• Physics 18, 135
A proposed experiment could shed light on the unknown interplay of quantum theory and general relativity.
Quantum theory has been remarkably successful ever since its inception 100 years ago. And yet, there is a glaring mismatch between the discrete, quantum nature of matter and the apparent continuous, classical nature of spacetime, in which matter resides and interacts. This disparity raises profound questions. Does spacetime have indivisible units, or quanta, even though it does not seem to be divisible like matter [1, 2]? And if so, do these quanta have observable signatures, and do they influence other areas of physics? Now Jacob Covey at the University of Illinois Urbana-Champaign and his colleagues have proposed a way to address these questions [3]. Their strategy involves using a widely distributed quantum state to probe the essential features of quantum theory in the curved spacetime of Earth’s gravitational field.
The team’s proposal is relevant to the problem of quantum gravity—that is, how to coherently and logically combine quantum theory and the general theory of relativity [4]. Many researchers consider this problem to be one of the greatest unsolved puzzles in physics (although some still think that gravity should not be quantized and that the whole concept of quantum gravity might be fundamentally misguided [5]). But compared with other thriving areas of quantum theory and its manifold applications, quantum gravity remains an almost entirely theoretical enterprise that is pursued through string theory, loop quantum gravity, and many other approaches [4]. It is thus inherently nonempirical and speculative, constrained only by our current knowledge of quantum theory and general relativity.
In the 1950s, physicists began suggesting experimental probes of quantum gravity that were, at that time, largely impractical. Over the past few years, researchers have started to pursue such ideas more seriously, turning to various powerful contemporary techniques in quantum optics, gravitational interferometry, and multimessenger astronomy. Physicists have considered many possible signatures of quantum gravity, such as gravitationally induced quantum entanglement of masses [6, 7], fluctuating quantum spacetime in gravitational interferometry [8], and intrinsic quantum interference effects of third order or higher in gravitational fields [9]. (Such interference effects are inherently impossible in quantum theory without gravity because of the quadratic nature of the so-called Born rule that governs the computation of quantum probabilities.) It is within this emerging research on quantum gravity that the proposal of Covey and his colleagues belongs.
Quantum theory in curved spacetime can be theoretically understood beyond the limit at which Newtonian physics no longer provides accurate descriptions. Covey and his colleagues aimed to devise an experiment that could, for the first time, offer empirical evidence that quantum theory holds in this extreme regime and probe the fundamental properties of quantum theory in such post-Newtonian curved spacetime. As the team points out, the main practical challenge of such an endeavor is the minuscule difference in spacetime curvature across the typical length scale of quantum effects.
To overcome this challenge, Covey and his colleagues propose constructing a widely distributed quantum state that is sensitive to the post-Newtonian curved spacetime of Earth’s gravitational field. Specifically, they consider delocalizing a single optical atomic clock between three atomic systems that are situated at different elevations separated by kilometer-scale distances (Fig. 1). This delocalization is achieved by encoding the presence or absence of the clock into the state of each system, resulting in the systems sharing a collective, entangled state. The researchers show that this collective state’s properties depend on the differences in so-called proper time and, in turn, in spacetime curvature between the locations of the three atomic systems.
Perhaps most excitingly, Covey and his colleagues discuss how their proposed experiment could probe fundamental facets of quantum theory in curved spacetime. These facets include the theory’s linearity, unitarity, and probabilistic nature (encoded by the Born rule). Such aspects are central to the structure, evolution, and measurement of quantum states. The main novelty of the team’s approach is that it combines several advances made in the past decade on neutral atoms and trapped ions to achieve a new, unique quantum probe of curved spacetime.
What’s next? Implementing the scheme proposed by Covey and his colleagues is currently at the limit of what is experimentally possible [3]. The main difficulty is the inevitable fragility of the required collective, entangled state. Similar challenges are faced by the other, previously mentioned experimental probes of quantum gravity [6–9]. The weakest current test of the underlying principles of quantum theory is probably that of the Born rule, which has been tested poorly in nongravitational situations and not at all in the presence of gravity [3, 9]. That area is where we might expect the greatest surprises for quantum gravity [10]. Whatever the outcomes of such investigations, they will undoubtedly deepen our understanding of quantum theory and thus help the development of quantum technologies.
References
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- J. P. Covey et al., “Probing curved spacetime with a distributed atomic processor clock,” PRX Quantum 6, 030310 (2025).
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