Experimental characterization of quantum-correlated twin beams and a quantum channel with twin beams

We report the generation and characterization of quantum-correlated twin beams and their application to a quantum channel. Compared with our previous results, the characterization of twin beams and the quantum channel have been studied with 7.0-dB intensity difference squeezing. The measured inference variance of twin beams is decreased to 0.40 ± 0.02. For the quantum channel, the bit error rate will be 0.035 by using 7.0-dB intensity difference squeezing compared with our previous result of 0.067 by using 4.9-dB intensity difference squeezing.     

Realization of quantum correlation transfer between twin beams by conditional preparation technique

A conditional quantum correlation transfer is demonstrated experimentally for continuous variables by employing two pairs of twin-beams states. In present paper, the quantum correlation of twin beams is indicated by loss coefficient or “gemellity”, which is the remaining noise on the intensity difference normalized to the shot noise level. The quantum correlation between two initial idler beams with loss coefficient of 0.4 was conditionally transferred from two pairs of initially independent twin beams with loss coefficient of 0.2. A theory of conditional quantum correlation transfer and comparison between experimental results and theoretical prediction are also presented. The experimental results agree with the theory well.     

Remote state preparation of a photonic quantum state via quantum teleportation

We demonstrate an experimental realization of remote state preparation via the quantum teleportation algorithm, using an entangled photon pair in the polarization degree of freedom as the quantum resource. The input state is encoded on the path of one of the photons from the pair. The improved experimental scheme allows us to control the preparation and teleportation of a state over the entire Bloch sphere with a resolution of the degree of mixture given by the coherence length of the photon pair. Both the preparation of the input state and the implementation of the quantum gates are performed in a pair of chained displaced Sagnac interferometers, which contribute to the overall robustness of the setup. An average fidelity above 0.9 is obtained for the remote state preparation process. This scheme allows for a prepared state to be transmitted on every repetition of the experiment, thus giving an intrinsic success probability of 1.     

Manipulating the quantum properties of continuous laser beams

The quantum properties of light play an ever increasing role in optics. Thanks to the ability to generate and use special light with modified quantum noise and to the generation of optical entanglement, we can now avoid the conventional limits imposed by the shot noise. In addition, we can create new quantum information protocols. This article concentrates on the case of continuous laser beams, which are used in many precision measurements and applications. The article summarizes some of the concepts, reviews the technical development, provides a simple and reliable model and shows some of the current directions of using optical quantum correlations, such as for the storage of quantum information. PACS 42.50.Dv     

The crossover from classical to quantum regime in the problem of the decay of the metastable state

The probability of the decay of the metastable state has been found as a function of viscosity and temperature. At low temperatures, a classical overbarrier transition changes to the quantum tunneling. At low viscosity in classical and transitive semiquantum region of temperatures a depopulation of the distribution function is significant. The distribution function is shown to satisfy the integral kinetic equation, the kernel of which equals the transition probability. The probability of transitions, induced by the red noise, with the frequency comparable to the transition frequency, is found.     

Analysis of continuous generation of electron beams in medium-pressure gases

Stationary and stable open discharges are obtained in air, helium, and helium with an admixture of a few percent of air. The structure of the generator of an electron beam with a wide set of cathode and insulator materials is described. The experiments are performed under an air pressure of hundreds of pascals and under a helium pressure of thousands of pascals. The current-voltage characteristics of the discharge are measured and the results on the maximal current density attained are compared with the available data.     

Edge state propagation direction in the fractional quantum Hall regime: multi-terminal magnetocapacitance experiment

The propagation direction of fractional quantum Hall effect (FQHE) edge states has been investigated experimentally via the symmetry properties of the multi-terminal capacitances of a two-dimensional electron gas. Although strong asymmetries with respect to zero magnetic field appear, no asymmetries with respect to even denominator Landau level filling factor ν are seen. This indicates that current-carrying FQHE edge states propagate in the same direction as integer QHE edge states. In addition, anomalous capacitance features, indicative of enhanced bulk conduction, are observed at and .     

Possible generation of electric multipole radiation from solid state quantum rings

The self-organized crystal growth of semiconductor quantum rings has opened a new possibility to study and exploit optical transitions between ring-shaped quantum states. In such states, orbital angular momenta of particle envelope functions are well-defined. We investigate theoretically the intraband interlevel transitions between such states and examine the possibility of electrical multipole radiations (EMRs). Selections rules due to envelope function quantum numbers are deduced. To enhance the EMR efficiency, we propose a novel coupled dot–ring structure, by which the lowest allowed EMR can be selected and manipulated, allowing the efficient radiation of multipole photons.     

Probabilistic joint remote preparation of a high-dimensional equatorial quantum state

<正>This paper proposes a scheme for probabilistic joint remote preparation of an arbitrary high-dimensional equatorial quantum state by using high-dimensional single-particle orthogonal projective measurement and appropriate unitary operation.As a special case,a scheme of joint remote preparation of a single-qutrit equatorial state is presented in detail.The scheme is also generalized to the multi-party high-dimensional case.It shows that,only if when all the senders collaborate with each other,the receiver can reconstruct the original state with a certain probability.     

