arXiv subjects

Jared H. Cole

Publications and source records attributed to Jared H. Cole.

At least 55 records · Page 3Linked to original sources

arXiv2019

Linear response theory of Josephson junction arrays in a microwave cavity

Recent experiments on Josephson junction arrays (JJAs) in microwave cavities have opened up a new avenue for investigating the properties of these devices while minimising the amount of external noise coming from the measurement apparatus itself. These experiments have already shown promise for probing many-body quantum effects in JJAs. In this work, we develop a general theoretical description of such experiments by deriving a quantum phase model for planar JJAs containing quantized vortices. The dynamical susceptibility of this model is calculated for some simple circuits, and signatures of the injection of additional vortices are identified. The effects of decoherence are considered via a Lindblad master equation.

cond-mat.mes-hall↗

arXiv2018

The effects of thermal and correlated noise on magnons in a quantum ferromagnet

The dynamics and thermal equilibrium of spin waves (magnons) in a quantum ferromagnet as well as the macroscopic magnetisation are investigated. Thermal noise due to an interaction with lattice phonons and the effects of spatial correlations in the noise are considered. We first present a Markovian master equation approach with analytical solutions for any homogeneous spatial correlation function of the noise. We find that spatially correlated noise increases the decay rate of magnons with low wave vectors to their thermal equilibrium, which also leads to a faster decay of the ferromagnet's magnetisation to its steady-state value. For long correlation lengths and higher temperature we find that additionally there is a component of the magnetisation which decays very slowly, due to a reduced decay rate of fast magnons. This effect could be useful for fast and noise-protected quantum or classical information transfer and magnonics. We further compare ferromagnetic and antiferromagnetic behaviour in noisy environments and find qualitatively similar behaviour in Ohmic but fundamentally different behaviour in super-Ohmic environments.

quant-ph↗

arXiv2018

Magnetic-field-induced parity effect in insulating Josephson junction chains

We report the experimental manifestation of even-odd parity effects in the transport characteristics of insulating Josephson junction chains which occur as the superconducting gap is suppressed by applied magnetic fields at millikelvin temperatures. The primary signature is a non-monotonic dependence of the critical voltage, $V_c$, for the onset of charge transport through the chain, with the parity crossover indicated by a maximum of $V_c$ at the parity field $B^*$. We also observe a distinctive change in the transport characteristics across the parity transition, indicative of Cooper-pair dominated transport below $B^*$, giving way to single-electron dominated transport above $B^*$. For fields applied in the plane of the superconducting aluminum films, the parity effect is found to occur at the field, $B^*_{||}$, such that the superconducting gap equals the single-electron charging energy, $Δ(B^*_{||})=E_C$. On the contrary, the parity effect for perpendicularly applied fields can occur at relatively lower fields, $B^*_\perp\simeq 2Φ_0/A_I$, depending only on island area, $A_I$. Our results suggest a novel explanation for the insulating peak observed in disordered superconducting films and one-dimensional strips patterned from such films.

cond-mat.mes-hall↗

arXiv2018

Breaking time-reversal symmetry with a superconducting flux capacitor

We present the design of a passive, on-chip microwave circulator based on a ring of superconducting tunnel junctions. We investigate two distinct physical realisations, based on either Josephson junctions (JJ) or quantum phase slip elements (QPS), with microwave ports coupled either capacitively (JJ) or inductively (QPS) to the ring structure. A constant bias applied to the center of the ring provides the symmetry breaking (effective) magnetic field, and no microwave or rf bias is required. We find that this design offers high isolation even when taking into account fabrication imperfections and environmentally induced bias perturbations and find a bandwidth in excess of 500 MHz for realistic device parameters.

quant-ph↗

arXiv2017

Büttiker probes and the Recursive Green's Function; an efficient approach to include dissipation in general configurations

An efficient and compact approach to the inclusion of dissipative effects in Non-Equilibrium Green's Function (NEGF) simulations of electronic systems is introduced. The algorithm is based on two well known methods in the literature, firstly that of the so-called Recursive Green's Function (RGF) and secondly that of Büttiker probes. Numerical methods for exact evaluation of the Jacobian are presented by a direct extension to RGF which can be modularly included in any codebase that uses it presently. Then using both physical observations and numerical methods, the computation time of the Büttiker probe Jacobian is improved significantly. An improvement to existing phonon models within Büttiker probes is then demonstrated in the simulation of fully atomistic graphene nanoribbon based field effect transistors in n-i-n and p-i-n operation.

