49 research outputs found
Generating Macroscopic Superpositions with Interacting Bose-Einstein Condensates: Multi-Mode Speed-Ups and Speed Limits
We theoretically investigate the effect of multi-mode dynamics on the
creation of macroscopic superposition states (spin-cat states) in Bose-Einstein
condensates via one-axis twisting. A two-component Bose-Einstein condensate
naturally realises an effective one-axis twisting interaction, under which an
initially separable state will evolve toward a spin-cat state. However, the
large evolution times necessary to realise these states is beyond the scope of
current experiments. This evolution time is proportional to the degree of
asymmetry in the relative scattering lengths of the system, which results in
the following trade-off; faster evolution times are associated with an increase
in multi-mode dynamics, and we find that generally multi-mode dynamics reduce
the degree of entanglement present in the final state. However, we find that
highly entangled cat-like states are still possible in the presence of
significant multi-mode dynamics, and that these dynamics impose a speed-limit
on the evolution such states
Mean-field dynamics and Fisher information in matter wave interferometry
There has been considerable recent interest in the mean-field dynamics of various atom-interferometry schemes designed for precision sensing. In the field of quantum metrology, the standard tools for evaluating metrological sensitivity are the classical and quantum Fisher information. In this Letter, we show how these tools can be adapted to evaluate the sensitivity when the behavior is dominated by mean-field dynamics. As an example, we compare the behavior of four recent theoretical proposals for gyroscopes based on matter-wave interference in toroidally trapped geometries. We show that while the quantum Fisher information increases at different rates for the various schemes considered, in all cases it is consistent with the well-known Sagnac phase shift after the matter waves have traversed a closed path. However, we argue that the relevant metric for quantifying interferometric sensitivity is the classical Fisher information, which can vary considerably between the schemes
A dynamic scheme for generating number squeezing in Bose-Einstein condensates through nonlinear interactions
We develop a scheme to generate number squeezing in a Bose-Einstein
condensate by utilizing interference between two hyperfine levels and nonlinear
atomic interactions. We describe the scheme using a multimode quantum field
model and find agreement with a simple analytic model in certain regimes. We
demonstrate that the scheme gives strong squeezing for realistic choices of
parameters and atomic species. The number squeezing can result in noise well
below the quantum limit, even if the initial noise on the system is classical
and much greater than that of a poisson distribution.Comment: 4 pages, 3 figure
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
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
A hybrid method of generating spin-squeezed states for quantum-enhanced atom interferometry
We introduce a new spin-squeezing technique that is a hybrid of two well
established spin-squeezing techniques, quantum nondemolition measurement (QND)
and one-axis twisting (OAT). This hybrid method aims to improve spin-squeezing
over what is currently achievable using QND and OAT. In practical situations,
the strength of both the QND and OAT interactions is limited. We found that in
these situations, the hybrid scheme performed considerably better than either
OAT or QND used in isolation. As QND and OAT have both been realised
experimentally, this technique could be implemented in current atom
interferometry setups with only minor modifications to the experiment