Z. Kis

Continuous variable remote state preparation

We extend exact deterministic remote state preparation (RSP) with minimal classical communication to quantum systems of continuous variables. We show that, in principle, it is possible to remotely prepare states of an ensemble that is parametrized by infinitely many real numbers---i.e., by a real function---while the classical communication cost is one real number only. We demonstrate continuous variable RSP in three examples using (i) quadrature measurement and phase-space displacement operations, (ii) measurement of the optical phase and unitaries shifting the same, and (iii) photon counting and photon number shift.

Pulse propagation in a dressed, degenerate system

We study the propagation of coherent light pulses in a medium of three-level atoms with degenerate ground- and excited-state sublevels in an electromagnetically induced transparency (EIT)-type configuration. Both the strong control field and the weak probe pulse have elliptical polarization, which gives rise to concurrent multipath couplings between the ground-state sublevels and the auxiliary stable state. We derive the probe field susceptibility and show that in general, the probe field propagates in two separate polarization modes, one of which is attenuated, the other of which displays EIT. This generic result is valid provided the atomic medium is prepared to be in a pure quantum state over the ground-state sublevels initially. We also investigate the case when the initial state of the medium is described by an incoherent mixture of ground-state sublevels and show how EIT-like pulse propagation degrades. The possibility of controlling the probe susceptibility matrix with control field polarization provides a convenient tool for probing the quantum state of the medium on the degenerate ground-state sublevels.

Engineering mixed states in a degenerate four-state system

A method is proposed for preparing any pure and wide class of mixed quantum states in the decoherence-free ground-state subspace of a degenerate multilevel lambda system. The scheme is a combination of optical pumping and a series of coherent excitation processes, and for a given pulse sequence the same final state is obtained regardless of the initial state of the system. The method is robust with respect to the fluctuation of the pulse areas, as in adiabatic methods; however, the field amplitude can be adjusted in a larger range.

Collapse and revival in the vibronic dynamics of laser-driven diatomic molecules

Abstract Resonant driving of certain electronic transitions in diatomic molecules is shown to yield collapses and revivals of the electronic populations due to the quantized vibrational motion of the nuclei. We analyze the properties of the vibrational potentials, the frequency and power of the classical driving laser field for which this effect can be observed.

Quantum circuit-based modeling of continuous-variable quantum key distribution system

The second generation Continuous-Variable Quantum Key Distribution is one of the most promising application of quantum mechanics in communications. This paper investigates a state-of-the-art Continuous-Variable Quantum Key Distribution solution from circuit modeling point of view in order to build an appropriate simulation framework. This framework allows detailed evaluation of the systems performance and adjusting the system parameters. Copyright

Photon statistics determination for single photon based quantum key distribution

In practice the most advanced field of quantum information science is quantum cryptography. The best known example of quantum cryptography is quantum key distribution (QKD) which offers an information-theoretically secure solution to the key exchange problem. There are QKD protocols which rely on a quantum channel that is able to transmit single photons. This can be implemented by using very low-power coherent light field, where values of bits are encoded into the polarization state of photons. Therefore, it is important to know the photon number distribution of the light source. In this paper we present the reconstruction of photon statistics for different pulsed light sources radiating at 850 nm (a common choice for free space quantum communication), using only a single on-off photon counting detector.

Real-time dynamic calibration of a tunable frequency laser source using a Fabry-Pérot interferometer

We report on a method for real-time dynamic calibration of a tunable external cavity diode laser by using a partially mode-matched plano-concave Fabry-Pérot interferometer in reflection geometry. Wide range laser frequency scanning is carried out by piezo-driven tilting of a diffractive grating playing the role of a frequency selective mirror in the laser cavity. The grating tilting system has a considerable mechanical inertness, so static laser frequency calibration leads to false results. The proposed real-time dynamic calibration based on the identification of primary- and Gouy-effect type secondary interference peaks with known frequency and temporal history can be used for a wide scanning range (from 0.2 GHz to more than 1 GHz). A concave spherical mirror with a radius of R = 100 cm and a plain 1% transmitting mirror was used as a Fabry-Pérot interferometer with various resonator lengths to investigate and demonstrate real-time calibration procedures for two kinds of laser frequency scanning functions.

Robust state preparation in a degenerate four-state system

A robust method utilizing a combination of optical pumping and a series of coherent excitation processes is developed for preparing any pure and a wide class of mixed quantum states in the decoherence-free ground-state subspace of a degenerate four-state system. For a given pulse sequence the same final state is obtained regardless of the initial state of the system. An example is presented where a pure state is prepared by a series of excitation processes in the two-dimensional dark subspace of the atom.

Photon number teleportation

Abstract It is shown that a superposition state | Φ 〉 of one and two photons can be teleported using an optical teleportation scheme. A Bell-state analysis for entangled one- and two-photon states is discussed. A Gedankenexperiment is presented in which the state | Φ 〉 is teleported with 50% efficiency.

Second generation QKD system over commercial fibers

Security of the communications could be ensured using cryptography protocols. While asymmetrical protocols can be cracked using quantum computers, the symmetrical protocols stand against quantum attacks. However, the keys need to be exchanged in a secure way. A method is offered by the quantum key distribution (QKD) protocols. The QKD system should operate close to the fundamental quantum noise level. A possible attack is to steal some photons during the communication, but every change will result in some excess noise. This is why it is important to know the base noise level of the system, and every possible solutions need to be considered to reduce the noise of the system. To foster the research in this field, we started to develop a second generation QKD system over a 16 km long single mode, ordinary telecommunications fiber and focused on noise reduction.