308 research outputs found
Pure dephasing in flux qubits due to flux noise with spectral density scaling as
For many types of superconducting qubits, magnetic flux noise is a source of
pure dephasing. Measurements on a representative dc superconducting quantum
interference device (SQUID) over a range of temperatures show that , where is the flux noise spectral density,
is of the order of 1 and ; is the flux quantum. For a qubit with an energy level
splitting linearly coupled to the applied flux, calculations of the dependence
of the pure dephasing time of Ramsey and echo pulse sequences on
for fixed show that decreases rapidly as is
reduced. We find that is relatively insensitive to the noise
bandwidth, , for all provided the ultraviolet
cutoff frequency . We calculate the ratio of the echo () and Ramsey () sequences, and the dependence
of the decay function on and . We investigate the case in which
is fixed at the "pivot frequency" Hz while
is varied, and find that the choice of can greatly influence the
sensitivity of and to the value of .
Finally, we present calculated values of in a qubit corresponding
to the values of and measured in our SQUID.Comment: 7 pages, 8 figures, 1 tabl
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
Multilevel effects in the Rabi oscillations of a Josephson phase qubit
We present Rabi oscillation measurements of a Nb/AlOx/Nb dc superconducting
quantum interference device (SQUID) phase qubit with a 100 um^2 area junction
acquired over a range of microwave drive power and frequency detuning. Given
the slightly anharmonic level structure of the device, several excited states
play an important role in the qubit dynamics, particularly at high power. To
investigate the effects of these levels, multiphoton Rabi oscillations were
monitored by measuring the tunneling escape rate of the device to the voltage
state, which is particularly sensitive to excited state population. We compare
the observed oscillation frequencies with a simplified model constructed from
the full phase qubit Hamiltonian and also compare time-dependent escape rate
measurements with a more complete density-matrix simulation. Good quantitative
agreement is found between the data and simulations, allowing us to identify a
shift in resonance (analogous to the ac Stark effect), a suppression of the
Rabi frequency, and leakage to the higher excited states.Comment: 14 pages, 9 figures; minor corrections, updated reference
Single and double qubit gates by manipulating degeneracy
A novel mechanism is proposed for single and double qubit state manipulations
in quantum computation with four-fold degenerate energy levels. The principle
is based on starting with a four fold degeneracy, lifting it stepwise
adiabatically by a set of control parameters and performing the quantum gate
operations on non-degenerate states. A particular realization of the proposed
mechanism is suggested by using inductively coupled rf-squid loops in the
macroscopic quantum tunnelling regime where the energy eigen levels are
directly connected with the measurable flux states. The one qubit and two qubit
controlled operations are demonstrated explicitly. The appearance of the flux
states also allows precise read-in and read-out operations by the measurement
of flux.Comment: 6 pages + 5 figures (separately included
Paramagnetic Meissner Effect in Multiply-Connected Superconductors
We have measured a paramagnetic Meissner effect in Nb-Al2O3-Nb Josephson
junction arrays using a scanning SQUID microscope. The arrays exhibit
diamagnetism for some cooling fields and paramagnetism for other cooling
fields. The measured mean magnetization is always less than 0.3 flux quantum
(in terms of flux per unit cell of the array) for the range of cooling fields
investigated. We demonstrate that a new model of magnetic screening, valid for
multiply-connected superconductors, reproduces all of the essential features of
paramagnetism that we observe and that no exotic mechanism, such as d-wave
superconductivity, is needed for paramagnetism.Comment: 4 pages, 3 figures, LaTe
Dephasing of Electrons in Mesoscopic Metal Wires
We have extracted the phase coherence time of electronic
quasiparticles from the low field magnetoresistance of weakly disordered wires
made of silver, copper and gold. In samples fabricated using our purest silver
and gold sources, increases as when the temperature
is reduced, as predicted by the theory of electron-electron interactions in
diffusive wires. In contrast, samples made of a silver source material of
lesser purity or of copper exhibit an apparent saturation of
starting between 0.1 and 1 K down to our base temperature of 40 mK. By
implanting manganese impurities in silver wires, we show that even a minute
