808 research outputs found
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
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
Quantum logic with weakly coupled qubits
There are well-known protocols for performing CNOT quantum logic with qubits
coupled by particular high-symmetry (Ising or Heisenberg) interactions.
However, many architectures being considered for quantum computation involve
qubits or qubits and resonators coupled by more complicated and less symmetric
interactions. Here we consider a widely applicable model of weakly but
otherwise arbitrarily coupled two-level systems, and use quantum gate design
techniques to derive a simple and intuitive CNOT construction. Useful
variations and extensions of the solution are given for common special cases.Comment: 4 pages, Revte
Magnetism in SQUIDs at Millikelvin Temperatures
We have characterized the temperature dependence of the flux threading dc
SQUIDs cooled to millikelvin temperatures. The flux increases as 1/T as
temperature is lowered; moreover, the flux change is proportional to the
density of trapped vortices. The data is compatible with the thermal
polarization of surface spins in the trapped fields of the vortices. In the
absence of trapped flux, we observe evidence of spin-glass freezing at low
temperature. These results suggest an explanation for the "universal" 1/f flux
noise in SQUIDs and superconducting qubits.Comment: 4 pages, 4 figure
Giant oscillations of the current in a dirty 2D electron system flowing perpendicular to a lateral barrier under magnetic field
The charge transport in a dirty 2-dimensional electron system biased in the
presence of a lateral potential barrier under magnetic field is theoretically
studied. The quantum tunneling across the barrier provides the quantum
interference of the edge states localized on its both sides that results in
giant oscillations of the charge current flowing perpendicular to the lateral
junction. Our theoretical analysis is in a good agreement with the experimental
observations presented in Ref.8. In particular, positions of the conductance
maxima coincide with the Landau levels while the conductance itself is
essentially suppressed even at the energies at which the resonant tunneling
occurs and hence these puzzling observations can be resolved without taking
into account the electron-electron interaction.Comment: 6 pages, 4 figure
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