arXiv subjects

James Q. Quach

Publications and source records attributed to James Q. Quach.

32 records · Page 2Linked to original sources

arXiv2020

Quantum Weak Equivalence Principle and the Gravitational Casimir Effect in Superconductors

We will use Fisher information to properly analyze the quantum weak equivalence principle. We argue that gravitational waves will be partially reflected by superconductors. This will occur as the violation of the weak equivalence principle in Cooper pairs is larger than the surrounding ionic lattice. Such reflections of virtual gravitational waves by superconductors can produce a gravitational Casimir effect, which may be detected using currently available technology.

gr-qc↗

arXiv2020

Fisher information and the weak equivalence principle of a quantum particle in a gravitational wave

We show that the weak equivalence principle (WEP) is violated for a quantum particle in a gravitational wave (GW) background, in the sense that extra mass information can be extracted in the presence of the GW. We quantify the degree of violation with the Fisher information of mass. This provides a precise characterisation of WEP violation by quantum systems in a GW, that should be useful in formalising other works that have argued for such violations heuristically.

gr-qc↗

arXiv2020

Using dark states to charge and stabilise open quantum batteries

We introduce an open quantum battery protocol using dark states to achieve both superextensive capacity and power density, with non-interacting spins coupled to a reservoir. Further, our power density actually scales with the of number of spins $N$ in the battery. We show that the enhanced capacity and power is correlated with entanglement. Whilst connected to the charger, the charged state of the battery is a steady state, stabilized through quantum interference in the open system.

quant-ph↗

arXiv2019

Dual to the anomalous weak value effect of photon-polarisation separation

The quantum Cheshire cat (QCC) thought experiment proposes that a quantum object's property (\textit{e.g} polarisation, spin, etc.) can be separated from its physical body or \textit{disembodied}. This conclusion arose from an argument that interprets a zero weak value (WV) of polarisation as no polarisation. We show that this argument is incomplete in the sense that a zero WV reading could equally be interpreted as linear polarisation. Nevertheless, through a generalisation of the QCC, we complete their argument by excluding the possibility of linear polarisation as a consistent interpretation. We go further, and introduce the dual of the generalised QCC. The dual QCC exhibits an intriguing effect, where a horizontally-polarised interferometer with just one arm, can give rise to interference which is vertically-polarised. The interference appears to arise as the result of the phase difference between the physical arm and a \textit{phantom} arm. This peculiar effect arises from the interplay between the pre-selected and post-selected states, which characterises WVs. The QCC has not yet been unambiguously experimentally demonstrated. The QCC dual offers an alternative pathway to experimental realisation.

quant-ph↗

arXiv2019

Quantum chaos and entanglement in ergodic and non-ergodic systems

We study entanglement entropy (EE) as a signature of quantum chaos in ergodic and non-ergodic systems. In particular we look at the quantum kicked top and kicked rotor as multi-qubit systems, and investigate the single qubit EE which characterizes bipartite entanglement of this qubit with the rest of the system. We study the correspondence of the Kolmogorov-Sinai entropy of the classical kicked systems with the EE of their quantum counterparts. We find that EE is a signature of global chaos in ergodic systems, and local chaos in non-ergodic systems. In particular, we show that EE can be maximised even when systems are highly non-ergodic, when the corresponding classical system is locally chaotic. In contrast, we find evidence that the quantum analogue of Kolmogorov-Arnol'd-Noser (KAM) tori are tori of low entanglement entropy. We conjecture that entanglement should play an important role in any quantum KAM theory.

quant-ph↗

arXiv2017

Which-way double slit experiments and Born rule violation

In which-way double-slit experiments with perfect detectors, it is assumed that having a second detector at the slits is redundant, as it will not change the interference pattern. We however show that if higher-order or non-classical paths are accounted for, the presence of the second detector will have an effect on the interference pattern. Accounting for these non-classical paths also means that the Sorkin parameter in triple-slit experiments is only an approximate measure of Born rule violation. Using the difference between single and double which-way detectors, we give an alternative parameter which is an exact measure of Born rule violation.

quant-ph↗

arXiv2016

Beats and Expansion of Two-Component Bose-Einstein Condensates in the Thomas-Fermi Limit

A unique feature of multi-component BECs is the possibility of beating frequencies in collective oscillations. We analytically determine this beating frequency for the two-component BEC in one-dimension. We also show that the Thomas-Fermi approximation, where the quantum pressure is neglected, describes well the expansion of the two-component condensate released from an harmonic trap.

