1,896 research outputs found

### Charge sensitivity of radio frequency single-electron transistor (RF-SET)

A theoretical analysis of the charge sensitivity of the RF-SET is presented.
We use the ``orthodox'' approach and consider the case when the carrier
frequency is much less than $I/e$ where $I$ is the typical current through
RF-SET. The optimized noise-limited sensitivity is determined by the
temperature $T$, and at low $T$ it is only 1.4 times less than the sensitivity
of conventional single-electron transistor.Comment: 3 pages, 4 figure

### Nonideal quantum detectors in Bayesian formalism

The Bayesian formalism for a continuous measurement of solid-state qubits is
derived for a model which takes into account several factors of the detector
nonideality. In particular, we consider additional classical output and
backaction noises (with finite correlation), together with quantum-limited
output and backaction noises, and take into account possible asymmetry of the
detector coupling. The formalism is first derived for a single qubit and then
generalized to the measurement of entangled qubits.Comment: 10 page

### Spectrum of qubit oscillations from Bloch equations

We have developed a formalism suitable for calculation of the output spectrum
of a detector continuously measuring quantum coherent oscillations in a
solid-state qubit, starting from microscopic Bloch equations. The results
coincide with that obtained using Bayesian and master equation approaches. The
previous results are generalized to the cases of arbitrary detector response
and finite detector temperature.Comment: 8 page

### Violating the Modified Helstrom Bound with Nonprojective Measurements

We consider the discrimination of two pure quantum states with three allowed
outcomes: a correct guess, an incorrect guess, and a non-guess. To find an
optimum measurement procedure, we define a tunable cost that penalizes the
incorrect guess and non-guess outcomes. Minimizing this cost over all
projective measurements produces a rigorous cost bound that includes the usual
Helstrom discrimination bound as a special case. We then show that
nonprojective measurements can outperform this modified Helstrom bound for
certain choices of cost function. The Ivanovic-Dieks-Peres unambiguous state
discrimination protocol is recovered as a special case of this improvement.
Notably, while the cost advantage of the latter protocol is destroyed with the
introduction of any amount of experimental noise, other choices of cost
function have optima for which nonprojective measurements robustly show an
appreciable, and thus experimentally measurable, cost advantage. Such an
experiment would be an unambiguous demonstration of a benefit from
nonprojective measurements.Comment: 5 pages, 2 figure

### Efficient algorithm for current spectral density calculation in single-electron tunneling and hopping

This write-up describes an efficient numerical method for the Monte Carlo
calculation of the spectral density of current in the multi-junction
single-electron devices and hopping structures. In future we plan to expand
this write-up into a full-size paper.Comment: 4 page

### Implementing generalized measurements with superconducting qubits

We describe a method to perform any generalized purity-preserving measurement
of a qubit with techniques tailored to superconducting systems. First, we
consider two methods for realizing a two-outcome partial projection: using a
thresholded continuous measurement in the circuit QED setup, or using an
indirect ancilla qubit measurement. Second, we decompose an arbitrary
purity-preserving two-outcome measurement into single qubit unitary rotations
and a partial projection. Third, we systematically reduce any multiple-outcome
measurement to a sequence of such two-outcome measurements and unitary
operations. Finally, we consider how to define suitable fidelity measures for
multiple-outcome generalized measurements.Comment: 13 pages, 3 figure

### Decoherence suppression by uncollapsing

We show that the qubit decoherence due to zero-temperature energy relaxation
can be almost completely suppressed by using the quantum uncollapsing
procedure. To protect a qubit state, a partial quantum measurement moves it
towards the ground state, where it is kept during the storage period, while the
second partial measurement restores the initial state. This procedure
preferentially selects the cases without energy decay events. Stronger
decoherence suppression requires smaller selection probability; a desired point
in this trade-off can be chosen by varying the measurement strength. The
experiment can be realized in a straightforward way using the superconducting
phase qubit.Comment: 4 page

### Quantum theory of a bandpass Purcell filter for qubit readout

The readout fidelity of superconducting transmon and Xmon qubits is partially
limited by the qubit energy relaxation through the resonator into the
transmission line, which is also known as the Purcell effect. One way to
suppress this energy relaxation is to employ a filter which impedes microwave
propagation at the qubit frequency. We present semiclassical and quantum
analyses for the bandpass Purcell filter realized by E.\ Jeffrey \textit{et
al}.\ [Phys.\ Rev.\ Lett.\ 112, 190504 (2014)]. For typical experimental
parameters, the bandpass filter suppresses the qubit relaxation rate by up to
two orders of magnitude while maintaining the same measurement rate. We also
show that in the presence of a microwave drive the qubit relaxation rate
further decreases with increasing drive strength.Comment: 15 pages, 4 figures; published versio

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