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Mohammad H. Ansari

Publications and source records attributed to Mohammad H. Ansari.

28 records · Page 2Linked to original sources
arXiv2015

Rate of tunneling nonequilibrium quasiparticles in superconducting qubits

In superconducting qubits the lifetime of quantum states cannot be prolonged arbitrarily by decreasing temperature. At low temperature quasiparticles tunneling between electromagnetic environment and superconducting islands takes the condensate state out of equilibrium due to charge imbalance. We obtain the tunneling rate from a phenomenological model of non-equilibrium, where nonequilibrium quasiparticle tunnelling stimulates a temperature-dependent chemical potential shift in superconductor. As a result we obtain a non-monotonic behavior for relaxation rate as function of temperature. Depending on the fabrication parameters for some qubits the lowest tunneling rate of nonequilibrium quasiparticles can take place only near the onset temperature below which nonequilibrium quasiparticles dominate over equilibrium one. Our theory also indicates that such tunnelings can influence the probability of transitions in qubits through a coupling to the zero-point energy of phase fluctuations.

cond-mat.mes-hall↗
arXiv2014

Josephson Interference due to Orbital States in a Nanowire Proximity Effect Junction

The Josephson supercurrent in a Nb-InAs nanowire-Nb junction was studied experimentally. The nanowire goes superconducting due to the proximity effect, and can sustain a phase coherent supercurrent. An unexpected modulation of the junction critical current in an axial magnetic field is observed, which we attribute to a novel form of Josephson interference, due to the multi-band nature of the nanowire. Andreev pairs occupying states of different orbital angular momentum acquire different superconducting phases, producing oscillations of the critical current versus magnetic flux. We develop a semi-classical multi-band model that reproduces the experimental data well. While spin-orbit and Zeeman effects are predicted to produce similar behaviour, the orbital effects are dominant in the device studied here. This interplay between orbital states and magnetic field should be accounted for in the study of multi-band nanowire Josephson junctions, in particular, regarding the search for signatures of topological superconductivity in such devices.

cond-mat.mes-hall↗
arXiv2013

The effect of environmental coupling on tunneling of quasiparticles in Josephson junctions

We study quasiparticle tunneling in Josephson tunnel junctions embedded in an electromagnetic environment. We identify tunneling processes that transfer electrical charge and couple to the environment in a way similar to that of normal electrons, and processes that mix electrons and holes and are thus creating charge superpositions. The latter are sensitive to the phase difference between the superconductor and are thus limited by phase diffusion even at zero temperature. We show that the environmental coupling is suppressed in many environments, thus leading to lower quasiparticle decay rates and thus better superconductor qubit coherence than previously expected. Our approach is nonperturbative in the environmental coupling strength.

cond-mat.mes-hall↗
arXiv2012

A gas of elongated objects; an analytical approach

We calculate a collective number of thermodynamic quantities in a one-dimensional gas of hard elongated objects (such as needles) whose centers mobile on a line. Our formalism uses an approximation for the probabilities of contact between the objects. We show that in moderate pressures the quantities extracted from the noncentral potential do not rely on its noncentrality, instead we can extract them analytically from a central potential. Our formalism reproduces the nontrivial features of a gas of elongated objects. Finally, we show below a crossover pressure $p_o$ the rotational couplings causes quantities proportional to inverse distance (such as density) are on average deviated from the inverse of average distance.

cond-mat.stat-mech↗
arXiv2011

Noise and microresonance of critical current in Josephson junction induced by Kondo trap states

We analyze the impact of trap states in the oxide layer of a superconducting tunnel junctions, on the fluctuation of the Josephson critical current, thus on coherence in superconducting qubits. Two mechanisms are usually considered: the current blockage due to repulsion at the occupied trap states, and the noise from electrons hopping across a trap. We extend previous studies of noninteracting traps to the case where the traps have on-site electron repulsion inside one ballistic channel. The repulsion not only allows the appropriate temperature dependence of 1/f noise, but also is a control to the coupling between the computational qubit and the spurious two-level systems inside the oxide dielectric. We use second order perturbation theory which allows to obtain analytical formulae for the interacting bound states and spectral weights, limited to small and intermediate repulsions. Remarkably, it still reproduces the main features of the model as identified from the Numerical Renormalization Group. We present analytical formulations for the subgap bound state energies, the singlet-doublet phase boundary, and the spectral weights. We show that interactions can reverse the supercurrent across the trap. We finally work out the spectrum of junction resonators for qubits in the presence of on-site repulsive electrons and analyze its dependence on microscopic parameters that may be controlled by fabrication.

cond-mat.mes-hall↗
arXiv2008

Spectroscopy of a canonically quantized horizon

Deviations from Hawking's thermal black hole spectrum, observable for macroscopic black holes, are derived from a model of a quantum horizon in loop quantum gravity. These arise from additional area eigenstates present in quantum surfaces excluded by the classical isolated horizon boundary conditions. The complete spectrum of area unexpectedly exhibits evenly spaced symmetry. This leads to an enhancement of some spectral lines on top of the thermal spectrum. This can imprint characteristic features into the spectra of black hole systems. It most notably gives the signature of quantum gravity observability in radiation from primordial black holes, and makes it possible to test loop quantum gravity with black holes well above Planck scale.

hep-th↗
arXiv2008

Generic degeneracy and entropy in loop quantum gravity

Without imposing the trapping boundary conditions and only from within the very definition of area it is shown that the loop quantization of area manifests an unexpected degeneracy in area eigenvalues. This could lead to a deeper understanding of the microscopic description of a quantum black hole. If a certain number of semi-classically expected properties of black holes are imposed on a quantum surface its entropy coincides with the Bekenstein-Hawking entropy.

gr-qc↗
arXiv2007

Area, ladder symmetry, degeneracy and fluctuations of a horizon

Loop quantum gravity admits a kind of area quantization that is characterized by three quantum numbers. We show the complete spectrum of area is the union of equidistant subsets and a universal reformulation with fewer parameters is possible. Associated with any area there is also another number that determines its degeneracy. One application is that a quantum horizon manifests harmonic modes in vacuum fluctuations. It is discussed the physical fluctuations of a space-time horizon should include all the excluded area eigenvalues, where quantum amplification effect occurs. Due to this effect the uniformity of transition matrix elements between near levels could be assumed. Based on these, a modification to the previous method of analyzing the radiance intensities in hep-th/0607081 is presented that makes the result one step further precise. A few of harmonic modes appear to be extremely amplified on top of the Hawking's radiation. They are expected to form a few brightest lines with the wavelength not larger than the black hole size.

hep-th↗
arXiv2005

Self-organized criticality in quantum gravity

We study a simple model of spin network evolution motivated by the hypothesis that the emergence of classical space-time from a discrete microscopic dynamics may be a self-organized critical process. Self organized critical systems are statistical systems that naturally evolve without fine tuning to critical states in which correlation functions are scale invariant. We study several rules for evolution of frozen spin networks in which the spins labelling the edges evolve on a fixed graph. We find evidence for a set of rules which behaves analogously to sand pile models in which a critical state emerges without fine tuning, in which some correlation functions become scale invariant.

hep-th↗