons cannot be effectively stored e.g. in the quantum memory (represented as a white box). The new converter enables to modify the properties of photons so that virtually …more
Using the new device, the researchers achieved a six-fold lengthening of the duration of a single-photon pulse without disrupting the quantum superposition, which automatically means a narrowing of its spectrum. What is particularly important is that the whole operation was carried out while preserving very high conversion efficiency. Existing converters have operated only under laboratory conditions and were only able to modify one in several tens of photons. The new device works with efficiency in excess of 30 percent, up to 200 times better than certain existing solutions, while retaining a low level of noise.

"In essence, we process every photon entering the crystal. The efficiency is less than 100 percent not because of the physics of the phenomenon, but on account of hard-to-avoid losses of a purely technical nature, appearing, for instance, when light enters or exits optical fibers," explains Ph.D. student Michal Jachura (FUW).

The new converter is not only efficient and low-noise, but also stable and compact. The device can be contained in a box around 10 cm (4 in.), easy to install in an optical fiber system channeling individual photons. Such a device could enable building such things as hybrid quantum computers, the individual subcomponents of which would process information using different physical platforms and quantum phenomena. At present, attempts are being made to build quantum computers using things like trapped ions, electron spins in diamond, quantum dots, superconducting electric circuits, and atomic clouds. Each system interacts with light of different properties, which in practice rules out optical transmission of quantum information between different systems. The new converter, on the other hand, can efficiently transform single-photon pulses of light compatible with one system into pulses compatible with another. Scientists are therefore working toward quantum networks, both small ones within a single quantum computer (or subcomponent thereof), and global ones providing a way to send data completely securely between quantum computers situated in different parts of the world.



Read more at: http://phys.org/news/2016-11-single-photon-convertera-key-component-quantum.html#jCp

 

[Hamartia Antidote]

http://physicsworld.com/cws/article/news/2016/nov/25/new-optical-device-absorbs-just-one-photon


Lone photon: the new device will extract exactly one photon from a beam of light
Physicists in Germany have created a new optical device that can absorb exactly one photon. They say that this device, which exploits the physical properties of giant micron-sized atoms known as Rydberg atoms, could be used in optical quantum computing networks of the future.

Sebastian Hofferberth of the University of Stuttgart explains that the device first behaves like a dark sunglasses lens, but once it absorbs its first photon it becomes transparent to light. One important application of the device, says Hofferberth, could be to absorb single photons from a quantum network. Another potential application is a precise photon counter, which could be made by putting a number of the devices in series.

Atomic cloud
At the heart of the device is a micron-sized diffuse cloud of rubidium atoms cooled to near-absolute-zero temperature. To make the cloud only absorb a single photon, it is first illuminated with laser light with precisely enough energy to excite the atoms' outermost electron into the 121st energy level. There the electron is about a thousand times further from the nucleus than it would be in the atom's ground state. Such atoms have radii of more than a micron and are known as Rydberg atoms.

When a rubidium atom in the cloud absorbs a single photon to become the first Rydberg atom, no other atom can accept another photon from the laser beam. This is because the first Rydberg atom's outermost electron is so far from its nucleus that it overlaps with all the other atoms in the cloud, changing their electronic structures. "The presence of the first Rydberg atom has such a strong influence that it changes the resonance conditions for all the other atoms," Hofferberth says, adding "Rydberg atoms can interact with its neighbours about 10 microns away". Because no other atoms can absorb photons, the cloud becomes transparent.

To verify that only one photon had been captured, the researchers use the fact that the outer electron is loosely bound to the Rydberg atom's nucleus. "They're very fragile," Hofferberth says. So to verify that the cloud only absorbed one atom, he and his colleagues converted the Rydberg atom into a rubidium ion by knocking the outermost electron away. Then, they counted how many rubidium ions were present – and measured only one.

Delicate process
Creating this photon absorber was experimentally difficult, Hofferberth says. While laser cooling and trapping the rubidium atoms is a standard technique, creating the atomic cloud and the single Rydberg atom is still a very delicate process.

The concept behind this single-photon absorber was first proposed in 2011, says Alexey Gorshkov, a physicist at the University of Maryland who has collaborated with Hofferberth in the past, but was not involved in this most recent work. "These guys have implemented it, which is pretty cool," Gorshkov says. However, he points out that when the device absorbs a single photon, it also distorts the signal of subsequent photons passing through it, which may complicate its use in quantum information applications.

Hofferberth explains that his team's overarching goal is to create an array of general tools to precisely add, subtract, and control individual photons. "We have now built the most primitive version of such a tool for manipulating light," he says. "We can subtract exactly one photon." A similar single photon absorber based on a different physical mechanism was unveiled in 2015 by Barak Dayan and colleagues at the Weizmann Institute of Science in Israel and it is too early to tell which will be a more effective tool. The next step, according to Hofferberth, is to create a device that does the reverse – a collection of atoms that can produce exactly one photon.