Analysis of atmospheric effects on the continuous variable quantum key distribution

Atmospheric effects have significant influence on the performance of a free-space optical continuous variable quantum key distribution (CVQKD) system. In this paper, we investigate how the transmittance, excess noise and interruption probability caused by atmospheric effects affect the secret-key rate (SKR) of the CVQKD. Three signal wavelengths, two weather conditions, two detection schemes, and two types of attacks are considered in our investigation. An expression aims at calculating the interruption probability is proposed based on the Kolmogorov spectrum model. The results show that a signal using long working wavelength can propagate much further than that of using short wavelength. Moreover, as the wavelength increases, the influence of interruption probability on the SKR becomes more significant, especially within a certain transmission distance. Therefore, interruption probability must be considered for CVQKD by using long-signal wavelengths. Furthermore, different detection schemes used by the receiver will result in different transmission distances when subjected to individual attacks and collective attacks, respectively.     

Thermal activation in the quantum regime and macroscopic quantum tunnelling in the thermal regime in a metabistable system consisting of a superconducting ring interrupted by a weak junction : Part II: Thermal activation in the quantum regime (continuation)

In this short contribution we present a commentary on the interpretation of our thermal activation data obtained in the quantum regime of a SQUID, as discussed in part I [Bol et al., Physica B 133 (1985) 196]. Under certain circumstances a superconducting ring containing a weak superconducting junction, a SQUID, has two metastable magnetic flux states separated by a potential energy barrier ΔV. In this metabistable system stochastic magnetic flux transitions were observed due to intrinsic thermal activation. It was found that the transition rate was strongly reduced compared with the predictions of the classical thermal activation theory of Kramers or with the modern thermal activation theory of Grabert and Weiss which is an extension to the quantum regime where kT ω₀ (ω₀ being the free oscillation frequency corresponding to the metastable potential well). In these theories the transition rate is proportional to exp(-ΔV/kT), in which V is treated as a temperature independence potential just as in the case in microscopic activated processes. In fact, however, from the thermodynamic point of view the relevant quantity in the exponent is the magnetic availability of the system with respect to the surroundings fixed by the temperature of the heat bath and the external magnetic field. Only when the system is completely isothermal can the potential V be identified with the Gibbs function. But in general when a flux transition takes place between the metastable potential wells, some energy will be dissipated possibly causing a temporary temperature rise due to self-heating. In principle, therefore, the system behaves neither perfectly isothermal nor adiabatic.     

Nonequilibrium state of the two-dimensional electron gas in the integer quantum Hall effect regime

We have compared the properties of the nonequilibrium state of the two-dimensional electron gas observed in the samples of different types by means of magnetotransport and magnetization measurements in the quantum Hall effect regime at integer filling factors n. It has been found that the range of filling factors corresponding to the nonequilibrium state is universal for the samples of different types and different measurement techniques and varies from 0.1 to 0.3 for n changing from 1 to 4. The comparison indicates that the observed nonequilibrium state is not directly caused by the appearance of eddy currents and the dielectric phase in the two-dimensional electron gas but is probably associated with the magnetic field-induced phase transition.     

Conductance of a quantum wire in the Wigner-crystal regime

We study the effect of Coulomb interactions on the conductance of a single-mode quantum wire connecting two bulk leads. When the density of electrons in the wire is very low, they arrange in a finite-length Wigner crystal. In this regime the electron spins form an antiferromagnetic Heisenberg chain with an exponentially small coupling J. An electric current in the wire perturbs the spin chain and gives rise to a temperature-dependent contribution of the spin subsystem to the resistance. At low temperature TJ the spin effect reduces the conductance to e2/h.     

Time-resolved entanglement and photon number correlation of twin beams: Application to quantum key distribution scheme

Quantum entanglement and intensity correlations in continuous variable (CV) regimes have investigated in the time domain in addition to many analogous investigations performed in the frequency domain. The specific signatures of time-resolved CV quadrature entanglement and intensity difference squeezing have been analyzed for periodically-pulsed optical parametric oscillator (OPO). An application to quantum key distribution scheme based on intensity correlation of twin beams has also been considered. The text was submitted by the author in English.     

Radiation-pressure acceleration of ion beams from nanofoil targets: the leaky light-sail regime

A new ion radiation-pressure acceleration regime, the "leaky light sail," is proposed which uses sub-skin-depth nanometer foils irradiated by circularly polarized laser pulses. In the regime, the foil is partially transparent, continuously leaking electrons out along with the transmitted laser field. This feature can be exploited by a multispecies nanofoil configuration to stabilize the acceleration of the light ion component, supplementing the latter with an excess of electrons leaked from those associated with the heavy ions to avoid Coulomb explosion. It is shown by 2D particle-in-cell simulations that a monoenergetic proton beam with energy 18 MeV is produced by circularly polarized lasers at intensities of just 101? W/cm2. 100 MeV proton beams are obtained by increasing the intensities to 2 × 102? W/cm2.     

Tolerance of continuous-variables quantum key distribution to the noise in state preparation

We address the applicability of the continuous-variables quantum key distribution in the realistic conditions of noisy preparation, channel loss and detection noise and investigate the possibility to increase its tolerance to the state preparation noise. The two types of preparation noise, either phase-sensitive or phase-insensitive excess noise, are considered in the assumption of optimal attacks performed by an eavesdropper within the setup based on the entangled source and either homodyne or heterodyne measurements. We show that preparation noise is destructive for the secure channel upon even low noise variances in the conditions of channel loss, while detection noise just decreases the key rate. We propose the method of sender-side attenuation to suppress the preparation noise in the entanglements-based scheme and show that it enables the secure key transmission upon arbitrary high preparation noise of both types and any pure channel loss against both individual and collective attacks.