Active noise control for a quantum drum

Active noise control for a quantum drum
The central pad of a holey silicon nitride membrane (yellow, inside red silicon frame) vibrates like a “quantum drum,” thanks to the extreme acoustic isolation provided by the hole pattern invented at the Schliesser lab. Laser-based measurement of the drum’s vibrations then allows control of its motional quantum state, eliminating all noise—including the quantum perturbation by the measurement itself—analogous to noise-cancellation headphones. Earbuds in the background provide a size reference.

Researchers of the Schliesser Lab at the Niels Bohr Institute and the Centre for Hybrid Quantum Networks (Hy-Q), University of Copenhagen, have demonstrated a new way to address a central problem in quantum physics: at the quantum scale, any measurement disturbs the measured object. This disturbance limits, for example, the precision with which the motion of an object can be tracked. But in a millimeter-sized membrane that vibrates like a drumhead, the researchers have managed to precisely monitor the motion with a laser—and to undo the quantum disturbance by the measurement. This allows them to control the membrane’s motion at the quantum level. The result has potential applications in ultraprecise sensors of position, velocity and force, and the architecture of a future quantum computer.

Living in a quantum world
At the quantum level, making measurements disturbs the object measured: using a laser beam to determine the position or velocity of an object requires bombarding it with many photons. The photons will kick it with every impact, and the object will start moving accordingly. As the photons arrive randomly, this results in additional random motion on top of the original movements, degrading the ability to measure and control the actual motional state. If the laser intensity is turned down, in order to reduce such measurement “backaction”, the signal-to-noise ratio in the detector goes down and the measurement becomes imprecise – again. “A strong measurement is needed, even though it results in quantum backaction. All we have to do is to measure and undo the quantum backaction. And that is basically what we’ve succeeded in doing”, Professor Albert Schliesser explains.

The experiment
“Our experiments offer us a really unique opportunity: our data very clearly show quantum effects, such as quantum backaction, in the measurement of mechanical motion. So we can test in our labs if clever modifications of the measurement apparatus can improve precision—using tricks that in the last few decades could only be theorized,” he continues.

The experimental system is a ca. 3×3 mm-sized membrane made of the ceramic silicon nitride (Fig 1). It is under high tension and vibrates when struck—just like a drumhead. A special hole pattern invented in Schliesser’s lab isolates these vibrations extremely well: once it vibrates, it undergoes a billion oscillation cycles before it loses a significant fraction of its energy to its surroundings. (For a normal drum, that number would be about one hundred.) An additional advantage of silicon nitride is that it does not absorb any of the laser light used to interrogate its motion—so the membrane does not heat up , which would again lead to some uncontrolled motion of the membrane.

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