70 research outputs found
Time-Continuous Bell Measurements
We combine the concept of Bell measurements, in which two systems are
projected into a maximally entangled state, with the concept of continuous
measurements, which concerns the evolution of a continuously monitored quantum
system. For such time-continuous Bell measurements we derive the corresponding
stochastic Schr\"odinger equations, as well as the unconditional feedback
master equations. Our results apply to a wide range of physical systems, and
are easily adapted to describe an arbitrary number of systems and measurements.
Time-continuous Bell measurements therefore provide a versatile tool for the
control of complex quantum systems and networks. As examples we show show that
(i) two two-level systems can be deterministically entangled via homodyne
detection, tolerating photon loss up to 50%, and (ii) a quantum state of light
can be continuously teleported to a mechanical oscillator, which works under
the same conditions as are required for optomechanical ground state cooling.Comment: 4+4 pages, 4 figure
A quantum volume hologram
We propose a new scheme for parallel spatially multimode quantum memory for
light. The scheme is based on counter-propagating quantum signal wave and
strong classical reference wave, like in a classical volume hologram, and
therefore can be called a quantum volume hologram. The medium for the hologram
consists of a spatially extended ensemble of atoms placed in a magnetic field.
The write-in and read-out of this quantum hologram is as simple as that of its
classical counterpart and consists of a single pass illumination. In addition
we show that the present scheme for a quantum hologram is less sensitive to
diffraction and therefore is capable of achieving higher density of storage of
spatial modes as compared to previous proposals. A quantum hologram capable of
storing entangled images can become an important ingredient in quantum
information processing and quantum imaging.Comment: 8 pages, 2 figure
Dissipative versus Conditional Generation of Gaussian Entanglement and Spin Squeezing
Spin squeezing of collective atomic spins can be achieved conditionally via
probing with light and subsequent homodyne detection, as is done in a Quantum
Nondemolition measurement. Recently it has been shown that squeezing can also
be created unconditionally by a properly designed dissipative dynamics. We
compare the two approaches in a Gaussian description, and optimize over all
Gaussian light-matter interactions. We find that in the optimal unconditional
scheme based on dissipation the level of squeezing scales as . In
contrast, the optimal conditional scheme based on measurement of light -- which
in fact is not a Quantum Nondemolition measurement -- can provide squeezing
which scales as in the most relevant regime of moderate optical
depths. Our results apply directly also to the creation of entanglement in the
form of non-local spin squeezing of two atomic ensembles.Comment: 9 pages, 7 figure
Unconditional steady-state entanglement in macroscopic hybrid systems by coherent noise cancellation
The generation of entanglement between disparate physical objects is a key
ingredient in the field of quantum technologies, since they can have different
functionalities in a quantum network. Here we propose and analyze a generic
approach to steady-state entanglement generation between two oscillators with
different temperatures and decoherence properties coupled in cascade to a
common unidirectional light field. The scheme is based on a combination of
coherent noise cancellation and dynamical cooling techniques for two
oscillators with effective masses of opposite signs, such as quasi-spin and
motional degrees of freedom, respectively. The interference effect provided by
the cascaded setup can be tuned to implement additional noise cancellation
leading to improved entanglement even in the presence of a hot thermal
environment. The unconditional entanglement generation is advantageous since it
provides a ready-to-use quantum resource. Remarkably, by comparing to the
conditional entanglement achievable in the dynamically stable regime, we find
our unconditional scheme to deliver a virtually identical performance when
operated optimally.Comment: Final version; 6 pages, 3 figures + Supplemental Materia
Quantum memory for images - a quantum hologram
Matter-light quantum interface and quantum memory for light are important
ingredients of quantum information protocols, such as quantum networks,
distributed quantum computation, etc. In this Letter we present a spatially
multimode scheme for quantum memory for light, which we call a quantum
hologram. Our approach uses a multi-atom ensemble which has been shown to be
efficient for a single spatial mode quantum memory. Due to the multi-atom
nature of the ensemble it is capable of storing many spatial modes, a feature
critical for the present proposal. A quantum hologram has a higher storage
capacity compared to a classical hologram, and is capable of storing quantum
features of an image, such as multimode superposition and entangled quantum
states, something that a standard hologram is unable to achieve. Due to optical
parallelism, the information capacity of the quantum hologram will obviously
exceed that of a single-mode scheme.Comment: 5 pages, 3 figure
Optimal and Variational Multi-Parameter Quantum Metrology and Vector Field Sensing
We study multi-parameter sensing of 2D and 3D vector fields within the
Bayesian framework for quantum interferometry. We establish a method to
determine the optimal quantum sensor, which establishes the fundamental limit
on the precision of simultaneously estimating multiple parameters with an
-atom sensor. Keeping current experimental platforms in mind, we present
sensors that have limited entanglement capabilities, and yet, significantly
outperform sensors that operate without entanglement and approach the optimal
quantum sensor in terms of performance. Furthermore, we show how these sensors
can be implemented on current programmable quantum sensors with variational
quantum circuits by minimizing a metrological cost function. The resulting
circuits prepare tailored entangled states and perform measurements in an
appropriate entangled basis to realize the best possible quantum sensor given
the native entangling resources available on a given sensor platform. Notable
examples include a 2D and 3D quantum ``compass'' and a 2D sensor that provides
a scalable improvement over unentangled sensors. Our results on optimal and
variational multi-parameter quantum metrology are useful for advancing
precision measurements in fundamental science and ensuring the stability of
quantum computers, which can be achieved through the incorporation of optimal
quantum sensors in a quantum feedback loop.Comment: 20 pages, 8 figure
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