1,379 research outputs found
Optimal synchronization deep in the quantum regime: resource and fundamental limit
We develop an analytical framework to study the synchronization of a quantum
self-sustained oscillator to an external signal. Our unified description allows
us to identify the resource on which quantum synchronization relies, and to
compare quantitatively the synchronization behavior of different limit cycles
and signals. We focus on the most elementary quantum system that is able to
host a self-sustained oscillation, namely a single spin 1. Despite the spin
having no classical analogue, we first show that it can realize the van der Pol
limit cycle deep in the quantum regime, which allows us to provide an
analytical understanding to recently reported numerical results. Moving on to
the equatorial limit cycle, we then reveal the existence of an
interference-based quantum synchronization blockade and extend the classical
Arnold tongue to a snake-like split tongue. Finally, we derive the maximum
synchronization that can be achieved in the spin-1 system, and construct a
limit cycle that reaches this fundamental limit asymptotically.Comment: 15 pages, 9 figures, equivalent to published versio
Unraveling nonclassicality in the optomechanical instability
Conditional dynamics due to continuous optical measurements has successfully
been applied for state reconstruction and feedback cooling in optomechanical
systems. In this article, we show that the same measurement techniques can be
used to unravel nonclassicality in optomechanical limit cycles. In contrast to
unconditional dynamics, our approach gives rise to nonclassical limit cycles
even in the sideband-unresolved regime, where the cavity decay rate exceeds the
mechanical frequency. We predict a significant reduction of the mechanical
amplitude fluctuations for realistic experimental parameters.Comment: 8 pages, 5 figures, equivalent to published versio
Creating photon-number squeezed strong microwave fields by a Cooper-pair injection laser
The use of artificial atoms as an active lasing medium opens a way to
construct novel sources of nonclassical radiation. An example is the creation
of photon-number squeezed light. Here we present a design of a laser consisting
of multiple Cooper-pair transistors coupled to a microwave resonator. Over a
broad range of experimentally realizable parameters, this laser creates
photon-number squeezed microwave radiation, characterized by a Fano factor , at a very high resonator photon number. We investigate the impact of
gate-charge disorder in a Cooper-pair transistor and show that the system can
create squeezed strong microwave fields even in the presence of maximum
disorder.Comment: extended and revised version, equivalent to the published article. 11
pages, 3 figure
When does a one-axis-twist-untwist quantum sensing protocol work?
Spin squeezing can increase the sensitivity of interferometric measurements
of small signals in large spin ensembles beyond the standard quantum limit. In
many practical settings, the ideal metrological gain is limited by imperfect
readout of the sensor. To overcome this issue, protocols based on time reversal
of unitary one-axis-twist (OAT) spin-squeezing dynamics have been proposed.
Such protocols mitigate readout noise and, when implemented using cavity
feedback, have been argued to also be robust against dissipation as long as the
collective cooperativity of the system is sufficiently large [Davis et al., PRL
116, 053601 (2016)]. Here, we perform a careful systematic study of dissipative
effects on three different implementations of a OAT twist-untwist sensing
scheme (based on symmetric as well as asymmetric cavity feedback and on a
Tavis-Cummings interaction). Our full treatment shows that the three approaches
have markedly different properties and resilience when subject to dissipation.
Moreover, the metrological gain for an implementation using symmetric cavity
feedback is more sensitive to undesired dissipation than was previously
appreciated.Comment: 11+7 pages, 5+4 figure
Quantum Synchronization on the IBM Q System
We report the first experimental demonstration of quantum synchronization.
This is achieved by performing a digital simulation of a single spin-
limit-cycle oscillator on the quantum computers of the IBM Q System. Applying
an external signal to the oscillator, we verify typical features of quantum
synchronization and demonstrate an interference-based quantum synchronization
blockade. Our results show that state-of-the-art noisy intermediate-scale
quantum computers are powerful enough to implement realistic dissipative
quantum systems. Finally, we discuss limitations of current quantum hardware
and define requirements necessary to investigate more complex problems.Comment: equivalent to published version, 8 pages, 5 figure
Geometric Phase in Quantum Synchronization
We consider a quantum limit-cycle oscillator implemented in a spin system
whose quantization axis is slowly rotated. Using a kinematic approach to define
geometric phases in nonunitary evolution, we show that the quantum limit-cycle
oscillator attains a geometric phase when the rotation is sufficiently slow. In
the presence of an external signal, the geometric phase as a function of the
signal strength and the detuning between the signal and the natural frequency
of oscillation shows a structure that is strikingly similar to the Arnold
tongue of synchronization. Surprisingly, this structure vanishes together with
the Arnold tongue when the system is in a parameter regime of synchronization
blockade. We derive an analytic expression for the geometric phase of this
system, valid in the limit of slow rotation of the quantization axis and weak
external signal strength, and we provide an intuitive interpretation for this
surprising effect
Squeezed superradiance enables robust entanglement-enhanced metrology even with highly imperfect readout
Quantum metrology protocols using entangled states of large spin ensembles
attempt to achieve measurement sensitivities surpassing the standard quantum
limit (SQL), but in many cases they are severely limited by even small amounts
of technical noise associated with imperfect sensor readout. Amplification
strategies based on time-reversed coherent spin-squeezing dynamics have been
devised to mitigate this issue, but are unfortunately very sensitive to
dissipation, requiring a large single-spin cooperativity to be effective. Here,
we propose a new dissipative protocol that combines amplification and squeezed
fluctuations. It enables the use of entangled spin states for sensing well
beyond the SQL even in the presence of significant readout noise. Further, it
has a strong resilience against undesired single-spin dissipation, requiring
only a large collective cooperativity to be effective.Comment: 6+9 pages, 3+3 figures; equivalent to published version; a reference
to a previously unpublished manuscript has been update
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