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The procedure of quantum measurement—unambiguous discrimination—of four two-mode coherent states of the optical field which are of interest for information transmission by the optical communication channel is studied. It is shown that complex conjugation of the amplitude of one of the modes leads to better discriminability between the states. An interferometric scheme for unambiguous discrimination between such states is proposed and the probability of successful discrimination is found. Applications of the considered set of states in quantum cryptography, quantum teleportation, and optical communication with a high level of losses are discussed. [ABSTRACT FROM AUTHOR]

It is shown that the temporal intensity correlation function of laser light backscattered by two isotropic atoms displays the photon antibunching property in a helicity-conserving polarization channel. A simple qualitative explanation to this effect is given, results of numerical calculations that support the qualitative analysis are presented, and possibilities of the experimental observation of this effect are discussed. [ABSTRACT FROM AUTHOR]

A model of the propagation of a quantized electromagnetic field in a one-dimensional photonic crystal that contains two-level atoms that interact with the field is proposed. The model separately considers the interaction of the quantized radiation with the quantum system and with the photonic crystal. The use of the model is exemplified by its application to the study of the decay of excited states of one- and two-level atoms placed in the photonic crystal. The possibility of the transformation of the front of a single photon by a finite dielectric is demonstrated. [ABSTRACT FROM AUTHOR]

The dynamics of the elementary photoelectric conversion in the interaction of a single-mode electromagnetic field with an arbitrary photon number distribution and a bound electron in a 1D rectangular potential well is considered. It is shown that if the frequency of the coherent field exceeds the frequency corresponding to the electron work function, the photoexcitation of this electron is incomplete. [ABSTRACT FROM AUTHOR]

The possibility of using the long-lived entangled state between the emitter and field near the band edge of the density of states of a photonic crystal to create an efficient generator of single photons is considered. The generator can be efficiently excited by incoherent pumping, excludes simultaneous generation of two photons, and ensures a high probability of extracting the photon into a specified mode. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The article discusses various reports published within the issue, including one by M. Zukowski on the problems of creating and detecting multiphoton entangled states, one by A. Nizovtsev and colleagues presenting the results of calculations of a diamond quantum register by the spin Hamilton method, and one by A. Zeilinger on the fundamentals and use of entanglement in photonic systems.

The spin Hamiltonian method in combination with ab initio calculations of the spin characteristics of quantum registers that include an electron spin S = 1 of a single NV center in the ground electronic state and nuclear spins I = 1/2 of several atoms 13C located at different lattice sites near the vacancy of the NV center is applied to find eigenvalues and eigenfunctions of spin systems NV + n13C for cases where the lattice sites nearest to the vacancy of the NV center contain one, two, or three 13C nuclear spins, as well as for cases where 13C atoms are located at sites more distant from the vacancy. For these single spin NV + n13C systems, the spectra of optically detected magnetic resonance (ODMR) are calculated, which agree well with available experimental data. [ABSTRACT FROM AUTHOR]

A method for creating an entangled state between two NV centers in a diamond that do not interact directly with each other is studied. The method is based on the excitation of centers via a half divider (a 50% beamsplitter) with the subsequent coherent inversion of the spin of the excited center and the detection of a photon emitted by the excited center. The parameters of a multimode single-photon pulse that make it possible to achieve a maximally possible excitation of the centers at a given solid angle of the focusing of the light on the center are found. The probability to create an entangled state in relation to the parameters of the initial state of a photon, the turn-on and turn-off times of the microwave pulse and the light focusing solid angle is determined. Optimal values of these parameters that ensure a maximal probability of the entanglement creation are determined. [ABSTRACT FROM AUTHOR]

The effect of the surface of diamond on atomic, electronic, and spin properties of diamond nanocrystals containing single nitrogen-vacancy defects ([NV]− centers) is studied. The surface was modeled with clusters C33H30[NV]−, C66H72[NV]−, which were constructed based on bulk clusters C33H36[NV]− and C69H84[NV]−, respectively. In all cases, clusters in the triplet state S = 1 are considered with the cluster charge being −1. The geometric structure of clusters is optimized using the principle of minimization of the total energy of the system; then, the electronic and spin characteristics of clusters are calculated by the density functional theory. The isotropic and anisotropic hyperfine interaction constants of the electron spin of the NV center with the nuclear spin of the nitrogen atom and 13C atoms located at different sites in the cluster are calculated. It is found that, in contrast to bulk clusters with [NV]-centers in which the spin density is mainly localized at the three carbon atoms that are the nearest neighbors of the vacancy of the center, upon arrangement of the NV center in the immediate proximity to the surface, the spin density is redistributed such that it is mainly localized at the three carbon atoms that are the nearest neighbors of the nitrogen atom of the center and at C atoms that form the first atomic layer of the (111) surface of the nanocrystal. [ABSTRACT FROM AUTHOR]

