839 research outputs found
Communicating Josephson Qubits
We propose a scheme to implement a quantum information transfer protocol with
a superconducting circuit and Josephson charge qubits. The information exchange
is mediated by an L-C resonator used as a data bus. The main decoherence
sources are analyzed in detail.Comment: 4 pages, 2 figure
Decoherence by a nonlinear environment: canonical vs. microcanonical case
We compare decoherence induced in a simple quantum system (qubit) for two
different initial states of the environment: canonical (fixed temperature) and
microcanonical (fixed energy), for the general case of a fully interacting
oscillator environment. We find that even a relatively compact oscillator bath
(with the effective number of degrees of freedom of order 10), initially in a
microcanonical state, will typically cause decoherence almost indistinguishable
from that by a macroscopic, thermal environment, except possibly at
singularities of the environment's specific heat (critical points). In the
latter case, the precise magnitude of the difference between the canonical and
microcanonical results depends on the critical behavior of the dissipative
coefficient, characterizing the interaction of the qubit with the environment.Comment: 18 pages, revtex, 2 figures; minor textual changes, corrected typo in
eq. (53) (v2); textual changes, mostly in the introduction (v3
Spin swap gate in the presence of qubit inhomogeneity in a double quantum dot
We study theoretically the effects of qubit inhomogeneity on the quantum
logic gate of qubit swap, which is an integral part of the operations of a
quantum computer. Our focus here is to construct a robust pulse sequence for
swap operation in the simultaneous presence of Zeeman inhomogeneity for quantum
dot trapped electron spins and the finite-time ramp-up of exchange coupling in
a double dot. We first present a geometric explanation of spin swap operation,
mapping the two-qubit operation onto a single-qubit rotation. We then show that
in this geometric picture a square-pulse-sequence can be easily designed to
perform swap in the presence of Zeeman inhomogeneity. Finally, we investigate
how finite ramp-up times for the exchange coupling negatively affect the
performance of the swap gate sequence, and show how to correct the problems
numerically.Comment: published versio
Coherent dynamics of a Josephson charge qubit
We have fabricated a Josephson charge qubit by capacitively coupling a
single-Cooper-pair box (SCB) to an electrometer based upon a single-electron
transistor configured for radio-frequency readout (RF-SET). Charge quantization
of 2e is observed and microwave spectroscopy is used to extract the Josephson
and charging energies of the box. We perform coherent manipulation of the SCB
by using very fast DC pulses and observe quantum oscillations in time of the
charge that persist to ~=10ns. The observed contrast of the oscillations is
high and agrees with that expected from the finite E_J/E_C ratio and finite
rise-time of the DC pulses. In addition, we are able to demonstrate nearly 100%
initial charge state polarization. We also present a method to determine the
relaxation time T_1 when it is shorter than the measurement time T_{meas}.Comment: accepted for publication in Phys. Rev.
Quantum Search with Two-atom Collisions in Cavity QED
We propose a scheme to implement two-qubit Grover's quantum search algorithm
using Cavity Quantum Electrodynamics. Circular Rydberg atoms are used as
quantum bits (qubits). They interact with the electromagnetic field of a
non-resonant cavity . The quantum gate dynamics is provided by a
cavity-assisted collision, robust against decoherence processes. We present the
detailed procedure and analyze the experimental feasibility.Comment: 4 pages, 2 figure
An asymptotical von-Neumann measurement strategy for solid-state qubits
A measurement on a macroscopic quantum system does in general not lead to a
projection of the wavefunction in the basis of the detector as predicted by
von-Neumann's postulate. Hence, it is a question of fundametal interest, how
the preferred basis onto which the state is projected is selected out of the
macroscopic Hilbert space of the system. Detector-dominated von-Neumann
measurements are also desirable for both quantum computation and verification
of quantum mechanics on a macroscopic scale. The connection of these questions
to the predictions of the spin-boson modelis outlined. I propose a measurement
strategy, which uses the entanglement of the qubit with a weakly damped
harmonic oscillator. It is shown, that the degree of entanglement controls the
degree of renormalization of the qubit and identify, that this is equivalent to
the degree to which the measurement is detector-dominated. This measurement
very rapidly decoheres the initial state, but the thermalization is slow. The
implementation in Josephson quantum bits is described and it is shown that this
strategy also has practical advantages for the experimental implementation.Comment: 4 pages, 3 figures, accepted for publication as a rapid communication
in Phys. Rev.
Entanglement of solid-state qubits by measurement
We show that two identical solid-state qubits can be made fully entangled
(starting from completely mixed state) with probability 1/4 just measuring them
by a detector, equally coupled to the qubits. This happens in the case of
repeated strong (projective) measurements as well as in a more realistic case
of weak continuous measurement. In the latter case the entangled state can be
identified by a flat spectrum of the detector shot noise, while the
non-entangled state (probability 3/4) leads to a spectral peak at the Rabi
frequency with the maximum peak-to-pedestal ratio of 32/3.Comment: 5 pages, 2 figure
Gate errors in solid state quantum computer architectures
We theoretically consider possible errors in solid state quantum computation
due to the interplay of the complex solid state environment and gate
imperfections. In particular, we study two examples of gate operations in the
opposite ends of the gate speed spectrum, an adiabatic gate operation in
electron-spin-based quantum dot quantum computation and a sudden gate operation
in Cooper pair box superconducting quantum computation. We evaluate
quantitatively the non-adiabatic operation of a two-qubit gate in a
two-electron double quantum dot. We also analyze the non-sudden pulse gate in a
Cooper-pair-box-based quantum computer model. In both cases our numerical
results show strong influences of the higher excited states of the system on
the gate operation, clearly demonstrating the importance of a detailed
understanding of the relevant Hilbert space structure on the quantum computer
operations.Comment: 6 pages, 2 figure
Decoherence and Relaxation of a Quantum Bit in the Presence of Rabi Oscillations
Dissipative dynamics of a quantum bit driven by a strong resonant field and
interacting with a heat bath is investigated. We derive generalized Bloch
equations and find modifications of the qubit's damping rates caused by Rabi
oscillations. Nonequilibrium decoherence of a phase qubit inductively coupled
to a LC-circuit is considered as an illustration of the general results. It is
argued that recent experimental results give a clear evidence of effective
suppression of decoherence in a strongly driven flux qubit.Comment: 14 pages; misprints correcte
Measurement of coherent charge transfer in an adiabatic Cooper pair pump
We study adiabatic charge transfer in a superconducting Cooper pair pump,
focusing on the influence of current measurement on coherence. We investigate
the limit where the Josephson coupling energy between the various parts
of the system is small compared to the Coulomb charging energy . In this
case the charge transferred in a pumping cycle , the charge of one
Cooper pair: the main contribution is due to incoherent Cooper pair tunneling.
We are particularly interested in the quantum correction to , which is due
to coherent tunneling of pairs across the pump and which depends on the
superconducting phase difference between the electrodes: . A measurement of tends to destroy the phase
coherence. We first study an arbitrary measuring circuit and then specific
examples and show that coherent Cooper pair transfer can in principle be
detected using an inductively shunted ammeter
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