arXiv subjects

P. C. E. Stamp

Publications and source records attributed to P. C. E. Stamp.

At least 19 recordsLinked to original sources

arXiv2025

Vacuum tunneling of vortices in two-dimensional $^4$He superfluid films

At low temperature T we expect vacuum tunneling processes to occur in superfluid $^{4}$ He films. We distinguish between extrinsic processes, in which single vortices nucleate by tunneling off boundaries in the system, and intrinsic processes, in which vortex/anti-vortex pairs nucleate far from boundaries. It is crucial to incorporate the varying effective mass of the vortex in tunneling calculations. The intrinsic processes are the superfluid analogue of the Schwinger mechanism in quantum field theory; here they appear as a quantum phase transition at T = 0, driven by an external supercurrent. We calculate the tunneling rate for these processes, and describe a means of testing the predictions using a specific vortex counting experiment.

cond-mat.quant-gas↗

arXiv2024

A Gallery of Soft Modes: Theory and Experiment at a Ferromagnetic Quantum Phase Transition

We examine the low-energy excitations in the vicinity of the quantum critical point in LiHoF$_4$, a physical realization of the Transverse Field Ising Model, focusing on the long-range fluctuations which soften to zero energy at the ferromagnetic quantum phase transition. Microwave spectroscopy in tunable loop-gap resonator structures identifies and characterizes the soft mode and higher-energy electronuclear states. We study these modes as a function of frequency and magnetic fields applied transverse and parallel to the Ising axis. These are understood in the context of a theoretical model of a soft electronuclear mode that interacts with soft photons as well as soft phonons. We identify competing infrared divergences at the quantum critical point, coming from the photons and the electronuclear soft mode. It is an incomplete cancellation of these divergences that leads to the muted but distinct signatures observed in the experiments. The application of a longitudinal magnetic field gaps the soft mode. Measurements well away from the quantum critical point reveal a set of ``Walker'' modes associated with ferromagnetic domain dynamics.

cond-mat.str-el↗

arXiv2024

Decoherence of a 2-Path System by Infrared Photons

We calculate the decoherence caused by photon emission for a charged particle travelling through an interferometer; the decoherence rate gives a quantitative measure of how much "which-path" quantum information is gained by the electromagnetic field. We isolate the quantum information content of both leading and sub-leading soft photons, and show that it can be extracted entirely from information about the endpoints of the particle's paths. When infrared dressing is used to cure the infrared divergences in the theory, the leading order soft photons then give no contribution to decoherence, and carry no quantum information. The sub-leading soft photons in contrast may carry finite which-path information, and the sub-leading contribution to decoherence takes an extremely simple, time-independent form depending only on the size of the interferometer. An interesting open question is whether or not dressing should also be applied at sub-leading order; we discuss the possibility of answering this question experimentally.

quant-ph↗

arXiv2023

Testing Quantum Gravity using Pulsed Optomechanical Systems

An interesting idea, dating back to Feynman, argues that quantum mechanics may break down for large masses if one entertains the possibility that gravity can be "classical", thereby leading to predictions different from conventional low-energy quantum gravity. Despite the technical difficulty in testing such deviations, a large number of experimental proposals have been put forward due to the high level of fundamental interest. Here, we consider the Schrödinger-Newton (SN) theory and the Correlated Worldline (CWL) theory, and show that they can be distinguished from conventional quantum mechanics, as well as each other, by performing pulsed optomechanics experiments. For CWL specifically we develop a framework resembling the commonly used "Heisenberg-picture" treatment of coupled oscillators, allowing one to perform simple calculations for such systems without delving into the deeper path-integral formalism. We find that discriminating between the theories will be very difficult until experimental control over low frequency quantum optomechanical systems is pushed much further. However, the predicted departures of SN and CWL from quantum mechanics occur at the same scale, so both alternative models could in principle be probed by a single experiment.

quant-ph↗

arXiv2023

Gauge Invariant Propagators and States in Quantum Electrodynamics

We study gauge invariant states in QED, where states are understood in terms of data living on the boundary of gauge invariant path-integrals. This is done for both scalar and spinor QED, and for boundaries that are either time slices, or the boundaries of a 'causal diamond'. We discuss both the case where the gauge field falls off to zero at the boundaries, and the case of 'large gauge transformations', where it remains finite at the boundaries. The dynamics are discussed using the gauge-invariant propagator, describing motion of both the particles and the field between the boundaries. We demonstrate how the path-integral naturally generates a 'Coulomb-field' dressing factor for states living on time-slices, and how this is done without fixing any gauge. We show that the form of the dressing depends only on the nature of the boundaries. We also derive the analogous dressing for states defined on null infinity, showing both its Coulombic parts as well as soft-photon parts.

