12,626 research outputs found
Rabi Oscillations in Systems with Small Anharmonicity
When a two-level quantum system is irradiated with a microwave signal, in
resonance with the energy difference between the levels, it starts Rabi
oscillation between those states. If there are other states close, in energy,
to the first two, the Rabi signal will also induce transition to those. Here,
we study the probability of transition to the third state, in a three-level
system, while a Rabi oscillation between the first two states is performed. We
investigate the effect of pulse shaping on the probability and suggest methods
for optimizing pulse shapes to reduce transition probability.Comment: 7 pages, 7 figure
Mesoscopic multiterminal Josephson structures: I. Effects of nonlocal weak coupling
We investigate nonlocal coherent transport in ballistic four-terminal
Josephson structures (where bulk superconductors (terminals) are connected
through a clean normal layer, e.g., a two-dimensional electron gas).
Coherent anisotropic superposition of macroscopic wave functions of the
superconductors in the normal region produces phase slip lines (2D analogs to
phase slip centres) and time-reversal symmetry breaking 2D vortex states in it,
as well as such effects as phase dragging and magnetic flux transfer. The
tunneling density of local Andreev states in the normal layer was shown to
contain peaks at the positions controlled by the phase differences between the
terminals.
We have obtained general dependence of these effects on the controlling
supercurrent/phase differences between the terminals of the ballistic
mesoscopic four-terminal SQUID.Comment: 18 pages, 11 figure
Macroscopic Resonant Tunneling in the Presence of Low Frequency Noise
We develop a theory of macroscopic resonant tunneling of flux in a
double-well potential in the presence of realistic flux noise with significant
low-frequency component. The rate of incoherent flux tunneling between the
wells exhibits resonant peaks, the shape and position of which reflect
qualitative features of the noise, and can thus serve as a diagnostic tool for
studying the low-frequency flux noise in SQUID qubits. We show, in particular,
that the noise-induced renormalization of the first resonant peak provides
direct information on the temperature of the noise source and the strength of
its quantum component.Comment: 4 pages, 1 figur
Persistent currents in mesoscopic rings and boundary conformal field theory
A tight-binding model of electron dynamics in mesoscopic normal rings is
studied using boundary conformal field theory. The partition function is
calculated in the low energy limit and the persistent current generated as a
function of an external magnetic flux threading the ring is found. We study the
cases where there are defects and electron-electron interactions separately.
The same temperature scaling for the persistent current is found in each case,
and the functional form can be fitted, with a high degree of accuracy, to
experimental data.Comment: 6 pages, 4 enclosed postscript figure
Decoherence in adiabatic quantum computation
We have studied the decoherence properties of adiabatic quantum computation
(AQC) in the presence of in general non-Markovian, e.g., low-frequency, noise.
The developed description of the incoherent Landau-Zener transitions shows that
the global AQC maintains its properties even for decoherence larger than the
minimum gap at the anticrossing of the two lowest energy levels. The more
efficient local AQC, however, does not improve scaling of the computation time
with the number of qubits as in the decoherence-free case. The scaling
improvement requires phase coherence throughout the computation, limiting the
computation time and the problem size n.Comment: 4 pages, 2 figures, published versio
Non-Markovian incoherent quantum dynamics of a two-state system
We present a detailed study of the non-Markovian two-state system dynamics
for the regime of incoherent quantum tunneling. Using perturbation theory in
the system tunneling amplitude , and in the limit of strong system-bath
coupling, we determine the short time evolution of the reduced density matrix
and thereby find a general equation of motion for the non-Markovian evolution
at longer times. We relate the nonlocality in time due to the non-Markovian
effects with the environment characteristic response time. In addition, we
study the incoherent evolution of a system with a double-well potential, where
each well consists several quantized energy levels. We determine the crossover
temperature to a regime where many energy levels in the wells participate in
the tunneling process, and observe that the required temperature can be much
smaller than the one associated with the system plasma frequency. We also
discuss experimental implications of our theoretical analysis.Comment: 10 pages, published versio
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