Among other important tasks of quantum photonics there is a problem of building an interface between atoms and light photons. Joint think-tank of physicists from Russia and United States studies how single photons interact with quantum objects and has already built a prototype device for transferring information from an atom to a photon. The device is an integrated chip in an artificial atom with a fibre port. The simplest application of future device is a memory cell, as for more complicated things, this device can become a single-photon transistor, working on the level of individual quanta, for building complex logic systems.

Main aim of the group is building an interface between light and atoms or artificial atoms. An interface is a way of effectively transferring information from one object to another. Building such an interface means learning to create some given kind of a state in a system (such state is called superposition), read it without damaging and transfer to another object. Mentioned given state – a superposition of two or more atom’s energy states, which can be registered with some certain probability – is what researchers call quantum information. This information is transferred to light (an individual photon serves as a carrier), which in its turn can be reliably detected – information on a photon can be “read”. In other words, there exists a channel, through which quantum information is transferred from memory cell to another cell or an out port.

 

 

A system, working with quantum information, can be “conveniently” built on atoms, which can be universal memory cells for superposition – it has only weak interactions with an environment and can store information for some period of time. The think-tank, we are talking about in this article, has already created an interface with artificial atoms – quantum dots and colour centres in a diamond. These are structures in diamond’s crystal lattice, where a carbon atom is replaced with a nitrogen atom. These structures have a position of energy levels similar to that of an atom, and can host state of superposition. Artificial atoms, especially colour centres in a diamond, have a long-storage memory, having a nucleus spin’s lifetime of about a second. Nothing like a hard drive, of course, but enough for random access memory, since all operations require only microseconds for being performed.

A device, developed by Russian and American researchers, is a chip made of silicon. The chip hosts an artificial atom with a diamond crystal, containing a colour centre. Mentioned crystal, 50x50 nanometres in size, is sitting on a silver wire (100 nm in diameter), combined with a light-conducting dielectric waveguide. All work is performed at room temperature, using specially designed confocal microscope for observations. One channel of the microscope shows sample image for choosing desired object and an interesting spot on this object. The laser radiation is focused on this spot, after that colour centre produces individual photons, registered during an experiment. Another channel of the microscope scans sample’s environment and collects information from any glowing spot – no matter if it is a waveguide’s end, or a wire’s end. Excitation beam can move along the sample and collect radiation from different colour centers.

Researchers successfully created an interface between a photon and a quantum object (an atom) – they have a technique of creating stable working chips, they can register individual photons and calculate correlation functions. However, scientists can register only 60% of photons, but no one can currently register more. Authors of the research claim that they know how to fix this problem. Further studies are aimed at making a resonator out of the wire – this function raises the chances of interaction between atom and light, and chances of radiation getting into the wire, correspondingly.

As for practical applications of a new research – there are plenty, from memory cells to transistors and complex elements of single-photon logic. New technologies for communication and quantum computers are closer, than we think.

Source: the Institute of Physics

Kizilova Anna