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Cavity Optomechanics : Union of Engineering and Quantum Physics

Often in physics, new discoveries are made by improving the sensitivities of measurements, such as the recent example of the gravitational wave detector. One way to improve the sensitivities for the measurement and transduction of physical forces is cavity optomechanics. Cavity-optomechanics is an interdisciplinary area of mechanical engineering, electrical engineering, optics and quantum physics.It emerged as an independent field of its own only very recently, and utilizes the interaction between mechanical motion and light. Recently featured as the ‘milestone of photon history’ in nature photonics, cavity optomechanics is also one of the chosen fields of interest for Dr. Vibhor Singh, Assistant Professor at the Department of Physics, Indian Institute of Science, Bangalore. Dr. Vibhor has worked extensively in nanomechanical systems during his graduate as well as post doctoral career and has recently joined IISc. He is currently setting up an experimental laboratory to explore various nanomechanical and optomechanical systems.

Cavity optomechanics is about utilizing the interactions between light and motion to control and manipulate their quantum states. Light carries momentum and hence can produce radiation pressure force. Each photon, a particle of light, bouncing off a mirror, for example, imparts some momentum to the mirror. Yet, in our everyday experience, we do not observe mirrors moving when reflecting light. This is because the transferred momentum is small compared to the size of the mirror. However, by increasing the light intensity and using lighter mirrors, the effects of the light radiation can be magnified.

Arrangement of mirrors in a particular way can be used to amplify light by letting the photons from the light source bounce back and forth between the mirrors. In such a system, if one of the mirrors is made extremely light (micrometer thickness or lower) and movable, we obtain an “optomechanical resonator” where the radiation forces of the reflected photons cause mechanical movement of the mirror. A feedback mechanism kicks into action since a change in the mirror position affects the length of the cavity which in turn changes the intensity of light inside it. Thus, the radiation pressure force on the mirror is dependent on its position, leading to a coupling between the optical and mechanical modes. There are several different implementations of such interaction with variations in the size and material of the movable mirror, its placement and the frequency of light used.

So, what are these optomechanical systems good for? As it turns out, there are quite a variety of tricks to be played with this system.

First off is the one involving the infamous Schrödinger’s cat, the popular face of quantum theory. The Schrödinger’s cat refers to the postulate of quantum theory that quantum states can exist in a superposition of various states at the same time. But, such quantum superpositions are observable only in microscopic systems isolated from outside interference since interactions with the environment destroy the superposition. For example, in real life, we do not see a cat being simultaneously dead and alive, like the hypothetical Schrodinger’s cat is capable of. Objects seem to lose their ‘quantumness’ once we are in the macroscopic realm. The mechanism of this decay of quantum states into normal classical states, termed decoherence, is of interest from a fundamental physics point of view. This is where optomechanical resonators step in by presenting us with quantum control over the motion of macroscopic objects (the mirror) via the optical field, thus enabling researchers to prepare superpositions of macroscopic mechanical states. Dr. Vibhor’s lab aims to take advantage of this capability to carry out fundamental tests of quantum theory. These experiments will be carried out at very low temperatures to remove thermal as well as Brownian motion of mechanical systems thus enabling the study of purely quantum mechanical effects.

Dr. Vibhor is also interested in pursuing the potential applications of optomechanical resonators in quantum information technology. Optomechanical resonators act as efficient transducers – devices effecting interconversion between different types of signals – because of the versatility of both the light field and the nanomechanical oscillations to couple to a variety of systems. Such transducers are required in today’s ‘hybrid circuits’ which aim to integrate physical systems such as light and sound along with electronic components in circuits, and specifically find applications in quantum computing.

Quantum computing refers to computing carried out by exploiting quantum mechanical effects.In the hunt for physical systems that can be used as qubits or quantum bits (which are the building blocks of quantum computing just as the two state bits are the basis of classical computing), superconducting circuits have emerged as a worthy competitor. Due to inherent mechanical compliance, optomechanical systems have the potential to act as a transducer to convert quantum information from a superconducting qubit to the optical photons which are good information careers over long distances. Just as optical cables connect the nodes of today’s networks, such interfaces are necessary in connecting the quantum nodes of a future quantum internet.

In charting the future course for his laboratory, Dr. Singh envisions a lab exploring the field of cavity optomechanics from both a fundamental physics as well as an application point of view, especially looking at various implementations in quantum information technology.


About the scientist:

Dr. Vibhor Singh is currently an assistant professor at the Department of Physics, Indian Institute of Science, Bangalore.