cond-mat.quant-gas↗

arXiv2016

Spin gravitational resonance and graviton detection

We develop a gravitational analogue of spin magnetic resonance, called spin gravitational resonance, whereby a gravitational wave interacts with a magnetic field to produce a spin transition. In particular, an external magnetic field separates the energy spin states of a spin-1/2 particle, and the presence of the gravitational wave produces a perturbation in the components of the magnetic field orthogonal to the gravitational wave propagation. In this framework we test Dyson's conjecture that individual gravitons cannot be detected. Although we find no fundamental laws preventing single gravitons being detected with spin gravitational resonance, we show that it cannot be used in practice, in support of Dyson's conjecture.

gr-qc↗

arXiv2015

Foldy-Wouthuysen transformation of the generalised Dirac Hamiltonian in a gravitational-wave background

Goncalves et al. derived a non-relativistic limit of the generalised Dirac Hamiltonian in the presence of a gravitational wave, using the exact Foldy-Wouthuysen transformation. This gave rise to the intriguing notion that spin-precession may occur even in the absence of a magnetic field. We argue that this effect is not physical as it is the result of a gauge-variant term that was an artefact of a flawed application of the exact Foldy-Wouthuysen transformation. In this paper we derive the correct non-relativistic limit of the generalised Dirac Hamiltonian in the presence of a gravitational wave, using both the exact and standard Foldy-Wouthuysen transformation. We show that both transformations consistently produce a Hamiltonian where all terms are gauge-invariant. Unfortunately however, we also show that this means the novel spin-precession effect does not exist.

gr-qc↗

arXiv2015

Gravitational Casimir effect

We derive the gravitonic Casimir effect with non-idealised boundary conditions. This allows the quantification of the gravitonic contribution to the Casimir effect from real bodies. We quantify the meagreness of the gravitonic Casimir effect in ordinary matter. We also quantify the enhanced effect produced by the speculated Heisenberg-Couloumb (H-C) effect in superconductors, thereby providing a test for the validity of the H-C theory, and consequently the existence of gravitons.

gr-qc↗

arXiv2013

Disorder Correlation Frequency Controlled Diffusion in the Jaynes-Cummings-Hubbard Model

We investigate time-dependent stochastic disorder in the one-dimensional Jaynes-Cummings-Hubbard model and show that it gives rise to diffusive behaviour. We find that disorder correlation frequency is effective in controlling the level of diffusivity. In the defectless system the mean squared displacement (MSD), which is a measure of the diffusivity, increases with increasing disorder frequency. Contrastingly, when static defects are present the MSD increases with disorder frequency only at lower frequencies; at higher frequencies, increasing disorder frequency actually reduces the MSD.

quant-ph↗

arXiv2013

Transformation optics for cavity array metamaterials

Cavity array metamaterials (CAMs), composed of optical microcavities in a lattice coupled via tight-binding interactions, represent a novel architecture for engineering metamaterials. Since the size of the CAMs' constituent elements are commensurate with the operating wavelength of the device, it cannot directly utilise classical transformation optics in the same way as traditional metamaterials. By directly transforming the internal geometry of the system, and locally tuning the permittivity between cavities, we provide an alternative framework suitable for tight-binding implementations of metamaterials. We develop a CAM-based cloak as the case study.

quant-ph↗

arXiv2012

Domain structures in quantum graphity

Quantum graphity offers the intriguing notion that space emerges in the low energy states of the spatial degrees of freedom of a dynamical lattice. Here we investigate metastable domain structures which are likely to exist in the low energy phase of lattice evolution. Through an annealing process we explore the formation of metastable defects at domain boundaries and the effects of domain structures on the propagation of bosons. We show that these structures should have observable background independent consequences including scattering, double imaging, and gravitational lensing-like effects.

gr-qc↗

arXiv2011

Reconfigurable quantum metamaterials

By coupling controllable quantum systems into larger structures we introduce the concept of a quantum metamaterial. Conventional meta-materials represent one of the most important frontiers in optical design, with applications in diverse fields ranging from medicine to aerospace. Up until now however, metamaterials have themselves been classical structures and interact only with the classical properties of light. Here we describe a class of dynamic metamaterials, based on the quantum properties of coupled atom-cavity arrays, which are intrinsically lossless, reconfigurable, and operate fundamentally at the quantum level. We show how this new class of metamaterial could be used to create a reconfigurable quantum superlens possessing a negative index gradient for single photon imaging. With the inherent features of quantum superposition and entanglement of metamaterial properties, this new class of dynamic quantum metamaterial, opens a new vista for quantum science and technology.

quant-ph↗