127 research outputs found
On the connection between quantum nonlocality and phase sensitivity of two-mode entangled Fock state superpositions
In two-mode interferometry, for a given total photon number , entangled
Fock state superpositions of the form have been considered for phase
estimation. Indeed all such states are maximally mode-entangled and violate a
Clauser-Horne-Shimony-Holt (CHSH) inequality. However, they differ in their
optimal phase estimation capabilities as given by their quantum Fisher
informations. The quantum Fisher information is the largest for the
state and
decreases for the other states with decreasing photon number difference between
the two modes. We ask the question whether for any particular Clauser-Horne
(CH) (or CHSH) inequality, the maximal values of the CH (or the CHSH)
functional for the states of the above type follow the same trend as their
quantum Fisher informations, while also violating the classical bound whenever
the states are capable of sub-shot-noise phase estimation, so that the
violation can be used to quantify sub-shot-noise sensitivity. We explore CH and
CHSH inequalities in a homodyne setup. Our results show that the amount of
violation in those nonlocality tests may not be used to quantify sub-shot-noise
sensitivity of the above states.Comment: Published online in Quantum Information Processin
The parity operator in quantum optical metrology
Photon number states are assigned a parity of if their photon number is even
and a parity of if odd. The parity operator, which is minus one to the power of
the photon number operator, is a Hermitian operator and thus a quantum
mechanical observable though it has no classical analog, the concept being
meaningless in the context of classical light waves. In this paper we review
work on the application of the parity operator to the problem of quantum
metrology for the detection of small phase shifts with quantum optical
interferometry using highly entangled field states such as the so-called N00N
states, and states obtained by injecting twin Fock states into a beam splitter.
With such states and with the performance of parity measurements on one of the
output beams of the interferometer, one can breach the standard quantum limit,
or shot-noise limit, of sensitivity down to the Heisenberg limit, the greatest
degree of phase sensitivity allowed by quantum mechanics for linear phase
shifts. Heisenberg limit sensitivities are expected to eventually play an
important role in attempts to detect gravitational waves in interferometric
detection systems such as LIGO and VIRGO.Comment: to be published in Contemporary Physic
Quantum-enhanced measurements without entanglement
Quantum-enhanced measurements exploit quantum mechanical effects for increasing the sensitivity of measurements of certain physical parameters and have great potential for both fundamental science and concrete applications. Most of the research has so far focused on using highly entangled states, which are, however, difficult to produce and to stabilize for a large number of constituents. In the following we review alternative mechanisms, notably the use of more general quantum correlations such as quantum discord, identical particles, or non-trivial Hamiltonians; the estimation of thermodynamical parameters or parameters characterizing non-equilibrium states; and the use of quantum phase transitions. We describe both theoretically achievable enhancements and enhanced sensitivities, not primarily based on entanglement, that have already been demonstrated experimentally, and indicate some possible future research directions
Sub-Planck structures and sensitivity of the superposed photon-added or photon-subtracted squeezed-vacuum states
The Wigner function of the compass state (a superposition of four coherent
states) develops phase-space structures of dimension much less than the Planck
scale, which are crucial in determining the sensitivity of these states to
phase-space displacements. In the present work, we introduce compass-like
states that may have connection to the contemporary experiments, which are
obtained by either adding photons to or subtracting photons from the
superposition of two squeezed-vacuum states. We show that, when a significant
quantity of photons is added (or subtracted), the Wigner function of these
states are shown to have phase-space structures of an area that is
substantially smaller than the Planck scale. In addition, these states exhibit
sensitivity to displacements that is much higher than the standard quantum
limit. Finally, we show that both the size of the sub-Planck structures and the
sensitivity of our states are strongly influenced by the average photon number,
with the photon addition case having a higher average photon number leading to
the smaller sub-Planck structures and, consequently, being more sensitive to
displacement than the photon subtraction case. Our states offer unprecedented
resolution to the external perturbations, making them suitable for quantum
sensing applications.Comment: PHYSICAL REVIEW A 107, 052614 (2023), 15 Figures, 20 page
Entanglement: Quantum or Classical?
From its seemingly non-intuitive and puzzling nature, most evident in
numerous EPR-like gedankenexperiments to its almost ubiquitous presence in
quantum technologies, entanglement is at the heart of modern quantum physics.
First introduced by Erwin Schr\"{o}dinger nearly a century ago, entanglement
has remained one of the most fascinating ideas that came out of quantum
mechanics. Here, we attempt to explain what makes entanglement fundamentally
different from any classical phenomenon. To this end, we start with a
historical overview of entanglement and discuss several hidden variables models
that were conceived to provide a classical explanation and demystify quantum
entanglement. We discuss some inequalities and bounds that are violated by
quantum states thereby falsifying the existence of some of the classical hidden
variables theories. We also discuss some exciting manifestations of
entanglement, such as N00N states and the non-separable single particle states.
We conclude by discussing some contemporary results regarding quantum
correlations and present a future outlook for the research of quantum
entanglement
From Quantum Optics to Quantum Technologies
Quantum optics is the study of the intrinsically quantum properties of light.
During the second part of the 20th century experimental and theoretical
progress developed together; nowadays quantum optics provides a testbed of many
fundamental aspects of quantum mechanics such as coherence and quantum
entanglement. Quantum optics helped trigger, both directly and indirectly, the
birth of quantum technologies, whose aim is to harness non-classical quantum
effects in applications from quantum key distribution to quantum computing.
Quantum light remains at the heart of many of the most promising and
potentially transformative quantum technologies. In this review, we celebrate
the work of Sir Peter Knight and present an overview of the development of
quantum optics and its impact on quantum technologies research. We describe the
core theoretical tools developed to express and study the quantum properties of
light, the key experimental approaches used to control, manipulate and measure
such properties and their application in quantum simulation, and quantum
computing.Comment: 20 pages, 3 figures, Accepted, Prog. Quant. Ele
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