1,349 research outputs found
QND measurement of a superconducting qubit in the weakly projective regime
Quantum state detectors based on switching of hysteretic Josephson junctions
biased close to their critical current are simple to use but have strong
back-action. We show that the back-action of a DC-switching detector can be
considerably reduced by limiting the switching voltage and using a fast
cryogenic amplifier, such that a single readout can be completed within 25 ns
at a repetition rate of 1 MHz without loss of contrast. Based on a sequence of
two successive readouts we show that the measurement has a clear quantum
non-demolition character, with a QND fidelity of 75 %.Comment: submitted to PR
Safeguarding research staff “in the field”: a blind spot in ethics guidelines
Across disciplines there is a large and increasing number of research projects that rely on data collection activities in low- and middle-income countries (LMICs). However, these are accompanied by an extensive range of ethical challenges. While the safeguarding of study participants is the primary aim of existing ethics guidelines, this paper argues that this “do no harm” principle should be extended to include research staff. This study is a comprehensive review of more than 80 existing ethics guidelines and protocols that reveals a lack of safeguarding research staff regarding the ethical challenges faced during data collection activities in LMICs. This is particularly the case when it comes to issues such as power imbalances, political risk, staff’s emotional wellbeing or dealing with feelings of guilt. Lead organizations are called upon to develop guiding principles that encompass the safeguarding of research staff, which are then to be adapted and translated into specific protocols and tools by institutions
Interqubit coupling mediated by a high-excitation-energy quantum object
We consider a system composed of two qubits and a high-excitation-energy
quantum object used to mediate coupling between the qubits. We treat the entire
system quantum mechanically and analyze the properties of the eigenvalues and
eigenstates of the total Hamiltonian. After reproducing well-known results
concerning the leading term in the mediated coupling, we obtain an expression
for the residual coupling between the qubits in the off state. We also analyze
the entanglement between the three objects, i.e. the two qubits and the
coupler, in the eigenstates of the total Hamiltonian. Although we focus on the
application of our results to the recently realized parametric-coupling scheme
with two qubits, we also discuss extensions of our results to
harmonic-oscillator couplers, couplers that are near resonance with the qubits
and multi-qubit systems. In particular, we find that certain errors that are
absent for a two-qubit system arise when dealing with multi-qubit systems.Comment: 15 pages (two-column
Role of relaxation in the quantum measurement of a superconducting qubit using a nonlinear oscillator
We analyze the relaxation of a superconducting flux qubit during measurement.
The qubit state is measured with a nonlinear oscillator driven across the
threshold of bifurcation, acting as a switching dispersive detector. This
readout scheme is of quantum non-demolition type. Two successive readouts are
used to analyze the evolution of the qubit and the detector during the
measurement. We introduce a simple transition rate model to characterize the
qubit relaxation and the detector switching process. Corrected for qubit
relaxation the readout fidelity is at least 95%. Qubit relaxation strongly
depends on the driving strength and the state of the oscillator.Comment: 4 pages, 4 figure
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