56 research outputs found
Creation of macroscopic superpositions of flow states with Bose-Einstein condensates
We present a straightforward scheme for creating macroscopic superpositions
of different superfluid flow states of Bose-Einstein condensates trapped in
optical lattices. This scheme has the great advantage that all the techniques
required are achievable with current experiments. Furthermore, the relative
difficulty of creating cats scales favorably with the size of the cat. This
means that this scheme may be well-suited to creating superpositions involving
large numbers of particles. Such states may have interesting technological
applications such as making quantum-limited measurements of angular momentum.Comment: 9 pages, 7 figure
Transformation properties and entanglement of relativistic qubits under space-time and gauge transformations
We revisit the properties of qubits under Lorentz transformations and, by considering Lorentz invariant quantum states in the Heisenberg formulation, clarify some misleading notation that has appeared in the literature on relativistic quantum information theory. We then use this formulation to consider the transformation properties of qubits and density matrices under space-time and gauge transformations. Finally we use our results to understand the behaviour of entanglement between different partitions of quantum systems. Our approach not only clarifies the notation, but provides a more intuitive and simple way of gaining insight into the behaviour of relativistic qubits. In particular, it allows us to greatly generalize the results in the current literature as well as substantially simplifying the calculations that are needed
Bayesian multiparameter quantum metrology with limited data
A longstanding problem in quantum metrology is how to extract as much information as possible in realistic scenarios with not only multiple unknown parameters, but also limited measurement data and some degree of prior information. Here we present a practical solution to this: We derive a Bayesian multi-parameter quantum bound, construct the optimal measurement when our bound can be saturated for a single shot, and consider experiments involving a repeated sequence of these measurements. Our method properly accounts for the number of measurements and the degree of prior information, and we illustrate our ideas with a qubit sensing network and a model for phase imaging, clarifying the nonasymptotic role of local and global schemes. Crucially, our technique is a powerful way of implementing quantum protocols in a wide range of practical scenarios that tools such as the Helstrom and Holevo Cramér-Rao bounds cannot normally access
Entanglement and nonlocality of a single relativistic particle
Recent work has argued that the concepts of entanglement and nonlocality must
be taken seriously even in systems consisting of only a single particle. These
treatments, however, are nonrelativistic and, if single particle entanglement
is fundamental, it should also persist in a relativistic description. Here we
consider a spin-1/2 particle in a superposition of two different velocities as
viewed by an observer in a different relativistically-boosted inertial frame.
We show that the entanglement survives right up to the speed of light and that
the boosted observer would see single-particle violations of Bell's inequality.
We also discuss how quantum gates could be implemented in this way and the
possible implications for quantum information processing.Comment: 4 page
Quantum metrology in the presence of limited data
Quantum metrology protocols are typically designed around the assumption that we have an abundance of measurement data, but recent practical applications are increasingly driving interest in cases with very limited data. In this regime the best approach involves an interesting interplay between the amount of data and the prior information. Here we propose a new way of optimising these schemes based on the practically-motivated assumption that we have a sequence of identical and independent measurements. For a given probe state we take our measurement to be the best one for a single shot and we use this sequentially to study the performance of different practical states in a Mach-Zehnder interferometer when we have moderate prior knowledge of the underlying parameter. We find that we recover the quantum Cramér-Rao bound asymptotically, but for low data counts we find a completely different structure. Despite the fact that intra-mode correlations are known to be the key to increasing the asymptotic precision, we find evidence that these could be detrimental in the low data regime and that entanglement between the paths of the interferometer may play a more important role. Finally, we analyse how close realistic measurements can get to the bound and find that measuring quadratures can improve upon counting photons, though both strategies converge asymptotically. These results may prove to be important in the development of quantum enhanced metrology applications where practical considerations mean that we are limited to a small number of trials
Generation of maximally entangled states with sub-luminal Lorentz boost
Recent work has studied entanglement between the spin and momentum components
of a single spin-1/2 particle and showed that maximal entanglement is obtained
only when boosts approach the speed of light. Here we extend the boost scenario
to general geometries and show that, intriguingly, maximal entanglement can be
achieved with boosts less than the speed of light. Boosts approaching the speed
of light may even decrease entanglement. We also provide a geometric
explanation for this behavior
Heisenberg scaling with classical long-range correlations
The Heisenberg scaling is typically associated with nonclassicality and entanglement. In this work, however, we discuss how classical long-range correlations between lattice sites in many-body systems may lead to a 1=N scaling in precision with the number of probes in the context of quantum optical dissipative systems. In particular, we show that networks of coupled single qubit lasers can be mapped onto a classical XY model, and a Heisenberg scaling with the number of sites appears when estimating the amplitude and phase of a weak periodic driving field
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Maps generated by entangled momenta: exploring spin entanglement in relativity
We study relativistic entanglement of a bipartite system consisting of massive spin-1=2 particles with momenta. The spin state is described by the maximally entangled Bell state and momenta are given by entangled Gaussian distributions. We conceptualize the dependency between spin and momentum in relativity along the lines of controlled operations in quantum information theory. This leads to a systematic study of maps that Wigner rotations generate on the spin degree of freedom of the total system in different boost scenarios. We use a visualization tool from quantum information theory in order to get better insight into how and why the entanglement changes in different boost geometries
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