Mario Stipcevic, Ph.D.

Quantum Communication and Quantum Information group

Five focuses of this group are: entanglement, quantum cryptography, quantum randomness, logic gates for quantum computing and single photon detectors.

We plan to begin with research of the phenomenon of entanglement. To that end we will build a laboratory setup for production of type-II polarization entangled photon pairs. The goal of this would be to investigate new methods for efficient photon entanglement with help of new nonlinear crystals, new sources of light, repeated usage of the same crystal, resonators and technology of femtosecond laser pulses. For some of this techniques it will be necessary to gain further theoretical insight and/or perform extensive numerical simulations, in which our group is well educated and experienced. We expect to achieve a setup for entangled photon production with fairly improved performances in terms of intensity, entanglement purity, spectral width and walk-off.

In collaboration with our colleagues from the LMU Muenchen, we will investigate a new techniques for multi-photon entanglement. This research is importat for better understanding of the entanglement, and perhaps it may lead to the solution of the problem of limited range of the quantum communication.

Very important element of our research are new techniques of detection of single photons, because the photons are our basic tool for probing the laws of quantum mechanics. So far we already have a very promising result in technique for active quenching in silicon avalanche photo diodes. Our preliminary prototype exhibits very small dead time (better than commercial devices) and very small autocorrelation, however additional measurements are required to verify the result and to further optimize the circuit. Both parameters (dead time, autocorrelation) are important for both loophole-free testing of basic laws of quantum physics and for generating of random numbers based on measurements of random quantum processes which result in photon emission.

Single-photon avalanche diodes (SPADs) and associated active and passive quenching circuits, and gated operation will be examined. Instrumentation and methods suitable for single-photon-counting and time-correlated photon-counting will be defined.

For characterization of single photon detectors we will use the following methods: measurement of dead time, measurement of the response time and its dispersion, afterpulsing, autocorrelation and other.

Time interval measurement methods, with picosecond resolution for timing characterization (time response, time spread, propagation time, timing jitter etc.) of high speed pulsed signals will be used.

Our previous experience in the field of timing measurement is precondition for expected applications in development of high-end electronic and optoelectronic devices for scientific measurement, quantum information and quantum communication applications. We will continue our previous and very successful theoretical and experimental research of random number (random bit) generators. We will use advanced randomness tests for testing randomness of binary sequences produced by such generators. The key element for practical realization of these generators are single photon detectors.

For a loophole-free Bell test with entangled atoms Phys. Rev. Lett. 91 (2003); 110405, in which we collaborate with the group of prof. H. Weinfurter from the LMU Muenchen, in is necessary to develop a new type of generator that would be capable of producing a random number in a very short time upon a request. For that particular application high repetitiveness, that is production rate, is not necessary. Most existing methods for generation of random numbers follow the exponential statistics and therefore there is no guarantee that the random number will be generated within a specified time period, a feature that is most desirable for this purpose. We expect to be able to devise such a generator.

Efficient photon detectors with a low dead time are also important for our next goal: apparatus for quantum cryptography. Quantum cryptography, as opposed to classical cryptography which is currently in wide use, has a potential of making possible a perfectly secret communication. But its big unsolved problems are limited communication range and speed. Because of exponential weakening of efficiency of communication , these problems cannot be solved solely on technological basis but the additional scientific research is required, in which we plan to give our contribution.

For start, we plan to build a quantum cryptographic setup based upon a modified BB84 protocol for which only one electro optical modulator is required. By modifying this basic setup we will be able to investigate various new experimental techniques and new protocols, some of which include use of entangled photon pairs, reference to classical (Shannon) information theory and quantum information theory, and theoretical results in the field of binary noisy channels.

We plan to realize free-space quantum communication during the night and also investigate possibilities to establish the quantum communication during the day. Namely, communication during the day, in the presence of a large background from the sunlight is a big challenge and up to now only pioneering attempts exist in that area.

Also we plan to investigate, at least theoretically, ways to realize "quantum switchboard" that would make possible quantum communication among a number of participants without quadratic expansion of required number of quantum channels.

Our noted theoretical work on quantum computing Phys. Lett. A 223 (1996); 241, concerning CNOT quantum logic gates has not yet been fully exploited experimentally. In the heart of these gates is a photonic resonator whose experimental realization is fairly demanding. This proposal opens some deep fundamental questions to which only experiment can give a decisive answer.

To that end we will consider building a quantum interrogation CNOT as well as a probabilistic all-optical CNOT gate which can be used for building a quantum repeater.

Finally it is important to remark that some of the proposed research can lead to generation of innovations. In such event adequate measures for protection of the IPR will be taken.