arXiv subjects

L. Stodolsky

Publications and source records attributed to L. Stodolsky.

At least 109 records · Page 6Linked to original sources

arXiv2003

Adiabatic evolution of a coupled-qubit Hamiltonian

We present a general method for studying coupled qubits driven by adiabatically changing external parameters. Extended calculations are provided for a two-bit Hamiltonian whose eigenstates can be used as logical states for a quantum CNOT gate. From a numerical analysis of the stationary Schroedinger equation we find a set of parameters suitable for representing CNOT, while from a time-dependent study the conditions for adiabatic evolution are determined. Specializing to a concrete physical system involving SQUIDs, we determine reasonable parameters for experimental purposes. The dissipation for SQUIDs is discussed by fitting experimental data. The low dissipation obtained supports the idea that adiabatic operations could be performed on a time scale shorter than the decoherence time.

cond-mat↗

arXiv2003

Presence On Earth of Non-Identical Particles

We speculate on some novel consequences possibly associated with a change of elementary particle constants with time. These concern the question as to whether particles reaching the earth from distant parts of the universe reflect their properties at their point of origin and so can be different from local particles. Assuming that such particles would be present in very high energy cosmic rays, we estimate their density on the earth by two different methods, finding on this hypothesis a density of roughly $10^{-28}-10^{-29}$. Various possibilities for detection are discussed, including the formation of unusual nuclear isotopes.

hep-ph↗

arXiv2003

A Textured Silicon Calorimetric Light Detector

We apply the standard photovoltaic technique of texturing to reduce the reflectivity of silicon cryogenic calorimetric light detectors. In the case of photons with random incidence angles, absorption is compatible with the increase in surface area. For the geometrically thin detectors studied, energy resolution from athermal phonons, dominated by position dependence, is proportional to the surface-to-volume ratio. With the CaWO4 scintillating crystal used as light source, the time constants of the calorimeter should be adapted to the relatively slow light-emission times.

physics.ins-det↗

arXiv2003

Inversion of the Photon Number Integral

We consider the behavior of the photon number integral under inversion, concentrating on euclidean space. The discussion may be framed in terms of an additive differential $I$ which arises under inversions. The quantity $\int \int I$ is an interesting integral invariant whose value characterizes different configurations under inversion.

hep-th↗

arXiv2002

Defining < A^2 > in the finite volume hamiltonian formalism

It is shown how in principle for non-abelian gauge theories it is possible in the finite volume hamiltonian framework to make sense of calculating the expectation value of ||A||^2=\int d^3x(A^a_i(x))^2. Gauge invariance requires one to replace ||A||^2 by its minimum over the gauge orbit, which makes it a highly non-local quantity. We comment on the difficulty of finding a gauge invariant expression for ||A||^2_{min} analogous to that found for the abelian case, and the relation of this question to Gribov copies. We deal with these issues by implementing the hamiltonian on the so-called fundamental domain, with appropriate boundary conditions in field space, essential to correctly represent the physics of the problem.

hep-th↗

arXiv2002

Design of Adiabatic Logic for a Quantum CNOT Gate

We examine the realization of a quantum CNOT gate by adiabatic operations.The principles of such systems and their analysis are briefly discussed and a model consisting of two weakly coupled double- potential well qubits is studied numerically. Regions of the parameter space with suitable well-defined sets of wavefunctions are found, in which then an adiabatic sweep of an external bias produces the switching behavior of CNOT. Results are presented on the adiabatic condition and the identification with the parameters of a flux-coupled two-SQUID system is given.For typical parameters adiabatic times in the nanosecond regime are obtained.

cond-mat↗

arXiv2002

Lessons of Coherence and Decoherence--From Neutrinos to SQUIDS

We indicate some of the lessons learned from our work on coherence and decoherence in various fields and mention some recent work with sold state devices as elements of the ``quantum computer'', including the realization of simple logic gates controlled by adiabatic processes. We correct a commonly held misconception concerning decoherence for a free particle.

cond-mat↗

arXiv2002

Classical Radiation of a Finite Number of Photons

Under certain conditions the number of photons radiated classically by a charged particle following a prescribed trajectory can be finite. An interesting formula for this number is presented and discussed.

hep-th↗

arXiv2001

Results and plans of the CRESST dark matter search

Data taken by CRESST in 2000 with a cryogenic detector system based on 262 g sapphire crystals is used to place limits on WIMP dark matter in the Galactic Halo. The detector is especially sensitive for low-mass WIMPS with spin-dependent cross sections and improves on existing limits in this region. CRESST is now preparing for a second phase, which will use a 10 kg detector consisting of 300 g CaWO4 crystals with simultaneous detection of phonons and scintillation light to reduce background.

astro-ph↗

arXiv2000

The CRESST Experiment: Recent Results and Prospects

The CRESST experiment seeks hypothetical WIMP particles that could account for the bulk of dark matter in the Universe. The detectors are cryogenic calorimeters in which WIMPs would scatter elastically on nuclei, releasing phonons. The first phase of the experiment has successfully deployed several 262 g sapphire devices in the Gran Sasso underground laboratories. A main source of background has been identified as microscopic mechanical fracturing of the crystals, and has been eliminated, improving the background rate by up to three orders of magnitude at low energies, leaving a rate close to one count per day per kg and per keV above 10 keV recoil energy. This background now appears to be dominated by radioactivity, and future CRESST scintillating calorimeters which simultaneously measure light and phonons will allow rejection of a great part of it.

