Synopsis: Chilled Cavity Reaches New Level of Quiet

A crystal cavity for light and sound has been chilled close to its motional ground state.
Synopsis figure
O. Painter/California Institute of Technology

A small patterned structure on a silicon chip can act as a resonant cavity for both light and sound. Such an optomechanical crystal (OMC) could store and transfer quantum information in the form of photons and phonons within a quantum network. To explore this potential, a research team has used laser pulses to measure the phonon population in an OMC cooled to subkelvin temperatures. The results show that the phonon cavity has a record low thermal phonon occupancy of 0.02, which means it is in the quantum ground state (with zero phonons) 98% of the time.

In the last few years, physicists have developed optomechanical crystals as a way to localize—and thereby couple—photons and phonons within the same structure. The basic OMC design is a thin film of silicon with a pattern of holes that acts as a cavity for specific light and sound waves traveling along the film’s surface. One vexing problem has been that optical signals entering the cavity can cause heating that decoheres the resonant acoustic modes.

To reduce this heating, Oskar Painter of the California Institute of Technology in Pasadena and his colleagues used laser pulses (rather than a continuous beam) to probe the phonon population of an OMC made from a silicon nanobeam. The team placed their device in a dilution refrigerator and fired in infrared laser pulses along an optical fiber. The outgoing signal gave a measure of the number of resonant phonons stored in the OMC cavity. Besides the low phonon occupancy, the data also revealed a very long coherence time of 450 microseconds, implying that resonant 5.6-gigahertz phonons are stored for roughly 2.5 million mechanical oscillation periods.

–Michael Schirber

This research is published in Physical Review X


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