Physics-Listen to the Differences in Quantum

Physics 15, 87

At very low volumes, quantum optics microphones perform better than traditional devices, and humans can hear the difference.

Romolo Tavani /

I believe in hearing. Quantum microphones are advantageous for understanding speech when the volume is extremely low. Researchers have used microphone tests to demonstrate techniques that may be useful in experiments that require high-frequency, high-sensitivity measurements.I believe in hearing. Quantum microphones are advantageous for understanding speech when the volume is extremely low. Researchers have used microphone tests to demonstrate techniques that may be useful in experiments where sensitive measurements are made … Show more

Quantum devices are often touted as performing better than traditional devices, but the impact can seem far from our daily lives.Researchers have now demonstrated quantum optics microphones that listeners produce clearer sounds than their classic counterparts. [1].. The microphone has been tested under certain conditions (low volume and high noise), but otherwise the quantum benefits are not noticeable. Despite this limitation, new quantum technologies may be useful elsewhere. Ultimately, it could be used to improve the imaging of biological samples.

Many precision measurements, such as gravity wave detection, rely on interferometers to measure the effects of interference, such as fringes, that occur when photons travel through two possible paths. Using a pair of quantum mechanically entangled photons reduces random variation (shot noise) in such measurements and improves the sensitivity of the measurement. However, some common techniques measure both photons in an intertwined pair. This is a slow selection process that limits the measurement speed to 1Hz. When using entangled photons to track fast actions such as the movement of a single molecule within a living cell, the speed is too slow.

Florian Kaiser and his colleagues at the University of Stuttgart in Germany have come up with a way to increase the measurement speed of such quantum optics experiments by 10,000 times. In those setups, the input laser light first passes through a non-linear crystal, creates a stream of intertwined photon pairs, and then feeds into the two paths (or arms) of the interferometer.

To eliminate the need to measure both photons at the output of the interferometer, the team added an optical component called a wavelength selection waveplate. This rotates the polarization of the light that passes through one of the arms of the interferometer. You can see that this simple operation encodes the two-photon information (the quantum phase of the pair) into only one photon.

When the information is transferred to a single photon, it is easier to measure the interference signal. Just get the difference in light intensity between the two outputs in the same way as traditional interferometry. The team has shown that the quantum augmented signal-to-noise ratio can be obtained at a high sampling rate of 100 kHz. This frequency is high enough to produce high quality audio, allowing researchers to demonstrate their technique in recording experiments. “We wanted to see if humans could actually hear the quantum improvement,” says Kaiser.

JCM Gebhardt / Ulm University

Cellular network. The techniques used in quantum microphones may ultimately be useful in biological experiments such as this single molecule tracking of fluorescently labeled chromatin-binding molecules in living zebrafish embryos.

Researchers turned the interferometer into an optical microphone by attaching one of the mirrors to a film that vibrates in response to sound waves. As the mirror moves back and forth, the length of one of the interferometer arms changes, resulting in an observable change in the light reaching the detector. The team used a microphone in a standardized hearing test. The selected word was recorded with a microphone and played by a series of human listeners who were asked to identify the word. A similar test was done on a “classical” optical microphone and the same interferometer was used, but with no photon entanglement. Subjects had slightly better success in recognizing quantum recorded words.

Kaiser immediately admits that the test was fraudulent. “Our microphone has a quantum advantage in the artificial situation we created here,” he says. In that situation, I turned down the volume during the recording session so that the measured shot noise was higher than the other noise contributions. Kaiser compares the noise level to the garbled transmission between the racing driver and the pit crew. Only about half of the words are correctly understood here.

However, even if new quantum technologies do not revolutionize audio recording, they may be useful for other types of measurements such as biological imaging. Kaiser explains that most cells can behave abnormally or be damaged under strong lighting. Quantum microscopes that use the researcher’s entanglement scheme can improve high-resolution imaging techniques by allowing them to function better with fewer photons.

“In the context of the development of practical sensors, this new study stands as an elegant demonstration of the’quantum benefits’ of using quantum states of entangled light,” said Nice Sophia, France. Laurent Labonté, a quantum optics expert at the University of Antipolis, says. ..

“This is a very novel and original integration of the concept of quantum measurement,” says Bill Prick of the University of Dayton, Ohio, who is studying the basics of quantum mechanics. “I don’t think this work is” fundamentally quantum perception “, but it provides people with a way to get quantum effects and see if they can have a recognizable effect. increase. This is really great. “

– Michael Schirber

Michael Schirber, Physics magazine Based in Lyon, France.


  1. R. Nord et al.“Quantum optical microphone in audio band” PRX Quantum 3020358 (2022).

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