778 research outputs found
Josephson charge-phase qubit with radio frequency readout: coupling and decoherence
The charge-phase Josephson qubit based on a superconducting single charge
transistor inserted in a low-inductance superconducting loop is considered. The
loop is inductively coupled to a radio-frequency driven tank circuit enabling
the readout of the qubit states by measuring the effective Josephson inductance
of the transistor. The effect of qubit dephasing and relaxation due to electric
and magnetic control lines as well as the measuring system is evaluated.
Recommendations for operation of the qubit in magic points producing minimum
decoherence are given.Comment: 11 pages incl. 6 fig
Interference effects in isolated Josephson junction arrays with geometric symmetries
As the size of a Josephson junction is reduced, charging effects become
important and the superconducting phase across the link turns into a periodic
quantum variable. Isolated Josephson junction arrays are described in terms of
such periodic quantum variables and thus exhibit pronounced quantum
interference effects arising from paths with different winding numbers
(Aharonov-Casher effects). These interference effects have strong implications
for the excitation spectrum of the array which are relevant in applications of
superconducting junction arrays for quantum computing. The interference effects
are most pronounced in arrays composed of identical junctions and possessing
geometric symmetries; they may be controlled by either external gate potentials
or by adding/removing charge to/from the array. Here we consider a loop of N
identical junctions encircling one half superconducting quantum of magnetic
flux. In this system, the ground state is found to be non-degenerate if the
total number of Cooper pairs on the array is divisible by N, and doubly
degenerate otherwise (after the stray charges are compensated by the gate
voltages).Comment: 9 pages, 6 figure
Active suppression of dephasing in Josephson-junction qubits
Simple majority code correcting dephasing errors by encoding a qubit of
information into physical qubits is studied quantitatively. We derive an
equation for quasicontinuous evolution of the density matrix of encoded quantum
information under the error correction procedure in the presence of dephasing
noise that in general can be correlated at different qubits. Specific design of
the Josephson-junction circuit implementing this scheme is suggested.Comment: 4 pages, 1 figur
Eigenstates of a Small Josephson Junction Coupled to a Resonant Cavity
We carry out a quantum-mechanical analysis of a small Josephson junction
coupled to a single-mode resonant cavity. We find that the eigenstates of the
combined junction-cavity system are strongly entangled only when the gate
voltage applied at one of the superconducting islands is tuned to certain
special values. One such value corresponds to the resonant absorption of a
single photon by Cooper pairs in the junction. Another special value
corresponds to a {\em two-photon} absorption process. Near the single-photon
resonant absorption, the system is accurately described by a simplified model
in which only the lowest two levels of the Josephson junction are retained in
the Hamiltonian matrix. We noticed that this approximation does not work very
well as the number of photons in the resonator increases. Our system shows also
the phenomenon of ``collapse and revival'' under suitable initial conditions,
and our full numerical solution agrees with the two level approximation result.Comment: 7 pages, and 6 figures. To be published in Phys. Rev.
Continuous weak measurement of quantum coherent oscillations
We consider the problem of continuous quantum measurement of coherent
oscillations between two quantum states of an individual two-state system. It
is shown that the interplay between the information acquisition and the
backaction dephasing of the oscillations by the detector imposes a fundamental
limit, equal to 4, on the signal-to-noise ratio of the measurement. The limit
is universal, e.g., independent of the coupling strength between the detector
and system, and results from the tendency of quantum measurement to localize
the system in one of the measured eigenstates
Properties of mesoscopic superconducting thin-film rings. London approach
Superconducting thin-film rings smaller than the film penetration depth (the
Pearl length) are considered. The current distribution, magnetic moment, and
thermodynamic potential for a flat, washer-shaped annular
ring in a uniform applied field perpendicular to the film are solved
analytically within the London approach for a state with winding number and
a vortex at radius between the inner and outer radii.Comment: Submitted to Phys. Rev.
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
Quantum state engineering with Josephson-junction devices
We review recent theoretical and experimental progress in quantum state
engineering with Josephson junction devices. The concepts of quantum computing
have stimulated an increased activity in the field. Either charges or phases
(fluxes) of the Josephson systems can be used as quantum degrees of freedom,
and their quantum state can be manipulated coherently by voltage and current
pulses. They thus can serve as qubits, and quantum logic gates can be
performed. Their phase coherence time, which is limited, e.g., by the
electromagnetic fluctuations in the control circuit, is long enough to allow a
series of these manipulations. The quantum measurement process performed by a
single-electron transistor, a SQUID, or further nanoelectronic devices is
analyzed in detail.Comment: An article prepared for Reviews of Modern Physics, 46 pages, 23
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