5,869 research outputs found
Atomic-state diagnostics and optimization in cold-atom experiments
We report on the creation, observation and optimization of superposition
states of cold atoms. In our experiments, rubidium atoms are prepared in a
magneto-optical trap and later, after switching off the trapping fields,
Faraday rotation of a weak probe beam is used to characterize atomic states
prepared by application of appropriate light pulses and external magnetic
fields. We discuss the signatures of polarization and alignment of atomic spin
states and identify main factors responsible for deterioration of the atomic
number and their coherence and present means for their optimization, like
relaxation in the dark with the strobe probing. These results may be used for
controlled preparation of cold atom samples and in situ magnetometry of static
and transient fieldsComment: 15 pages and 9 figures (including supplementary information
Pump-Enhanced Continuous-Wave Magnetometry using Nitrogen-Vacancy Ensembles
Ensembles of nitrogen-vacancy centers in diamond are a highly promising
platform for high-sensitivity magnetometry, whose efficacy is often based on
efficiently generating and monitoring magnetic-field dependent infrared
fluorescence. Here we report on an increased sensing efficiency with the use of
a 532-nm resonant confocal cavity and a microwave resonator antenna for
measuring the local magnetic noise density using the intrinsic nitrogen-vacancy
concentration of a chemical-vapor deposited single-crystal diamond. We measure
a near-shot-noise-limited magnetic noise floor of 200 pT/
spanning a bandwidth up to 159 Hz, and an extracted sensitivity of
approximately 3 nT/, with further enhancement limited by the
noise floor of the lock-in amplifier and the laser damage threshold of the
optical components. Exploration of the microwave and optical pump-rate
parameter space demonstrates a linewidth-narrowing regime reached by virtue of
using the optical cavity, allowing an enhanced sensitivity to be achieved,
despite an unoptimized collection efficiency of <2 %, and a low
nitrogen-vacancy concentration of about 0.2 ppb.Comment: 10 pages and 5 figure
Restoring the encoding properties of a stochastic neuron model by an exogenous noise
Here we evaluate the possibility of improving the encoding properties of an impaired neuronal system by superimposing an exogenous noise to an external electric stimulation signal. The approach is based on the use of mathematical neuron models consisting of stochastic HH-like circuit, where the impairment of the endogenous presynaptic inputs is described as a subthreshold injected current and the exogenous stimulation signal is a sinusoidal voltage perturbation across the membrane. Our results indicate that a correlated Gaussian noise, added to the sinusoidal signal can significantly increase the encoding properties of the impaired system, through the Stochastic Resonance (SR) phenomenon. These results suggest that an exogenous noise, suitably tailored, could improve the efficacy of those stimulation techniques used in neuronal systems, where the presynaptic sensory neurons are impaired and have to be artificially bypassed
The dressed atom as binary phase modulator: towards attojoule/edge optical phase-shift keying
Nanophotonic technologies offer great promise for ultra-low power optical
signal processing, but relatively few nonlinear-optical phenomena have yet been
explored as bases for robust digital
modulation/switching~\cite{Yang07,Fara08,Liu10,Noza10}. Here we show that a
single two-level system (TLS) coupled strongly to an optical resonator can
impart binary phase modulation on a saturating probe beam. Our experiment
relies on spontaneous emission to induce occasional transitions between
positive and negative phase shifts---with each such edge corresponding to a
dissipated energy of just one photon ( aJ)---but an optical
control beam could be used to trigger additional phase switching at signalling
rates above this background. Although our ability to demonstrate controlled
switching in our atom-based experiment is limited, we discuss prospects for
exploiting analogous physics in a nanophotonic device incorporating a quantum
dot as the TLS to realize deterministic binary phase modulation with control
power in the aJ/edge regime.Comment: 7 pages, 4 figure
Leptophilic dark matter from gauged lepton number: Phenomenology and gravitational wave signatures
New gauge symmetries often appear in theories beyond the Standard Model. Here
we study a model where lepton number is promoted to a gauge symmetry. Anomaly
