115 research outputs found
Observing and Verifying the Quantum Trajectory of a Mechanical Resonator
Continuous weak measurement allows localizing open quantum systems in state
space, and tracing out their quantum trajectory as they evolve in time.
Efficient quantum measurement schemes have previously enabled recording quantum
trajectories of microwave photon and qubit states. We apply these concepts to a
macroscopic mechanical resonator, and follow the quantum trajectory of its
motional state conditioned on a continuous optical measurement record. Starting
with a thermal mixture, we eventually obtain coherent states of 78%
purity--comparable to a displaced thermal state of occupation 0.14. We
introduce a retrodictive measurement protocol to directly verify state purity
along the trajectory, and furthermore observe state collapse and decoherence.
This opens the door to measurement-based creation of advanced quantum states,
and potential tests of gravitational decoherence models.Comment: 20 pages, 4 figure
Measurement-based quantum control of mechanical motion
Controlling a quantum system based on the observation of its dynamics is
inevitably complicated by the backaction of the measurement process. Efficient
measurements, however, maximize the amount of information gained per
disturbance incurred. Real-time feedback then enables both canceling the
measurement's backaction and controlling the evolution of the quantum state.
While such measurement-based quantum control has been demonstrated in the clean
settings of cavity and circuit quantum electrodynamics, its application to
motional degrees of freedom has remained elusive. Here we show
measurement-based quantum control of the motion of a millimetre-sized membrane
resonator. An optomechanical transducer resolves the zero-point motion of the
soft-clamped resonator in a fraction of its millisecond coherence time, with an
overall measurement efficiency close to unity. We use this position record to
feedback-cool a resonator mode to its quantum ground state (residual thermal
occupation n = 0.29 +- 0.03), 9 dB below the quantum backaction limit of
sideband cooling, and six orders of magnitude below the equilibrium occupation
of its thermal environment. This realizes a long-standing goal in the field,
and adds position and momentum to the degrees of freedom amenable to
measurement-based quantum control, with potential applications in quantum
information processing and gravitational wave detectors.Comment: New version with corrected detection efficiency as determined with a
NIST-calibrated photodiode, added references and revised structure. Main
conclusions are identical. 41 pages, 18 figure
Continuous Force and Displacement Measurement Below the Standard Quantum Limit
Quantum mechanics dictates that the precision of physical measurements must
be subject to certain constraints. In the case of inteferometric displacement
measurements, these restrictions impose a 'standard quantum limit' (SQL), which
optimally balances the precision of a measurement with its unwanted backaction.
To go beyond this limit, one must devise more sophisticated measurement
techniques, which either 'evade' the backaction of the measurement, or achieve
clever cancellation of the unwanted noise at the detector. In the half-century
since the SQL was established, systems ranging from LIGO to ultracold atoms and
nanomechanical devices have pushed displacement measurements towards this
limit, and a variety of sub-SQL techniques have been tested in
proof-of-principle experiments. However, to-date, no experimental system has
successfully demonstrated an interferometric displacement measurement with
sensitivity (including all relevant noise sources: thermal, backaction, and
imprecision) below the SQL. Here, we exploit strong quantum correlations in an
ultracoherent optomechanical system to demonstrate off-resonant force and
displacement sensitivity reaching 1.5dB below the SQL. This achieves an
outstanding goal in mechanical quantum sensing, and further enhances the
prospects of using such devices for state-of-the-art force sensing
applications.Comment: 18 pages, 7 figure
Investigation on the Compressibility Characteristics of Low Mach Number Laminar Flow in Rotating Channel
In high-speed rotating channels, significant compressive effects are
observed, resulting in distinct flow characteristics compared to incompressible
flows. In this study, we employed a finite volume method based on the simple
algorithm to solve for low-speed compressible laminar flow within rotating
channels using an orthogonal uniform grid. The governing equations include the
full Navier-Stokes equations and the energy equation. Contrary to stationary
channel, the alterations in flow within rotating channel are primarily
influenced by the compressive effects of centrifugal force and the
compressibility of fluid within the flow's normal section. The first effect
involves a reduction in the velocity due to centrifugal force, leading to an
increasing influence of the Coriolis force compared to inertial forces along
the flow direction. This trend in axial changes aligns closely with the
increase in rotation speed. The second effect arises from the increase in Mach
number and the Coriolis compression, resulting in slight density differences
within the cross-section. Strong centrifugal forces generate significant
centrifugal additional force (buoyancy force). Consequently, under the same
local rotation number, the velocity profiles of the mainstream experience
considerable changes. Additionally, higher Mach number significantly impact
wall shear stress, with the leading side being notably affected. For instance,
at a cross-sectional Ro = 0.6 and Ma = 0.035, the dimensionless shear stress on
the leading side decreased by 13%. Furthermore, while an increase in Mach
number has minimal impact on the cross-sectional secondary flow structure,
changes in mainstream velocity profiles influence secondary flow intensity,
resulting in an enhanced velocity peak and a shift towards the trailing side
Study of the Effect of a Novel Dimensionless Parameter -- the Centrifugal Work Number(CW), on Spanwise Rotating channel Low-speed Compressible Flow
In the study of similarity of rotating channel flow, we found that the flow
obtained from the enlarged model under the theory of incompressible rotation
similarity (maintaining geometric similarity and the same Reynolds number,
rotation number, Prandtl number and buoyancy number) differs significantly from
the flow inside the original channel. Through theoretical derivation and
dimensional analysis, we discovered another significant parameter -- the
centrifugal work number(CW), which characterizes the ratio of centrifugal work
to gas enthalpy in the rotating channel and is an important parameter for
measuring the compressibility of fluids inside the rotating channel. Moreover,
we have verified the effect of the centrifugal work ratio on the flow state of
the rotating channel using numerical methods, further improving the similarity
theory of rotating channel compressible flow.Comment: 16 pages, 6 figure
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