99 research outputs found
Photon Qubit is Made of Two Colors
Single particles of light can be prepared in a quantum superposition of two different colors, an achievement that could prove useful for quantum information processing
Light-Mediated Collective Atomic Motion in an Optical Lattice Coupled to a Membrane
We observe effects of collective atomic motion in a one-dimensional optical
lattice coupled to an optomechanical system. In this hybrid atom-optomechanical
system, the lattice light generates a coupling between the lattice atoms as
well as between atoms and a micromechanical membrane oscillator. For large atom
numbers we observe an instability in the coupled system, resulting in
large-amplitude atom-membrane oscillations. We show that this behavior can be
explained by light-mediated collective atomic motion in the lattice, which
arises for large atom number, small atom-light detuning and asymmetric pumping
of the lattice, in agreement with previous theoretical work. The model connects
the optomechanical instability to a phase delay in the global atomic
back-action onto the lattice light, which we observe in a direct measurement.Comment: new introduction, title and outlook; small modifications of the main
text and figure
Frequency-tunable microwave field detection in an atomic vapor cell
We use an atomic vapor cell as a frequency tunable microwave field detector operating at frequencies from GHz to tens of GHz. We detect microwave magnetic fields from 2.3 GHz to 26.4 GHz, and measure the amplitude of the sigma+ component of an 18 GHz microwave field. Our proof-of-principle demonstration represents a four orders of magnitude extension of the frequency tunable range of atomic magnetometers from their previous dc to several MHz range. When integrated with a high resolution microwave imaging system, this will allow for the complete reconstruction of the vector components of a microwave magnetic field and the relative phase between them. Potential applications include near-field characterisation of microwave circuitry and devices, and medical microwave sensing and imaging
Mesoscopic quantum superpositions in bimodal Bose-Einstein condensates: decoherence and strategies to counteract it
We study theoretically the interaction-induced generation of mesoscopic
coherent spin state superpositions (small cat states) from an initial coherent
spin state in bimodal Bose-Einstein condensates and the subsequent phase
revival, including decoherence due to particle losses and fluctuations of the
total particle number. In a full multimode description, we propose a
preparation procedure of the initial coherent spin state and we study the
effect of preexisting thermal fluctuations on the phase revival, and on the
spin and orbito-spinorial cat fidelities.Comment: 26 pages, 12 figures; published versio
Does a large quantum Fisher information imply Bell correlations?
The quantum Fisher information (QFI) of certain multipartite entangled
quantum states is larger than what is reachable by separable states, providing
a metrological advantage. Are these nonclassical correlations strong enough to
potentially violate a Bell inequality? Here, we present evidence from two
examples. First, we discuss a Bell inequality designed for spin-squeezed states
which is violated only by quantum states with a large QFI. Second, we relax a
well-known lower bound on the QFI to find the Mermin Bell inequality as a
special case. However, a fully general link between QFI and Bell correlations
is still open.Comment: 4 pages, minor edit
Quantum metrology with nonclassical states of atomic ensembles
Quantum technologies exploit entanglement to revolutionize computing,
measurements, and communications. This has stimulated the research in different
areas of physics to engineer and manipulate fragile many-particle entangled
states. Progress has been particularly rapid for atoms. Thanks to the large and
tunable nonlinearities and the well developed techniques for trapping,
controlling and counting, many groundbreaking experiments have demonstrated the
generation of entangled states of trapped ions, cold and ultracold gases of
neutral atoms. Moreover, atoms can couple strongly to external forces and light
fields, which makes them ideal for ultra-precise sensing and time keeping. All
these factors call for generating non-classical atomic states designed for
phase estimation in atomic clocks and atom interferometers, exploiting
many-body entanglement to increase the sensitivity of precision measurements.
The goal of this article is to review and illustrate the theory and the
experiments with atomic ensembles that have demonstrated many-particle
entanglement and quantum-enhanced metrology.Comment: 76 pages, 40 figures, 1 table, 603 references. Some figures bitmapped
at 300 dpi to reduce file siz
Widefield microwave imaging in alkali vapor cells with sub-100 mum resolution
We report on widefield microwave vector field imaging with sub-100 mu m resolution using a microfabricated alkali vapor cell. The setup can additionally image dc magnetic fields, and can be configured to image microwave electric fields. Our camera-based widefield imaging system records 2D images with a 6 x 6 mm(2) field of view at a rate of 10 Hz. It provides up to 50 mu m spatial resolution, and allows imaging of fields as close as 150 mu m above structures, through the use of thin external cell walls. This is crucial in allowing us to take practical advantage of the high spatial resolution, as feature sizes in near-fields are on the order of the distance from their source, and represent an order of magnitude improvement in surface-feature resolution compared to previous vapor cell experiments. We present microwave and dc magnetic field images above a selection of devices, demonstrating a microwave sensitivity of 1.4 mu T Hz(-1/2) per 50 x 50 x 140 mu m(3) voxel, at present limited by the speed of our camera system. Since we image 120 x 120 voxels in parallel, a single scanned sensor would require a sensitivity of at least 12 nT Hz(-1/2) to produce images with the same sensitivity. Our technique could prove transformative in the design, characterization, and debugging of microwave devices, as there are currently no satisfactory established microwave imaging techniques. Moreover, it could find applications in medical imaging
Simple atomic quantum memory suitable for semiconductor quantum dot single photons
Quantum memories matched to single photon sources will form an important
cornerstone of future quantum network technology. We demonstrate such a memory
in warm Rb vapor with on-demand storage and retrieval, based on
electromagnetically induced transparency. With an acceptance bandwidth of
= 0.66~GHz the memory is suitable for single photons emitted by
semiconductor quantum dots. In this regime, vapor cell memories offer an
excellent compromise between storage efficiency, storage time, noise level, and
experimental complexity, and atomic collisions have negligible influence on the
optical coherences. Operation of the memory is demonstrated using attenuated
laser pulses on the single photon level. For 50 ns storage time we measure
\emph{end-to-end efficiency}
of the fiber-coupled memory, with an \emph{total intrinsic efficiency}
. Straightforward technological improvements can
boost the end-to-end-efficiency to ; beyond
that increasing the optical depth and exploiting the Zeeman substructure of the
atoms will allow such a memory to approach near unity efficiency.
In the present memory, the unconditional readout noise level of photons is dominated by atomic fluorescence, and for input pulses
containing on average photons the signal to noise level would
be unity
Bell correlations in a many-body system with finite statistics
A recent experiment reported the first violation of a Bell correlation
witness in a many-body system [Science 352, 441 (2016)]. Following discussions
in this paper, we address here the question of the statistics required to
witness Bell correlated states, i.e. states violating a Bell inequality, in
such experiments. We start by deriving multipartite Bell inequalities involving
an arbitrary number of measurement settings, two outcomes per party and one-
and two-body correlators only. Based on these inequalities, we then build up
improved witnesses able to detect Bell-correlated states in many-body systems
using two collective measurements only. These witnesses can potentially detect
Bell correlations in states with an arbitrarily low amount of spin squeezing.
We then establish an upper bound on the statistics needed to convincingly
conclude that a measured state is Bell-correlated.Comment: 5+12 pages, 3+4 figure
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