cond-mat.mes-hall↗

arXiv2017

Approximate solutions to Mathieu's equation

Mathieu's equation has many applications throughout theoretical physics. It is especially important to the theory of Josephson junctions, where it is equivalent to Schrodinger's equation. Mathieu's equation can be easily solved numerically, however there exists no closed-form analytic solution. Here we collect various approximations which appear throughout the physics and mathematics literature and examine their accuracy and regimes of applicability. Particular attention is paid to quantities relevant to the physics of Josephson junctions, but the arguments and notation are kept general so as to be of use to the broader physics community.

quant-ph↗

arXiv2017

Coulomb drag and depinning in bilinear Josephson junction arrays

Coulomb drag and depinning are electronic transport phenomena that occur in low-dimensional nanostructures. Recently, both phenomena have been reported in bilinear Josephson junction arrays. These devices provide a unique opportunity to study the interplay of Coulomb drag and depinning in a system where all relevant parameters can be controlled experimentally. We explain the Coulomb drag and depinning characteristics in the I-V curve of the bilinear Josephson junction array by adopting a quasicharge model which has previously proven useful in describing threshold phenomena in linear Josephson junction arrays. Simulations are performed for a range of coupling strengths, where numerically obtained I-V curves match well with what has been previously observed experimentally. Analytic expressions for the ratio between the active and passive currents are derived from depinning arguments. Novel phenomena are predicted at voltages higher than those for which experimental results have been reported to date.

cond-mat.mes-hall↗

arXiv2016

Signatures of spatially correlated noise and non-secular effects in two-dimensional electronic spectroscopy

We investigate how correlated fluctuations affect oscillatory features in rephasing and non-rephasing two-dimensional (2D) electronic spectra of a model dimer system. Based on a beating map analysis, we show that non-secular environmental couplings induced by uncorrelated fluctuations lead to oscillations centered at both cross- and diagonal-peaks in rephasing spectra as well as in non-rephasing spectra. Using an analytical approach, we provide a quantitative description of the non-secular effects in terms of the Feynman diagrams and show that the environment-induced mixing of different inter-excitonic coherences leads to oscillations in the rephasing diagonal-peaks and non-rephasing cross-peaks. We demonstrate that as correlations in the noise increase, the lifetime of oscillatory 2D signals is enhanced at rephasing cross-peaks and non-rephasing diagonal-peaks, while the other non-secular oscillatory signals are suppressed. We discuss that the asymmetry of 2D lineshapes in the beating map provides information on the degree of correlations in environmental fluctuations. Finally we investigate how the oscillatory features in 2D spectra are affected by inhomogeneous broadening.

quant-ph↗

arXiv2016

Single electron relativistic clock interferometer

Although time is one of the fundamental notions in physics, it does not have a unique description. In quantum theory time is a parameter ordering the succession of the probability amplitudes of a quantum system, while according to relativity theory each system experiences in general a different proper time, depending on the system's world line, due to time to time dilation. It is therefore of fundamental interest to test the notion of time in the regime where both quantum and relativistic effects play a role, for example, when different amplitudes of a single quantum clock experience different magnitudes of time dilation. Here we propose a realization of such an experiment with a single electron in a Penning trap. The clock can be implemented in the electronic spin precession and its time dilation then depends on the radial (cyclotron) state of the electron. We show that coherent manipulation and detection of the electron can be achieved already with present day technology. A single electron in a Penning trap is a technologically ready platform where the notion of time can be probed in a hitherto untested regime, where it requires a relativistic as well as quantum description.

