Anupam Mazumdar

Canadian Institute for Theoretical Astrophysics (CITA), University of Toronto, ON M5S 3H8, Ontario, Canada
Emails:  anupam.mazumdar@utoronto.ca,    mazumdar@cita.utoronto.ca

I have authored approximately 240 scientific papers spanning particle physics, cosmology, classical and quantum gravity, and quantum information science. These include major review articles published in Physics Reports (2003, 2011), Annual Review of Nuclear and Particle Science (2010), and Reports on Progress in Physics (2019). More recently, I contributed to a 2025 white paper devoted to the experimental protocol for testing the quantum nature of gravity in the laboratory.

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Throughout my career, I have worked across several areas of theoretical physics, including particle physics and cosmology, classical and quantum gravity, and quantum information theory.

 

I regard one of my most significant contributions as the proposal of a tabletop protocol to test whether gravity possesses genuinely quantum degrees of freedom, despite the extraordinary weakness of the gravitational interaction: “Spin Entanglement Witness for Quantum Gravity,” Physical Review Letters 119, 240401 (2017).

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The central idea is to place two mesoscopic masses in spatial quantum superpositions and determine whether their mutual gravitational interaction can generate entanglement. Under appropriate assumptions, observing such entanglement would provide evidence that the mediator responsible for the interaction cannot be described by a classical channel. This proposal initiated what is now commonly referred to as the Quantum Gravity–Induced Entanglement of Masses (QGEM) programme.

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Building on this idea, in 2022 I proposed a quantum analogue of the gravitational light-bending experiment in a quantum-optics setting: “Gravitational Optomechanics: Photon–Matter Entanglement via Graviton Exchange,” arXiv:2209.09273 [gr-qc]. In this protocol, entanglement between light and matter provides a complementary route for probing the quantum character of the gravitational interaction. Together, these proposals aim to connect fundamental questions about spacetime and quantum gravity with experimentally accessible quantum-information observables.

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The illustration below depicts the QGEM concept using two nanodiamonds containing embedded nitrogen-vacancy (NV) centres. Each nanodiamond is prepared in a spatial quantum superposition, and the two masses are brought sufficiently close that their branch-dependent gravitational interaction can generate an observable entangling phase. In a perturbative quantum-gravity description, this interaction may be represented as being mediated by virtual massless spin-2 gravitons. At the same time, the nanodiamonds are subject to electromagnetic interactions arising from virtual-photon exchange, including higher-order multipolar contributions.

 

A central experimental challenge is therefore to distinguish the extremely weak gravitational signal from electromagnetic backgrounds. Electromagnetic interactions can, in principle, be screened or strongly suppressed, whereas gravity cannot be shielded. The experiment consequently becomes a carefully controlled “hide-and-seek” problem: electromagnetic, magnetic, thermal, vibrational, and environmental backgrounds must be systematically mitigated until the gravitationally induced quantum phase can be isolated.

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In the envisioned experimental platform, the nanodiamonds would be levitated above a current-carrying chip and cooled both internally and in their centre-of-mass motion toward the quantum regime. Rotational control, potentially including rapid spinning, could provide gyroscopic stabilisation of the particles. The embedded NV spin would then be used in a Stern–Gerlach-type protocol to create and recombine a spatial superposition of the nanodiamond, enabling closed-loop matter-wave interferometry over an interaction time of approximately 0.1–1s. Achieving this requires exceptional control of the NV-spin coherence, motional dynamics, magnetic-field gradients, and environmental decoherence.

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Operating on characteristic timescales of roughly 1–10 Hz, such a platform would constitute an exceptionally sensitive quantum sensor for acceleration, gravity gradients, magnetic and electric field fluctuations, and other weak forces. Beyond its primary objective of probing the quantum nature of gravity, the same experimental architecture could provide a versatile discovery platform for searches for axions, neutrino-induced effects, fifth forces, and other new weakly coupled interactions.

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Ultimately, the QGEM programme seeks to address a fundamental question experimentally: what does it mean for spacetime and the gravitational field themselves to participate in a quantum superposition? By combining quantum sensing, mesoscopic interferometry, gravity, and quantum information science, these experiments offer a route toward investigating aspects of quantum gravity that were traditionally thought to lie far beyond the reach of laboratory experiments.