hep-ex↗

arXiv2000

Adiabatic Inversion in the SQUID, Macroscopic Coherence and Decoherence

A procedure for demonstrating quantum coherence and measuring decoherence times between different fluxoid states of a SQUID by using ``adiabatic inversion'' is discussed. One fluxoid state is smoothly transferred into the other, like a spin reversing direction by following a slowly moving magnetic field. This is accomplished by sweeping an external applied flux, and depends on a well-defined quantum phase between the two macroscopic states. Varying the speed of the sweep relative to the decoherence time permits one to move from the quantum regime, where such a well-defined phase exists, to the classical regime where it is lost and the inversion is inhibited. Thus observing whether inversion has taken place or not as a function of sweep speed offers the possibility of measuring the decoherence time. Estimates with some typical SQUID parameters are presented and it appears that such a procedure should be experimentally possible. The main requirement for the feasibility of the scheme appears to be that the low temperature relaxation time among the quantum levels of the SQUID be long compared to other time scales of the problem, including the readout time. Applications to the ``quantum computer'', with the level system of the SQUID playing the role of the qbit, are briefly examined.

cond-mat↗

arXiv2000

On the Significance of the Quantity "A Squared"

We consider the gauge potential A and argue that the minimum value of the volume integral of A squared (in Euclidean space) may have physical meaning, particularly in connection with the existence of topological structures. A lattice simulation comparing compact and non-compact ``photodynamics'' shows a jump in this quantity at the phase transition, supporting this idea.

hep-ph↗

arXiv2000

Study of Macroscopic Coherence and Decoherence in the SQUID by Adiabatic Inversion

We suggest a procedure for demonstrating quantum coherence and measuring decoherence times between different fluxoid states of a SQUID by using ``adiabatic inversion'', where one macroscopic fluxoid state is smoothly transferred into the other, like a spin reversing direction by following a slowly moving magnetic field. This is accomplished by sweeping an external applied flux, and depends on a well-defined quantum phase between the two macroscopic states. Varying the speed of the sweep relative to the decoherence time permits one to move from the quantum regime, where such a well-defined phase exists, to the classical regime where it is lost and the inversion is inhibited. Thus observing whether inversion has taken place or not as a function of sweep speed offers the possibility of measuring the decoherence time. The main requirement for the feasibility of the scheme appears to be that the low temperature relaxation time among the quantum levels of the SQUID be long compared to other time scales of the problem. Applications to the ``quantum computer'', with the level system of the SQUID playing the role of the qbit, are briefly discussed.

cond-mat↗

arXiv1999

On the Relation Between Quantum Mechanical and Classical Parallel Transport

We explain how the kind of ``parallel transport'' of a wavefunction used in discussing the Berry or Geometrical phase induces the conventional parallel transport of certain real vectors. These real vectors are associated with operators whose commutators yield diagonal operators; or in Lie algebras those operators whose commutators are in the (diagonal) Cartan subalgebra.

quant-ph↗

arXiv1999

Discrimination between Nuclear Recoils and Electron Recoils by Simultaneous Detection of Phonons and Scintillation Light

We have developed a detector, consisting of a cryogenic calorimeter with a scintillating crystal as absorber, and a second calorimeter for the detection of the scintillation light, both operated at 12 mK. Using a CaWO4 crystal with a mass of 6g as scintillating absorber, we have achieved a discrimination of nuclear recoils against electron recoils with a suppression factor of 99.7% at energies above 15 keV. This novel method will be applied for background rejection in the CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) experiment looking for dark matter Weakly Interacting Massive Particles (WIMPs).

physics.ins-det↗

arXiv1999

Decoherence - Fluctuation Relation and Measurement Noise

We discuss fluctuations in the measurement process and how these fluctuations are related to the dissipational parameter characterising quantum damping or decoherence. On the example of the measuring current of the variable-barrier or QPC problem we show there is an extra noise or fluctuation connected with the possible different outcomes of a measurement. This noise has an enhanced short time component which could be interpreted as due to ``telegraph noise'' or ``wavefunction collapses''. Furthermore the parameter giving the the strength of this noise is related to the parameter giving the rate of damping or decoherence.

quant-ph↗

arXiv1998

Measurement Process In a Two-Barrier System

The description of a measuring process, such as that which occurs when a quantum point contact (QPC) detector is influenced by a nearby external electron which can take up two possible positions, provides a interesting application of the method of quantum damping. We find a number of new effects, due to the complete treatment of phases afforded by the formalism, although our results are generally similiar to those of other treatments, particularly to those of Buks et al. These are effects depending on the phase shift in the detector, effects which depend on the direction of the measuring current, and in addition to damping or dissipative effects, an energy shift of the measured system. In particular, the phase shift effect leads to the conclusion that there can be effects of "observation" even when the two barriers in question pass the same current. The nature of the current through the barriers and its statistics is discussed, giving a description of correlations in the current due to "measurement" and of the origin of "telegraphic" signals.

quant-ph↗

arXiv1998

When the Wavepacket is Unnecessary

We point out that many wavepacket discussions for the coherence properties of particle beams are unnecessary since they deal with stationary sources; and when the problem is stationary, essentially all information is in the energy spectrum. This recognition allows a simple answer to a number of long-debated points, usually framed in terms of "the length of the wavepacket". In particular we discuss neutrino oscillations, and some issues in neutron physics. The question as to whether two simple beams with the same energy spectrum are distinguishable is answered negatively for stationary situations. The question as to whether neutrino oscillations should be thought of as taking place between states of the same energy or the same momentum is answered in favor of energy for stationary situations. Consequences for proposals involving the 7Be neutrino line of the sun, the observation of oscillations in supernova neutrinos and wavepacket studies with neutrons are briefly discussed, as well as the connection with the coherence notions of quantum optics.

hep-ph↗