SUPREME-QG: Space-borne Ultra-Precise Measurement of the Equivalence Principle Signature of Quantum Gravity

NASA · January 10

4 min read

Preparations for Next Moonwalk Simulations Underway (and Underwater)

Artist concept highlighting the novel approach proposed by the 2025 NIAC awarded selection of SUPREME-QG: Space-borne Ultra-Precise Measurement of the Equivalence Principle Signature of Quantum Gravity

NASA/Selim Shahriar

Selim Shahriar
Northwestern University, Evanston

Progress in physics has largely been driven by the development and verification of new theories that unify different fundamental forces of nature. For example, Maxwell revolutionized physics with his unified theory of electricity and magnetism, and the Standard Model of particle physics provides a consistent description of all fundamental forces (electromagnetic, strong, and weak) except for gravity. The major barrier to completing the quest for unification is that General Relativity (GR), the current theory of gravity, cannot be reconciled with QM. Theories of Quantum Gravity (TQG), which are yet untested, prescribe modifications of both GR and QM in a manner that makes them consistent with each other. Tests of TQG represent arguably the greatest challenge facing our understanding of the Universe. The most promising way to test TQG is to search for violation of the Equivalence Principle (EP), a fundamental tenet of GR which states that all objects experience the same acceleration in a gravitational field. Violation of EP is characterized by a nonzero Eotvos parameter, Eta, defined as the ratio of the relative acceleration to the mean acceleration experienced by two objects with different inertial masses in a gravitational field. EP violations at the level of Eta < 10^(-18) arise in many versions of TQG (e.g., string theory). The most precise test of the EP to date has been carried out under the space-borne MICROSCOPE experiment employing classical accelerometers, constraining the value of Eta to <1.5×10^(-15). We propose to investigate the use of a radically new method that leverages quantum entanglement to test the EP with extreme precision, at the level of Eta ~ 10^(-20), using a space-borne platform. This method is described in a recent paper by us (PRD 108, 024011, ’23). It makes use of simultaneous Schroedinger Cat (SC) state atom interferometers (AIs) with two isotopes of Rb. Consisting of N=10^6 atoms, the SC state, which is a maximally entangled quantum state generated via spin-squeezing of cold atoms in an optical cavity, acts as a single particle, in a superposition of two collective states, enhancing the sensitivity by a factor of ~root(N)=10^3. Such large-N SC states are difficult to create and have not been observed yet, let alone leveraged for precision metrology. In another recent paper, we described a novel protocol, namely the generalized echo squeezing protocol (GESP), to overcome the challenges of creating such a state (PRA 107, 032610, ’23). We will demonstrate the functionality of this method in a testbed to enable a follow-on space-borne mission capable of testing the EP at the level of Eta ~ 10^(-20). If EP violation is observed, the version of TQG that agrees most closely with the result would form the foundation for a complete theory governing the universe, including its birth: the Big Bang. A null result would force physicists to conceive an entirely new approach to addressing the irreconcilability of GR and QM, fundamentally altering the course of theoretical physics. Either outcome would represent one of the greatest developments in our quest for understanding nature. The SC-state AI (SCAI), also holds the promise of revolutionary improvements in the precision of gravitational cartography and inertial navigation, when configured for simultaneous accelerometry and rotation sensing. The sensitivity of such a sensor, for one second averaging time, would be ~0.9 femto-g for accelerometry, and ~0.5 pico-degree/hour for rotation sensing. This would represent an improvement by a factor of ~10^5 over the best conventional accelerometer, and a factor of ~10^4 over the best conventional gyroscopes. As such, the SCAI would find widespread usage in defense as well as non-defense sectors, including deep-space exploration, for inertial navigation. A space-borne SCAI would be able to carry out gravitational cartography with a resolution far greater than that achieved using the GRACE-FO satellites.

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