In Print

A collection of my most cited research papers, and a selection of my books.

My Research

Quantum mechanics with real numbers: entanglement, superselection rules and gauges

We show how imaginary numbers in quantum physics can be eliminated by enlarging the Hilbert Space followed by an imposition of – what effectively amounts to – a superselection rule. We illustrate this procedure with a qubit and apply it to the Mach-Zehnder interferometer. The procedure is somewhat reminiscent of the constrained quantization of the electromagnetic field, where, in order to manifestly comply with relativity, one enlargers the Hilbert Space by quantizing the longitudinal and scalar modes, only to subsequently introduce a constraint to make sure that they are actually not directly observable.

Read in full

Observing ghost entanglement beyond scattering amplitudes in quantum electrodynamics

A fully local quantum account of the interactions experienced between charges requires us to use all the four modes of the electromagnetic vector potential, in the Lorenz gauge. However, it is frequently stated that only the two transverse modes of the vector potential are “real” in that they contain photons that can actually be detected. The photons present in the other two modes, the scalar and the longitudinal, are considered unobservable, and are referred to as “virtual particles” or “ghosts”. Here we argue that this view is erroneous and that even these modes can, in fact, be observed. We present…

Read in full

Temporal witnesses of non-classicality in a macroscopic biological system

Exciton transfer along a polymer is essential for many biological processes, for instance light harvesting in photosynthetic biosystems. Here we apply a new witness of non-classicality to this phenomenon, to conclude that, if an exciton can mediate the coherent quantum evolution of a photon, then the exciton is non-classical. We then propose a general qubit model for the quantum transfer of an exciton along a polymer chain, also discussing the effects of environmental decoherence. The generality of our results makes them ideal candidates to design new tests of quantum features in complex bio-molecules.

Read in full

Probing spectral features of quantum many-body systems with quantum simulators

The efficient probing of spectral features of quantum many-body systems is important for characterising and understanding the structure and dynamics of quantum materials. In this work, we establish a framework for probing the excitation spectrum of quantum many-body systems with quantum simulators. Our approach effectively realises a spectral detector by processing the dynamics of observables with time intervals drawn from a defined probability distribution, which only requires native time evolution governed by the Hamiltonian without any ancilla. The critical element of our method is the engineered emergence of frequency resonance such that the excitation spectrum can be probed. We show…

Read in full

Pionic Entanglement in Femtoscopy: A Lesson in Interference and Indistinguishability

We present an analysis of recent experiments in femtoscopy by the STAR collaboration in terms of the protocol of entanglement witnessing involving purity measurements. The entanglement is between the charge and momentum degrees of freedom of pions and the state purity measurements ultimately rely on the bosonic nature of the detected pions. The pion experiment is intended to measure the size of nuclei and the distance between the nuclei involved, however it indirectly confirms that the states of differently charged pions are entangled through an entanglement witness based on the purity of various pionic states. The entangled state of pions…

Read in full

Quantum Causal Inference with Extremely Light Touch

We consider the quantum version of inferring the causal relation between events. There has been recent progress towards identifying minimal interventions and observations needed. We here show, by means of constructing an explicit scheme, that quantum observations alone are sufficient for quantum causal inference for the case of a bipartite quantum system with measurements at two times. Our scheme involves the derivation of a closed-form expression for the space-time pseudo-density matrix associated with many times and qubits. This matrix can be determined by coarse-grained quantum observations alone. We show that from this matrix one can infer the causal structure via…

Read in full

The Everything-is-a-Quantum-Wave Interpretation of Quantum Physics

In this paper I would like to outline what I think is the most natural interpretation of quantum mechanics. By natural, I simply mean that it requires the least amount of excess baggage and that it is universal in the sense that it can be consistently applied to all the observed phenomena including the universe as a whole. I call it the “Everything is a Quantum Wave” Interpretation (EQWI) because I think this is a more appropriate name than the Many Worlds Interpretation (MWI). The paper explains why this is so.

