120 research outputs found
A Deterministic and Nondestructively-Verifiable Photon Number Source
We present a deterministic approach based on continuous measurement and
real-time quantum feedback control to prepare arbitrary photon number states of
a cavity mode. The procedure passively monitors the number state actually
achieved in each feedback stabilized measurement trajectory, thus providing a
nondestructively verifiable photon source. The feasibility of a possible cavity
QED implementation in the many-atom good-cavity coupling regime is analyzed
Collective Uncertainty in Partially-Polarized and Partially-Decohered Spin-1/2 Systems
It has become common practice to model large spin ensembles as an effective
pseudospin with total angular momentum J = N x j, where j is the spin per
particle. Such approaches (at least implicitly) restrict the quantum state of
the ensemble to the so-called symmetric Hilbert space. Here, we argue that
symmetric states are not generally well-preserved under the type of decoherence
typical of experiments involving large clouds of atoms or ions. In particular,
symmetric states are rapidly degraded under models of decoherence that act
identically but locally on the different members of the ensemble. Using an
approach [Phys. Rev. A 78, 052101 (2008)] that is not limited to the symmetric
Hilbert space, we explore potential pitfalls in the design and interpretation
of experiments on spin-squeezing and collective atomic phenomena when the
properties of the symmetric states are extended to systems where they do not
apply.Comment: 13 pages, 7 figure
Robust quantum parameter estimation: coherent magnetometry with feedback
We describe the formalism for optimally estimating and controlling both the
state of a spin ensemble and a scalar magnetic field with information obtained
from a continuous quantum limited measurement of the spin precession due to the
field. The full quantum parameter estimation model is reduced to a simplified
equivalent representation to which classical estimation and control theory is
applied. We consider both the tracking of static and fluctuating fields in the
transient and steady state regimes. By using feedback control, the field
estimation can be made robust to uncertainty about the total spin number
Efficient feedback controllers for continuous-time quantum error correction
We present an efficient approach to continuous-time quantum error correction
that extends the low-dimensional quantum filtering methodology developed by van
Handel and Mabuchi [quant-ph/0511221 (2005)] to include error recovery
operations in the form of real-time quantum feedback. We expect this paradigm
to be useful for systems in which error recovery operations cannot be applied
instantaneously. While we could not find an exact low-dimensional filter that
combined both continuous syndrome measurement and a feedback Hamiltonian
appropriate for error recovery, we developed an approximate reduced-dimensional
model to do so. Simulations of the five-qubit code subjected to the symmetric
depolarizing channel suggests that error correction based on our approximate
filter performs essentially identically to correction based on an exact quantum
dynamical model
Generalized Limits for Single-Parameter Quantum Estimation
We develop generalized bounds for quantum single-parameter estimation
problems for which the coupling to the parameter is described by intrinsic
multi-system interactions. For a Hamiltonian with -system
parameter-sensitive terms, the quantum limit scales as where is the
number of systems. These quantum limits remain valid when the Hamiltonian is
augmented by any parameter independent interaction among the systems and when
adaptive measurements via parameter-independent coupling to ancillas are
allowed.Comment: 4 pages, 1 figure. v2 typos correcte
Distinguishing between optical coherent states with imperfect detection
Several proposed techniques for distinguishing between optical coherent
states are analyzed under a physically realistic model of photodetection.
Quantum error probabilities are derived for the Kennedy receiver, the Dolinar
receiver and the unitary rotation scheme proposed by Sasaki and Hirota for
sub-unity detector efficiency. Monte carlo simulations are performed to assess
the effects of detector dark counts, dead time, signal processing bandwidth and
phase noise in the communication channel. The feedback strategy employed by the
Dolinar receiver is found to achieve the Helstrom bound for sub-unity detection
efficiency and to provide robustness to these other detector imperfections
making it more attractive for laboratory implementation than previously
believed
- …