Temperature-Dependent Periodicity of the Persistent Current in Strongly Interacting Systems.

The persistent current in small isolated rings enclosing magnetic flux is the current circulating in equilibrium in the absence of an external excitation. While initially studied in superconducting and normal metals, recently, atomic persistent currents have been generated in ultracold gases spurring a new wave of theoretical investigations. Nevertheless, our understanding of the persistent currents in interacting systems is far from complete, especially at finite temperatures.

Localizing Fractional Quasiparticles on Graphene Quantum Hall Antidots.

We report localization of fractional quantum Hall (QH) quasiparticles on graphene antidots. By studying coherent tunneling through the localized QH edge modes on the antidot, we measured the QH quasiparticle charges to be approximately ±e/3 at fractional fillings of ν=±1/3. The Dirac spectrum in graphene allows large energy scales and robust quasiparticle localization against thermal excitation.

Suppression of Dephasing by Qubit Motion in Superconducting Circuits.

We suggest and demonstrate a protocol which suppresses the low-frequency dephasing by qubit motion, i.e., transfer of the logical qubit of information in a system of n≥2 physical qubits.

Experimental realization of a Szilard engine with a single electron.

The most succinct manifestation of the second law of thermodynamics is the limitation imposed by the Landauer principle on the amount of heat a Maxwell demon (MD) can convert into free energy per single bit of information obtained in a measurement. We propose and realize an electronic MD based on a single-electron box operated as a Szilard engine, where kBT ln 2 of heat is extracted from the reservoir at temperature T per one bit of created information. The information is encoded in the position of an extra electron in the box.

Decoherence induced deformation of the ground state in adiabatic quantum computation.

Despite more than a decade of research on adiabatic quantum computation (AQC), its decoherence properties are still poorly understood. Many theoretical works have suggested that AQC is more robust against decoherence, but a quantitative relation between its performance and the qubits' coherence properties, such as decoherence time, is still lacking. While the thermal excitations are known to be important sources of errors, they are predominantly dependent on temperature but rather insensitive to the qubits' coherence.

Test of the Jarzynski and Crooks fluctuation relations in an electronic system.

Recent progress on micro- and nanometer-scale manipulation has opened the possibility to probe systems small enough that thermal fluctuations of energy and coordinate variables can be significant compared with their mean behavior. We present an experimental study of nonequilibrium thermodynamics in a classical two-state system, namely, a metallic single-electron box. We have measured with high statistical accuracy the distribution of dissipated energy as single electrons are transferred between the box electrodes.

Real-time observation of discrete Andreev tunneling events.

We provide a direct proof of two-electron Andreev transitions in a superconductor-normal-metal tunnel junction by detecting them in a real-time electron counting experiment. Our results are consistent with ballistic Andreev transport with an order of magnitude higher rate than expected for a uniform barrier, suggesting that only part of the interface is effectively contributing to the transport. These findings are quantitatively supported by our direct current measurements in single-electron transistors with similar tunnel barriers.

Violation of the fluctuation-dissipation theorem in time-dependent mesoscopic heat transport.

We have analyzed the spectral density of fluctuations of the energy flux through a mesoscopic constriction between two equilibrium reservoirs. It is shown that at finite frequencies, the fluctuating energy flux is not related to the thermal conductance of the constriction by the standard fluctuation-dissipation theorem, but contains additional noise. The main physical consequence of this extra noise is that the fluctuations do not vanish at zero temperature together with the vanishing thermal conductance.

Probing noise in flux qubits via macroscopic resonant tunneling.

Macroscopic resonant tunneling between the two lowest lying states of a bistable rf SQUID is used to characterize noise in a flux qubit. Measurements of the incoherent decay rate as a function of flux bias revealed a Gaussian-shaped profile that is not peaked at the resonance point but is shifted to a bias at which the initial well is higher than the target well. The rms amplitude of the noise, which is proportional to the dephasing rate 1/tauphi, was observed to be weakly dependent on temperature below 70 mK.

Nonadiabatic charge pumping in a hybrid single-electron transistor.

We study theoretically current quantization in the charge turnstile based on the superconductor-normal-metal single-electron transistor. The quantization accuracy is limited by either Andreev reflection or by Cooper-pair-electron cotunneling. The rates of these processes are calculated in the "above-the-threshold" regime when they compete directly with the lowest-order tunneling.

Macroscopic resonant tunneling in the presence of low frequency noise.

