Alexander R. H. Smith

Selected Works & 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.

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)