66 research outputs found
Measurement scheme for the Lamb shift in a superconducting circuit with broadband environment
Motivated by recent experiments on quantum mechanical charge pumping in a
Cooper pair sluice, we present a measurement scheme for observing shifts of
transition frequencies in two-level quantum systems induced by broadband
environmental fluctuations. In contrast to quantum optical and related set-ups
based on cavities, the impact of a thermal phase reservoir is considered. A
thorough analysis of Lamb and Stark shifts within weak-coupling master
equations is complemented by non-perturbative results for the model of an
exactly solvable harmonic system. The experimental protocol to measure the Lamb
shift in experimentally feasible superconducting circuits is analysed in detail
and supported by numerical simulations.Comment: 8 pages, 4 figure
Cooper pair current in the presence of flux noise
We study the effect of the flux noise on the Cooper pair current and pumping.
We generalize the definition of the current in order to take into account the
contribution induced by the environment. It turns out that this dissipative
current vanishes for charge noise but it is finite in general for noise
operators that do not commute with the charge operator. We discuss in a generic
framework the effect of flux noise and present a way to engineer it by coupling
the system to an additional external circuit. We calculate numerically the
pumped charge through the device by solving the master equation for the reduced
density matrix of the system and show how it depends on the coupling to the
artificial environment.Comment: 10 pages, 6 figure
Single-electron current sources: towards a refined definition of ampere
Controlling electrons at the level of elementary charge has been
demonstrated experimentally already in the 1980's. Ever since, producing an
electrical current , or its integer multiple, at a drive frequency has
been in a focus of research for metrological purposes. In this review we first
discuss the generic physical phenomena and technical constraints that influence
charge transport. We then present the broad variety of proposed realizations.
Some of them have already proven experimentally to nearly fulfill the demanding
needs, in terms of transfer errors and transfer rate, of quantum metrology of
electrical quantities, whereas some others are currently "just" wild ideas,
still often potentially competitive if technical constraints can be lifted. We
also discuss the important issues of read-out of single-electron events and
potential error correction schemes based on them. Finally, we give an account
of the status of single-electron current sources in the bigger framework of
electric quantum standards and of the future international SI system of units,
and briefly discuss the applications and uses of single-electron devices
outside the metrological context.Comment: 55 pages, 38 figures; (v2) fixed typos and misformatted references,
reworded the section on AC pump
Equivalent qubit dynamics under classical and quantum noise
We study the dynamics of quantum systems under classical and quantum noise,
focusing on decoherence in qubit systems. Classical noise is described by a
random process leading to a stochastic temporal evolution of a closed quantum
system, whereas quantum noise originates from the coupling of the microscopic
quantum system to its macroscopic environment. We derive deterministic master
equations describing the average evolution of the quantum system under
classical continuous-time Markovian noise and two sets of master equations
under quantum noise. Strikingly, these three equations of motion are shown to
be equivalent in the case of classical random telegraph noise and proper
quantum environments. Hence fully quantum-mechanical models within the Born
approximation can be mapped to a quantum system under classical noise.
Furthermore, we apply the derived equations together with pulse optimization
techniques to achieve high-fidelity one-qubit operations under random telegraph
noise, and hence fight decoherence in these systems of great practical
interest.Comment: 5 pages, 2 figures; converted to PRA format, added Fig. 2, corrected
typo
as parameter of Minkowski metric in effective theory
With the proper choice of the dimensionality of the metric components, the
action for all fields becomes dimensionless. Such quantities as the vacuum
speed of light c, the Planck constant \hbar, the electric charge e, the
particle mass m, the Newton constant G never enter equations written in the
covariant form, i.e., via the metric g^{\mu\nu}. The speed of light c and the
Planck constant are parameters of a particular two-parametric family of
solutions of general relativity equations describing the flat isotropic
Minkowski vacuum in effective theory emerging at low energy:
g^{\mu\nu}=diag(-\hbar^2, (\hbar c)^2, (\hbar c)^2, (\hbar c)^2). They
parametrize the equilibrium quantum vacuum state. The physical quantities which
enter the covariant equations are dimensionless quantities and dimensionful
quantities of dimension of rest energy M or its power. Dimensionless quantities
include the running coupling `constants' \alpha_i; topological and geometric
quantum numbers (angular momentum quantum number j, weak charge, electric
charge q, hypercharge, baryonic and leptonic charges, number of atoms N, etc).
Dimensionful parameters include the rest energies of particles M_n (or/and mass
matrices); the gravitational coupling K with dimension of M^2; cosmological
constant with dimension M^4; etc. In effective theory, the interval s has the
dimension of 1/M; it characterizes the dynamics of particles in the quantum
vacuum rather than geometry of space-time. We discuss the effective action, and
the measured physical quantities resulting from the action, including
parameters which enter the Josepson effect, quantum Hall effect, etc.Comment: 18 pages, no figures, extended version of the paper accepted in JETP
Letter
Coherent superconducting quantum pump
We demonstrate non-adiabatic charge pumping utilizing a sequence of coherent
oscillations between a superconducting island and two reservoirs. Our method,
based on pulsed quantum state manipulations, allows to speedup charge pumping
to a rate which is limited by the coupling between the island and the
reservoirs given by the Josephson energy. Our experimental and theoretical
studies also demonstrate that relaxation can be employed to reset the pump and
avoid accumulation of errors due to non-ideal control pulses.Comment: 4 pages, 3 figure
Distribution of Entropy Production in a Single-Electron Box
Recently, the fundamental laws of thermodynamics have been reconsidered for
small systems. The discovery of the fluctuation relations has spurred
theoretical and experimental studies on thermodynamics of systems with few
degrees of freedom. The concept of entropy production has been extended to the
microscopic level by considering stochastic trajectories of a system coupled to
a heat bath. However, the experimental observation of the microscopic entropy
production remains elusive. We measure distributions of the microscopic entropy
production in a single-electron box consisting of two islands with a tunnel
junction. The islands are coupled to separate heat baths at different
temperatures, maintaining a steady thermal non-equilibrium. As Jarzynski
equality between work and free energy is not applicable in this case, the
entropy production becomes the relevant parameter. We verify experimentally
that the integral and detailed fluctuation relations are satisfied.
Furthermore, the coarse-grained entropy production from trajectories of
electronic transitions is related to the bare entropy production by a universal
formula. Our results reveal the fundamental roles of irreversible entropy
production in non-equilibrium small systems
Quantum algorithm for simulating the dynamics of an open quantum system
In the study of open quantum systems, one typically obtains the decoherence
dynamics by solving a master equation. The master equation is derived using
knowledge of some basic properties of the system, the environment and their
interaction: one basically needs to know the operators through which the system
couples to the environment and the spectral density of the environment. For a
large system, it could become prohibitively difficult to even write down the
appropriate master equation, let alone solve it on a classical computer. In
this paper, we present a quantum algorithm for simulating the dynamics of an
open quantum system. On a quantum computer, the environment can be simulated
using ancilla qubits with properly chosen single-qubit frequencies and with
properly designed coupling to the system qubits. The parameters used in the
simulation are easily derived from the parameters of the system+environment
Hamiltonian. The algorithm is designed to simulate Markovian dynamics, but it
can also be used to simulate non-Markovian dynamics provided that this dynamics
can be obtained by embedding the system of interest into a larger system that
obeys Markovian dynamics. We estimate the resource requirements for the
algorithm. In particular, we show that for sufficiently slow decoherence a
single ancilla qubit could be sufficient to represent the entire environment,
in principle.Comment: 5 figures, two table
- âŠ