The protocol for creating entangled states of electromagnetic field modes at a distance of 100 km using a small Kerr nonlinearity (χmin ≈ 0.01) is proposed. This technique corresponds to the modern level of experimental equipment and can be used for practical purposes, such as quantum key distribution. [ABSTRACT FROM AUTHOR]

Details of the application of the spin Hamiltonian method for studying spin characteristics of a quantum register that includes an electron spin S = 1 of a single NV center in the ground electronic state and nuclear spins I = 1/2 of several isotopic atoms 13C located at different lattice sites near the vacancy of the NV center. Two methods of finding the hyperfine interaction tensors for these NV + n13C spin systems are considered, one of which is based on the conventional electron spin resonance (ESR) method, while the other involves methods of quantum chemistry. The results of the latter method are compared with ESR data and with spectra of optically detected magnetic resonance (ODMR) and with the character of the modulation of the ODMR echo decay observed in single NV + n13C systems. This comparison shows that the ab initio modeling of the spin characteristics of diamond nanoclusters containing NV centers makes it possible to obtain quantitative spin characteristics of the quantum registers under study. [ABSTRACT FROM AUTHOR]

The article discusses various papers on quantum optics and quantum information technologies published within the issue including one by P. Drummond and colleagues on phase representations and another by L. Magdenko and colleagues on quantum teleportation.

Dynamics of the elementary photoelectric conversion upon interaction of a single-photon mode with a bound electron in a one-dimensional rectangular potential well is considered. The spatial and temporal dynamics of the wave packet corresponding to a free photoelectron is investigated. [ABSTRACT FROM AUTHOR]

We give a definition of a universal entangling machine that entangles a system in an unknown state to a specially prepared ancilla. We describe explicitly such a machine for one qubit; i.e., we show how a qubit in an a priori unknown state can be entangled to a four-level ancilla by means of a unitary transformation. As a result, a fixed state-independent amount of entanglement is generated in exchange for a fixed degradation of qubit state fidelity. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

In an effort to realize a two-bit processor for a quantum computer on the basis of single nitrogen–vacancy defect centers (NV centers) in diamond, the optically detected nutations of the electron spin of a single NV center in the ground state and of the nuclear spin of a 13C atom located at a diamond lattice site nearest to the NV center are studied. The photodynamics of NV and NV + 13C centers under different temperatures and optical excitation conditions is discussed. A seven-level model of a center excited by radiation from an Ar+ laser at room temperature is proposed. On the basis of this model, the experimental spectra of optically detected electron paramagnetic and electron–nuclear double resonances of single NV and NV + 13C centers in diamond nanocrystals, as well as experimental data on the optically detected nutations of the electron and nuclear spins of these centers caused by the actions of pulsed microwave and radiofrequency fields, respectively, are interpreted. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

The electronic and spin properties of different nanocrystals of carbon are studied. The properties of these cluster systems are modeled in terms of the ab initio (Hartree–Fock) and semiempirical (PM3, AM1) quantum-chemical methods. The calculations are performed for different carbon nanocluster systems: defect-free and with [NV]– centers, hydrogen passivated (C38H42, C71H84, C86H78), and with a free (unpassivated) surface (C38, C71, C86). The spin properties of unhydrated nanoclusters were studied for the first time. The structure of all the clusters under study was optimized using the total energy minimization principle. It is shown that, in the case of hydrated carbon nanocrystals passivated by hydrogen atoms, diamond-like clusters are formed. The atomic structure of an unpassivated nanocrystal depends on the number of atoms in the cluster, as well as on its initial geometrical parameters. In some cases, clusters with a fullerene-like surface are formed. In hydrogen-passivated diamond nanocrystals with [NV]– centers, the spin density is localized at the nuclei of C atoms nearest to the center vacancies. For the unpassivated counterparts, the spin density is localized at the nuclei of C atoms forming the surface of the corresponding nanocrystal. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

The effect of thermal photons on the radiative properties of a one-atom maser is investigated. Stationary P and Q distribution functions are determined analytically in the approximation of strong interaction between an atom and a cavity field mode. The generation conditions are found for a one-atom maser. It is shown that taking into account thermal photons leads to a decrease in the threshold value of incoherent pumping. It is demonstrated that generation may occur in the absence of population inversion in the atomic subsystem and when losses exceed the intensity of incoherent pumping. A formula is proposed for the Fano factor of a one-atom maser in the form of the sum of the Fano factors of the microcavity radiation in the absence of thermal photons and the thermal radiation. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

