7,845 research outputs found
Observation and interpretation of motional sideband asymmetry in a quantum electro-mechanical device
Quantum electro-mechanical systems offer a unique opportunity to probe
quantum noise properties in macroscopic devices, properties which ultimately
stem from the Heisenberg Uncertainty Principle. A simple example of this is
expected to occur in a microwave parametric transducer, where mechanical motion
generates motional sidebands corresponding to the up and down
frequency-conversion of microwave photons. Due to quantum vacuum noise, the
rates of these processes are expected to be unequal. We measure this
fundamental imbalance in a microwave transducer coupled to a radio-frequency
mechanical mode, cooled near the ground state of motion. We also discuss the
subtle origin of this imbalance: depending on the measurement scheme, the
imbalance is most naturally attributed to the quantum fluctuations of either
the mechanical mode or of the electromagnetic field
Covariant Helicity-Coupling Amplitudes: A New Formulation
We have worked out covariant amplitudes for any two-body decay of a resonance
with an arbitrary non-zero mass, which involves arbitrary integer spins in the
initial and the final states. One key new ingredient for this work is the
application of the total intrinsic spin operator which is given
directly in terms of the generators of the Poincar\'e group.
Using the results of this study, we show how to explore the Lorentz factors
which appear naturally, if the momentum-space wave functions are used to form
the covariant decay amplitudes. We have devised a method of constructing our
covariant decay amplitudes, such that they lead to the Zemach amplitudes when
the Lorentz factors are set one
Quantum squeezing of motion in a mechanical resonator
As a result of the quantum, wave-like nature of the physical world, a
harmonic oscillator can never be completely at rest. Even in the quantum ground
state, its position will always have fluctuations, called the zero-point
motion. Although the zero-point fluctuations are unavoidable, they can be
manipulated. In this work, using microwave frequency radiation pressure, we
both prepare a micron-scale mechanical system in a state near the quantum
ground state and then manipulate its thermal fluctuations to produce a
stationary, quadrature-squeezed state. We deduce that the variance of one
motional quadrature is 0.80 times the zero-point level, or 1 dB of
sub-zero-point squeezing. This work is relevant to the quantum engineering of
states of matter at large length scales, the study of decoherence of large
quantum systems, and for the realization of ultra-sensitive sensing of force
and motion
Mechanically Detecting and Avoiding the Quantum Fluctuations of a Microwave Field
During the theoretical investigation of the ultimate sensitivity of
gravitational wave detectors through the 1970's and '80's, it was debated
whether quantum fluctuations of the light field used for detection, also known
as photon shot noise, would ultimately produce a force noise which would
disturb the detector and limit the sensitivity. Carlton Caves famously answered
this question with "They do." With this understanding came ideas how to avoid
this limitation by giving up complete knowledge of the detector's motion. In
these back-action evading (BAE) or quantum non-demolition (QND) schemes, one
manipulates the required quantum measurement back-action by placing it into a
component of the motion which is unobserved and dynamically isolated. Using a
superconducting, electro-mechanical device, we realize a sensitive measurement
of a single motional quadrature with imprecision below the zero-point
fluctuations of motion, detect both the classical and quantum measurement
back-action, and demonstrate BAE avoiding the quantum back-action from the
microwave photons by 9 dB. Further improvements of these techniques are
expected to provide a practical route to manipulate and prepare a squeezed
state of motion with mechanical fluctuations below the quantum zero-point
level, which is of interest both fundamentally and for the detection of very
weak forces
COGNITIVE BEHAVIOURAL THERAPY IN SOUTH AFRICA: COUNSELLORS' EXPERIECES FOLLOWING A TRAINING PROGRAMME
Many South African communities experience high level of violence and other phenomena that potentially provoke symptoms of traumatic stress among residents. Cognitive behaviour therapy (CBT) has been demonstrated to be an effective psychological intervention to ameliorate symptom of trauma, but is seldom practised in South African community mental health settings. In order to determine the barriers to implementing CBT. 12 credentialed community counsellors participated in a two-day training workshop focused on CBT. Counsellor were asked to implement CBT with their clients who presented with ymptom of PTSD. The counsellors were then asked to complete a questionnaire six-months after the training workshop in order to identify the barriers they experienced in implementing the treatment model. The chief barriers that counsellors identified included high workload and limited time, unsuitable clients, client drop out, and an inappropriate match between the counsellor's theoretical paradigm and the CBT model. These result are considered in the context of community mental health care in post-apartheid South Africa
Fracture toughness and crack-resistance curve behavior in metallic glass-matrix composites
Nonlinear-elastic fracture mechanics methods are used to assess the fracture toughness of bulk metallic glass (BMG) composites; results are compared with similar measurements for other monolithic and composite BMG alloys. Mechanistically, plastic shielding gives rise to characteristic resistance-curve behavior where the fracture resistance increases with crack extension. Specifically, confinement of damage by second-phase dendrites is shown to result in enhancement of the toughness by nearly an order of magnitude relative to unreinforced glass
Exploring the Oxygen Order in Hg-1223 and Hg-1201 by 199Hg MAS NMR
We demonstrate the use of a high-resolution solid-state fast (45 kHz) magic
angle spinning (MAS) NMR for mapping the oxygen distribution in Hg-based
cuprate superconductors. We identify observed three peaks in 199Hg spectrum as
belonging to the different chemical environments in the HgO? layer with no
oxygen neighbors, single oxygen neighbor, and two oxygen neighbors. We discuss
observed differences between Hg-1201 and Hg-1223 materials.Comment: 4 pages, 2 figures included. Submitted to NATO Advanced Research
Workshop Proceedings (Miami January 2004
Spin Dynamics in the LTT Phase of ~1/8 Doped Single Crystal La_{1.67}Eu_{0.2}Sr_{0.13}CuO_4
We present La and Cu NMR relaxation measurements in single crystal
La_{1.67}Eu_{0.2}Sr_{0.13}CuO_4. A strong peak in the La spin-lattice
relaxation rate observed in the spin ordered state is well-described by the BPP
mechanism[1] and arises from continuous slowing of electronic spin fluctuations
with decreasing temperature; these spin fluctuations exhibit XY-like anisotropy
in the ordered state. The spin pseudogap is enhanced by the static
charge-stripe order in the LTT phase.Comment: Four pages, three figure
Suppression of Antiferromagnetic Order by Light Hole Doping in La_2Cu_{1-x}Li_xO_4: A ^{139}La NQR Study
^{139}La nuclear quadrupole resonance measurements in lightly doped
La_2Cu_{1-x}Li_xO_4 have been performed to reveal the dependence of the
magnetic properties of the antiferromagnetic CuO_2 planes on the character of
the doped holes and their interactions with the dopant. A detailed study shows
that the magnetic properties are remarkably insensitive to the character of the
dopant impurity. This indicates that the added holes form previously
unrecognized collective structures.Comment: 4 pages, 3 figures. Slightly modified version, as accepted for
publication in Physical Review Letter
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