# Alexander R. H. Smith

## Junior Fellow, Society of Fellows and NSERC Postdoctoral Fellow

My work takes an information-theoretic approach to investigating questions at the intersection of quantum theory and gravitational physics. My research interests include quantum field theory in curved spacetime, interferometric tests of the equivalence principle, quantum reference frames and resource theories, and the problem of time in quantum gravity.

As an undergraduate I studied at the University of Waterloo and then moved to the University of Toronto to complete an MSc. I then enrolled in a cotutelle PhD program at the University of Waterloo and Macquarie University, earning a doctorate degree from both institutions. I joined the Dartmouth community as a Junior Fellow in the Society of Fellows in 2017 and was awarded an NSERC Postdoctoral Fellowship to work in the area of relativistic quantum information. During the Winter 2019 term, I designed and taught a new course in the physics and astronomy department at Dartmouth titled “Introductory Mathematical Methods for Physicists”.

## Selected works and activities

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.

**Satellite experiments to test general relativity:**

Recent advances in satellite and quantum technologies have ushered in a new era of experimental physics in outer 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, 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, D.R. Terno, Physical Review D 91 (6), 064041 (2015)

**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 lead 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.

Harvesting entanglement from the black hole vacuum, L.J. Henderson, R.A. Hennigar, R.B. Mann, A.R.H. Smith, 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, J. Zhang arXiv:1809.06862 [quant-ph] (2018)

Massive Unruh particles cannot be directly observed, F. Kiałka, A.R.H. Smith, M. Ahmadi, 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, A. Dragan, Physical Review D 93 (12), 124031 (2016)

Spacetime structure and vacuum entanglement, E. Martín-Martínez, A.R.H. Smith, D.R. Terno, Physical Review D 93 (4), 044001 (2016)

Looking inside a black hole, A.R.H. Smith, R.B. Mann, Classical Quantum Gravity 31, 082001 (2014)

Persistence of tripartite nonlocality for noninertial observers, A.R.H. Smith, R.B. Mann, Physical Review A 86 (1), 012306 (2012)

**Quantum reference frames:**

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 for which the associated group would be the noncompact Poincaré group.

Communicating without shared reference frames, A.R.H. Smith, arXiv:1812.08053 [quant-ph] (2018)

Communication between inertial observers with partially correlated reference frames, M. Ahmadi, A.R.H. Smith, 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, R.B. Mann, Physical Review A 94 (1), 012333 (2016)

**The problem of time:**

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, which constitutes 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 are 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 the ensuing empirical consequences.

Quantizing time: Interacting clocks and systems, A.R.H. Smith, M. Ahmadi, arXiv:1712.00081 [quant-ph] (2017)

## Recent Events

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.

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.

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.