The spin-selective photokinetics of a single matrix-isolated impurity molecule with a triplet-triplet optical transition, T[SUB0]-T[SUB1], is considered and the manifestations of the photokinetics in the fluorescence excitation spectra and intensity autocorrelation functions g[SUP(2)](τ) of the molecule undergoing narrow-band optical excitation is studied to resolve the fine structure of the transition. The rates of intersystem crossings (ISCs) T[SUB1] S → T[SUB0] to and from a nonradiating singlet state S of the molecule and the rate of population relaxation among the ground (T[SUB0]) state sublevels can be obtained from the spectra and g[SUP(2)](τ) using the analytical expressions obtained. New experiments on an individual NV defect center in nanocrystals of diamond, where, for the first time, the fine structure of its triplet-triplet [SUP3]A-[SUP3]E zero-phonon optical transition (∼637 nm) at 1.4 K was resolved, are interpreted. It is concluded that the rate of the ISC transition from the m[SUBS] = 0 sublevel of the excited [SUP3]E state to the singlet [SUP1]A state (∼1 kHz) is much slower than the rates from the m[SUBS] = ±1 substates, while the rates of ISC transitions to different m[SUBS] substates of the ground [SUP3]A state are close to each other (∼1 Hz). As a result, only the optical transition between m[SUBS] = 0 sublevels in the [SUP3]A-[SUP3]E manifold contributes strongly to the fluorescence. The experimentally observed double-exponential decay of the g[SUP(2)](τ) function is explained by the two pathways available to the center for it to leave the S state: (i) the S → T[SUB0] (m[SUBS] = 0) transition and (ii) the S → T[SUB0] (m[SUBS] = ±1) transitions followed by the slow spin-lattice relaxation T[SUB0](m[SUBS] = ±1) → T[SUB0](m[SUBS] = 0) (rate ∼0.1 Hz). The work is important for studies where the NV center is used as a single photon source or for quantum information processing. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A quantum system consisting of a two-level atom interacting with a single field mode of a high-Q cavity under influence of a coherent pump is considered. The analytical solutions for the P and Q distribution functions are obtained in the limit of large Rabi frequencies. In the presence of thermal photons, the P distribution function loses its property of restriction by the range on the complex plane and becomes an analytical function. When the ratio of the atomic decay rate to the cavity mode damping rate is smaller than 4, the effect of phase bistability appears. Absorptive optical bistability is absent in this case. On the basis of the system of Fokker-Planck equations for the quasi-probabilities corresponding to the atom being on the upper and lower atomic levels, computer simulation of the stochastic trajectory of motion for the system is presented. [ABSTRACT FROM AUTHOR]

Properties of a laser formed by a high-Q cavity and a single incoherently excited atom are studied on the basis of analytical solutions obtained for the quasi-probability P-function. Expressions for the boundaries separating various operating regimes of a one-atom laser (below threshold, above threshold, and selfquenching) are found. It is shown that lasing without inversion and lasing with the decay rate of the excited state exceeding the pump intensity are possible in the system. The degree of coherence of laser radiation is studied. [ABSTRACT FROM AUTHOR]

Spectrally resolved detection of single atom resonance fluorescence in the limit of well-separated spectral lines is considered. By using a special type of correlation measurements over the fluorescent field, in which a filtered photon detection is followed by an unfiltered photon detection, we obtain a conditioned atomic state following the filtered photon detection. The properties of the atomic state following detection of the reflected photon are studied and interpreted on the basis of quantum interference between the dressed states. Measurement operators associated with the detection of the passed and reflected photons are derived and used to construct the master equation for the atomic density matrix subjected to continuous spectral detection, the filter tuning being arbitrary. [ABSTRACT FROM AUTHOR]

We discuss the possibility of realizing quantum computation on the basis of a cluster of single interacting nuclear spins in solids. This idea seems to be feasible because of the combination of two techniques-Single Molecule Spectroscopy and Optically Detected Electron Nuclear Double Resonance. Compared to the well-known bulk Nuclear Magnetic Resonance (NMR), the proposed method of quantum computation has the advantage that quantum computation is performed with pure spin states and the quantum processor is more easily scalable. At the same time, the advantages of NMR quantum computation are kept: long coherence time and easy construction of quantum gates. As a specific system to implement the above idea, we discuss the [sup 13]C-nuclear spins in the nearest vicinity of a single nitrogen-vacancy (NV) defect center in diamond, which can be optically detected using the technique of scanning confocal microscopy. Owing to the hyperfine coupling of the ground state electron paramagnetic spin S = 1 of the center to [sup 13]C nuclear spins in a diamond lattice, the states of nuclear spins in the vicinity of the defect-center can be addressed individually. Preliminary consideration shows that it should be possible to address up to 12 individual [sup 13]C nuclear spins. The dephasing time of the nuclear spin states at low temperatures allows realization up to 10[sup 5] gates. [ABSTRACT FROM AUTHOR]

Introduces a series of articles on quantum optics presented at the Ninth International Conference 'Quantum Optics 2002.' Investigation of specific features of entangled states; Studies on nonclassical states of light; Nanophotonics.

Comments on articles on quantum optics and quantum information in the September 2001 issue of 'Optics and Spectroscopy.' Computational resources provided by quantum physics; Possibility of quantum teleportation; Realization of quantum computations on the basis of optical interactions of atoms.