Effect of the shot-noise on a Coulomb blockaded single Josephson junction

We have investigated how the Coulomb blockade of a mesoscopic Josephson junction in a high-impedance environment is suppressed by shot noise from an adjacent junction. The presented theoretical analysis is an extension of the phase correlation theory for the case of a non-Gaussian noise. Asymmetry of the non-Gaussian noise should result in the shift of the conductance minimum from zero voltage and the ratchet effect (nonzero current at zero voltage), which have been experimentally observed. The analysis demonstrates that a Coulomb blockaded tunnel junction in a high impedance environment can be used as an effective noise detector.Comment: 4 pages, 1 figure; figure and typos corrected, added reference

Influence of a Random Telegraph Process on the Transport through a Point Contact

We describe the transport properties of a point contact under the influence of a classical two-level fluctuator. We employ a transfer matrix formalism allowing us to calculate arbitrary correlation functions of the stochastic process by mapping them on matrix products. The result is used to obtain the generating function of the full counting statistics of a classical point contact subject to a classical fluctuator, including extensions to a pair of two-level fluctuators as well as to a quantum point contact. We show that the noise in the quantum point contact is a sum of the (quantum) partitioning noise and the (classical) noise due to the two-level fluctuator. As a side result, we obtain the full counting statistics of a quantum point contact with time-dependent transmission probabilities.Comment: 8 pages, 2 figure; a new section about experiments and a figure showing the crossover from sub- to superpoissonian noise have been adde

Entanglement in Mesoscopic Structures: Role of Projection

We present a theoretical analysis of the appearance of entanglement in non-interacting mesoscopic structures. Our setup involves two oppositely polarized sources injecting electrons of opposite spin into the two incoming leads. The mixing of these polarized streams in an ideal four-channel beam splitter produces two outgoing streams with particular tunable correlations. A Bell inequality test involving cross-correlated spin-currents in opposite leads signals the presence of spin-entanglement between particles propagating in different leads. We identify the role of fermionic statistics and projective measurement in the generation of these spin-entangled electrons.Comment: 5 pages, 1 figur

Quantum-enhanced magnetometry by phase estimation algorithms with a single artificial atom

Phase estimation algorithms are key protocols in quantum information processing. Besides applications in quantum computing, they can also be employed in metrology as they allow for fast extraction of information stored in the quantum state of a system. Here, we implement two suitably modified phase estimation procedures, the Kitaev and the semiclassical Fourier-transform algorithms, using an artificial atom realized with a superconducting transmon circuit. We demonstrate that both algorithms yield a flux sensitivity exceeding the classical shot-noise limit of the device, allowing one to approach the Heisenberg limit. Our experiment paves the way for the use of superconducting qubits as metrological devices which are potentially able to outperform the best existing flux sensors with a sensitivity enhanced by few orders of magnitude

Time ordering and counting statistics

The basic quantum mechanical relation between fluctuations of transported charge and current correlators is discussed. It is found that, as a rule, the correlators are to be time-ordered in an unusual way. Instances where the difference with the conventional ordering matters are illustrated by means of a simple scattering model. We apply the results to resolve a discrepancy concerning the third cumulant of charge transport across a quantum point contact.Comment: 19 pages, 1 figure; inconsequential mistake and typos correcte

Entanglement in mesoscopic structures: Role of projection

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Photo--assisted current and shot noise in the fractional quantum Hall effect

The effect of an AC perturbation on the shot noise of a fractional quantum Hall fluid is studied both in the weak and the strong backscattering regimes. It is known that the zero-frequency current is linear in the bias voltage, while the noise derivative exhibits steps as a function of bias. In contrast, at Laughlin fractions, the backscattering current and the backscattering noise both exhibit evenly spaced singularities, which are reminiscent of the tunneling density of states singularities for quasiparticles. The spacing is determined by the quasiparticle charge $\nu e$ and the ratio of the DC bias with respect to the drive frequency. Photo--assisted transport can thus be considered as a probe for effective charges at such filling factors, and could be used in the study of more complicated fractions of the Hall effect. A non-perturbative method for studying photo--assisted transport at $\nu=1/2$ is developed, using a refermionization procedure.Comment: 14 pages, 6 figure

Spin-Flip Noise in a Multi-Terminal Spin-Valve

We study shot noise and cross correlations in a four terminal spin-valve geometry using a Boltzmann-Langevin approach. The Fano factor (shot noise to current ratio) depends on the magnetic configuration of the leads and the spin-flip processes in the normal metal. In a four-terminal geometry, spin-flip processes are particular prominent in the cross correlations between terminals with opposite magnetization.Comment: 4 pages, 3 figure

Momentum noise in a quantum point contact

Ballistic electrons flowing through a constriction can transfer momentum to the lattice and excite a vibration of a free-standing conductor. We show (both numerically and analytically) that the electromechanical noise power P does not vanish on the plateaus of quantized conductance -- in contrast to the current noise. The dependence of $P$ on the constriction width can be oscillatory or stepwise, depending on the geometry. The stepwise increase amounts to an approximate quantization of momentum noise.Comment: 4 pages including 4 figure

Bell inequalities and entanglement in solid state devices

Bell-inequality checks constitute a probe of entanglement -- given a source of entangled particles, their violation are a signature of the non-local nature of quantum mechanics. Here, we study a solid state device producing pairs of entangled electrons, a superconductor emitting Cooper pairs properly split into the two arms of a normal-metallic fork with the help of appropriate filters. We formulate Bell-type inequalities in terms of current-current cross-correlators, the natural quantities measured in mesoscopic physics; their violation provides evidence that this device indeed is a source of entangled electrons.Comment: 4 pages, 1 figur