35 research outputs found
Optimal and Robust Quantum Metrology Using Interaction-Based Readouts
Useful quantum metrology requires nonclassical states with a high particle
number and (close to) the optimal exploitation of the state's quantum
correlations. Unfortunately, the single-particle detection resolution demanded
by conventional protocols, such as spin squeezing via one-axis twisting, places
severe limits on the particle number. Additionally, the challenge of finding
optimal measurements (that saturate the quantum Cram{\'e}r-Rao bound) for an
arbitrary nonclassical state limits most metrological protocols to only
moderate levels of quantum enhancement. "Interaction-based readout" protocols
have been shown to allow optimal interferometry \emph{or} to provide robustness
against detection noise at the expense of optimality. In this Letter, we prove
that one has great flexibility in constructing an optimal protocol, thereby
allowing it to also be robust to detection noise. This requires the full
probability distribution of outcomes in an optimal measurement basis, which is
typically easily accessible and can be determined from specific criteria we
provide. Additionally, we quantify the robustness of several classes of
interaction-based readouts under realistic experimental constraints. We
determine that optimal \emph{and} robust quantum metrology is achievable in
current spin-squeezing experiments.Comment: 7 pages, 3 figure
Quantum metrology with mixed states: when recovering lost information is better than never losing it
Quantum-enhanced metrology can be achieved by entangling a probe with an auxiliary system, passing the probe through an interferometer, and subsequently making measurements on both the probe and auxiliary system. Conceptually, this corresponds to performing metrology with the purification of a (mixed) probe state. We demonstrate via the quantum Fisher information how to design mixed states whose purifications are an excellent metrological resource. In particular, we give examples of mixed states with purifications that allow (near) Heisenberg-limited metrology and provide examples of entangling Hamiltonians that can generate these states. Finally, we present the optimal measurement and parameter-estimation procedure required to realize these sensitivities (i.e., that saturate the quantum Cramér-Rao bound). Since pure states of comparable metrological usefulness are typically challenging to generate, it may prove easier to use this approach of entanglement and measurement of an auxiliary system. An example where this may be the case is atom interferometry, where entanglement with optical systems is potentially easier to engineer than the atomic interactions required to produce nonclassical atomic states
Ignorance is bliss: General and robust cancellation of decoherence via no-knowledge quantum feedback
A "no-knowledge" measurement of an open quantum system yields no information
about any system observable; it only returns noise input from the environment.
Surprisingly, performing such a no-knowledge measurement can be advantageous.
We prove that a system undergoing no-knowledge monitoring has reversible noise,
which can be cancelled by directly feeding back the measurement signal. We show
how no-knowledge feedback control can be used to cancel decoherence in an
arbitrary quantum system coupled to a Markovian reservoir that is being
monitored. Since no-knowledge feedback does not depend on the system state or
Hamiltonian, such decoherence cancellation is guaranteed to be general, robust
and can operate in conjunction with any other quantum control protocol. As an
application, we show that no-knowledge feedback could be used to improve the
performance of dissipative quantum computers subjected to local loss.Comment: 6 pages + 2 pages supplemental material, 3 figure
Controlling chaos in the quantum regime using adaptive measurements
The continuous monitoring of a quantum system strongly influences the
emergence of chaotic dynamics near the transition from the quantum regime to
the classical regime. Here we present a feedback control scheme that uses
adaptive measurement techniques to control the degree of chaos in the
driven-damped quantum Duffing oscillator. This control relies purely on the
measurement backaction on the system, making it a uniquely quantum control, and
is only possible due to the sensitivity of chaos to measurement. We quantify
the effectiveness of our control by numerically computing the quantum Lyapunov
exponent over a wide range of parameters. We demonstrate that adaptive
measurement techniques can control the onset of chaos in the system, pushing
the quantum-classical boundary further into the quantum regime
Improving cold-atom sensors with quantum entanglement: Prospects and challenges
Quantum entanglement has been generated and verified in cold-atom experiments
and used to make atom-interferometric measurements below the shot-noise limit.
