124 research outputs found
Modelling and feedback control design for quantum state preparation
The goal of this article is to provide a largely self-contained introduction to the modelling of controlled quantum systems under continuous observation, and to the design of feedback controls that prepare particular quantum states. We describe a bottom-up approach, where a field-theoretic model is subjected to statistical inference and is ultimately controlled. As an example, the formalism is applied to a highly idealized interaction of an atomic ensemble with an optical field. Our aim is to provide a unified outline for the modelling, from first principles, of realistic experiments in quantum control
Feedback control of quantum state reduction
Feedback control of quantum mechanical systems must take into account the probabilistic nature of quantum measurement. We formulate quantum feedback control as a problem of stochastic nonlinear control by considering separately a quantum filtering problem and a state feedback control problem for the filter. We explore the use of stochastic Lyapunov techniques for the design of feedback controllers for quantum spin systems and demonstrate the possibility of stabilizing one outcome of a quantum measurement with unit probability
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
Adaptive homodyne measurement of optical phase
We present an experimental demonstration of the power of real-time feedback
in quantum metrology, confirming a theoretical prediction by Wiseman regarding
the superior performance of an adaptive homodyne technique for single-shot
measurement of optical phase. For phase measurements performed on weak coherent
states with no prior knowledge of the signal phase, we show that the variance
of adaptive homodyne estimation approaches closer to the fundamental quantum
uncertainty limit than any previously demonstrated technique. Our results
underscore the importance of real-time feedback for reaching quantum
performance limits in coherent telecommunication, precision measurement and
information processing.Comment: RevTex4, color PDF figures (separate files), submitted to PR
Deterministic Dicke state preparation with continuous measurement and control
We characterize the long-time projective behavior of the stochastic master
equation describing a continuous, collective spin measurement of an atomic
ensemble both analytically and numerically. By adding state based feedback, we
show that it is possible to prepare highly entangled Dicke states
deterministically.Comment: Additional information is available at
http://minty.caltech.edu/Ensemble
Scattering of polarized laser light by an atomic gas in free space: a QSDE approach
We propose a model, based on a quantum stochastic differential equation
(QSDE), to describe the scattering of polarized laser light by an atomic gas.
The gauge terms in the QSDE account for the direct scattering of the laser
light into different field channels. Once the model has been set, we can
rigorously derive quantum filtering equations for balanced polarimetry and
homodyne detection experiments, study the statistics of output processes and
investigate a strong driving, weak coupling limit.Comment: 9 pages, 2 figure
Tensor polarizability and dispersive quantum measurement of multilevel atoms
Optimally extracting information from measurements performed on a physical
system requires an accurate model of the measurement interaction. Continuously
probing the collective spin of an Alkali atom cloud via its interaction with an
off-resonant optical probe is an important example of such a measurement where
realistic modeling at the quantum level is possible using standard techniques
from atomic physics. Typically, however, tutorial descriptions of this
technique have neglected the multilevel structure of realistic atoms for the
sake of simplification. In this paper we account for the full multilevel
structure of Alkali atoms and derive the irreducible form of the polarizability
Hamiltonian describing a typical dispersive quantum measurement. For a specific
set of parameters, we then show that semiclassical predictions of the theory
are consistent with our experimental observations of polarization scattering by
a polarized cloud of laser-cooled Cesium atoms. We also derive the
signal-to-noise ratio under a single measurement trial and use this to predict
the rate of spin-squeezing with multilevel Alkali atoms for arbitrary detuning
of the probe beam.Comment: Significant corrections to theory and data. Full quality figures and
other information available from http://minty.caltech.edu/papers.ph
Characterizing the entanglement of symmetric many-particle spin-1/2 systems
Analyzing the properties of entanglement in many-particle spin-1/2 systems is
generally difficult because the system's Hilbert space grows exponentially with
the number of constituent particles, . Fortunately, it is still possible to
investigate many-particle entanglement when the state of the system possesses
sufficient symmetry. In this paper, we present a practical method for
efficiently computing various bipartite entanglement measures for states in the
symmetric subspace and perform these calculations for . By
considering all possible bipartite splits, we construct a picture of the
multiscale entanglement in large symmetric systems. In particular, we
characterize dynamically generated spin-squeezed states by comparing them to
known reference states (e.g., GHZ and Dicke states) and new families of states
with near-maximal bipartite entropy. We quantify the trade-off between the
degree of entanglement and its robustness to particle loss, emphasizing that
substantial entanglement need not be fragile.Comment: Updated version reflects changes made in January 200
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