99 research outputs found
Quantum Signatures of the Optomechanical Instability
In the past few years, coupling strengths between light and mechanical motion
in optomechanical setups have improved by orders of magnitude. Here we show
that, in the standard setup under continuous laser illumination, the steady
state of the mechanical oscillator can develop a non-classical, strongly
negative Wigner density if the optomechanical coupling is large at the
single-photon level. Because of its robustness, such a Wigner density can be
mapped using optical homodyne tomography. These features are observed near the
onset of the instability towards self-induced oscillations. We show that there
are also distinct signatures in the photon-photon correlation function
in that regime, including oscillations decaying on a time scale
not only much longer than the optical cavity decay time, but even longer than
the \emph{mechanical} decay time.Comment: 6 pages including 1 appendix. 6 Figures. Correcte
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
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
Ab initio quantum theory of mass defect and time dilation in trapped-ion optical clocks
We derive a Hamiltonian for the external and internal dynamics of an
electromagnetically bound, charged two-particle system in external
electromagnetic and gravitational fields, including leading-order relativistic
corrections. We apply this Hamiltonian to describe the relativistic coupling of
the external and internal dynamics of cold ions in Paul traps, including the
effects of micromotion, excess micromotion, and trap imperfections. This
provides a systematic and fully quantum-mechanical treatment of relativistic
frequency shifts in atomic clocks based on single trapped ions. Our approach
reproduces well-known formulas for the second-order Doppler shift for thermal
states, which were previously derived on the basis of semiclassical arguments.
We complement and clarify recent discussions in the literature on the role of
time dilation and mass defect in ion clocks
Quadrupole transitions and quantum gates protected by continuous dynamic decoupling
Dynamical decoupling techniques are a versatile tool for engineering quantum
states with tailored properties. In trapped ions, nested layers of continuous
dynamical decoupling by means of radio-frequency field dressing can cancel
dominant magnetic and electric shifts and therefore provide highly prolonged
coherence times of electronic states. Exploiting this enhancement for frequency
metrology, quantum simulation or quantum computation, poses the challenge to
combine the decoupling with laser-ion interactions for the quantum control of
electronic and motional states of trapped ions. Ultimately, this will require
running quantum gates on qubits from dressed decoupled states. We provide here
a compact representation of nested continuous dynamical decoupling in trapped
ions, and apply it to electronic and states and optical quadrupole
transitions. Our treatment provides all effective transition frequencies and
Rabi rates, as well as the effective selection rules of these transitions. On
this basis, we discuss the possibility of combining continuous dynamical
decoupling and M{\o}lmer-S{\o}rensen gates
Lower bounds on the complexity of simulating quantum gates
We give a simple proof of a formula for the minimal time required to simulate
a two-qubit unitary operation using a fixed two-qubit Hamiltonian together with
fast local unitaries. We also note that a related lower bound holds for
arbitrary n-qubit gates.Comment: 6 page
Analysis of Elliptically Polarized Maximally Entangled States for Bell Inequality Tests
When elliptically polarized maximally entangled states are considered, i.e.,
states having a non random phase factor between the two bipartite polarization
components, the standard settings used for optimal violation of Bell
inequalities are no longer adapted. One way to retrieve the maximal amount of
violation is to compensate for this phase while keeping the standard Bell
inequality analysis settings. We propose in this paper a general theoretical
approach that allows determining and adjusting the phase of elliptically
polarized maximally entangled states in order to optimize the violation of Bell
inequalities. The formalism is also applied to several suggested experimental
phase compensation schemes. In order to emphasize the simplicity and relevance
of our approach, we also describe an experimental implementation using a
standard Soleil-Babinet phase compensator. This device is employed to correct
the phase that appears in the maximally entangled state generated from a
type-II nonlinear photon-pair source after the photons are created and
distributed over fiber channels.Comment: 8 page
Topological phase transitions in the non-Abelian honeycomb lattice
Ultracold Fermi gases trapped in honeycomb optical lattices provide an
intriguing scenario, where relativistic quantum electrodynamics can be tested.
Here, we generalize this system to non-Abelian quantum electrodynamics, where
massless Dirac fermions interact with effective non-Abelian gauge fields. We
show how in this setup a variety of topological phase transitions occur, which
arise due to massless fermion pair production events, as well as pair
annihilation events of two kinds: spontaneous and strongly-interacting induced.
Moreover, such phase transitions can be controlled and characterized in optical
lattice experiments.Comment: RevTex4 file, color figure
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