hep-th↗

arXiv2023

Quantum Barkhausen Noise Induced by Domain Wall Co-Tunneling

Most macroscopic magnetic phenomena (including magnetic hysteresis) are typically understood classically. Here, we examine the dynamics of a uniaxial rare-earth ferromagnet deep within the quantum regime, so that domain wall motion, and the associated hysteresis, is dominated by large-scale quantum tunneling of spins, rather than classical thermal activation over a potential barrier. The domain wall motion is found to exhibit avalanche dynamics, observable as an unusual form of Barkhausen noise. We observe non-critical behavior in the avalanche dynamics that only can be explained by going beyond traditional renormalization group methods or classical domain wall models. We find that this ``quantum Barkhausen noise'' exhibits two distinct mechanisms for domain wall movement, each of which is quantum-mechanical, but with very different dependences on an external magnetic field applied transverse to the spin (Ising) axis. These observations can be understood in terms of the correlated motion of pairs of domain walls, nucleated by co-tunneling of plaquettes (sections of domain wall), with plaquette pairs correlated by dipolar interactions; this correlation is suppressed by the transverse field. Similar macroscopic correlations may be expected to appear in the hysteresis of other systems with long-range interactions.

cond-mat.dis-nn↗

arXiv2022

Ultrafast non-equilibrium dynamics of rotons in superfluid helium

Superfluid 4He, the first superfluid ever discovered, is in some ways the least well understood. Unlike 3He superfluid, or the variety of Bose-Einstein condensates of ultracold gases, superfluid 4He is a very dense liquid of strongly interacting quasiparticles. The theory is then necessarily phenomenological: the quasiparticle properties are found from experiment, and controversies over their description still remain, notably regarding vortex dynamics and the nature of rotons and roton pair creation. It is therefore important to develop new experimental tools for probing the system far from equilibrium. Here we describe a method for locally perturbing the density of superfluid helium through the excitation of roton pairs with ultrashort laser pulses. By measuring the time dependence of this perturbation, we track the non-equilibrium evolution of the two-roton states on a picosecond timescale. Our results reveal an ultrafast cooling of hot roton pairs as they thermalize with the colder gas of other quasiparticles. We anticipate that these findings, as well as future applications of the introduced ultrafast laser technique to different temperature and pressure regimes in bulk liquid 4He, will stimulate further experimental and theoretical investigations towards better understanding of superfluidity.

cond-mat.quant-gas↗

arXiv2022

Propagators in the Correlated Worldline Theory of Quantum Gravity

Starting from a formulation of Correlated Worldline (CWL) theory in terms of functional integrals over paths, we define propagators for particles and matter fields in this theory. We show that the most natural formulation of CWL theory involves a rescaling of the generating functional for the theory; correlation functions then simplify, and all loops containing gravitons disappear from perturbative expansions. The spacetime metric obeys the Einstein equation, sourced by all of the interacting CWL paths. The matter paths are correlated by gravitation, thereby violating quantum mechanics for large masses. We derive exact results for the generating functional and the matter propagator, and for linearized weak field theory. For the example of a two-path experiment, we derive the CWL matter propagator, and show how the results compare with conventional quantum theory and with semiclassical gravity. We also exhibit the structure of low-order perturbation theory for the CWL matter propagator.

gr-qc↗

arXiv2021

Direct Observation of Collective Electronuclear Modes About a Quantum Critical Point

We directly measure the low energy excitation modes of the quantum Ising magnet LiHoF$_4$ using microwave spectroscopy. Instead of a single electronic mode, we find a set of collective electronuclear modes, in which the spin-$1/2$ Ising electronic spins hybridize with the bath of spin-$7/2$ Ho nuclear spins. The lowest-lying electronuclear mode softens at the approach to the quantum critical point, even in the presence of disorder. This softening is rapidly quenched by a longitudinal magnetic field. Similar electronuclear structures should exist in other spin-based quantum Ising systems.

cond-mat.str-el↗

arXiv2020

Correlated Worldline theory: Structure and Consistency

We give a formal treatment of the "Correlated Worldline" theory of quantum gravity. The generating functional is written as a product over multiple copies of the coupled matter and gravitational fields; paths for fields are correlated via gravity itself. In the limit where the gravitational coupling $G \rightarrow 0$, conventional quantum field theory is recovered; in the classical limit $\hbar \rightarrow 0$, General Relativity is recovered. A formal loop expansion is derived, with all terms up to one-loop order $\sim O(l_P^2)$ given explicitly, where $l_P$ is the Planck length. We then derive the form of a perturbation expansion in $l_P^2$ around a background field, with the correlation functions given explicitly up to $\sim O(l_P^2)$. Finally, we explicitly demonstrate the on-shell gauge independence of the theory, to order $l_P^2$ in gravitational coupling and to all orders in matter loops, and derive the relevant Ward identities.

gr-qc↗

arXiv2017

Dynamical Quantum Phase Transitions in presence of a spin bath

We derive an effective time independent Hamiltonian for the transverse Ising model coupled to a spin bath, in the presence of a high frequency AC magnetic field. We show that the spin blocking mechanism that removes the quantum phase transition can be suppressed by the AC field, allowing high tunability of the quantum critical point. Finally, we calculate the phase diagram within the RPA approximation for the case of spin $7/2$ nuclei, appropriate to the $LiHo_{x}Y_{1-x}F_{4}$ system.

