1,302 research outputs found
Full distribution of work done on a quantum system for arbitrary initial states
We propose a novel approach to define and measure the statistics of work,
internal energy and dissipated heat in a driven quantum system. In our
framework the presence of a physical detector arises naturally and work and its
statistics can be investigated in the most general case. In particular, we show
that the quantum coherence of the initial state can lead to measurable effects
on the moments of the work done on the system. At the same time, we recover the
known results if the initial state is a statistical mixture of energy
eigenstates. Our method can also be applied to measure the dissipated heat in
an open quantum system. By sequentially coupling the system to a detector, we
can track the energy dissipated in the environment while accessing only the
system degrees of freedom.Comment: 8 pages, 2 figures. Title change
Geometric Landau-Zener interferometry in a superconducting charge pump
We propose a new type of interferometry, based on geometric phases
accumulated by a periodically driven two-level system undergoing multiple
Landau-Zener transitions. As a specific example, we study its implementation in
a superconducting charge pump. We find that interference patterns appear as a
function of the pumping frequency and the phase bias, and clearly manifest
themselves in the pumped charge. We also show that the effects described should
persist in the presence of realistic decoherence.Comment: 5 pages, 3 figure
Radiation comb generation with extended Josephson junctions
We propose the implementation of a Josephson radiation comb generator (JRCG)
based on an extended Josephson junction subject to a time dependent magnetic
field. The junction critical current shows known diffraction patterns and
determines the position of the critical nodes when it vanishes. When the
magnetic flux passes through one of such critical nodes, the superconducting
phase must undergo a -jump to minimize the Josephson energy.
Correspondingly a voltage pulse is generated at the extremes of the junction.
Under periodic driving this allows us to produce a comb-like voltage pulses
sequence. In the frequency domain it is possible to generate up to hundreds of
harmonics of the fundamental driving frequency, thus mimicking the frequency
comb used in optics and metrology. We discuss several implementations through a
rectangular, cylindrical and annular junction geometries, allowing us to
generate different radiation spectra and to produce an output power up to
~pW at ~GHz for a driving frequency of ~MHz.Comment: 4+ pages, 4 color figure
Single Cooper-pair pumping in the adiabatic limit and beyond
We demonstrate controlled pumping of Cooper pairs down to the level of a
single pair per cycle, using an rf-driven Cooper-pair sluice. We also
investigate the breakdown of the adiabatic dynamics in two different ways. By
transferring many Cooper pairs at a time, we observe a crossover between pure
Cooper-pair and mixed Cooper-pair-quasiparticle transport. By tuning the
Josephson coupling that governs Cooper-pair tunneling, we characterize
Landau-Zener transitions in our device. Our data are quantitatively accounted
for by a simple model including decoherence effects.Comment: 5 pages, 5 figure
Adiabatically steered open quantum systems: Master equation and optimal phase
We introduce an alternative way to derive the generalized form of the master
equation recently presented by J. P. Pekola et al. [Phys. Rev. Lett. 105,
030401 (2010)] for an adiabatically steered two-level quantum system
interacting with a Markovian environment. The original derivation employed the
effective Hamiltonian in the adiabatic basis with the standard interaction
picture approach but without the usual secular approximation. Our approach is
based on utilizing a master equation for a non-steered system in the first
super-adiabatic basis. It is potentially efficient in obtaining higher-order
equations. Furthermore, we show how to select the phases of the adiabatic
eigenstates to minimize the local adiabatic parameter and how this selection
leads to states which are invariant under a local gauge change. We also discuss
the effects of the adiabatic noncyclic geometric phase on the master equation.Comment: 8 pages, no figures, final versio
Photonic heat conduction in Josephson-coupled Bardeen-Cooper-Schrieffer superconductors
We investigate the photon-mediated heat flow between two Josephson-coupled
Bardeen-Cooper-Schrieffer (BCS) superconductors. We demonstrate that in
standard low temperature experiments involving temperature-biased
superconducting quantum interference devices (SQUIDs), this radiative
contribution is negligible if compared to the direct galvanic one, but it
largely exceeds the heat exchanged between electrons and the lattice phonons.
The corresponding thermal conductance is found to be several orders of
magnitude smaller, for real experiments setup parameters, than the universal
quantum of thermal conductance, kappa_0(T)=pi k_B^2T/6hbar.Comment: 8 pages, 6 figure
Coherent caloritronics in Josephson-based nanocircuits
We describe here the first experimental realization of a heat interferometer,
thermal counterpart of the well-known superconducting quantum interference
device (SQUID). These findings demonstrate, on the first place, the existence
of phase-dependent heat transport in Josephson-based superconducting circuits
and, on the second place, open the way to novel ways of mastering heat at the
nanoscale. Combining the use of external magnetic fields for phase biasing and
different Josephson junction architectures we show here that a number of heat
interference patterns can be obtained. The experimental realization of these
architectures, besides being relevant from a fundamental physics point of view,
might find important technological application as building blocks of
phase-coherent quantum thermal circuits. In particular, the performance of two
different heat rectifying devices is analyzed.Comment: 34 pages, 15 figures, review article for Ultra-low temperatures and
nanophysics ULTN2013. Microkelvin Proceeding
Measurement-dependent corrections to work distributions arising from quantum coherences
For a quantum system undergoing a unitary process work is commonly defined
based on the Two Projective Measurement (TPM) protocol which measures the
energies of the system before and after the process. However, it is well known
that projective measurements disregard quantum coherences of the system with
respect to the energy basis, thus removing potential quantum signatures in the
work distribution. Here we consider weak measurements of the system's energy
difference and establish corrections to work averages arising from initial
system coherences. We discuss two weak measurement protocols that couple the
system to a detector, prepared and measured either in the momentum or the
position eigenstates. Work averages are derived for when the system starts in
the proper thermal state versus when the initial system state is a pure state
with thermal diagonal elements and coherences characterised by a set of phases.
We show that by controlling only the phase differences between the energy
eigenstate contributions in the system's initial pure state, the average work
done during the same unitary process can be controlled. By changing the phases
alone one can toggle from regimes where the systems absorbs energy, i.e. a work
cost, to the ones where it emits energy, i.e. work can be drawn. This suggests
that the coherences are additional resources that can be used to manipulate or
store energy in a quantum system.Comment: 9 pages, 3 figure
Non-Abelian geometric phases in ground state Josephson devices
We present a superconducting circuit in which non-Abelian geometric
transformations can be realized using an adiabatic parameter cycle. In contrast
to previous proposals, we employ quantum evolution in the ground state. We
propose an experiment in which the transition from non-Abelian to Abelian
cycles can be observed by measuring the pumped charge as a function of the
period of the cycle. Alternatively, the non-Abelian phase can be detected using
a single-electron transistor working as a charge sensor.Comment: 5 pages, 3 figures; added references and clarified discussion about
earlier research on the fiel
Coherent diffraction of thermal currents in Josephson tunnel junctions
We theoretically investigate heat transport in temperature-biased Josephson
tunnel junctions in the presence of an in-plane magnetic field. In full analogy
with the Josephson critical current, the phase-dependent component of the heat
flux through the junction displays coherent diffraction. Thermal transport is
analyzed in three prototypical junction geometries highlighting their main
differences. Notably, minimization of the Josephson coupling energy requires
the quantum phase difference across the junction to undergo \pi-slips in
suitable intervals of magnetic flux. An experimental setup suited to detect
thermal diffraction is proposed and analyzed.Comment: 6.5 pages, 4 color figures, updated versio
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