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## Adjunct Assistant Professor

## Assistant Professor of Physics at Saint Anselm College

My work takes an information-theoretic approach to investigating questions at the intersection of quantum theory and gravitational physics. My research interests include quantum information science, quantum field theory on curved spacetimes, interferometric tests of the equivalence principle, and relational formulations of quantum mechanics.

Wilder 201

HB 6127

- PhD University of Waterloo, Canada (2017)
- PhD Macquarie University, Australia (2017)
- MSc University of Toronto, Canada (2012)
- BSc (Hons) University of Waterloo, Canada (2011)

John Wheeler advocated a __radically conservative__ approach to physics: Insist on adhering to well-established physical laws (be conservative), but follow those laws into their most extreme domains (be radical), where unexpected insights into nature might be found. Our most well-established laws of physics today are encapsulated by quantum theory and general relativity. Taking an information-theoretic approach, my research pushes these theories to their extremes, into an arena in which they must confront one another, in the hopes of uncovering new insights into Nature.

**Quantum Time Dilation**

Einstein's theory of relativity explained what is known as time dilation: Clocks appear to slow down as determined by an observer in relative motion. Time dilation is not common at our everyday human speeds, however, when objects start to move close to the speed of light, time dilation becomes significant. The discovery of this counter-interview phenomenon overthrew Newton's long-held conviction that time was absolute and has since been observed for clocks moving as slow as 10 m/s.

Quantum mechanics allows for objects to be placed into a nonclassical state known as a superposition — a quantum combination of two classical states — so it is natural to examine time dilation when a clock moves in a superposition of different speeds/momenta. My collaborators and I have suggested that a clock moving in a superposition of different speeds experiences quantum corrections to the time dilation it observes, revealing the phenomenon of quantum time dilation. We hope to witness the quantum time dilation effect in the experiment, and in doing so test fundamental physics at the intersection of quantum mechanics and relativity.

__Quantum time dilation in atomic spectra__

P.T. Grochowski, A.R.H. Smith, A. Dragan, and K. Dębski

Physical Review Research 3, 023053 (2021)

__Quantum time dilation: A new test of relativistic quantum theory__

A.R.H. Smith

arXiv:2004.10810 [quant-ph] (2020)

__Quantum clocks observe classical and quantum time dilation__

A.R.H. Smith and M. Ahmadi

Nature Communications 11, 5360 (2020)

**Relational Quantum Physics**

When we describe the configuration of a system, we almost always make use of a classical reference frame; a common instance of this being that we usually specify the speed of a car with respect to the surface of the earth. The same is true in quantum theory. For example, we commonly describe the spin of an electron with respect to the orientation of a large Stern-Gerlach device.

This state of affairs is not fully satisfactory for one notable reason: a quantum system is being described with respect to a classical system, mixing elements from conceptually different frameworks. We must remember that a reference frame is a physical object, and as such it too is subject to the laws of quantum mechanics. This leads to the study of quantum reference frames, which has found practical applications in classical and quantum communication protocols and proven useful in the construction of relational quantum theories inspired by quantum gravity. My contributions in this area have focused on describing quantum reference frames associated with noncompact groups, like those associated with positional reference frames, with the aim of developing a relativistic theory of quantum reference frames.

__Communicating without shared reference frames__

A.R.H. Smith

Phys. Rev. A 99, 052315 (2019)

__Communication between inertial observers with partially correlated reference frames__

M. Ahmadi, A.R.H. Smith, and A. Dragan

Physical Review A 92 (6), 062319 (2015)

__Quantum reference frames associated with noncompact groups: The case of translations and boosts and the role of mass__

A.R.H. Smith, M. Piani, and R.B. Mann

Physical Review A 94 (1), 012333 (2016)

**Relational Quantum Dynamics**

In quantum theory, time enters through its appearance as an external classical parameter in the Schrödinger equation, as opposed to other physical quantities, such as position or momentum, which are associated with self-adjoint operators and treated dynamically. However, the canonical quantization of gravity leads to the Wheeler-DeWitt equation in which this notion of time disappears, constituting one aspect of what is known as the problem of time. The conditional probability interpretation (CPI) of time offers a solution. Built from the kinematical structure of standard quantum theory, the CPI posits that the dynamics of a system of interest should be specified with respect to a physical clock, and that the system's dynamics emerge from correlations between the joint state describing the clock and system. The unitary dynamics described by the Schrödinger equation is only recovered when one makes use of a perfect classical clock. I am interested in understanding the more general quantum dynamics the CPI suggests and its foundational implications.

__The trinity of relational quantum dynamics__

P.A. Höhn, A.R.H. Smith, and M.P.E. Lock

Physicsal Review D 104, 066001(2021)

__Generalized probability rules from a timeless formulation of Wigner's friend scenarios__

V. Baumann, F. Del Santo, A.R.H. Smith, F. Giacomini, E. Castro-Ruiz, and Č. Brukner

Quantum 5, 524 (2021)

__Equivalence of approaches to relational quantum dynamics in relativistic settings__

P.A. Höhn, A.R. H. Smith, and M.P.E. Lock

Frontiers in Physics 9, 181 (2021)

__Quantizing time: Interacting clocks and systems__

A.R.H. Smith and M. Ahmadi

Quantum 3, 160 (2019)

**Quantum Field Theory in Curved Spacetime**

Given the tremendous success of the standard model, our best description of matter at its most fundamental level is given by quantum field theory. However, the standard model is intimately connected to the symmetries of flat space and thus ignores the effects of gravity. This leads to the investigation of quantum fields in curved spacetime — a paradigm in which the quantum nature of fields and the effects of gravity are both important, but gravity itself can be treated classically and described by Einstein's field equations. This paradigm has led to our best clues as to what we should expect from a full-fledged theory of quantum gravity, such as black hole thermodynamics and phenomena occurring in the early universe. My research in this area is focused on operational probes of quantum fields, such as Unruh-DeWitt detectors and more general measurement models, and how these probes can witness the effects of different spacetime structures (e.g., topology and curvature) on quantum field theories.

