arXiv subjects

Philip C. E. Stamp

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

9 recordsLinked to original sources

arXiv2026

Coherent spectroscopy of collective excitations in superfluid helium far from equilibrium

Ultrafast dynamics of collective excitations in superfluids remains largely unexplored beyond the roton region of the Landau excitation spectrum, despite the importance of such dynamics for understanding nonequilibrium processes in these systems. Here, we employ ultrafast coherent control with sequences of femtosecond pulses to perform spectroscopy of multiple quasiparticles in superfluid helium far from equilibrium. By measuring the time-resolved optical birefringence, we track the nonequilibrium dynamics of quasiparticle pairs associated with rotons, maxons and the Pitaevskii plateau region. The spectral lineshape of the roton peak is explained by an ab initio theoretical analysis of the roton-roton interaction. We also reveal strong energy shifts and short lifetimes of both maxon and Pitaevskii-plateau pairs, as well as an influence of the quasiparticle effective mass on the phase of their coherent response to laser pulses. These results demonstrate the ability to extract previously inaccessible information about collective excitations in a strongly interacting quantum fluid by probing its nonequilibrium dynamics on picosecond and sub-picosecond timescales.

cond-mat.other↗

arXiv2025

The Infra-Red Road to Quantum Gravity

We review work in areas ranging from condensed matter physics to quantum gravity, with the following interconnected questions in mind: (i) what is the nature of the vacuum in condensed matter systems, in quantum field theory, and in classical and quantum gravity; (ii) how do analogies between these systems work, how well do they work, and how useful are they; (iii) what modifications can we make to quantum mechanics to deal with quantum gravity, and (iv) how and why low-energy theories of quantum gravity are, in our view, the right way to make progress in this field. We use many different examples to illustrate our arguments.

cond-mat.quant-gas↗

arXiv2022

Snowmass 2021 White Paper: Tabletop experiments for infrared quantum gravity

Progress in the quantum readout and control of mechanical devices from single atoms to large masses may enable a first generation of experiments probing the gravitational interaction in the quantum regime, conceivably within the next decade. In this Snowmass whitepaper, we briefly outline the possibilities and challenges facing the realization of these experiments. In particular, we emphasize the need for detailed theories of modifications to the usual effective QFT of gravitons in the infrared regime $E/L^3 \ll m_{\rm Pl}/\ell_{\rm Pl}^3$ in which these experiments operate, and relations to possible UV completions.

gr-qc↗

arXiv2018

Tabletop experiments for quantum gravity: a user's manual

Recent advances in cooling, control, and measurement of mechanical systems in the quantum regime have opened the possibility of the first direct observation of quantum gravity, at scales achievable in experiments. This paper gives a broad overview of this idea, using some matter-wave and optomechanical systems to illustrate the predictions of a variety of models of low-energy quantum gravity. We first review the treatment of perturbatively quantized general relativity as an effective quantum field theory, and consider the particular challenges of observing quantum effects in this framework. We then move on to a variety of alternative models, such as those in which gravity is classical, emergent, or responsible for a breakdown of quantum mechanics.

quant-ph↗

arXiv2018

Partitioned Density Matrices and Entanglement Correlators

The density matrix of a non-relativistic quantum system, divided into $N$ sub-systems, is rewritten in terms of the set of all partitioned density matrices for the system. For the case where the different sub-systems are distinguishable, we derive a hierarchy of equations of motion linking the dynamics of all the partitioned density matrices, analogous to the "Schwinger-Dyson" hierarchy in quantum field theory. The special case of a set of $N$ coupled spin-$1/2$ "qubits" is worked out in detail. The equations are then rewritten in terms of a set of "entanglement correlators", which comprise all the possible correlation functions for the system - this case is worked out for coupled spin systems. The equations of motion for these correlators can be written in terms of a first-order differential equation for an entanglement correlator supervector.

quant-ph↗

arXiv2018

Structure of Correlated Worldline Theories of Quantum Gravity

We consider the general form of "Correlated Worldline" (CWL) theories of quantum gravity. We show that one can have 2 different kinds of CWL theory, in which the generating functional is written as either a sum or a product over multiple copies of the coupled matter and gravitational fields. In both versions, the paths in a functional formulation are correlated via gravity itself, causing a breakdown of the superposition principle; however, the product form survives consistency tests not satisfied by the summed form. To better understand the structure of these two theories, we show how to perform diagrammatic expansions in the gravitational coupling for each version of CWL theory, using particle propagation and scalar fields as examples. We explicitly calculate contributions to 2-point and 4-point functions, again for each version of the theory, up to 2nd-order in the gravitational coupling.

gr-qc↗

arXiv2017

Quantum Vortex Dynamics: Results for a 2-d Superfluid

We model vortex dynamics in a 2-dimensional Bose superfluid using the Thompson-Stamp (TS) equations of motion, which describes both the classical Hall-Vinen-Iordanskii (HVI) dynamical regime and the fully developed quantum regime, and the crossover between them. The TS equations can be written in the form of a quantum Langevin equation. Analytic solutions are given for all of these regimes for a single vortex in a 2-dimensional system. In the classical regime we include the vortex inertial and Langevin noise terms, which are dropped in the usual HVI analysis. In the quantum and crossover regimes the effect of memory terms is important, and leads to clear differences from the classical predictions.

cond-mat.mes-hall↗

arXiv2016

The polaron paradigm: a dual coupling effective band model

Non-diagonal couplings to a bosonic bath completely change polaronic dynamics, from the usual diagonally-coupled paradigm of smoothly-varying properties. We study, using analytic and numerical methods, a model having both diagonal Holstein and non-diagonal Su-Schrieffer-Heeger (SSH) couplings. The critical coupling found previously in the pure SSH model, at which the $k=0$ effective mass diverges, now becomes a transition line in the coupling constant plane - the form of the line depends on the adiabaticity parameter. Detailed results are given for the quasiparticle and ground state properties, over a wide range of couplings and adiabaticity ratios. The new paradigm involves a destabilization, at the transition line, of the simple Holstein polaron to one with a finite ground-state momentum, but with everywhere a continuously evolving band shape. No 'self-trapping transition' exists in any of these models. The physics may be understood entirely in terms of competition between different hopping terms in a simple renormalized effective band theory. The possibility of further transitions is suggested by the results.

cond-mat.str-el↗

arXiv2008

Spin-Based Quantum Computers made by Chemistry: Hows and Whys

This introductory review discusses the main problems facing the attempt to build quantum information processing systems (like quantum computers) from spin-based qubits. We emphasize 'bottom-up' attempts using methods from chemistry. The essentials of quantum computing are explained, along with a description of the qubits and their interactions in terms of physical spin qubits. The main problem to be overcome is decoherence - how this works is described, along with ways to suppress contributions from spin bath and oscillator bath environments, and from dipolar interactions. Finally we discuss various strategies for making chemistry-based spin qubits, using both magnetic molecules and magnetic ions.

quant-ph↗