237 research outputs found
Number-Phase Wigner Representation for Scalable Stochastic Simulations of Controlled Quantum Systems
Simulation of conditional master equations is important to describe systems
under continuous measurement and for the design of control strategies in
quantum systems. For large bosonic systems, such as BEC and atom lasers, full
quantum field simulations must rely on scalable stochastic methods whose
convergence time is restricted by the use of representations based on coherent
states. Here we show that typical measurements on atom-optical systems have a
common form that allows for an efficient simulation using the number-phase
Wigner (NPW) phase-space representation. We demonstrate that a stochastic
method based on the NPW can converge over an order of magnitude longer and more
precisely than its coherent equivalent. This opens the possibility of realistic
simulations of controlled multi-mode quantum systems.Comment: 5 pages, 1 figur
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
Compensating for non-linear distortions in controlled quantum systems
Predictive design and optimization methods for controlled quantum systems
depend on the accuracy of the system model. Any distortion of the input fields
in an experimental platform alters the model accuracy and eventually disturbs
the predicted dynamics. These distortions can be non-linear with a strong
frequency dependence so that the field interacting with the microscopic quantum
system has limited resemblance to the input signal. We present an effective
method for estimating these distortions which is suitable for non-linear
transfer functions of arbitrary lengths and magnitudes provided the available
training data has enough spectral components. Using a quadratic estimation, we
have successfully tested our approach for a numerical example of a single
Rydberg atom system. The transfer function estimated from the presented method
is incorporated into an open-loop control optimization algorithm allowing for
high-fidelity operations in quantum experiments.Comment: 15 pages, 10 figure
Coherent Imaging Spectroscopy of a Quantum Many-Body Spin System
Quantum simulators, in which well controlled quantum systems are used to
reproduce the dynamics of less understood ones, have the potential to explore
physics that is inaccessible to modeling with classical computers. However,
checking the results of such simulations will also become classically
intractable as system sizes increase. In this work, we introduce and implement
a coherent imaging spectroscopic technique to validate a quantum simulation,
much as magnetic resonance imaging exposes structure in condensed matter. We
use this method to determine the energy levels and interaction strengths of a
fully-connected quantum many-body system. Additionally, we directly measure the
size of the critical energy gap near a quantum phase transition. We expect this
general technique to become an important verification tool for quantum
simulators once experiments advance beyond proof-of-principle demonstrations
and exceed the resources of conventional computers
Quantum sensing with arbitrary frequency resolution
Quantum sensing takes advantage of well controlled quantum systems for
performing measurements with high sensitivity and precision. We have
implemented a concept for quantum sensing with arbitrary frequency resolution,
independent of the qubit probe and limited only by the stability of an external
synchronization clock. Our concept makes use of quantum lock-in detection to
continuously probe a signal of interest. Using the electronic spin of a single
nitrogen vacancy center in diamond, we demonstrate detection of oscillating
magnetic fields with a frequency resolution of 70 uHz over a MHz bandwidth. The
continuous sampling further guarantees an excellent sensitivity, reaching a
signal-to-noise ratio in excess of 10,000:1 for a 170 nT test signal measured
during a one-hour interval. Our technique has applications in magnetic
resonance spectroscopy, quantum simulation, and sensitive signal detection.Comment: Manuscript resubmitted to Science. Includes Supplementary Material
Quantum computing and the entanglement frontier - Rapporteur talk at the 25th Solvay Conference
Quantum information science explores the frontier of highly complex quantum states,
the "entanglement frontier". This study is motivated by the observation (widely believed
but unproven) that classical systems cannot simulate highly entangled quantum systems
efficiently, and we hope to hasten the day when well controlled quantum systems can
perform tasks surpassing what can be done in the classical world. One way to achieve
such "quantum supremacy" would be to run an algorithm on a quantum computer which
solves a problem with a super-polynomial speedup relative to classical computers, but
there may be other ways that can be achieved sooner, such as simulating exotic quantum
states of strongly correlated matter. To operate a large scale quantum computer reliably
we will need to overcome the debilitating effects of decoherence, which might be done
using "standard" quantum hardware protected by quantum error-correcting codes, or by
exploiting the nonabelian quantum statistics of anyons realized in solid state systems,
or by combining both methods. Only by challenging the entanglement frontier will we
learn whether Nature provides extravagant resources far beyond what the classical world
would allow
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