1,189 research outputs found
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
Temperature dependence of coherent oscillations in Josephson phase qubits
We experimentally investigate the temperature dependence of Rabi oscillations
and Ramsey fringes in superconducting phase qubits driven by microwave pulses.
In a wide range of temperatures, we find that both the decay time and the
amplitude of these coherent oscillations remain nearly unaffected by thermal
fluctuations. The oscillations are observed well above the crossover
temperature from thermally activated escape to quantum tunneling for undriven
qubits. In the two-level limit, coherent qubit response rapidly vanishes as
soon as the energy of thermal fluctuations kT becomes larger than the energy
level spacing of the qubit. Our observations shed new light on the origin of
decoherence in superconducting qubits. The experimental data suggest that,
without degrading already achieved coherence times, phase qubits can be
operated at temperatures much higher than those reported till now.Comment: 4 pages, 4 figure
Quantum escape of the phase in a strongly driven Josephson junction
A quantum mechanical analysis of the Josephson phase escape in the presence
of both dc and ac bias currents is presented. We find that the potential
barrier for the escape of the phase is effectively suppressed as the resonant
condition occurs, i.e. when the frequency of the ac bias matches the
Josephson junction energy level separation. This effect manifests itself by a
pronounced drop in the dependence of the switching current on the power
of the applied microwave radiation and by a peculiar double-peak structure
in the switching current distribution . The developed theory is in a
good accord with an experiment which we also report in this paper. The obtained
features can be used to characterize certain aspects of the quantum-mechanical
behavior of the Josephson phase, such as the energy level quantization, the
Rabi frequency of coherent oscillations and the effect of damping.Comment: 4 pages, 3 figures, to be published in Physical Review B (Rapid
Communication
Generation and detection of NOON states in superconducting circuits
NOON states, states between two modes of light of the form
allow for super-resolution interformetry. We
show how NOON states can be efficiently produced in circuit quntum
electrodynamics using superconducting phase qubits and resonators. We propose a
protocol where only one interaction between the two modes is required, creating
all the necessary entanglement at the start of the procedure. This protocol
makes active use of the first three states of the phase qubits. Additionally,
we show how to efficiently verify the success of such an experiment, even for
large NOON states, using randomly sampled measurements and semidefinite
programming techniques.Comment: 15 pages and 3 figure
Decoherence in a Josephson junction qubit
The zero-voltage state of a Josephson junction biased with constant current
consists of a set of metastable quantum energy levels. We probe the spacings of
these levels by using microwave spectroscopy to enhance the escape rate to the
voltage state. The widths of the resonances give a measurement of the coherence
time of the two states involved in the transitions. We observe a decoherence
time shorter than that expected from dissipation alone in resonantly isolated
20 um x 5 um Al/AlOx/Al junctions at 60 mK. The data is well fit by a model
including dephasing effects of both low-frequency current noise and the escape
rate to the continuum voltage states. We discuss implications for quantum
computation using current-biased Josephson junction qubits, including the
minimum number of levels needed in the well to obtain an acceptable error limit
per gate.Comment: 4 pages, 6 figure
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