CQE PI Feature – Vivishek Sudhir

Doing quantum mechanics, literally

Featured in QSEC November newsletter 2023

The defining feature of our understanding of the physical universe is the disparity between quantum mechanics — the theory of the small — and general relativity — the theory of gravity at large. And yet, basic issues at the confluence of these two theories remain unanswered. The development of theory is stymied by a dearth of experiments, while experiments have lacked the precision to access phenomena at the interface of these two theories. 

Prof. Vivishek Sudhir, is currently the Class of 1957 Career Development Professor, and directs the Quantum & Precision Measurements Group (precision.mit.edu) whose aim is to close the gap between quantum mechanics and gravity through precision mechanical experiments. 

The gap in knowledge is best exemplified by the question: “Is gravity quantum?” If gravity is classical, then, in a thought experiment where massive quantum objects are interfered, the classical character of gravity would prevent the development of an interference pattern, thus violating quantum mechanics for massive objects. However, several experiments, including those pioneered by Prof. Sudhir over the past few years, have demonstrated that massive objects – ranging in size from the nanogram to the kilogram scale – can be coaxed into quantum states of their motion. 

Breaking this tension requires a new generation of ideas and experiments to directly test whether gravity is quantum or not. In turn, that calls for testable theories, and experiments precise enough to be decisive. The activities of Prof. Sudhir’s group span the full spectrum from developing fundamental theories and testable proposals, to the invention of tools and techniques to advance the state-of-the-art in precision measurements and quantum control, to their demonstration such as in the case where a kilogram-scale mechanical object was prepared in a quantum state of its motion.

Some of the techniques developed in this pursuit have fruitful collateral appeal. In fact, Vivishek’s PhD thesis, which was motivated by the fundamental quest to observe the quantum zero-point motion of a macroscopic object, resulted in the first demonstration of quantum feedback control of mechanical motion, and of quantum-enhanced mechanical force sensing. These techniques proved interesting for LIGO, which is how Vivishek arrived at MIT as a postdoc in late 2017. Since then he has been part of the MIT LIGO group, where his recent work has involved understanding the limitations in precision optical interferometry that arise from thermal fluctuations in crystalline optical materials, and the first correct understanding of mechanical quantum noise in limiting the sensitivity of LIGO and other future interferometric gravitational-wave detectors. Very recently, motivated by experimental issues in the test of gravity’s quantumness, Vivishek’s group got interested in the frequency stability of oscillators. This has led to a fundamental understanding of the quantum limits to the frequency stability of oscillators, and techniques to evade these limits using quantum resources.

Vivishek likes to say that to do beautiful science, one has to be sensitive to beauty in other spheres of activity. Well before science held any meaning, Vivishek was an oil painter. His passion for physics was lit by a chance gift from his high-school English teacher: Stephen Hawking’s A Brief History of Time; Vivishek is still trying to fully understand that book through his research at MIT.

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