However, current state-of-the-art cold-atom devices exploit separable (i.e.
unentangled) atomic states. This Perspective piece asks the question: can
entanglement usefully improve cold-atom sensors, in the sense that it gives new
sensing capabilities unachievable with current state-of-the-art devices? We
briefly review the state-of-the-art in precision cold-atom sensing, focussing
on clocks and inertial sensors, identifying the potential benefits entanglement
could bring to these devices, and the challenges that need to be overcome to
realize these benefits. We survey demonstrated methods of generating
metrologically-useful entanglement in cold-atom systems, note their relative
strengths and weaknesses, and assess their prospects for near-to-medium term
quantum-enhanced cold-atom sensing.Comment: Invited perspective; close to published version. Note the change in
title. 19 pages, 7 figure
Robustness of System-Filter Separation for the Feedback Control of a Quantum Harmonic Oscillator Undergoing Continuous Position Measurement
We consider the effects of experimental imperfections on the problem of
estimation-based feedback control of a trapped particle under continuous
position measurement. These limitations violate the assumption that the
estimator (i.e. filter) accurately models the underlying system, thus requiring
a separate analysis of the system and filter dynamics. We quantify the
parameter regimes for stable cooling, and show that the control scheme is
robust to detector inefficiency, time delay, technical noise, and miscalibrated
parameters. We apply these results to the specific context of a weakly
interacting Bose-Einstein condensate (BEC). Given that this system has
previously been shown to be less stable than a feedback-cooled BEC with strong
interatomic interactions, this result shows that reasonable experimental
imperfections do not limit the feasibility of cooling a BEC by continuous
measurement and feedback.Comment: 14 pages, 8 figure
Mutual friction and diffusion of two-dimensional quantum vortices
We present a microscopic open quantum systems theory of thermally-damped
vortex motion in oblate atomic superfluids that includes previously neglected
energy-damping interactions between superfluid and thermal atoms. This
mechanism couples strongly to vortex core motion and causes dissipation of
vortex energy due to mutual friction, as well as Brownian motion of vortices
due to thermal fluctuations. We derive an analytic expression for the
dimensionless mutual friction coefficient that gives excellent quantitative
agreement with experimentally measured values, without any fitted parameters.
Our work closes an existing two orders of magnitude gap between dissipation
theory and experiments, previously bridged by fitted parameters, and provides a
microscopic origin for the mutual friction and diffusion of quantized vortices
in two-dimensional atomic superfluids
Optimal matter-wave gravimetry
We calculate quantum and classical Fisher informations for gravity sensors based on matterwave interference, and find that current Mach-Zehnder interferometry is not optimally extracting the full metrological potential of these sensors. We show that by making measurements that resolve either the momentum or the position we can considerably improve the sensitivity. We also provide a simple modification that is capable of more than doubling the sensitivity
Thermally robust spin correlations between two Rb-85 atoms in an optical microtrap
The complex collisional properties of atoms fundamentally limit investigations into a range of processes in many-atom ensembles. In contrast, the bottom-up assembly of few- and manybody systems from individual atoms offers a controlled approach to isolating and studying such collisional processes. Here, we use optical tweezers to individually assemble pairs of trapped Rb-85 atoms, and study the spin dynamics of the two-body system in a thermal state. The spin-2 atoms show strong pair correlation between magnetic sublevels on timescales exceeding one second, with measured relative number fluctuations 11.9 +/- 0.3 dB below quantum shot noise, limited only by detection efficiency. Spin populations display relaxation dynamics consistent with simulations and theoretical predictions for Rb-85 spin interactions, and contrary to the coherent spin waves witnessed in finite-temperature many-body experiments and zero-temperature two-body experiments. Our experimental approach offers a versatile platform for studying two-body quantum dynamics and may provide a route to thermally robust entanglement generation