117 research outputs found
Decoherence effects on weak value measurements in double quantum dots
We study the effect of decoherence on a weak value measurement in a paradigm
system consisting of a double quantum dot continuously measured by a quantum
point contact. Fluctuations of the parameters controlling the dot state induce
decoherence. We find that, for measurements longer than the decoherence time,
weak values are always reduced within the range of the eigenvalues of the
measured observable. For measurements at shorter time scales, the measured weak
value strongly depends on the interplay between the decoherence dynamics of the
system and the detector backaction. In particular, depending on the
postselected state and the strength of the decoherence, a more frequent
classical readout of the detector might lead to an enhancement of weak values.Comment: published version, new figures and comments added; 15 pages, 7
figure
Weak measurement of cotunneling time
Quantum mechanics allows the existence of "virtual states" that have no
classical analogue. Such virtual states defy direct observation through strong
measurement, which would destroy the volatile virtual state. Here we show how a
virtual state of an interacting many-body system can be detected employing a
weak measurement protocol with postselection. We employ this protocol for the
measurement of the time it takes an electron to tunnel through a virtual state
of a quantum dot (cotunneling). Contrary to classical intuition, this
cotunneling time is independent of the strength of the dot-lead coupling and
may deviate from that predicted by time-energy uncertainty relation. Our
approach, amenable to experimental verification, may elucidate an important
facet of quantum mechanics which hitherto was not accessible by direct
measurements.Comment: 13 pages, 5 figures, 1 tabl
Full Counting Statistics of Cooper Pair Shuttling
The Cooper pair shuttle is a simple model system that combines features of
coherent and incoherent transport. We evaluate the full counting statistics
(FCS) of charge transfer via the shuttle in the incoherent regime. We describe
two limiting cases when the FCS allows for classical interpretation. Generally,
the classical interpretation fails yielding negative and imaginary
"probabilities". This signals that superconducting coherence survives even in
incoherent regime. We evaluate the current noise in some detail.Comment: 4 pages, 3 figures; v2 (published version) corrected misprint
Solid-State Quantum Communication With Josephson Arrays
Josephson junction arrays can be used as quantum channels to transfer quantum
information between distant sites. In this work we discuss simple protocols to
realize state transfer with high fidelity. The channels do not require
complicate gating but use the natural dynamics of a properly designed array. We
investigate the influence of static disorder both in the Josephson energies and
in the coupling to the background gate charges, as well as the effect of
dynamical noise. We also analyze the readout process, and its backaction on the
state transfer
Charge sensing amplification via weak values measurement
A protocol employing weak values (WVs) to obtain ultra sensitive
amplification of weak signals in the context of a solid state setup is
proposed. We consider an Aharonov-Bohm interferometer where both the orbital
and the spin degrees of freedom are weakly affected by the presence of an
external charge to be detected. The interplay between the spin and the orbital
WVs leads to a significant amplification even in the presence of finite
temperature, voltage, and external noise.Comment: 6 pages, 5 figure
Measuring cotunneling in its wake
We introduce a rate formalism to treat classically forbidden electron
transport through a quantum dot (cotunneling) in the presence of a coupled
measurement device. We demonstrate this formalism for a toy model case of
cotunneling through a single-level dot while being coupled to a strongly
pinched-off quantum point contact (QPC). We find that the detector generates
three types of back-action: the measurement collapses the coherent transport
through the virtual state, but at the same time allows for QPC-assisted
incoherent transport, and widens the dot level. Last, we obtain the measured
cotunneling time from the cross correlation between dot and QPC currents.Comment: 15 pages, 9 figures, 1 appendix, published versio
A scattering matrix formulation of the topological index of interacting fermions in one-dimensional superconductors
We construct a scattering matrix formulation for the topological
classification of one-dimensional superconductors with effective time reversal
symmetry in the presence of interactions. For a closed geometry, Fidkowski and
Kitaev have shown that such systems have a topological
classification. We show that in the weak coupling limit, these systems retain a
unitary scattering matrix at zero temperature, with a topological index given
by the trace of the Andreev reflection matrix, \mbox{tr}\, r_{\rm he}. With
interactions, \mbox{tr}\, r_{\rm he} generically takes on the finite set of
values , , , , and . We show that the two
topologically equivalent phases with \mbox{tr}\, r_{\rm he} = \pm 4 support
emergent many-body end states, which we identify to be a topologically
protected Kondo-like resonance. The path in phase space that connects these
equivalent phases crosses a non-fermi liquid fixed point where a multiple
channel Kondo effect develops. Our results connect the topological index to
transport properties, thereby highlighting the experimental signatures of
interacting topological phases in one dimension.Comment: 4 pages, 1 fi
Thermodynamics of weakly measured quantum systems
We consider continuously monitored quantum systems and introduce definitions
of work and heat along individual quantum trajectories that are valid for
coherent superpositions of energy eigenstates. We use these quantities to
extend the first and second laws of stochastic thermodynamics to the quantum
domain. We illustrate our results with the case of a weakly measured driven
two-level system and show how to distinguish between quantum work and heat
contributions. We finally employ quantum feedback control to suppress detector
backaction and determine the work statistics.Comment: 6 pages, 3 figure
Decoherence of Rabi oscillations of electronic spin states in a double quantum dot
We study the role of charge fluctuations in the decoherence of Rabi
oscillations between spin states |>, |> of two electrons in a double dot structure. We consider the effects
of fluctuations in energy and in the quantum state of the system, both in the
classical and quantum limit. The role of state fluctuations is shown to be of
leading order at sufficiently high temperature, applicable to actual
experiments. At low temperature the low frequency energy fluctuations are the
only dominant contribution.Comment: 5 pages, 2 figures; v2: (extended version of the published article)
added details of calculations, modified fig. 2, improved "readability
Crossover between strong and weak measurement in interacting many-body systems
Measurements with variable system-detector interaction strength, ranging from
weak to strong, have been recently reported in a number of electronic
nanosystems. In several such instances many-body effects play a significant
role. Here we consider the weak--to--strong crossover for a setup consisting of
an electronic Mach-Zehnder interferometer, where a second interferometer is
employed as a detector. In the context of a conditional which-path protocol, we
define a generalized conditional value (GCV), and determine its full crossover
between the regimes of weak and strong (projective) measurement. We find that
the GCV has an oscillatory dependence on the system-detector interaction
strength. These oscillations are a genuine many-body effect, and can be
experimentally observed through the voltage dependence of cross current
correlations.Comment: 5 pages, 3 figures, and appendices (9 pages, 3 figures, 2 tables
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