945 research outputs found
Multi-Qubit Joint Measurements in Circuit QED: Stochastic Master Equation Analysis
We derive a family of stochastic master equations describing homodyne
measurement of multi-qubit diagonal observables in circuit quantum
electrodynamics. In the regime where qubit decay can be neglected, our approach
replaces the polaron-like transformation of previous work, which required a
lengthy calculation for the physically interesting case of three qubits and two
resonator modes. The technique introduced here makes this calculation
straightforward and manifestly correct. Using this technique, we are able to
show that registers larger than one qubit evolve under a non-Markovian master
equation. We perform numerical simulations of the three-qubit, two-mode case
from previous work, obtaining an average post-measurement state fidelity of
, limited by measurement-induced decoherence and dephasing.Comment: 22 pages, 9 figures. Comments welcom
Cold Trapped Ions as Quantum Information Processors
In this tutorial we review physical implementation of quantum computing using
a system of cold trapped ions. We discuss systematically all the aspects for
making the implementation possible. Firstly, we go through the loading and
confining of atomic ions in the linear Paul trap, then we describe the
collective vibrational motion of trapped ions. Further, we discuss interactions
of the ions with a laser beam. We treat the interactions in the travelling-wave
and standing-wave configuration for dipole and quadrupole transitions. We
review different types of laser cooling techniques associated with trapped
ions. We address Doppler cooling, sideband cooling in and beyond the Lamb-Dicke
limit, sympathetic cooling and laser cooling using electromagnetically induced
transparency. After that we discuss the problem of state detection using the
electron shelving method. Then quantum gates are described. We introduce
single-qubit rotations, two-qubit controlled-NOT and multi-qubit controlled-NOT
gates. We also comment on more advanced multi-qubit logic gates. We describe
how quantum logic networks may be used for the synthesis of arbitrary pure
quantum states. Finally, we discuss the speed of quantum gates and we also give
some numerical estimations for them. A discussion of dynamics on off-resonant
transitions associated with a qualitative estimation of the weak coupling
regime and of the Lamb-Dicke regime is included in Appendix.Comment: 44 revtex pages, 23 figures, to appear in Journal of Modern Optic
Sliding Mode Control of Two-Level Quantum Systems
This paper proposes a robust control method based on sliding mode design for
two-level quantum systems with bounded uncertainties. An eigenstate of the
two-level quantum system is identified as a sliding mode. The objective is to
design a control law to steer the system's state into the sliding mode domain
and then maintain it in that domain when bounded uncertainties exist in the
system Hamiltonian. We propose a controller design method using the Lyapunov
methodology and periodic projective measurements. In particular, we give
conditions for designing such a control law, which can guarantee the desired
robustness in the presence of the uncertainties. The sliding mode control
method has potential applications to quantum information processing with
uncertainties.Comment: 29 pages, 4 figures, accepted by Automatic
Coherent versus measurement feedback: Linear systems theory for quantum information
To control a quantum system via feedback, we generally have two options in
choosing control scheme. One is the coherent feedback, which feeds the output
field of the system, through a fully quantum device, back to manipulate the
system without involving any measurement process. The other one is the
measurement-based feedback, which measures the output field and performs a
real-time manipulation on the system based on the measurement results. Both
schemes have advantages/disadvantages, depending on the system and the control
goal, hence their comparison in several situation is important. This paper
considers a general open linear quantum system with the following specific
control goals; back-action evasion (BAE), generation of a quantum
non-demolished (QND) variable, and generation of a decoherence-free subsystem
(DFS), all of which have important roles in quantum information science. Then
some no-go theorems are proven, clarifying that those goals cannot be achieved
by any measurement-based feedback control. On the other hand it is shown that,
for each control goal, there exists a coherent feedback controller
accomplishing the task. The key idea to obtain all the results is system
theoretic characterizations of BAE, QND, and DFS in terms of controllability
and observability properties or transfer functions of linear systems, which are
consistent with their standard definitions.Comment: 21 pages, 10 figures, to appear in Physical Review
- âŚ