Quantum optics and the search for new physics

In 1900, Lord Kelvin gave a celebrated talk entitled "Nineteenth Century Clouds over the Dynamical Theory of Heat and Light" in which he stated with remarkable insight that `The beauty and clearness of the dynamical theory, which asserts heat and light to be modes of motion, is at present obscured by two clouds.' He went on to explain that the first of these was the inability to experimentally detect the luminous ether -- the medium that was thought to be vibrating to create light waves; and the second was the so-called ultraviolet catastrophe of blackbody radiation -- the fact that Maxwell's theory utterly failed to predict the amount of ultraviolet radiation emitted by objects as a function of their temperature. As we know, these two clouds led to two earthshaking revolutions in physics: relativity and quantum mechanics.

The parallel with the current state of physics is actually quite striking. Despite the extraordinary successes of the Standard Model, one might argue that it is not just two clouds that obscure our horizon, but something more like a thick fog. Indeed, it is now well established that we understand only about 5 percent of the Universe. The particles included in the Standard Model and the associated force carriers of the weak, strong and electromagnetic interaction, plus the Higgs boson, properly describe ordinary matter. But they account neither for dark matter, which accounts for approximately 27 percent of the Universe composition, nor for dark energy, which accounts for another 68 percent. And in addition, the Standard Model also fails to unify gravity with the other fundamental interactions.

Shedding light on these issues is arguably the greatest current challenge in fundamental physics. Not surprisingly, this pursuit relies heavily on the use of high-energy particle colliders and astronomical observations, but there is no doubt that quantum optics and Atomic, Molecular and Optical (AMO) physics are in a position to also contribute essential resources and expertise [1]. (This is of course not a new state of affairs: AMO science has a long and distinguished history of doing so and has been at the heart of the development of fields from astronomy to quantum mechanics and to relativity, to mention just three examples.) A series of truly spectacular experiments published in the last few months and involving atom clocks, atom interferometry, and optomechanics illustrate this point beautifully.

Atomic clocks have the potential to provide profound experimental guidance in efforts to unify general relativity and quantum mechanics. They have long served to test the gravitational redshift at distance from 30 centimeters to thousands of kilometers, but extraordinary advances in their precision and accuracy give a taste of even more promising things to come. Very recently, atomic clocks based on a narrow electronic transition in Sr atoms trapped inside an optical lattice have succeeded in measuring frequency gradients consistent with the gravitational redshift within a single millimeter scale sample of ultracold strontium! [2] Once the clock uncertainty, already an astounding 7.6×10^-21, is further improved, one can hope that they will become sensitive to the finite wavefunction of quantum objects oscillating in curved spacetime.

Another promising tool in this general context is atom interferometry. For example, in an experiment just published [3], a beam of cold Rb atoms was split into two wave packets 25 cm apart, with one of them being then subjected to the influence of a large mass, demonstrating that gravity creates Aharonov-Bohm phase shifts analogous to those produced by electromagnetic interactions whereby an electrically charged particle is affected by the electromagnetic potential while in a region where both the electric and magnetic field are zero.

Even more exciting perhaps is the use of cooled, levitated particle sensors as now developed in several groups worldwide: The combination of cavity optomechanical control of levitated systems and free-fall situations may open the way to situations in which quantum systems can act as gravitational source masses. For example, a recent experiment [4] succeeded in measuring the gravitational coupling between two millimeter-sized masses. If future experiments where the source mass is prepared in a quantum state of motion can be shown to result in the two masses becoming quantum-mechanically entangled, then this would imply that gravity must be the culprit and must be inherently quantum, since only a quantum field can induce entanglement [5]. But to realize these experiments researchers will need to figure out how to create and maintain quantum superpositions of relatively massive objects and how to reduce the effects of forces other than gravity. Not an easy task…

In addition, levitated particles and other optomechanical systems offer much promise for the detection of a variety of proposed dark matter particles [6]. What we know for sure is that dark matter is massive, so that its coupling to normal matter can manifest itself in mechanical effects such as displacement, recoil kicks, or acceleration. Monitoring the mechanical motion of levitated particles could therefore give us unprecedented access to these minuscule signals. As discussed in Ref. [4] small source masses may also allow us to test the consequences of speculative scalar fields that have been discussed in the context of dark energy, or to investigate modifications of Newtonian dynamics that have been suggested as an alternative to dark-matter scenarios.

While 95% of the physical Universe remain a profound mystery to us, one could perhaps paraphrase Donald Rumsfeld and note that progress has been made in that at least we now “know what we don’t know.” And there is little doubt that quantum optics and AMO science will continue to significantly help in taking us past that point.

References:

[1] A recent review of AMO and quantum optics contributions to fundamental physics, including tests of general relativity and searches for dark matter and dark energy, can be found in M. S. Safronava, D. Budker, D. DeMille, D. F. Jackson Kimball, A. Derevianko, and C. W. Clark, `Search for new physics with atoms and molecules’, Rev. Mod. Phys. 90, 025008 (2018).

 [2] T. Bothwell, C. J. Kennedy, A. Aeppli, D. Kedar, J. M. Robinson, E. Oelker, A. Staron and J. Ye, `Resolving the gravitational redshift within a millimeter atomic sample., arXiv:2109.12238

[3] C. Overstreet, P. Asenbaum, M. Min, J. Curti, and M. A. Kasevich, `Observation of a gravitational Aharonov-Bohm effect’, Science 375, 226 (2022).

[4] T. Westphal, H. Hepach, J. Pfaff, and M. Aspelmeyer, `Measurement of gravitational coupling between millimetre-sized masses’ Nature 591, 225–228 (2021)

[5] C. Marletto and V. Vedral, `Gravitationally Induced Entanglement between Two Massive Particles is Sufficient Evidence of Quantum Effects in Gravity’, Phys. Rev. Lett. 119, 240402 (2017).

[6] For a general discussion see e.g  D. Carney,G. Krnjaic, D. C. Moore, C. A. Regal, G. Afek,  S. Bhave, B. Brubaker,T. Corbitt, J. Cripe, N. Crisosto, A. Geraci, S. Ghosh, J. G. E. Harris, A. Hook, E. W. Kolb, J. Kunjummen, R. F. Lang, T. Li, T. Lin, Z. Liu, J. Lykken, L. Magrini, J. Manley, N. Matsumoto, A. Monte, F. Monteiro, T. Purdy, C. J. Riedel, R. Singh, S. Singh, K. Sinha, J. M. Taylor, J. Qin, D. J. Wilson, and Y. Zhao, `Mechanical Quantum Sensing in the Search for Dark Matter’, Quant. Sci. Technol. 6, 024002 (2021)