14 research outputs found
Quiet SDS Josephson Junctions for Quantum Computing
Unconventional superconductors exhibit an order parameter symmetry lower than
the symmetry of the underlying crystal lattice. Recent phase sensitive
experiments on YBCO single crystals have established the d-wave nature of the
cuprate materials, thus identifying unambiguously the first unconventional
superconductor. The sign change in the order parameter can be exploited to
construct a new type of s-wave - d-wave - s-wave Josephson junction exhibiting
a degenerate ground state and a double-periodic current-phase characteristic.
Here we discuss how to make use of these special junction characteristics in
the construction of a quantum computer. Combining such junctions together with
a usual s-wave link into a SQUID loop we obtain what we call a `quiet' qubit
--- a solid state implementation of a quantum bit which remains optimally
isolated from its environment.Comment: 4 pages, 2 ps-figure
Ipsilateral common iliac artery plus femoral artery clamping for inducing sciatic nerve ischemia/reperfusion injury in rats: a reliable and simple method
The aim of this study was to develop a practical model of sciatic ischemia reperfusion (I/R) injury producing serious neurologic deficits and being technically feasible compared with the current time consuming or ineffective models. Thirty rats were divided into 6 groups (n = 5). Animal were anesthetized by using ketamine (50 mg/kg) and xylazine (4 mg/kg). Experimental groups included a sham-operated group and five I/R groups with different reperfusion time intervals (0 h, 3 h, 1 d, 4 d, 7 d). In I/R groups, the right common iliac artery and the right femoral artery were clamped for 3 hrs. Sham-operated animals underwent only laparotomy without induction of ischemia. Just before euthanasia, behavioral scores (based on gait, grasp, paw position, and pinch sensitivity) were obtained and then sciatic nerves were removed for light-microscopy studies (for ischemic fiber degeneration (IFD) and edema). Behavioral score deteriorated among the ischemic groups compared with the control group (p < 0.01), with maximal behavioral deficit occurring at 4 days of reperfusion. Axonal swelling and IFD were found to happen only after 4 and 7 days, respectively. Our observations led to an easy-to-use but strong enough method for inducing and studying I/R injury in peripheral nerves
Dissipative and Non-dissipative Single-Qubit Channels: Dynamics and Geometry
Single-qubit channels are studied under two broad classes: amplitude damping
channels and generalized depolarizing channels. A canonical derivation of the
Kraus representation of the former, via the Choi isomorphism is presented for
the general case of a system's interaction with a squeezed thermal bath. This
isomorphism is also used to characterize the difference in the geometry and
rank of these channel classes. Under the isomorphism, the degree of decoherence
is quantified according to the mixedness or separability of the Choi matrix.
Whereas the latter channels form a 3-simplex, the former channels do not form a
convex set as seen from an ab initio perspective. Further, where the rank of
generalized depolarizing channels can be any positive integer upto 4, that of
amplitude damping ones is either 2 or 4. Various channel performance parameters
are used to bring out the different influences of temperature and squeezing in
dissipative channels. In particular, a noise range is identified where the
distinguishability of states improves inspite of increasing decoherence due to
environmental squeezing.Comment: 12 pages, 4 figure
Back-action Evading Measurements of Nanomechanical Motion
When performing continuous measurements of position with sensitivity
approaching quantum mechanical limits, one must confront the fundamental
effects of detector back-action. Back-action forces are responsible for the
ultimate limit on continuous position detection, can also be harnessed to cool
the observed structure, and are expected to generate quantum entanglement.
Back-action can also be evaded, allowing measurements with sensitivities that
exceed the standard quantum limit, and potentially allowing for the generation
of quantum squeezed states. We realize a device based on the parametric
coupling between an ultra-low dissipation nanomechanical resonator and a
microwave resonator. Here we demonstrate back-action evading (BAE) detection of
a single quadrature of motion with sensitivity 4 times the quantum zero-point
motion, back-action cooling of the mechanical resonator to n = 12 quanta, and a
parametric mechanical pre-amplification effect which is harnessed to achieve
position resolution a factor 1.3 times quantum zero-point motion.Comment: 19 pages (double-spaced) including 4 figures and reference
On the measurement of a weak classical force coupled to a harmonic oscillator: Experimental progress
Several high-precision physics experiments are approaching a level of sensitivity at which the intrinsic quantum nature of the experimental apparatus is the dominant source of fluctuations limiting the sensitivity of the measurements. This quantum limit is embodied by the Heisenberg uncertainty principle, which prohibits arbitrarily precise simultaneous measurements of two conjugate observables of a system but allows one-time measurements of a single observable with any precision. The dynamical evolution of a system immediately following a measurement limits the class of observables that may be measured repeatedly with arbitrary precision, with the influence of the measurement apparatus on the system being confined strictly to the conjugate observables. Observables having this feature, and the corresponding measurements performed on them, have been named quantum nondemolition or back-action evasion observables. In a previous review (Caves el al., 1980, Rev. Mod. Phys. 52, 341) a quantum-mechanical analysis of quantum nondemolition measurements of a harmonic oscillator was presented. The present review summarizes the experimental progress on quantum nondemolition measurements and the classical models developed to describe and guide the development of practical implementations of quantum nondemolition measurements. The relationship between the classical and quantum theoretical models is also reviewed. The concept of quantum nondemolition and back-action evasion measurements originated in the context of measurements on a macroscopic mechanical harmonic oscillator, though these techniques may be useful in other experimental contexts as well, as is discussed in the last part of this review
MULTIPUMP AND QUASISTROBOSCOPIC BACK-ACTION EVASION MEASUREMENTS FOR RESONANT-BAR GRAVITATIONAL-WAVE ANTENNAS
A generalization of the back-action evasion (BAE) measurement technique, which we call multipump back-action evasion, has been demonstrated with a parametric electromechanical transducer similar to ones which are used on resonant-bar gravitational-wave detectors. The benefit of a BAE measurement is that the fluctuating back-action force of the transducer readout circuit acting on a test mass is squeezed, i.e., reduced in one of the quadrature phases of the test mass, thus improving the sensitivity for the detection of weak forces. The multipump BAE technique affords even greater noise squeezing than the BAE technique. Measured back-action noise squeezing factors are in quantitative agreement with the predictions of the model of a multipump BAE measurement. Using this system we have also demonstrated a quasistroboscopic position measurement when the number of pumps becomes large. The multipump BAE technique may be used to further improve the sensitivity of resonant-bar gravitational-wave antennas
UNCERTAINTY-PRINCIPLE NOISE IN VACUUM-TUNNELING TRANSDUCERS
The fundamental sources of noise in a vacuum-tunneling probe used as an electromechanical transducer to monitor the location of a test mass are examined using a first-quantization formalism. We show that a tunneling transducer enforces the Heisenberg uncertainty principle for the position and momentum of a test mass monitored by the transducer through the presence of two sources of noise: the shot noise of the tunneling current and the momentum fluctuations transferred by the tunneling electrons to the test mass. We analyze a number of cases including symmetric and asymmetric rectangular potential barriers and a barrier in which there is a constant electric field. Practical configurations for reaching the quantum limit in measurements of the position of macroscopic bodies with such a class of transducers are studied