__Gravitational waves affect vacuum entanglement__

Q. Xu, S. Ali Ahmad, and A.R.H. Smith

Physical Review D 102, 065019 (2020)

__Entangling detectors in anti-de Sitter space__

L.J. Henderson, R.A. Hennigar, R.B. Mann, A.R.H. Smith, and J. Zhang

Journal of High Energy Physics 178 (2019)

__The BTZ black hole exhibits anti-Hawking phenomena__

L.J. Henderson, R.A. Hennigar, R.B. Mann, A.R.H. Smith, and J. Zhang

Physics Letters B 135732 (2019)

__Harvesting entanglement from the black hole vacuum__

L.J. Henderson, R.A. Hennigar, R.B. Mann, A.R.H. Smith, and J. Zhang

Classical and Quantum Gravity 35 (21), 21LT02 (2018)

__Entangling detectors in anti-de Sitter space__

L.J. Henderson, R.A. Hennigar, R.B. Mann, A.R.H. Smith, and J. Zhang

Journal of High Energy Physics 178 (2019)

__Massive Unruh particles cannot be directly observed__

F. Kiałka, A.R.H. Smith, M. Ahmadi, and A. Dragan

Physical Review D 97 (6), 065010 (2018)

__Effect of relativistic acceleration on localized two-mode Gaussian quantum states__

M. Ahmadi, K. Lorek, A. Chęcińska, A.R.H. Smith, R.B. Mann, and A. Dragan

Physical Review D 93 (12), 124031 (2016)

__Spacetime structure and vacuum entanglement__

E. Martín-Martínez, A.R.H. Smith, and D.R. Terno

Physical Review D 93 (4), 044001 (2016)

__Looking inside a black hole__

A.R.H. Smith and R.B. Mann

Classical Quantum Gravity 31, 082001 (2014)

__Persistence of tripartite nonlocality for noninertial observers__

A.R.H. Smith and R.B. Mann

Physical Review A 86 (1), 012306 (2012)

**Satellite Experiments to Test General Relativity**

Recent advances in satellite and quantum technologies have ushered in a new era of experimental physics in space. Quantum states can now be teleported from the surface of the earth to satellites in low earth orbits and it is expected that one day this technology will allow us to establish a worldwide cryptographically secure network based on quantum key distribution. Together with my collaborators, we are interested in how this infrastructure can be used for fundamental science, in particular, the possibility of new experimental tests of general relativity.

__Proposal for an optical test of the Einstein equivalence principle__

D.R. Terno, F. Vedovato, M. Schiavon, A.R.H. Smith, P. Magnani, G. Vallone, and P. Villoresi

arXiv:1811.04835 [gr-qc] (2018)

__Post-Newtonian gravitational effects in optical interferometry__

A. Brodutch, A. Gilchrist, T. Guff, A.R.H. Smith, and D.R. Terno

Physical Review D 91 (6), 064041 (2015)

**Quantizing Time**

June 14th - 18th, 2021

Whatever the final theory of quantum gravity turns out to be, it will need to reconcile the incongruent ways in which time appears in quantum mechanics and general relativity. Quantum mechanics treats time as a classical background parameter, which is different than the way other observables, such as position and momentum, are treated. In stark contrast, general relativity promotes time to a dynamical quantity in the sense that Einstein’s equations relate how clocks behave in relative motion or differing gravitational fields. The aim of this conference is to discuss the full consequences of treating time as a quantum phenomenon in light of the recent progress on information-theoretic and operational descriptions of time as quantum observable. Topics discussed will include indefinite causal structures, the Page-Wootters formalism, relational quantum mechanics, quantum reference frames, the problem of time, and experimental implications.

**Gravity in the Quantum Regime**

June 17th and 18th, 2018

Physicists and philosophers from Austria, Canada, the Netherlands, and the United States, tackling issues at the intersection of quantum theory and gravitational issues, came together for the Gravity in the Quantum Regime conference. Topics discussed included tensor networks on curved spacetimes, gravitational decoherence, semiclassical gravity, and what happens to 'time' in a theory of quantum gravity.

**Quantum Frontiers**

February 8th, 2018

The Quantum Frontiers workshop brought together researchers from Europe and the United States working on a wide variety of areas at the forefront of quantum information science. Topics discussed included open quantum systems, resource theories, quantum metrology, quantum control, and gravitational quantum physics.

**Public Lecture: Quantum Theory versus Common Sense**

February 7th, 2018

Professor Andrzej Dragan from the University of Warsaw spoke about the counterintuitive features of quantum theory.

**Spacetime and Information**

June 17th - 22nd, 2017

The international Spacetime and Information Workshop brought together young researchers from around the world (Canada, USA, Australia, Austria, United Kingdom, and China) working on relativistic quantum information and related areas to discuss recent developments in the field and work on open problems.