Order parameter quantum fluctuations in a two-dimensional system of mesoscopic Josephson junctions

The boson lattice Hubbard model is used to study the role of quantum fluctuations of the phase and local density of the superfluid component in establishing a global superconducting state for a system of mesoscopic Josephson junctions or grains. The quantum Monte Carlo method is used to calculate the density of the superfluid component and fluctuations in the number of particles at sites of the two-dimensional lattice for various average site occupation numbers n ₀ (i.e., number of Cooper pairs per grain). For a system of strongly interacting bosons, the phase boundary of the ordered superconducting state lies above the corresponding boundary for its quasiclassical limit—the quantum XY-model—and approaches the latter as n ₀ increases. When the boson interaction is weak in the boson Hubbard model (i.e., the quantum fluctuations of the phase are small), the relative fluctuations of the order parameter modulus are significant when n ₀<10, while quantum fluctuations in the phase are significant when n ₀<8; this determines the region of mesoscopic behavior of the system. Comparison of the results of numerical modeling with theoretical calculations show that mean-field theory yields a qualitatively correct estimate of the difference between the phase diagrams of the quantum XY-model and the Hubbard model. For a quantitative estimate of this difference the free energy and thermodynamic averages of the Hubbard model are expanded in powers of 1/n ₀ using the method of functional integration. Zh. éksp. Teor. Fiz. 113, 261–277 (January 1998)     

Statistics of the work done on a quantum critical system by quenching a control parameter

We study the statistics of the work done on a quantum critical system by quenching a control parameter in the Hamiltonian. We elucidate the relation between the probability distribution of the work and the Loschmidt echo, a quantity emerging usually in the context of dephasing. Using this connection we characterize the statistics of the work done on a quantum Ising chain by quenching locally or globally the transverse field. We show that for local quenches starting at criticality the probability distribution of the work displays an interesting edge singularity.     

Quantum parameter estimation in a spin-boson dephasing quantum system by periodical projective measurements

In this paper,we explore how to estimate the phase damping parameter γ and the tunneling amplitude parameter ?from a spin-boson dephasing quantum model by periodical projective measurements.The preparation of initial states is accomplished by performing the period measurements in our scheme.The parameter γ can be always estimated when projective measurement bases are chosen as θ = π/2 and φ = 0.Based on the estimated value of γ and the interval information of ?,we can select another measurement bases(θ = π/4 and φ = π/2) to obtain the estimated value of ?.A coherent control is indispensable to estimate ? if γ is in the interval of ?;whereas the control is not necessary if γ is out of the known interval of ?.We establish the relation between the optimal period time and the parameter γ or ? in terms of Fisher information.Although the optimal measurement period cannot be selected beforehand,the aforementioned relation can be utilized to adjust the measurement period to approach the optimal one.     

Estimation of shift parameters of a quantum state

Estimation of shift parameters such as arrival time, phase, angle of rotation, position of a non-relativistic or relativistic particle is considered. An approach from the point of view of quantum estimation theory enables to give a proper definition of the time observable and the position observables of a massless relativistic particle, i.e. observables to which there do not correspond self-adjoint operators. Some new inequalities for estimates of shift parameters are obtained; in particular a rigorous uncertainty relation for coordinates of the photon is established.     

Enhancing parameter precision of optimal quantum estimation by quantum screening

We propose a scheme of quantum screening to enhance the parameter-estimation precision in open quantum systems by means of the dynamics of quantum Fisher information.The principle of quantum screening is based on an auxiliary system to inhibit the decoherence processes and erase the excited state to the ground state.By comparing the case without quantum screening,the results show that the dynamics of quantum Fisher information with quantum screening has a larger value during the evolution processes.     

Multi-objective optimization in quantum parameter estimation

We investigate quantum parameter estimation based on linear and Kerr-type nonlinear controls in an open quantum system, and consider the dissipation rate as an unknown parameter. We show that while the precision of parameter estimation is improved,it usually introduces a significant deformation to the system state. Moreover, we propose a multi-objective model to optimize the two conflicting objectives:(1) maximizing the Fisher information, improving the parameter estimation precision, and(2)minimizing the deformation of the system state, which maintains its fidelity. Finally, simulations of a simplified ε-constrained model demonstrate the feasibility of the Hamiltonian control in improving the precision of the quantum parameter estimation.     

Environment-mediated control of a quantum system

Recently, a new approach for the controllability of a two-dimensional quantum system S has been proposed, based on its interaction with an initially uncorrelated two-dimensional probe P whose initial state can be arbitrarily modified. Following this scheme and considering a particular model for the environment, we show that, in some specific cases, the environment-induced entanglement is rich enough to completely control the dynamics of S. Under suitable conditions on the interaction of S, P, and the environment, we prove that the state of S can be driven to an arbitrary target state by varying the initial state of P.     

