I. I. Ryabtsev

Experimental studies in quantum cryptography

A short survey on experimental works in quantum cryptography is presented. We describe experimental setups that were designed in the Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, for quantum key distribution through an air space and along a fiber-optic communication line. The results of the study of quantum efficiency parameters, probability of afterpulse appearance, and noise levels for different operation modes of InGaAs-InP avalanche photodiodes are presented.

Investigation of cold rubidium Rydberg atoms in a magneto-optical trap

This paper reports on the results of experiments with cold rubidium Rydberg atoms in a magneto-optical trap. The specific feature of the experiments is the excitation of Rydberg atoms in a small volume within a cloud of cold atoms and the sorting of measured signals and spectra according to the number of detected Rydberg atoms. The effective lifetime of the 37P Rydberg state and its polarizability in a weak electric field are measured. The results obtained are in good agreement with theoretical calculations. It is demonstrated that the localization of the excitation volume in the vicinity of the zero-magnetic-field point makes it possible to improve the spectral resolution and to obtain narrow microwave resonances in Rydberg atoms without switching off the quadrupole magnetic field of the trap. The dependence of the amplitude of dipole-dipole interaction resonances in Rydberg atoms on the number of atoms is measured. This dependence exhibits a linear behavior and agrees with the theory for a weak dipole-dipole interaction.

Spectroscopy of the three-photon laser excitation of cold Rubidium Rydberg atoms in a magneto-optical trap

The spectra of the three-photon laser excitation 5S1/2 → 5P3/2 → 6S1/2nP of cold Rb Rydberg atoms in an operating magneto-optical trap based on continuous single-frequency lasers at each stage are studied. These spectra contain two partly overlapping peaks of different amplitudes, which correspond to coherent three-photon excitation and incoherent three-step excitation due to the presence of two different ways of excitation through the dressed states of intermediate levels. A four-level theoretical model based on optical Bloch equations is developed to analyze these spectra. Good agreement between the experimental and calculated data is achieved by introducing additional decay of optical coherence induced by a finite laser line width and other broadening sources (stray electromagnetic fields, residual Doppler broadening, interatomic interactions) into the model.

Using single-photon detectors for quantum key distribution in an experimental fiber-optic communication system

This paper gives experimental results of quantum key distribution on a fiber-optic setup at a telecom wavelength of 1555 nm. A self-compensated two-channel optical circuit is used. Quantum key distribution was performed by coding the phase states of single photons radiated by a pulsed semiconductor laser in two alternative nonorthogonal bases. Specially developed single photon counters based on InGaAs: InP avalanche photodiodes were employed as high-sensitivity photodetectors. The results of investigation of the quantum efficiency, probability of afterpulses, and noise level for various operating modes of the detectors at temperatures from −40 to −60°C are given. A key distribution rate of 450 bit/s was obtained for a single-mode fiber-optic quantum communication channel between the receiver and sender 25 km long at a laser pulse clock frequency of 5 MHz and an average number of photons per pulse of about 0.2. For the achieved photodetector characteristics, the average number of errors in the quantum key did not exceed 3.7%.

Effect of photoions on the line shape of the Förster resonance lines and microwave transitions in cold rubidium Rydberg atoms

Experiments are carried out on the spectroscopy of the Förster resonance lines Rb(37P) + Rb(37P) → Rb(37S) + Rb(38S) and microwave transitions nP → n′S, n′D between Rydberg states of cold rubidium atoms in a magneto-optical trap (MOT). Under ordinary conditions, all spectra exhibit a linewidth of 2–3 MHz irrespective of the interaction time between atoms or between atoms and microwave radiation, although the limit resonance width should be determined by the inverse interaction time. The analysis of experimental conditions has shown that the main source of line broadening is the inhomogeneous electric field of cold photoions that are generated under the excitation of initial nP Rydberg states by broadband pulsed laser radiation. The application of an additional electric-field pulse that rapidly extracts photoions produced by a laser pulse leads to a considerable narrowing of lines of microwave resonances and the Förster resonance. Various sources of line broadening in cold Rydberg atoms are analyzed.

Single-mode vertical-cavity surface emitting lasers for 87Rb-based chip-scale atomic clock

The results of numerical simulation and study of lasing characteristics of semiconductor verticalcavity surface-emitting lasers based on AlxGa1 − xAs alloys are presented. Lasers exhibit stable single-mode lasing at a wavelength of 795 nm at low operating currents ∼1.5 mA and an output power of 350 μW, which offers prospects of their applications in next-generation chip-scale atomic clocks

Long-distance fiber-optic quantum key distribution using superconducting detectors

This paper presents the results of experimental studies on quantum key distribution in optical fiber using superconducting detectors. Key generation was obtained on an experimental setup based on a self-compensation optical circuit with an optical fiber length of 101.1 km. It was first shown that photon polarization encoding can be used for quantum key distribution in optical fiber over a distance in excess of 300 km.

Noise reduction methods of single photon detector based on InGaAs/InP avalanche photodiodes

This work is dedicated to the problem of noise reduction of single photon detector development based on InGaAs-InP avalanche photodiodes. Dark count probability and quantum efficiency of the detectors have been measured. We present the experimental fiber based quantum key distribution setup with phase coding of single photon states. The autocompensation two way optical scheme (plug&play) is used. The single photon source is the strongly attenuated laser pulse which goes through two paths of Mach-Zehnder interferometer where it undergoes the phase coding. Quantum channel is formed by 25 km single mode fiber. To generate the quantum key the four phases BB84 protocol is used.

Element base of quantum informatics II: Quantum communications with single photons

Quantum cryptography can provide almost complete security of the data transmitted in optical telecommunication systems by single photons based on the laws of quantum mechanics. The paper presents a brief overview of the element base and experimental investigations in the field of quantum cryptography and data transmission by single photons in atmospheric and optical fiber quantum communication lines. Two experimental setups for the single-photon quantum key distribution in the atmospheric and optical fiber quantum channels are described. The results of the quantum key distribution experiments performed on them are given.

A fiber quantum communication system based on an autocompensation optical circuit

An experimental setup for studying the distribution of a quantum key in a fiber optic communication line designed at the Institute of Semiconductor Physics, Russian Academy of Sciences, Siberian Branch is presented. The structure of a unified control unit for studying the distributed quantum key setup using the BB84 protocol with phase encoding is described. Experimental results on quantum key generation are given.