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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - GeNoSOS (Generation of Non-classical States in Optomechanical Systems)

Teaser

Summary

Quantum mechanics underpins all modern technology, such as lasers and microelectronics. Understanding its principles enabled such technologies, and further advances will usher in new technologies such as quantum computers. Harnessing quantum mechanical phenomena allows new capabilities, such as more sensitive measurements, more computational power or more data capacity. In addition it furthers our understanding of the universe. For example, the recent direct observation of gravitational waves from merging black holes followed decades of careful analysis of the limits imposed by quantum mechanics on such detection.

Future technologies will employ manifestly quantum mechanical entities, such as single atoms or quantized magnetic fluxes in superconductors. Another such entity is the motion of a mechanical oscillator, and only recently has it been possible to probe the quantum mechanical properties of carefully designed oscillators, in a field known as cavity optomechanics. Mechanical objects offer many advantages. They are often much less susceptible to environmental noises and can also be used to mediate between different systems, such as superconducting qubit and an optical data link. Moreover, the massiveness of mechanical oscillators provides a connection between quantum mechanics and the theory of relativity -- whose unification still baffles physicists.

We focus on a promising type of a mechanical oscillator, known as an optomechanical crystal. It is a silicon nanobeam (hundreds of nanometer in width and thickness, and tens of micrometers in length) that confines a localized mechanical oscillation as well as an optical resonance. The optics and the mechanics are coupled to each other by radiation pressure and electrostriction. Thus by injecting laser light into the devices, which can be done in an efficient manner, it is possible to probe and manipulate mechanical motion.

Many intriguing quantum-mechanical phenomena in mechanical systems were observed for the first time in optomechanical crystals. Using light it is possible to remove energy, or cool the mechanical oscillator down to its quantum ground state of motion. Quantum motion also exhibits intrinsic \'zero-point motion\' -- even in the absence of energy -- that has also been observed.
The quantization of motional energy of a mechanical system and even quantum entanglement between two such systems, has been demonstrated with optomechanical crystals. These experiments require operating at very low temperatures, usually of a few milli-Kelvin, with the oscillator situated in vacuum.

While these experiments constitute significant advance in understanding and manipulating mechanical oscillators in the quantum regime, they are encumbered by a severe limitation of technical heating of the system due to partial absorption of the interrogating light. Coupled with very low thermal conductivity, this limits many experiments to extremely low signals or short light pulses and long integration times, and prohibits experiments that require strong, continuous probing, such as avoiding measurement backaction or generating continuous non-classical (\'squeezed\') states of motion.

Building on expertise in optomechanics and cryogenics in the Laboratory of Photonics and Quantum Measurements at EPFL, we have set out to develop a platform that will provide the next step in continuous quantum measurements with mechanical oscillators in the optical domain. Our approach is based on a cryogenic system where ambient cold helium-3 gas serves as a buffer to facilitate the thermalization of the system and greatly reduce technical heating. Although operating at temperatures of a few Kelvin, the ability to use strong optical fields enables the use of optomechanical techniques to cool the oscillator to the ground state, and perform additional measurements.

The overall objectives of this project were to develop the above system and demonstrate the ability to cool the oscillator close to its ground st

Work performed

Over the course of the project, we have acquired the helium-3 buffer gas cryostat (Oxford Instruments HelioxTL) and adapted a method to place our sample inside the cryostat and couple light into the nanobeam using evanescent light coupling from a tapered optical fiber. We have then performed detailed characterization of our ability to cool down mechanical motion close to the quantum ground state, based on the base temperature and pressure of the buffer gas. The first generation of samples already enabled us to reach very low number of 1.3 mechanical quanta, or 40% ground state occupation.

One of the hallmarks of the quantum mechanical nature of oscillators is motional sideband asymmetry. Essentially this means that the rates of absorption and emission of energy quanta are different. We built an experimental setup that allowed us to probe both emission and absorption rates of the oscillator, while still cooling it close to the ground state, by pumping the device with three different, frequency-locked lasers. While analyzing the data we discovered a novel, formerly unnoticed phenomenon that has wide-ranging implications to any system that utilized multi-tone probing. Essentially, different laser frequencies can interact inside the optomechanical cavity due to a plethora of effects (thermal, Kerr-effect etc.). This interaction interferes with the optomechanical interaction to corrupt the sideband asymmetry and give wrong results. After carefully analyzing this effect in collaboration with the Nunnenkamp theory group in Cambridge, we have found a way to mitigate it and published our findings. We also presented it at various conferences.

This experience has proven that our system is robust and allows us to discern deviations from known theory, even under strong probing. Our next step was to demonstrate a Backaction-Evading (BAE) measurement. In short, BAE has been proposed many decades ago in the context of the aforementioned gravitational wave detection, as a way to overcome sensitivity limitations on measurement of motion imposed by quantum mechanics. This measurement has so far been demonstrated in nanomechanical oscillators coupled to microwave circuits, but remained elusive in the optical domain. Using our system, we have successfully demonstrated it for the first time in the optical domain. This work was also submitted for publication and presented at conferences.

Finally, in collaboration with IBM Zurich, we have designed and fabricated the next generation of optomechanical crystal devices. These new devices have optical quality improved by a factor of more than five, which both increases the optomechanical interaction and decreases the extraneous heating. Indeed, in preliminary tests we have been able to cool the mechanical oscillator down to 0.3 quanta, unprecedented in this kind of system.

Final results

Our system opens the way for many types of quantum measurements and quantum information protocols that have been so far inaccessible. In the near future we hope to be able to generate non-classical states of motion, robustly observe the quantization of mechanical energy and use our devices in quantum-computing platforms to connect quantum-optical datalinks and quantum-circuit processors.