concentration of magnetic impurities having a small Kondo temperature can lead
to a quasi saturation of over a broad temperature range, while
the resistance increase expected from the Kondo effect remains hidden by a
large background. We also measured the conductance of Aharonov-Bohm rings
fabricated using a very pure copper source and found that the amplitude of the
conductance oscillations increases strongly with magnetic field. This set
of experiments suggests that the frequently observed ``saturation'' of
in weakly disordered metallic thin films can be attributed to
spin-flip scattering from extremely dilute magnetic impurities, at a level
undetectable by other means.Comment: 16 pages, 11 figures, to be published in Physical Review
Phonon Emission from a 2D Electron Gas: Evidence of Transition to the Hydrodynamic Regime
Using as a thermometer the temperature dependent magneto-transport of a
two-dimensional electron gas, we find that effective temperature scales with
current as , where in the {\it Shubnikov
de-Haas} regime, and in both the {\it integer and fractional}
quantum Hall effect. This implies the phonon energy emission rate changes from
the expected to . We explain this, as well as the
dramatic enhancement in phonon emission efficiency using a hydrodynamic model.Comment: 4 pages, 2 Postscript figures uuencoded with TeX file uses psfig
macro. Submitted to Phys. Rev. Let
Geometric Quantum Computation on Solid-State Qubits
An adiabatic cyclic evolution of control parameters of a quantum system ends
up with a holonomic operation on the system, determined entirely by the
geometry in the parameter space. The operation is given either by a simple
phase factor (a Berry phase) or a non-Abelian unitary operator depending on the
degeneracy of the eigenspace of the Hamiltonian. Geometric quantum computation
is a scheme to use such holonomic operations rather than the conventional
dynamic operations to manipulate quantum states for quantum information
processing. Here we propose a geometric quantum computation scheme which can be
realized with current technology on nanoscale Josephson-junction networks,
known as a promising candidate for solid-state quantum computer.Comment: 6 figures; to appear in J. Phys.: Condens. Mat
Two-channel Kondo effect and renormalization flow with macroscopic quantum charge states
Many-body correlations and macroscopic quantum behaviors are fascinating
condensed matter problems. A powerful test-bed for the many-body concepts and
methods is the Kondo model which entails the coupling of a quantum impurity to
a continuum of states. It is central in highly correlated systems and can be
explored with tunable nanostructures. Although Kondo physics is usually
associated with the hybridization of itinerant electrons with microscopic
magnetic moments, theory predicts that it can arise whenever degenerate quantum
states are coupled to a continuum. Here we demonstrate the previously elusive
`charge' Kondo effect in a hybrid metal-semiconductor implementation of a
single-electron transistor, with a quantum pseudospin-1/2 constituted by two
degenerate macroscopic charge states of a metallic island. In contrast to other
Kondo nanostructures, each conduction channel connecting the island to an
electrode constitutes a distinct and fully tunable Kondo channel, thereby
providing an unprecedented access to the two-channel Kondo effect and a clear
path to multi-channel Kondo physics. Using a weakly coupled probe, we reveal
the renormalization flow, as temperature is reduced, of two Kondo channels
competing to screen the charge pseudospin. This provides a direct view of how
the predicted quantum phase transition develops across the symmetric quantum
critical point. Detuning the pseudospin away from degeneracy, we demonstrate,
on a fully characterized device, quantitative agreement with the predictions
for the finite-temperature crossover from quantum criticality.Comment: Letter (5 pages, 4 figures) and Methods (10 pages, 6 figures
The 1/3-shot noise suppression in diffusive nanowires
We report low-temperature shot noise measurements of short diffusive Au wires
attached to electron reservoirs of varying sizes. The measured noise
suppression factor compared to the classical noise value
strongly depends on the electric heat conductance of the reservoirs. For small
reservoirs injection of hot electrons increases the measured noise and hence
the suppression factor. The universal 1/3-suppression factor can only
asymptotically be reached for macroscopically large and thick electron
reservoirs. A heating model based on the Wiedemann-Franz law is used to explain
this effect.Comment: 10 figure
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