Read in full

Entanglement in a qubit-qubit-tardigrade system

Quantum and biological systems are seldom discussed together as they seemingly demand opposing conditions. Life is complex, “hot and wet” whereas quantum objects are small, cold and well controlled. Here, we overcome this barrier with a tardigrade — a microscopic multicellular organism known to tolerate extreme physicochemical conditions via a latent state of life known as cryptobiosis. We observe coupling between the animal in cryptobiosis and a superconducting quantum bit and prepare a highly entangled state between this combined system and another qubit. The tomographic data shows entanglement in the qubit-qubit-tardigrade system, with the tardigrade modelled as an ensemble of…

Read in full

Unpredictability is perfectly possible in a deterministic universe

We revisit the vexed question of how unpredictability can arise in a deterministic universe, focusing on unitary quantum theory. We discuss why quantum unpredictability is irrelevant for the possibility of what some people call `free-will’, and why existing `free-will’ arguments are themselves irrelevant to argue for or against a physical theory.

Read in full

Witnessing superpositions of causal orders by weak measurements at a given time

The questions we raise in this letter are as follows: What is the most general representation of a quantum state at a single time? Can we adapt the current representations to the scenarios in which the order of quantum operations are coherently or incoherently superposed? If so, what is the relation between the state at a given time and the uncertainty in the order of events before and after it? By establishing the relationship of two-state vector formalism with pseudo-density operators, we introduce the notion of single-time pseudo-state which can be constructed by either ideal or weak measurements. We show…

Read in full

Interference in quantum field theory: detecting ghosts with phases

We discuss the implications of the principle of locality for interference in quantum field theory. As an example, we consider the interaction of two charges via a mediating quantum field and the resulting interference pattern, in the Lorenz gauge. Using the Heisenberg picture, we propose that detecting relative phases or entanglement between two charges in an interference experiment is equivalent to accessing empirically the gauge degrees of freedom associated with the so-called ghost (scalar) modes of the field in the Lorenz gauge. These results imply that ghost modes are measurable and hence physically relevant, contrary to what is usually thought….

Read in full

A New Look at the C0-formulation of the Strong Cosmic Censorship Conjecture

We examine the C0-formulation of the strong cosmic censorship conjecture (SCC) from a quantum complexity-theoretic perspective and argue that for generic black hole parameters as initial conditions for the Einstein equations, corresponding to the expected geometry of a hyperbolic black hole, the metric is C0-extendable to a larger Lorentzian manifold across the Cauchy horizon. To demonstrate the pathologies associated with a hypothetical validity of the C0 SCC, we prove it violates the “complexity=volume” conjecture for a low-temperature hyperbolic AdSd+1 black hole dual to a CFT living on a (d−1)-dimensional hyperboloid Hd−1, where in order to preserve the gauge/gravity duality we…

Read in full
Visit the full research archive at Arxiv

My Books

Portals to a New Reality: Five Pathways to the Future of Physics
View Book
Modern Foundations Of Quantum Optics
View Book
Introductory Quantum Physics And Relativity
View Book
Solid State Quantum Information
View Book
Introduction to Quantum Information Science
View Book
Decoding Reality
View Book

My Blog

Substack

Flat Qubiters

Flat Earthers are a small but growing community of people who, as the name suggests, believe that the Earth is flat. They claim, despite overwhelming evidence to the contrary, that they can account using flat geometry for all the experiments revealing the Earth’s spherical shape.

Read More
Substack

Macroscopic Quantum Entanglement

The discovery of quantum mechanics in 1925 was followed by a fierce debate about its meaning and implications. And this debate still rages on, but there have been many twists and turns since the early days. This blog is about the twists that I think are about to happen.

Read More
Substack

Superposing Newton and Einstein

Discover how a recent quantum experiment confirmed a century-old theory, blending Einstein’s relativity with quantum mechanics, proving groundbreaking in validating the complex dynamics of particles in gravitational fields.

Read More

ASK ME ANYTHING!

If you'd like to ask me a question or discuss my research then please get in touch.

Get in touch