D. B. Horoshko

Quantum anonymous voting with anonymity check

Abstract We propose a new protocol for quantum anonymous voting having serious advantages over the existing protocols: it protects both the voters from a curious tallyman and all the participants from a dishonest voter in unconditional way. The central idea of the protocol is that the ballots are given back to the voters after the voting process, which gives a possibility for two voters to check the anonymity of the vote counting process by preparing a special entangled state of two ballots. Any attempt of cheating from the side of the tallyman results in destroying the entanglement, which can be detected by the voters.

Towards single-cycle squeezing in chirped quasi-phase-matched optical parametric down-conversion

We propose a method for generation of a single-cycle squeezed light by parametric down-conversion in a chirped quasi-phase-matched nonlinear crystal. We find an exact quantum solution for this process valid for an arbitrary parametric gain, and discover an ultrabroadband squeezing in the down-converted light with flat squeezing spectrum comprising the full optical octave. We describe a scheme for observation of this kind of squeezing using second-harmonic generation as an ultrafast correlator.

Decoherence slowing via feedback

Abstract It is shown that the decoherence of the Yurke–Stoler coherent state of a field in an open cavity can be significantly slowed down when the feedback is organized between the outgoing radiation and the intracavity field.

Generation of monocycle squeezed light in chirped quasi-phase-matched nonlinear crystals

We present a quantum theory of parametric down-conversion of light in chirped quasi-phase-matched second-order nonlinear crystals with undepleted quasi-monochromatic pump. This theory allows us to consider generation of ultrabroadband squeezed states of light and is valid for arbitrary, sufficiently slowly-varying nonlinear poling profiles. Using a first-order approximate quantum solution for the down-converted light field, we calculate the squeezing spectra and the characteristic squeezing angles. We compare the approximate solutions with the exact and numerical ones and find a very good agreement. This comparison validates our approximate solution in the regime of moderate gain, where the existing approaches are not applicable. Our results demonstrate that aperiodically poled crystals are very good candidates for generating ultrabroadband squeezed light with the squeezing bandwidth covering almost all the optical spectrum and the correlation time approaching a single optical cycle.

Resonance fluorescence excited by macroscopic superposition in a seedback soop

The resonance fluorescence of an individual atom excited by an optical field in a Yurke-Stoler state, consisting of a superposition of two coherent states with opposite phase, is studied. It is proposed that the decoherence of the field state be eliminated by means of electrooptic feedback [Phys. Rev. Lett. 78, 840 (1997)]. The master equation for the density operator of the atom-field system is derived and an analytic solution is obtained for the case where the change in the field is adiabatically slow. It is shown that the interaction entangles the atomic and field states. A new effect is predicted: there are no Rabi oscillations of the dipole moment and of the atomic populations with the excitation method described.

Optimal dimensionality for quantum cryptography

We perform a comparison of two protocols for generating a cryptographic key composed from d-valued symbols: one exploiting a string of independent qubits and another one utilizing d-level systems prepared in states belonging to d+1 mutually unbiased bases. We show that the protocol based on qubits is optimal for quantum cryptography, since it provides higher security and higher key generation rate.A comparison of two protocols for generating a cryptographic key composed from d-valued symbols, one using a string of independent qubits and another utilizing d-level systems prepared in states belonging to d + 1 mutually unbiased bases, is performed. The protocol based on qubits is shown to be optimal for quantum cryptography, since it provides higher security and a higher key generation rate.

Multimode unravelling of master equation and decoherence problem.

An unravelling of master equation for a set of fields interfering with one another is developed and conditions are found under which decoherence can be avoided for conditional and unconditional evolution of one of these fields.

Entanglement of quantum circular states of light

We present a general approach to calculating the entanglement of formation for superpositions of two-mode coherent states, placed equidistantly on a circle in the phase space. We show that in the particular case of rotationally-invariant circular states the Schmidt decomposition of two modes, and therefore the value of their entanglement, are given by analytical expressions. We analyse the dependence of the entanglement on the radius of the circle and number of components in the superposition. We also show that the set of rotationally-invariant circular states creates an orthonormal basis in the state space of the harmonic oscillator, and this basis is advantageous for representation of other circular states of light.

Time-shift quantum key distribution with decoy states

We suggest a protocol for quantum key distribution—a technology allowing two distant parties to create an unconditionally secure cryptographic key. For the creation of the key we suggest to use laser pulses weakened to the single-photon level of duration T, the pulse carrying the value “1” being shifted in time by T/2 compared to the pulse carrying the value “0”. The overlap of the pulses provides their non-orthogonality and, therefore, impossibility to discriminate between them with certainty. Besides the signal pulses the protocol uses coherent decoy pulses, having longer duration than the signal ones and providing a more effective protection from a wide class of attacks. Security of the protocol is based on interferometric control of the pulse coherence at the receiving station. We analyze the security of the protocol against a number of intercept-resend attacks and on the basis of this analysis substantiate the necessity of decoy state implementation.

Bistable vertical cavity laser as a truly random number generator

In this report we present a comprehensive statistical analysis of experimentally measured time intervals between spontaneous polarization switchings in a bistable VCSEL from standpoint of a possibility to generate random numbers. We propose an effective and robust algorithm of conversion of residence times into truly random binary numbers. This algorithm allows one to avoid the problem of biasing in the output bit stream as well as to obtain low correlation between bits.