144 research outputs found
Development of algorithms and next-generation sequencing data workflows for the analysis of gene regulatory networks
Word Measures on Wreath Products II
Every word in , the free group of rank , induces a probability
measure (the -measure) on every finite group , by substitution of random
-elements in the letters. This measure is determined by its Fourier
coefficients: the -expectations of the irreducible characters of
. For every finite group , every stable character of
(trace of a finitely generated -module), and every word , we
approximate up to an error term of , where
is the primitivity rank of . This generalizes previous works by Puder,
Hanany, Magee and the author. As an application we show that random Schreier
graphs of representation-stable actions of are close-to-optimal
expanders. The paper reveals a surprising relation between stable
representation theory of wreath products and not-necessarily connected
Stallings core graphs.Comment: 40 pages, 13 figure
Stable Invariants and Their Role in Word Measures on Groups
Every word in a free group induces a word measure -- a probability measure
defined via the word map -- on every compact group. This paper presents a
conjectural picture about the role of a plethora of stable invariants of words
in word measures on groups. These invariants generalize the stable commutator
length and include, among others, two invariants recently defined by Wilton:
the stable primitivity rank and a non-oriented analog of stable commutator
length we call stable square length. The conjectures say, roughly, that these
stable invariants control the asymptotics of the expected values of stable
characters, under word measures. We reinforce these conjectures by proving a
version for word measures on wreath products, and by introducing a related
formula for stable irreducible characters of the symmetric group.Comment: 53 pages, 3 figures, with an appendix by Danielle Ernst-West, Doron
Puder and Matan Seide
Optical Backaction-Evading Measurement of a Mechanical Oscillator
Quantum mechanics imposes a limit on the precision of a continuous position
measurement of a harmonic oscillator, as a result of quantum backaction arising
from quantum fluctuations in the measurement field. A variety of techniques to
surpass this standard quantum limit have been proposed, such as variational
measurements, stroboscopic quantum non-demolition and two tone
backaction-evading (BAE) measurements. The latter proceed by monitoring only
one of the two non-commuting quadratures of the motion. This technique,
originally proposed in the context of gravitational wave detection, has not
been implemented using optical interferometers to date. Here we demonstrate
continuous two-tone backaction-evading measurement in the optical domain of a
localized GHz frequency mechanical mode of a photonic crystal nanobeam
cryogenically and optomechanically cooled in a He buffer gas cryostat close
to the ground state. Employing quantum-limited optical heterodyne detection, we
explicitly show the transition from conventional to backaction-evading
measurement. We observe up to 0.67 dB (14%) reduction of total measurement
noise, thereby demonstrating the viability of BAE measurements for optical
ultrasensitive measurements of motion and force in nanomechanical resonators
Optical backaction-evading measurement of a mechanical oscillator.
Quantum mechanics imposes a limit on the precision of a continuous position measurement of a harmonic oscillator, due to backaction arising from quantum fluctuations in the measurement field. This standard quantum limit can be surpassed by monitoring only one of the two non-commuting quadratures of the motion, known as backaction-evading measurement. This technique has not been implemented using optical interferometers to date. Here we demonstrate, in a cavity optomechanical system operating in the optical domain, a continuous two-tone backaction-evading measurement of a localized gigahertz-frequency mechanical mode of a photonic-crystal nanobeam cryogenically and optomechanically cooled close to the ground state. Employing quantum-limited optical heterodyne detection, we explicitly show the transition from conventional to backaction-evading measurement. We observe up to 0.67 dB (14%) reduction of total measurement noise, thereby demonstrating the viability of backaction-evading measurements in nanomechanical resonators for optical ultrasensitive measurements of motion and force
Dissipative Quantum Feedback in Measurements Using a Parametrically Coupled Microcavity
Micro- and nanoscale optical or microwave cavities are used in a wide range
of classical applications and quantum science experiments, ranging from
precision measurements, laser technologies to quantum control of mechanical
motion. The dissipative photon loss via absorption, present to some extent in
any optical cavity, is known to introduce thermo-optical effects and thereby
impose fundamental limits on precision measurements. Here, we theoretically and
experimentally reveal that such dissipative photon absorption can result in
quantum feedback via in-loop field detection of the absorbed optical field,
leading to the intracavity field fluctuations to be squashed or antisquashed.
Strikingly, this modifies the optical cavity susceptibility in coherent
response measurements and causes excess noise and correlations in incoherent
interferometric optomechanical measurements using a cavity. We experimentally
observe such unanticipated dissipative dynamics in optomechanical spectroscopy
of sideband-cooled optomechanical crystal cavitiess at both cryogenic
temperature (approximately 8 K) and ambient conditions. The dissipative
feedback introduces effective modifications to the optical cavity linewidth and
the optomechanical scattering rate and gives rise to excess imprecision noise
in the interferometric quantum measurement of mechanical motion. Such
dissipative feedback differs fundamentally from a quantum nondemolition
feedback, e.g., optical Kerr squeezing. The dissipative feedback itself always
results in an antisqueezed out-of-loop optical field, while it can enhance the
coexisting Kerr squeezing under certain conditions. Our result has wide-ranging
implications for future dissipation engineering, such as dissipation enhanced
sideband cooling and Kerr squeezing, quantum frequency conversion, and
nonreciprocity in photonic systems
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