We develop a theory of macroscopic resonant tunneling of flux in a double-well potential in the presence of realistic flux noise with a significant low-frequency component. The rate of incoherent flux tunneling between the wells exhibits resonant peaks, the shape and position of which reflect qualitative features of the noise, and can thus serve as a diagnostic tool for studying the low-frequency flux noise in SQUID qubits. We show, in particular, that the noise-induced renormalization of the first resonant peak provides direct information on the temperature of the noise source and the strength of its quantum component.

Coulomb blockade of anyons in quantum antidots.

Coulomb interaction turns anyonic quasiparticles of a primary quantum Hall liquid with filling factor nu=1/(2m+1) into hard-core anyons. We have developed a model of coherent transport of such quasiparticles in systems of multiple antidots by extending the Wigner-Jordan description of 1D Abelian anyons to tunneling problems. We show that the anyonic exchange statistics manifests itself in tunneling conductance even in the absence of quasiparticle exchanges.

Mach-Zehnder interferometer in the fractional quantum Hall regime.

We consider tunneling between two edges of quantum Hall liquids of filling factors nu(0,1)=1/(2m(0,1)+1), with m(0)>or=m(1)>or=0, through two-point contacts forming a Mach-Zehnder interferometer. A quasiparticle description of the interferometer is derived explicitly through the instanton duality transformation. For m(0)+m(1)+1 identical withm>1, tunneling of quasiparticles of charge e/m leads to nontrivial m-state dynamics of effective flux through the interferometer, which restores the regular "electron" periodicity of the current in flux.

Counting statistics and detector properties of quantum point contacts.

Quantum detector properties of the quantum point contact (QPC) are analyzed for an arbitrary electron transparency and coupling strength to the measured system and are shown to be determined by the electron counting statistics. Conditions of the quantum-limited operation of the QPC detector, which prevent information loss through the scattering time and scattering phases, are found for arbitrary coupling. We show that the phase information can be restored and used for the quantum-limited detection by inclusion of the QPC detector in the electronic Mach-Zehnder interferometer.

Mesoscopic quadratic quantum measurements.

We develop a theory of quadratic quantum measurements by a mesoscopic detector. It is shown that the quadratic measurements should have nontrivial quantum information properties, providing, for instance, a simple way of entangling two noninteracting qubits. We also calculate the output spectrum of a detector with both linear and quadratic response, continuously monitoring two qubits.

Variable electrostatic transformer: controllable coupling of two charge qubits.

We propose and investigate a novel method for the controlled coupling of two Josephson charge qubits by means of a variable electrostatic transformer. The value of the coupling capacitance is given by the discretized curvature of the lowest energy band of a Josephson junction, which can be positive, negative, or zero. We calculate the charging diagram of the two-qubit system that reflects the transition from positive to negative through vanishing coupling.

Quantum oscillations in two coupled charge qubits.

A practical quantum computer, if built, would consist of a set of coupled two-level quantum systems (qubits). Among the variety of qubits implemented, solid-state qubits are of particular interest because of their potential suitability for integrated devices. A variety of qubits based on Josephson junctions have been implemented; these exploit the coherence of Cooper-pair tunnelling in the superconducting state.

Quantum nondemolition measurements of a qubit.

The concept of quantum nondemolition (QND) measurement is extended to coherent oscillations in an individual two-state system. Such a measurement enables direct observation of an intrinsic spectrum of these oscillations avoiding the detector-induced dephasing that affects the standard (non-QND) measurements. The suggested scheme can be realized in Josephson-junction qubits which combine flux and charge dynamics.

Aharonov-Casher-effect suppression of macroscopic tunneling of magnetic flux.

We suggest a system in which the amplitude of macroscopic flux tunneling can be modulated via the Aharonov-Casher effect. The system is an rf SQUID with the Josephson junction replaced by a Bloch transistor--two junctions separated by a small superconducting island on which the charge can be induced by an external gate voltage. When the Josephson coupling energies of the junctions are equal and the induced charge is q = e, destructive interference between tunneling paths brings the flux tunneling rate to zero.

Universal distribution of transparencies in highly conductive Nb/AlO(x)/Nb junctions.

We report the observation of the universal distribution of transparencies, predicted by Schep and Bauer [Phys. Rev. Lett.

Coulomb drag between one-dimensional conductors.

We have analyzed Coulomb drag between currents of interacting electrons in two parallel one-dimensional conductors of finite length L attached to external reservoirs. For strong coupling, the relative fluctuations of electron density in the conductors acquire energy gap M. At energies larger than gamma = constxv(-)exp(-LM/v(-))/L+gamma(+), where gamma(+) is the impurity scattering rate, and, for L>v(-)/M, where v(-) is the fluctuation velocity, the gap leads to an "ideal" drag with almost equal currents in the conductors.