cond-mat.str-el↗

arXiv2015

Rationale for a Correlated Worldline Theory of Quantum Gravity

It is argued that gravity should cause a breakdown of quantum mechanics, at low energies, accessible to table-top experiments. It is then shown that one can formulate a theory of quantum gravity in which gravitational correlations exist between worldline or worldsheet paths, for the particle or field of interest. Using a generalized equivalence principle, one can give a unique form for the correlators, yielding a theory with no adjustable parameters. A key feature of the theory is the "bunching" of quantum trajectories caused by the gravitational correlations - this is {\it not} a decoherence or a "collapse" mechanism. This bunching causes a breakdown of the superposition principle for large masses, with a very rapid crossover to classical behaviour at an energy scale which depends on the physical structure of the object. Formal details, and applications of the theory, are kept to a minimum in this paper; but we show how physical quantities can be calculated, and give a detailed discussion of the dynamics of a single particle.

gr-qc↗

arXiv2013

Low temperature universality in disordered solids

The low temperature universal properties in disordered and amorphous solids are considered. We introduce a model that includes two types of two level systems (TLSs), which, based on their local symmetry, interact weakly or strongly with the phonon field. This accounts well for the experimental results, and addresses some long-standing questions: the nature of the TLSs; the smallness and universality of the phonon attenuation, and the energy scale of $3$K below which universality is observed. Our model describes disordered lattices; we also discuss its application to amorphous solids.

cond-mat.dis-nn↗

arXiv2012

Inertial and fluctuational effects on the motion of a Bose superfluid vortex

We study the motion of a vortex under the influence of a harmonic force in an approximately two dimensional trapped Bose-condensed gas. The Hall-Vinen-Iordanskii equations, modified to include a fluctuational force and an inertial mass term, are solved for the vortex motion. The mass of the vortex has a strong influence on the time it takes the vortex to escape the trap. Since the vortex mass also depends on the trap size we have an additional dependence on the trap size in the escape time which we compare to the massless case.

cond-mat.quant-gas↗

arXiv2012

Vortex Dynamics: Quantum versus Classical Regimes

For many years the classical Hall-Vinen-Iordanski (HVI) equation has been used to analyse vortex dynamics in superfluids. Here we discuss the extension of the theory of vortex dynamics to the quantum regime, in which the characteristic vortex frequency is higher than the temperature. At the same time we justify, in the low-frequency classical regime, the use of the HVI equation, provided an inertial mass term and a noise fluctuation term are added to it. The crossover to the quantum regime is discussed, and an intuitive picture is given of the vortex dynamics, which in general is described by 2 equations (one for the vortex coordinate, and one for its quantum fluctuations); we also discuss the simple equation of motion found in the extreme quantum regime.

cond-mat.mes-hall↗

arXiv2012

Environmental Decoherence versus Intrinsic Decoherence

We review the difference between standard environmental decoherence and 'intrinsic decoherence', which is taken to be an ineluctable process of Nature. Environmental decoherence is typically modeled by spin bath or oscillator modes - we review some of the unanswered questions not captured by these models, and also the application of them to experiments. Finally, a sketch is given of a new theoretical approach to intrinsic decoherence, and this scheme is applied to the discussion of gravitational decoherence.

gr-qc↗

arXiv2012

Quantum Dynamics of a Bose Superfluid Vortex

We derive a fully quantum-mechanical equation of motion for a vortex in a 2-dimensional Bose superfluid, in the temperature regime where the normal fluid density $ρ_n(T)$ is small. The coupling between the vortex "zero mode" and the quasiparticles has no term linear in the quasiparticle variables -- the lowest-order coupling is quadratic. We find that as a function of the dimensionless frequency $\tilde Ω= \hbar Ω/k_BT$, the standard Hall-Vinen/Iordanskii equations are valid when $\tilde Ω\ll 1$ (the "classical regime"), but elsewhere, the equations of motion become highly retarded, with significant experimental implications when $\tilde Ω\gtrsim 1$.

cond-mat.mes-hall↗

arXiv2011

Decoherence in Crystals of Quantum Molecular Magnets

Decoherence in Nature has become one of the most pressing problems in physics. Many applications, including quantum information processing, depend on understanding it; and fundamental theories going beyond quantum mechanics have been suggested [1-3], where the breakdown of quantum theory appears as an 'intrinsic decoherence', mimicking environmental decoherence [4]. Such theories cannot be tested until we have a handle on ordinary environmental decoherence processes. Here we show that the theory for insulating electronic spin systems can make accurate predictions for environmental decoherence in molecular-based quantum magnets [5]. Experimental understanding of decoherence in molecular magnets has been limited by short decoherence times, which make coherent spin manipulation extremely difficult [6-9]. Here we reduce the decoherence by applying a strong magnetic field. The theory predicts the contributions to the decoherence from phonons, nuclear spins, and intermolecular dipolar interactions, for a single crystal of the Fe8 molecular magnet. In experiments we find that the decoherence time varies strongly as a function of temperature and magnetic field. The theoretical predictions are fully verified experimentally - there are no other visible decoherence sources. Our investigation suggests that the decoherence time is ultimately limited by nuclear spins, and can be extended up to about 500 microseconds, by optimizing the temperature, magnetic field, and nuclear isotopic concentrations.

cond-mat.mes-hall↗