gr-qc↗

arXiv2016

Stimulated emission from NV centres in diamond

Stimulated emission is the process fundamental to laser operation, thereby producing coherent photon output. Despite negatively-charged nitrogen-vacancy (NV$^-$) centres being discussed as a potential laser medium since the 1980's, there have been no definitive observations of stimulated emission from ensembles of NV$^-$ to date. Reasons for this lack of demonstration include the short excited state lifetime and the occurrence of photo-ionisation to the neutral charge state by light around the zero-phonon line. Here we show both theoretical and experimental evidence for stimulated emission from NV$^-$ states using light in the phonon-sidebands. Our system uses a continuous wave pump laser at 532 nm and a pulsed stimulating laser that is swept across the phononic sidebands of the NV$^-$. Optimal stimulated emission is demonstrated in the vicinity of the three-phonon line at 700 nm. Furthermore, we show the transition from stimulated emission to photoionisation as the stimulating laser wavelength is reduced from 700nm to 620 nm. While lasing at the zero-phonon line is suppressed by ionisation, our results open the possibility of diamond lasers based on NV centres, tuneable over the phonon-sideband. This broadens the applications of NV magnetometers from single centre nanoscale sensors to a new generation of ultra-precise ensemble laser sensors, which exploit the contrast and signal amplification of a lasing system.

quant-ph↗

arXiv2016

Charge filling factors in clean and disordered arrays of tunnel junctions

We simulate one-dimensional arrays of tunnel junctions using the kinetic Monte Carlo method to study charge filling behaviour in the large charging energy limit. By applying a small fixed voltage bias and varying the offset voltage, we investigate this behaviour in clean and disordered arrays (both weak and strong disorder effects). The offset voltage dependent modulation of the current is highly sensitive to background charge disorder and exhibits substantial variation depending on the strength of the disorder. We show that while small fractional charge filling factors are likely to be washed out in experimental devices due to strong background charge disorder, larger factors may be observable.

cond-mat.mes-hall↗

arXiv2016

Laser threshold magnetometry

We propose a new type of sensor, which uses diamond containing the optically active nitrogen-vacancy (NV$^-$) centres as a laser medium. The magnetometer can be operated at room-temperature and generates light that can be readily fibre coupled, thereby permitting use in industrial applications and remote sensing. By combining laser pumping with a radio-frequency Rabi-drive field, an external magnetic field changes the fluorescence of the NV$^-$ centres. We use this change in fluorescence level to push the laser above threshold, turning it on with an intensity controlled by the external magnetic field, which provides a coherent amplification of the readout signal with very high contrast. This mechanism is qualitatively different from conventional NV$^-$--based magnetometers which use fluorescence measurements, based on incoherent photon emission. We term our approach laser threshold magnetometry (LTM). We predict that an NV$^-$--based laser threshold magnetometer with a volume of 1mm$^3$ can achieve shot-noise limited d.c.~sensitivity of 1.86 fT$/\sqrt{\rm{Hz}}$ and a.c.~sensitivity of 3.97fT$/\sqrt{\rm{Hz}}$.

quant-ph↗

arXiv2015

Depinning of disordered bosonic chains

We consider one-dimensional bosonic chains with a repulsive boson-boson interaction that decays exponentially on large length-scales. This model describes transport of Cooper-pairs in a Josepshon junction array, or transport of magnetic flux quanta in quantum-phase-slip ladders, i.e. arrays of superconducting wires in a ladder-configuration that allow for the coherent tunnelling of flux quanta. In the low-frequency, long wave-length regime these chains can be mapped to an effective model of a one-dimensional elastic field in a disordered potential. The onset of transport in these systems, when biased by external voltage, is described by the standard depinning theory of elastic media in disordered pinning potentials. We numerically study the regimes that are of relevance for quantum-phase-slip ladders. These are (i) very short chains and (ii) the regime of weak disorder. For chains shorter than the typical pinning length, i.e., the Larkin length, the chains reach a saturation regime where the depinning voltage does not depend on the decay length of the repulsive interaction. In the regime of weak disorder we find an emergent correlation length-scale that depends on the disorder strength. For arrays shorter than this length the onset of transport is similar to the clean arrays, i.e., is due to the penetration of solitons into the array. We discuss the depinning scenarios for longer arrays in this regime.

