February 26, 2024

Study Finds More Stable Clocks Could Measure Quantum Phenomena, Including the Presence of Dark Matter

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Phase noise of quantum-enhanced feedback oscillators. Output phase quadrature spectra for four types of noise-limited quantum oscillators. Red shows the Schawlow-Townes spectrum of an oscillator with phase-insensitive amplifier and coupled and auxiliary modes in vacuum. The light and darker blues represent the case where these modes are compressed (light blue) and entangled (dark blue) (both with 12 dB of compression). Green shows the case where the loop amplifier is purely phase sensitive. Credit: Nature Communications (2023). DOI: 10.1038/s41467-023-42739-9

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Phase noise of quantum-enhanced feedback oscillators. Output phase quadrature spectra for four types of noise-limited quantum oscillators. Red shows the Schawlow-Townes spectrum of an oscillator with phase-insensitive amplifier and coupled and auxiliary modes in vacuum. The light and darker blues represent the case where these modes are compressed (light blue) and entangled (dark blue) (both with 12 dB of compression). Green shows the case where the loop amplifier is purely phase sensitive. Credit: Nature Communications (2023). DOI: 10.1038/s41467-023-42739-9

The practice of keeping time depends on stable oscillations. In a pendulum clock, the duration of one second is marked by a single swing of the pendulum. In a digital watch, the vibrations of a quartz crystal mark much smaller fractions of time. And in atomic clocks, the world’s next-generation timekeepers, the oscillations of a laser beam stimulate atoms to vibrate 9.2 billion times per second. These smaller, more stable divisions of time define the time for today’s satellite communications, GPS systems, and financial markets.

The stability of a clock depends on the noise in its environment. A light wind can desynchronize the swing of a pendulum. And heat can disrupt the oscillations of atoms in an atomic clock. Eliminating such environmental effects can improve watch accuracy. But only to a certain extent.

A new MIT study concludes that even if all noise from the outside world were eliminated, the stability of clocks, laser beams and other oscillators would still be vulnerable to the effects of quantum mechanics. The accuracy of the oscillators would ultimately be limited by quantum noise.

But in theory, there is a way to surpass this quantum limit. In their study, the researchers also show that by manipulating, or “compressing,” the states that contribute to quantum noise, the stability of an oscillator can be improved, even beyond its quantum limit.

“What we show is that there is actually a limit to the stability of oscillators like lasers and clocks, which is set not just by the environment but by the fact that quantum mechanics forces them to wobble a little,” says Vivishek. Sudhir, assistant professor of mechanical engineering at MIT. “So we showed that there are ways to get around this quantum mechanical jitter. But you have to be smarter than just isolating the thing from its environment. You have to play with the quantum states themselves.”

The team is working on an experimental test of their theory. If they can demonstrate that they can manipulate quantum states in an oscillating system, researchers predict that clocks, lasers and other oscillators could be tuned to super-quantum precision. These systems could then be used to track infinitely small differences in time, such as the fluctuations of a single qubit in a quantum computer or the presence of a dark matter particle floating between detectors.

“We plan to demonstrate several examples of lasers with quantum-enhanced timing capabilities over the next few years,” says Hudson Loughlin, a graduate student in the MIT Department of Physics. “We hope that our recent theoretical developments and future experiments will enhance our fundamental ability to keep time accurately and enable revolutionary new technologies.”

Loughlin and Sudhir detail their work in an open access paper published in the journal Nature Communications.

Laser precision

When studying the stability of oscillators, researchers first looked at the laser – an optical oscillator that produces a wave-like beam of highly synchronized photons. The invention of the laser is largely credited to physicists Arthur Schawlow and Charles Townes, who coined the name from its descriptive acronym: light amplification by stimulated emission of radiation.

The design of a laser centers on a “laser medium” – a collection of atoms, usually embedded in glass or crystals. In early lasers, a flash tube surrounding the laser medium would stimulate electrons in atoms to increase energy. When the electrons relax back to a lower energy, they emit some radiation in the form of a photon.

Two mirrors, at each end of the laser medium, reflect the emitted photon back to the atoms to stimulate more electrons and produce more photons. One mirror, together with the laser medium, acts as an “amplifier” to increase photon production, while the second mirror is partially transmissive and acts as a “coupler” to extract some photons as a concentrated beam of laser light.

Since the invention of the laser, Schawlow and Townes have hypothesized that laser stability should be limited by quantum noise. Since then, others have tested their hypotheses by modeling the microscopic characteristics of a laser. Through very specific calculations, they showed that, in fact, imperceptible quantum interactions between laser photons and atoms could limit the stability of their oscillations.

“But this work had to do with extremely detailed and delicate calculations, so that the limit was understood, but only for a specific type of laser”, notes Sudhir. “We wanted to simplify this enormously, to understand lasers and a wide range of oscillators.”

Putting on the ‘grip’

Rather than focusing on the physical complexities of the laser, the team sought to simplify the problem.

“When an electrical engineer thinks about making an oscillator, he takes an amplifier and feeds the amplifier’s output into his own input,” explains Sudhir. “It’s like a snake eating its own tail. It’s an extremely liberating way of thinking. You don’t need to know the details of a laser. Instead, you have an abstract picture, not just of a laser, but of all the oscillators. ”

In their study, the team devised a simplified representation of a laser-like oscillator. Their model consists of an amplifier (like the atoms in a laser), a delay line (for example, the time it takes for light to travel between the mirrors of a laser), and a coupler (like a partially reflective mirror).

The team then wrote the physics equations that describe the system’s behavior and performed calculations to see where quantum noise would arise in the system.

“By abstracting this problem to a simple oscillator, we can identify where quantum fluctuations enter the system, and they enter in two places: the amplifier and the coupler that allows us to get a signal from the oscillator,” says Loughlin. “If we know these two things, we know what the quantum limit is for the stability of this oscillator.”

Sudhir says scientists can use the equations they present in their studies to calculate the quantum limit in their own oscillators.

What’s more, the team showed that this quantum limit could be overcome, if the quantum noise in one of the two sources could be “compressed”. Quantum compression is the idea of ​​minimizing quantum fluctuations in one aspect of a system at the expense of proportionally increasing fluctuations in another aspect. The effect is similar to squeezing air from one part of a balloon to another.

In the case of a laser, the team found that if the quantum fluctuations in the coupler were compressed, this could improve the accuracy, or timing of the oscillations, in the output laser beam, even if the noise in the laser power increased as a result. .

“When you find some limit of quantum mechanics, there is always the question of how malleable is that limit?” Sudhir says. “Is it really a hard stop, or is there still some juice you can squeeze out by manipulating some quantum mechanics? In this case, we found that it did, which is a result applicable to a huge class of oscillators.”

More information:
Hudson A. Loughlin et al, Quantum noise and its evasion in feedback oscillators, Nature Communications (2023). DOI: 10.1038/s41467-023-42739-9

Diary information:
Nature Communications

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