Note on the quantum displacement of the critical point

The quantum corrections to the law of corresponding states are studied by calculating the critical pressure, temperature, and density to first order in Planck's constanth on an exactly soluble model. The ratio of the critical parameters to the corresponding classical values are found to be (p _c/p _c ⁰)^1/2=_c/_c ⁰ = T_c/T_c ⁰ = 1–0.67, with=h _c ^1/3(mkT _c)^–1/2. The critical ratio is independent ofh to first order. The results are compared with critical data for noble gases and hydrogen isotopes.     

Luminescence of a semiconductor quantum dot system

A microscopic theory is used to study photoluminescence of semiconductor quantum dots under the influence of Coulomb and carrier-photon correlation effects beyond the Hartree-Fock level. We investigate the emission spectrum and the decay properties of the time-resolved luminescence from initially excited quantum dots. The influence of the correlations is included within a cluster expansion scheme up to the singlet-doublet level.     

Mean-field dynamics of a quantum dot-microcavity system

Mean-field evolution equations for the exciton and photon populations and polarizations (Bloch–Lamb equations) are written and numerically solved in order to describe the dynamics of electronic states in a quantum dot coupled to the photon field of a microcavity. The equations account for phase space filling effects and Coulomb interactions among carriers, and include also (in a phenomenological way) incoherent pumping of the quantum dot, photon losses through the microcavity mirrors, and electron–hole population decay due to spontaneous emission of the dot. When the dot may support more than one electron–hole pair, asymptotic oscillatory states, with periods between 0.5 and 1.5 ps, are found almost for any values of the system parameters.     

Time evolution of a quantum lattice system

It is shown that for a quantum lattice system associated with a Hamiltonian with a kinetic part and a potential sufficiently decreasing in the particle number, the time evolution can be described, under certain assumptions, by automorphisms of a suitable algebra.     

Error-detected single-photon quantum routing using a quantum dot and a double-sided microcavity system

Based on a hybrid system consisting of a quantum dot coupled with a double-sided micropillar cavity, we investigate the implementation of an error-detected photonic quantum routing controlled by the other photon. The computational errors from unexpected experimental imperfections are heralded by single photon detections, resulting in a unit fidelity for the present scheme, so that this scheme is intrinsically robust. We discuss the performance of the scheme with currently achievable experimental parameters. Our results show that the present scheme is efficient. Furthermore, our scheme could provide a promising building block for quantum networks and distributed quantum information processing in the future.     

Dissipative dynamics of a quantum two-state system in presenceof nonequilibrium quantum noise

We analyze the real-time dynamics of a quantum two-state system in the presence ofnonequilibrium quantum fluctuations. The latter are generated by a coupling of thetwo-state system to a single electronic level of a quantum dot which carries anonequilibrium tunneling current. We restrict to the sequential tunneling regime andcalculate the dynamics of the two-state system, of the dot population, and of thenonequilibrium charge current on the basis of a diagrammatic perturbative method valid fora weak tunneling coupling. We find a nontrivial dependence of the relaxation and dephasingrates of the two-state system due to the nonequilibrium fluctuations which is directlylinked to the structure of the unperturbed central system. In addition, aHeisenberg-Langevin-equation of motion allows us to calculate the correlation function ofthe nonequilibrium fluctuations. By this, we obtain a generalized nonequilibriumfluctuation relation which includes the equilibrium fluctuation-dissipation theorem in thelimit of zero transport voltage. A straightforward extension to the case with atime-periodic ac voltage is shown.     

Improvement of a continuous-variable measurement-device-independent quantum key distribution system via quantum scissors

We propose a new scheme to enhance the performance of the Gussian-modulated coherent-state continuous-variable measurement-device-independent quantum key distribution (CV-MDI-QKD) system via quantum scissors (QS) operation at Bob's side. As an non-deterministic amplifying setup, we firstly introduce the QS-enhanced CV-MDI-QKD protocol and then investigate the success probability of the QS operation in accordance with the equivalent one-way scheme. Afterwards, we investigate the effect of the QS operation on the proposed scheme and analyze the performance of the QS-enhanced CV-MDI-QKD system under the extreme asymmetric circumstance. Simulation results show that the QS operation can indeed improve the performance of the CV-MDI-QKD system considerably. QS-enhanced CV-MDI-QKD protocol outperforms the original CV-MDI-QKD protocol in both the maximum transmission distance and the secret key rate. Moreover, the better the performance of QS operation, the more significant the improvement of performance of the system.