9,266 research outputs found
Creation of entanglement in a scalable spin quantum computer with long-range dipole-dipole interaction between qubits
Creation of entanglement is considered theoretically and numerically in an
ensemble of spin chains with dipole-dipole interaction between the spins. The
unwanted effect of the long-range dipole interaction is compensated by the
optimal choice of the parameters of radio-frequency pulses implementing the
protocol. The errors caused by (i) the influence of the environment,(ii)
non-selective excitations, (iii) influence of different spin chains on each
other, (iv) displacements of qubits from their perfect locations, and (v)
fluctuations of the external magnetic field are estimated analytically and
calculated numerically. For the perfectly entangled state the z component, M,
of the magnetization of the whole system is equal to zero. The errors lead to a
finite value of M. If the number of qubits in the system is large, M can be
detected experimentally. Using the fact that M depends differently on the
parameters of the system for each kind of error, varying these parameters would
allow one to experimentally determine the most significant source of errors and
to optimize correspondingly the quantum computer design in order to decrease
the errors and M. Using our approach one can benchmark the quantum computer,
decrease the errors, and prepare the quantum computer for implementation of
more complex quantum algorithms.Comment: 31 page
Dynamical Stability and Quantum Chaos of Ions in a Linear Trap
The realization of a paradigm chaotic system, namely the harmonically driven
oscillator, in the quantum domain using cold trapped ions driven by lasers is
theoretically investigated. The simplest characteristics of regular and chaotic
dynamics are calculated. The possibilities of experimental realization are
discussed.Comment: 24 pages, 17 figures, submitted to Phys. Rev
Radiative Tail in Decay and Some Comments on Universality
The result of lowest-order perturbation theory calculations of the photon and
positron spectra in radiative pion(e2) decay are generalized to all orders of
perturbation theory using the structure-function method. An additional source
of radiative corrections to the ratio of the positron and muon channels of pion
decay, due to emission of virtual and real photons and pairs, is considered. It
depends on details of the detection of the final particles and is large enough
to be taken into account in theoretical estimates with a level of accuracy of
0.1%.Comment: 5 pages, LaTeX, some misprints are corrected, submitted to Pisma Zh.
Eksp. Teor. Fi
Dynamics of a Quantum Control-Not Gate for an Ensemble of Four-Spin Molecules at Room Temperature
We investigate numerically a single-pulse implementation of a quantum
Control-Not (CN) gate for an ensemble of Ising spin systems at room
temperature. For an ensemble of four-spin ``molecules'' we simulate the
time-evolution of the density matrix, for both digital and superpositional
initial conditions. Our numerical calculations confirm the feasibility of
implementation of quantum CN gate in this system at finite temperature, using
electromagnetic -pulse.Comment: 7 pages 3 figure
Dispersion management using betatron resonances in an ultracold-atom storage ring
Specific velocities of particles circulating in a storage ring can lead to
betatron resonances at which static perturbations of the particles' orbit yield
large transverse (betatron) oscillations. We have observed betatron resonances
in an ultracold-atom storage ring by direct observation of betatron motion.
These resonances caused a near-elimination of the longitudinal dispersion of
atomic beams propagating at resonant velocities, an effect which can improve
the performance of atom interferometric devices. Both the resonant velocities
and the strength of the resonances were varied by deliberate modifications to
the storage ring.Comment: 4 pages, 5 figures. Also available at
http://physics.berkeley.edu/research/ultracol
Influence of External Fields and Environment on the Dynamics of Phase Qubit-Resonator System
We analyze the dynamics of a qubit-resonator system coupled with a thermal
bath and external electromagnetic fields. Using the evolution equations for the
set of Heisenberg operators, that describe the whole system, we derive an
expression for the resonator field, accounting for the resonator-drive,-bath,
and -qubit interaction. The renormalization of the resonator frequency, caused
by the qubit-resonator interaction, is accounted for. Using solutions for the
resonator field, we derive the equation describing qubit dynamics. The
influence of the qubit evolution during the measurement time on the fidelity of
a single-shot measurement is studied. The relation between the fidelity and
measurement time is shown explicitly. Also, an expression describing relaxation
of the superposition qubit state towards its stationary value is derived. The
possibility of controlling this state, by varying the amplitude and frequency
of drive, is shown.Comment: 15 page
Single electron capacitance spectroscopy of vertical quantum dots using a single electron transistor
We have incorporated an aluminum single electron transistor (SET) directly on
top of a vertical quantum dot, enabling the use of the SET as an electrometer
that is extremely responsive to the motion of charge into and out of the dot.
Charge induced on the SET central island from single electron additions to the
dot modulates the SET output, and we describe two methods for demodulation that
permit quantitative extraction of the quantum dot capacitance signal. The two
methods produce closely similar results for the determined single electron
capacitance peaks.Comment: Submitted to Applied Physics Letters (reformatted to fit correctly on
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