cond-mat.mes-hall↗

arXiv2015

Constructing ab initio models of ultra-thin Al-AlOx-Al barriers

The microscopic structure of ultra-thin oxide barriers often plays a major role in modern nano-electronic devices. In the case of superconducting electronic circuits, their operation depends on the electrical non-linearity provided by one or more such oxide layers in the form of ultra-thin tunnel barriers (also known as Josephson junctions). Currently available fabrication techniques manufacture an amorphous oxide barrier, which is attributed as a major noise source within the device. The nature of this noise is currently an open question and requires both experimental and theoretical investigation. Here, we present a methodology for constructing atomic scale computational models of Josephson junctions using a combination of molecular mechanics, empirical and ab initio methods. These junctions consist of ultra-thin amorphous aluminium-oxide layers sandwiched between crystalline aluminium. The stability and structure of these barriers as a function of density and stoichiometry are investigated, which we compare to experimentally observed parameters

cond-mat.mtrl-sci↗

arXiv2015

A 3D investigation of delocalised oxygen two-level defects in Josephson junctions

Environmental two-level systems (TLS) have been identified as significant decoherence sources in Josephson junction (JJ) based circuits. For such quantum devices to be functional, the removal or control of the TLS is a necessity. Understanding the microscopic origins of the 'strongly coupled' TLS type is one current path of investigation to that end. The delocalized oxygen model suggests the atomic position of an oxygen atom is spatially delocalized in the oxide forming the JJ barrier. In this report we extend this model from its previous 2+1D construction to a complete 3D description using a Wick-rotated time-dependent Schrodinger equation to solve for time-independent solutions in three dimensions. We compute experimentally observable parameters for phase qubits and compare the results to the 2+1D framework. We devise a Voronoi classification scheme to investigate oxygen atoms delocalizing within strained and non-strained crystalline lattices, as well as realistic atomic positions a JJ amorphous tunnel barrier constructed in previous density functional studies.

cond-mat.mes-hall↗

arXiv2015

Microwave quantum optics as a direct probe of the Overhauser field in a quantum-dot CQED device

We show theoretically that a quantum-dot circuit quantum electrodynamics (CQED) device can be used as a probe of the Overhauser field in quantum dots. By coupling a transmission line to the interdot tunneling gate, an electromagnetically-induced-transparency (EIT) scheme can be established, whose Fano-type interference leads to a sharp curvature in the reflection spectrum around resonance. This sharp feature persists even in the presence of the fluctuating spin bath, rendering a high-resolution method to extract the bath's statistical information. For strong nuclear spin fields, the reflection spectrum exhibits an Autler-Townes splitting, where the peak locations indicate the strengths of the Overhauser field gradient (OFG).

cond-mat.mes-hall↗

arXiv2015

Observation of directly interacting coherent two-level systems in a solid

Parasitic two-level tunneling systems originating from structural material defects affect the functionality of various microfabricated devices by acting as a source of noise. In particular, superconducting quantum bits may be sensitive to even single defects when these reside in the tunnel barrier of the qubit's Josephson junctions, and this can be exploited to observe and manipulate the quantum states of individual tunneling systems. Here, we detect and fully characterize a system of two strongly interacting defects using a novel technique for high-resolution spectroscopy. Mutual defect coupling has been conjectured to explain various anomalies of glasses, and was recently suggested as the origin of low frequency noise in superconducting devices. Our study provides conclusive evidence of defect interactions with full access to the individual constituents, demonstrating the potential of superconducting qubits for studying material defects. All our observations are consistent with the assumption that defects are generated by atomic tunneling.

cond-mat.mes-hall↗

arXiv2015

The parity effect in Josephson junction arrays

We study the parity effect and transport due to quasiparticles in circuits comprised of many superconducting islands. We develop a general approach and show that it is equivalent to previous methods for describing the parity effect in their more limited regimes of validity. As an example we study transport through linear arrays of Josephson junctions in the limit of negligible Josephson energy and observe the emergence of the parity effect with decreasing number of non-equilibrium quasiparticles. Due to the exponential increase in the number of relevant charge states with increasing length, in multi-junction arrays the parity effect manifests in qualitatively different ways to the two junction case. The role of charge disorder is also studied as this hides much of the parity physics which would otherwise be observed. Nonetheless, we see that the current through a multi-junction array at low bias is limited by the formation of meta-stable even-parity states.

cond-mat.supr-con↗