Research at GTQI - Quantum Communication

Quantum Repeaters

Atomic ensemble based systems will be developed for quantum repeaters that can relay quantum information through communication networks. Advances in storage times, efficiencies and throughput will require integration of atomic trapping, optical resonators and field control technologies.

Another approach to quantum repeaters based on trapped ions and parametric frequency converters is also being explored. Ions and atoms emit radiation in a wavelength range well outside the low optical fiber loss telecommunications bands at 1.3 and 1.5 µm wavelengths. High efficiency, low noise parametric converters are being developed to convert the ion/atom radiation to the communication bands and then back to the atom/ion wavelengths.

Novel optical resonator designs will likely be necessary in order to optimize the efficiency of network connectivity. For example, photonic bandgap resonators offer better prospects for integration and scalability than the traditional Fabry-Perot resonator. Photonic bandgap reflectors may be important for low loss UV mirrors required for optimizing laser control in the UV for trapped ions.

Some quantum repeater protocols suffer low entanglement connection rates over long distances. More efficient designs are currently being developed employing multiplexed quantum nodes.

Ali Adibi, Michael Chapman, Brian Kennedy, Alex Kuzmich, Dick Slusher, and Paul Voss

Basic Light-Matter Quantum Interfaces

The efficient transfer of quantum states between matter and light requires analysis of the basic atomic and optical physics properties of the components: photon storage and retrieval, atomic ensemble preparation and spatial modes of the photon emission. Interaction of light with atoms and ions can be strongly enhanced in optical resonant cavities to form interesting classes of quantum switches where the quantum states of the atom/ion can be transferred to the light.

Michael Chapman, Brian Kennedy, Alex Kuzmich, Dick Slusher, and Li You

Channel security, capacity and efficiency

The use of quantum states for the purpose of secure communications is an important field of study because of the perfect security it enables. It is now widely known that quantum systems can provide a level of security far exceeding that of any cryptographic protocol currently known. Great advances have been made in the last 10 years and commercial systems which implement quantum key distribution in fiber optics are now commercially available. Many important problems remain. First, fundamental information-theoretic security limits for such systems are not known and it is now widely believed (and early demonstrations have shown) that very high security is possible at data rates in the 10-100Mbps range. Second, the security provided by quantum systems sheds light on security of classical communication systems, like wireless communication. Early results by GT researchers have shown how to achieve perfect security in some limiting cases of entirely classical wireless communications, and prototypes (with applications to RFID and wireless LAN) are being developed. This area of research will be greatly expanded in the context of quantum and classical information theory.

Steve McLaughlin

Photon sources and detectors

Single photon sources are a basic element for quantum communication systems. Generation rate, bandwidth, storability, wavelength, efficiency and controllability are important properties that need more research effort.

High efficiency visible light photon counters (VLPCs) that distinguish precise photon count information are also important devices for future research. VLPC's are presently impurity band conduction silicon diodes, designed to convert single photons into many thousands of electrons with high quantum efficiency. In contrast to standard avalanche photo diodes, only one of the two carrier types participates in carrier multiplication, which takes place across a band gap of only 50 meV, thus reducing gain dispersion considerably. The wavelength response of this type of detector needs to be broadened and extended for quantum information work with ions and atoms.

High efficiency UV CCD detectors and optical fiber collection systems for the UV are another important research direction for large scale ion trap quantum information processors.

Ali Adibi and Dave Citrin