cancellation requires the introduction of additional leptons, the lightest of
which is a natural leptophilic dark matter candidate. We perform a
comprehensive study of both collider and dark matter phenomenology. Furthermore
we find that the model exhibits a first order lepton number breaking phase
transition in large regions of parameter space. The corresponding gravitational
wave signal is computed, and its detectability at LISA and other future GW
detectors assessed. Finally we comment on the complementarity of dark matter,
collider and gravitational wave observables, and on the potential reach of
future colliders.Comment: 36 pages + appendix, 24 figures. Version accepted for publication in
JHE
Measurement and control of a mechanical oscillator at its thermal decoherence rate
In real-time quantum feedback protocols, the record of a continuous
measurement is used to stabilize a desired quantum state. Recent years have
seen highly successful applications in a variety of well-isolated
micro-systems, including microwave photons and superconducting qubits. By
contrast, the ability to stabilize the quantum state of a tangibly massive
object, such as a nanomechanical oscillator, remains a difficult challenge: The
main obstacle is environmental decoherence, which places stringent requirements
on the timescale in which the state must be measured. Here we describe a
position sensor that is capable of resolving the zero-point motion of a
solid-state, nanomechanical oscillator in the timescale of its thermal
decoherence, a critical requirement for preparing its ground state using
feedback. The sensor is based on cavity optomechanical coupling, and realizes a
measurement of the oscillator's displacement with an imprecision 40 dB below
that at the standard quantum limit, while maintaining an
imprecision-back-action product within a factor of 5 of the Heisenberg
uncertainty limit. Using the measurement as an error signal and radiation
pressure as an actuator, we demonstrate active feedback cooling (cold-damping)
of the 4.3 MHz oscillator from a cryogenic bath temperature of 4.4 K to an
effective value of 1.10.1 mK, corresponding to a mean phonon number of
5.30.6 (i.e., a ground state probability of 16%). Our results set a new
benchmark for the performance of a linear position sensor, and signal the
emergence of engineered mechanical oscillators as practical subjects for
measurement-based quantum control.Comment: 24 pages, 10 figures; typos corrected in main text and figure
Pulsed excitation dynamics of an optomechanical crystal resonator near its quantum ground-state of motion
Using pulsed optical excitation and read-out along with single phonon
counting techniques, we measure the transient back-action, heating, and damping
dynamics of a nanoscale silicon optomechanical crystal cavity mounted in a
dilution refrigerator at a base temperature of 11mK. In addition to observing a
slow (~740ns) turn-on time for the optical-absorption-induced hot phonon bath,
we measure for the 5.6GHz `breathing' acoustic mode of the cavity an initial
phonon occupancy as low as 0.021 +- 0.007 (mode temperature = 70mK) and an
intrinsic mechanical decay rate of 328 +- 14 Hz (mechanical Q-factor =
1.7x10^7). These measurements demonstrate the feasibility of using short pulsed
measurements for a variety of quantum optomechanical applications despite the
presence of steady-state optical heating.Comment: 16 pages, 6 figure
Composite-pulse magnetometry with a solid-state quantum sensor
The sensitivity of quantum magnetometers is challenged by control errors and,
especially in the solid-state, by their short coherence times. Refocusing
techniques can overcome these limitations and improve the sensitivity to
periodic fields, but they come at the cost of reduced bandwidth and cannot be
applied to sense static (DC) or aperiodic fields. Here we experimentally
demonstrate that continuous driving of the sensor spin by a composite pulse
known as rotary-echo (RE) yields a flexible magnetometry scheme, mitigating
both driving power imperfections and decoherence. A suitable choice of RE
parameters compensates for different scenarios of noise strength and origin.
The method can be applied to nanoscale sensing in variable environments or to
realize noise spectroscopy. In a room-temperature implementation based on a
single electronic spin in diamond, composite-pulse magnetometry provides a
tunable trade-off between sensitivities in the microT/sqrt(Hz) range,
comparable to those obtained with Ramsey spectroscopy, and coherence times
approaching T1
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