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Quantum memory for light

Light is a robust and reliable carrier of quantum information, and it can be used to perform quantum information processing. However, certain quantum information processing tasks cannot be accomplished with light alone. For example, every computer, quantum or classical, needs memory, a means of interim and long-term storage of the information to be processed. It is clear that light is not well suitable for this task: photons don't like to stay put, they like to travel at the speed of light. So if we intend to design a light-based quantum computer we need to think of a medium that would allow storage of the quantum information light carries.

Importantly, optical memory is required not only for quantum computers, but also for quantum communication and quantum cryptography. Today's quantum communication range is limited by the distance a single photon can travel without being absorbed. About one-half of photons are absorbed every 10-15 km of optical fiber. As a result, modern quantum communication becomes unpractical when its range exceeds a few dozen kilometers. Development of quantum repeaters will lead to polynomial, rather than exponential, degradation in bit transfer rate with distance, and thus increase the maximum possible distance of quantum communication. An essential component of quantum repeaters is memory for light.

To be useful in quantum communications, optical memory must fit a set of criteria, such as long storage times, high efficiencies, low noise, compatibility with telecommunications wavelengths, etc. To date, all these benchmarks have been reached, but in separate experiments. The next goal is to bring these accomplishments together in a single setup. It is fair to expect a laboratory prototype of such memory to emerge within the next few years, followed by integration with optical communication lines and commercialization within a decade. This is why we include quantum-optical memory as a primary component of our research agenda.

What material can we use to store the quantum information carried by light? A very promising candidate for this role is atomic ensembles. Atoms can be made stationary by laser cooling (or placing them inside a solid host material), and they possess energy levels whose quantum state decays very slowly. At the same time, atoms interact strongly with light – and thus it is possible to enable exchange of quantum information between these two media.

Quantum-optical memory can be implemented by utilizing electromagnetically-induced transparency (EIT). The mechanism of such memory is illustrated in the tutorial. Recently, we have realized quantum memory for squeezed vacuum using this method. Along with EIT, we are working on a new method known as controlled reversible inhomogeneous broadening that has shown very high efficiencies in both solids and atomic gases. Our current goal is to demonstrate a system that would exhibit both a high efficiency and, at the same time, long storage lifetime. We plan to accomplish this by implementing the memory protocol in a dense cloud of ultracold atoms – so we can ensure that the atoms cannot leave the interaction region and no stored information is lost.

Our review paper on quantum optical memory

Giant optical nonlinearities

Interfacing quantum information between light and atoms will also allow us to realize nonlinear optical interaction at single-photon energy levels. This technology will open up a wide range of perspectives in quantum optical information processing, ranging from non-demolition measurements of photon numbers to photon number sources and "direct" processing of quantum optical information.

This task is extremely difficult, because most materials exhibit nonlinear optical properties only at relatively strong optical powers. This is where atomic ensembles, whose resonances feature high nonlinear susceptibilities, come in naturally. Especially high nonlinearities are exhibited by ensembles of so-called Rydberg atoms, i.e. atoms whose outermost electrons are excited to very high energy states, just barely below the ionization threshold. The orbitals, along which these electrons move, are extremely large: a few micrometers, i.e. millions of times larger than those of regular atoms. This is comparable to the average distance between atoms in a dense gas. Under these circumstances, the atoms start "talking" to each other: an atom excited, by a single photon, into a Rydberg state will affect the spectrum of its neighbors, thus producing giant optical nonlinearity.

An additional challenge in this project is to guide an optical wave so it remains focused over an extended length. We will address this by means of tapered optical fibers of submicron diameters. A significant fraction of the optical mode guided by such a fiber propagates as an evanescent field. In this way, if the fiber is placed inside a cloud of ultracold atoms, the atoms will have easy access to the guided light, thus facilitating strong coupling between these two quantum systems.