81 research outputs found
Conservation relation of nonclassicality and entanglement for Gaussian states in a beam-splitter
We study the relation between single-mode nonclassicality and two-mode
entanglement in a beam-splitter. We show that not all of the nonclassicality
(entanglement potential) is transformed into two-mode entanglement for an
incident single-mode light. Some of the entanglement potential remains as
single-mode nonclassicality in the two entangled output modes. Two-mode
entanglement generated in the process can be equivalently quantified as the
increase in the minimum uncertainty widths (or decrease in the squeezing) of
the output states compared to the input states. We use the nonclassical depth
and logarithmic negativity as single-mode nonclassicality and entanglement
measures, respectively. We realize that a conservation relation between the two
quantities can be adopted for Gaussian states, if one works in terms of
uncertainty width. This conservation relation is extended to many sets of
beam-splitters.Comment: 10 pages, 8 figure
Quantum States Preparation in Cavity Optomechanics
Quantum entanglement and quantum superposition are fundamental properties of quantum mechanics, which underline quantum information and quantum computation. Preparing quantum states in the macroscopic level is both conceptually interesting for extending quantum physics to a broader sense and fundamentally important for testing the validity of quantum mechanics. In this dissertation, schemes of preparing macroscopic entanglement and macroscopic superposition states in cavity optomechanics are studied using the unitary evolution method in the nonlinear regime or Lyapunov equation in the linearized regime. Quantum entanglement and quantum superposition states can be realized using experimentally feasible parameters with the proposals in this dissertation.
Firstly, a scheme of entangling two movable end mirrors in a Fabry-Perot cavity that are coupled to a common single photon superposition state is studied. It is shown that strong entanglement can be obtained either in the single-photon strong coupling regime deterministically or in the single-photon weak coupling regime conditionally.
Secondly, a scheme of entangling two movable end mirrors, that are coupled to two-mode entangled fields generated from a correlated-emission laser is investigated. By tuning the input driving laser frequencies at the Stokes sidebands of the cavity, the radiation-pressure coupling can be linearized as an effective beam-splitter-like interaction. Hence entanglement can be transferred from the two-mode fields to the two mechanical mirrors. Macroscopic entanglement between macroscopic mirrors persists at temperature ~ 1K.
Thirdly, a scheme of creating macroscopic quantum superpositions of a mechanical mirror via periodically flipping a photonic qubit is proposed. Quantum superposition states of a mechanical mirror can be obtained via the nonlinear radiation coupling with a single-photon superposition state. However, the difference between two superposed mechanical states is very small due to the weak single-photon coupling rate available in experiment. By periodically flipping the photonic qubit state, the difference can be magnified. It is shown in detail that this scheme is experimentally feasible under current technology
Classical-Nonclassical Polarity of Gaussian States
Gaussian states with nonclassical properties such as squeezing and
entanglement serve as crucial resources for quantum information processing.
Accurately quantifying these properties within multi-mode Gaussian states has
posed some challenges. To address this, we introduce a unified quantification:
the 'classical-nonclassical polarity', represented by . For a
single mode, a positive value of captures the reduced minimum
quadrature uncertainty below the vacuum noise, while a negative value
represents an enlarged uncertainty due to classical mixtures. For multi-mode
systems, a positive indicates bipartite quantum entanglement. We
show that the sum of the total classical-nonclassical polarity is conserved
under arbitrary linear optical transformations for any two-mode and three-mode
Gaussian states. For any pure multi-mode Gaussian state, the total
classical-nonclassical polarity equals the sum of the mean photon number from
single-mode squeezing and two-mode squeezing. Our results provide a new
perspective on the quantitative relation between single-mode nonclassicality
and entanglement, which may find applications in a unified resource theory of
nonclassical features
The Metrological Power of Nonclassical Single-Mode States
Nonclassical states enable metrology with a precision beyond that possible
with classical physics. Both for practical applications and to understand
non-classicality as a resource, it is useful to know the maximum quantum
advantage that can be provided by a nonclassical state when it is combined with
arbitrary classical states. This advantage has been termed the "metrological
power" of a quantum state. A key open question is whether the metrological
powers for the metrology of different quantities are related, especially
metrology of force (acceleration) and phase shifts (time). Here we answer this
question for all single-mode states, both for local and distributed metrology,
by obtaining complete expressions for the metrological powers for the metrology
of essentialy any quantity (any single-mode unitary transformation), as well as
the linear networks that achieve this maximal precision. This shows that the
metrological powers for all quantities are proportional to a single property of
the state, which for pure states is the quadrature variance, maximized over all
quadratures.Comment: 5 pages main text, 2 figures, 3 pages supplemental material
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