Squeezing as the source of inefficiency in the quantum Otto cycle

The availability of controllable macroscopic devices, which maintain quantum coherence over relatively long time intervals, for the first time allows an experimental realization of many effects previously considered only as Gedankenexperiments, such as the operation of quantum heat engines. The theoretical efficiency \eta of quantum heat engines is restricted by the same Carnot boundary \eta_C as for the classical ones: any deviations from quasistatic evolution suppressing \eta below \eta_C. Here we investigate an implementation of an analog of the Otto cycle in a tunable quantum coherent circuit and show that the specific source of inefficiency is the quantum squeezing of the thermal state due to the finite speed of compression/expansion of the system.Comment: 17 pages, 5 figure

Two-qubit parametric amplifier: large amplification of weak signals

Using numerical simulations, we show that two coupled qubits can amplify a weak signal about hundredfold. This can be achieved if the two qubits are biased simultaneously by this weak signal and a strong pump signal, both of which having frequencies close to the inter-level transitions in the system. The weak signal strongly affects the spectrum generated by the strong pumping drive by producing and controlling mixed harmonics with amplitudes of the order of the main harmonic of the strong drive. We show that the amplification is robust with respect to noise, with an intensity of the order of the weak signal. When deviating from the optimal regime (corresponding to strong qubit coupling and a weak-signal frequency equal to the inter-level transition frequency) the proposed amplifier becomes less efficient, but it can still considerably enhance a weak signal (by several tens). We therefore propose to use coupled qubits as a combined parametric amplifier and frequency shifter.Comment: 6 figure

Noise in a Quantum Point Contact due to a Fluctuating Impurity Configuration

We propose a theoretical model for the low-frequency noise observed in a quantum point contact (QPC) electrostatically defined in the 2D electron gas at a GaAs-AlGaAs interface. In such contacts electron scattering by soft impurity- or boundary potentials coherently splits an incoming wave function between different transverse modes. Interference between these modes have been suggested to explain observed non-linearities in the QPC-conductance. In this study we invoke the same mechanism and the time-dependent current due to soft dynamical impurity scattering in order to analyze the low-frequency (telegraph-like) noise which has been observed along with a nonlinear conductance. For the simplified case of a channel with two extended (current carrying) modes, a simple analytical formula for the noise intensity is derived. Generally we have found qualitative similarities between the noise and the square of the transconductance. Nevertheless, incidentally there may be situations when noise is suppressed but transconductance enhanced.Comment: 9 revte

Is a single photon's wave front observable?

The ultimate goal and the theoretical limit of weak signal detection is the ability to detect a single photon against a noisy background. [...] In this paper we show, that a combination of a quantum metamaterial (QMM)-based sensor matrix and quantum non-demolition (QND) readout of its quantum state allows, in principle, to detect a single photon in several points, i.e., to observe its wave front. Actually, there are a few possible ways of doing this, with at least one within the reach of current experimental techniques for the microwave range. The ability to resolve the quantum-limited signal from a remote source against a much stronger local noise would bring significant advantages to such diverse fields of activity as, e.g., microwave astronomy and missile defence. The key components of the proposed method are 1) the entangling interaction of the incoming photon with the QMM sensor array, which produces the spatially correlated quantum state of the latter, and 2) the QND readout of the collective observable (e.g., total magnetic moment), which characterizes this quantum state. The effects of local noise (e.g., fluctuations affecting the elements of the matrix) will be suppressed relative to the signal from the spatially coherent field of (even) a single photon.Comment: 13 pages, 4 figure

Dissymmetrical tunnelling in heavy fermion metals

A tunnelling conductivity between a heavy fermion metal and a simple metallic point is considered. We show that at low temperatures this conductivity can be noticeably dissymmetrical with respect to the change of voltage bias. The dissymmetry can be observed in experiments on the heavy fermion metals whose electronic system has undergone the fermion condensation quantum phase transition.Comment: 7 pages, Revte

Ultimate on-chip quantum amplifier

We report amplification of electromagnetic waves by a single artificial atom in open 1D space. Our three-level artificial atom -- a superconducting quantum circuit -- coupled to a transmission line presents an analog of a natural atom in open space. The system is the most fundamental quantum amplifier whose gain is limited by a spontaneous emission mechanism. The noise performance is determined by the quantum noise revealed in the spectrum of spontaneous emission, also characterized in our experiments.Comment: 4 pages, 4 figures + supplemenntary materials accepted for publication in Phys. Rev. Lett

Spin-polarized tunneling currents through a ferromagnetic insulator between two metallic or superconducting leads

Using the Keldysh formalism the tunneling current through a hybrid structure where a confined magnetic insulator (I) is sandwiched between two non-magnetic leads is calculated. The leads can be either normal metals (M) or superconductors (S). Each region is modelled as a single band in tight-binding approximation in order to understand the formation of the tunneling current as clearly as possible. The tunneling process itself is simulated by a hybridization between the lead and insulator conduction bands. The insulator is assumed to have localized moments which can interact with the tunneling electrons. This is described by the Kondo Lattice Model (KLM) and treated within an interpolating self-energy approach. For the superconductor the mean-field BCS theory is used. The spin polarization of the current shows a strong dependence both on the applied voltage and the properties of the materials. Even for this idealized three band model there is a qualitative agreement with experiment.Comment: 15 pages, 23 figures, accepted for publication in PR

Quantum theory as a relevant framework for the statement of probabilistic and many-valued logic

Based on ideas of quantum theory of open systems we propose the consistent approach to the formulation of logic of plausible propositions. To this end we associate with every plausible proposition diagonal matrix of its likelihood and examine it as density matrix of relevant quantum system. We are showing that all logical connectives between plausible propositions can be represented as special positive valued transformations of these matrices. We demonstrate also the above transformations can be realized in relevant composite quantum systems by quantum engineering methods. The approach proposed allows one not only to reproduce and generalize results of well-known logical systems (Boolean, Lukasiewicz and so on) but also to classify and analyze from unified point of view various actual problems in psychophysics and social sciences.Comment: 7 page

Impurity-induced Local Density of States in a D-wave Superconductor Carrying a Supercurrent

The local density of states (LDOS) and its Fourier component induced by a unitary impurity in a supercurrent-carrying d-wave superconductor are investigated. Both of these quantities possess a reflection symmetry about the line passing through the impurity site and along the supercurrent if it is applied along the antinodal or nodal direction. With increasing supercurrent, both the coherence and resonant peaks in the LDOS are suppressed and slightly broadened. Under a supercurrent along the antinodal direction, the coherence peaks split into double peaks. The modulation wavevectors associated with elastic scatterings of quasiparticles by the defect from one constant-energy piece of the Fermi surface to another are displayed as bright or dark spots in the Fourier space of the LDOS image, and they may be suppressed or enhanced, and shifted depending on the applied current and the bias voltage.Comment: 5 pages, 6 figure

Extended Ginzburg-Landau formalism: systematic expansion in small deviation from the critical temperature

Based on the Gor'kov formalism for a clean s-wave superconductor, we develop an extended version of the single-band Ginzburg-Landau (GL) theory by means of a systematic expansion in the deviation from the critical temperature T_c, i.e., tau=1-T/T_c. We calculate different contributions to the order parameter and the magnetic field: the leading contributions (~ tau^1/2 in the order parameter and ~ tau in the magnetic field) are controlled by the standard Ginzburg-Landau (GL) theory, while the next-to-leading terms (~ tau^3/2 in the gap and ~ tau^2 in the magnetic field) constitute the extended GL (EGL) approach. We derive the free-energy functional for the extended formalism and the corresponding expression for the current density. To illustrate the usefulness of our formalism, we calculate, in a semi-analytical form, the temperature-dependent correction to the GL parameter at which the surface energy becomes zero, and analytically, the temperature dependence of the thermodynamic critical field. We demonstrate that the EGL formalism is not just a mathematical extension to the theory - variations of both the gap and the thermodynamic critical field with temperature calculated within the EGL theory are found in very good agreement with the full BCS results down to low temperatures, which dramatically improves the applicability of the formalism compared to its standard predecessor