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An Indian-origin assistant professor of mechanical engineering at MIT along with a graduate student in MIT’s Department of Physics are carrying out experimental tests to demonstrate that quantum states in an oscillating clock system can be manipulated and tuned to super-quantum precision. These systems could be used to track minute differences in time – fluctuations of a single qubit in a quantum computer or the presence of a dark matter particle flitting between detectors.
This research, supported in part by the National Science Foundation, of Vivishek Sudhir, assistant professor of mechanical engineering at MIT, and Hudson Loughlin, a graduate student in MIT’s Department of Physics are working on a study to show that the stability of clocks, laser beams, and other oscillators would still be vulnerable to quantum mechanical effects but it is also possible to dodge these limitations.
“The practice of keeping time hinges on stable oscillations. In a grandfather clock, the length of a 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 state-of-the-art timekeepers, the oscillations of a laser beam stimulate atoms to vibrate at 9.2 billion times per second. These smallest, most stable divisions of time set the timing for today’s satellite communications, GPS systems, and financial markets. A clock’s stability depends on the noise in its environment. A slight wind can throw a pendulum’s swing out of sync. And heat can disrupt the oscillations of atoms in an atomic clock. Eliminating such environmental effects can improve a clock’s precision,” explained an MIT report on the study.
The new MIT study has found that even if all noise from the outside world is eliminated, the stability of clocks, laser beams, and other oscillators would still be vulnerable to quantum mechanical effects. The precision of oscillators would ultimately be limited by quantum noise. But Sudhir and Loughlin say that in theory, there’s a way to push past this quantum limit. In their study, the researchers also show that by manipulating, or “squeezing,” the states that contribute to quantum noise, the stability of an oscillator could be improved, even past its quantum limit.
“What we’ve shown is, there’s a limit to how stable oscillators like lasers and clocks can be, that’s set not just by their environment, but by the fact that quantum mechanics forces them to shake around a little bit,” says Sudhir. “Then, we’ve shown that there are ways you can even get around this quantum mechanical shaking. But you have to be cleverer 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 the quantum states in an oscillating system, the researchers envision that clocks, lasers, and other oscillators could be tuned to super-quantum precision. “We plan to demonstrate several instances of lasers with quantum-enhanced timekeeping ability over the next several years,” says Loughlin. “We hope that our recent theoretical developments and upcoming experiments will advance our fundamental ability to keep time accurately, and enable new revolutionary technologies.”
An open-access paper on this subject, written by Loughlin and Sudhir, detailing their work has been published in Nature Communications journal.
Since the invention of the laser, Schawlow and Townes put forth a hypothesis that a laser’s stability should be limited by quantum noise. Others have since tested their hypothesis by modeling the microscopic features of a laser. Through very specific calculations, they showed that indeed, imperceptible, quantum interactions among the laser’s photons and atoms could limit the stability of their oscillations.
“But this work had to do with extremely detailed, delicate calculations, such that the limit was understood, but only for a specific kind of laser,” Sudhir notes. “We wanted to enormously simplify this, to understand lasers and a wide range of oscillators.”
Rather than focus on a laser’s physical intricacies, the team looked to simplify the problem. “When an electrical engineer thinks of making an oscillator, they take an amplifier, and they feed the output of the amplifier into its input,” Sudhir explains. “It’s like a snake eating its tail. It’s an extremely liberating way of thinking. You don’t need to know the nitty-gritty of a laser. Instead, you have an abstract picture, not just of a laser, but of all oscillators.”
“By abstracting this problem to a simple oscillator, we can pinpoint where quantum fluctuations come into the system, and they come in in two places: the amplifier and the coupler that allows us to get a signal out of the oscillator,” Loughlin says. “If we know those two things, we know what the quantum limit on that oscillator’s stability is.”
Sudhir says scientists can use the equations they lay out in their study to calculate the quantum limit in their oscillators. The team also showed that this quantum limit might be overcome, if quantum noise in one of the two sources could be “squeezed.” Quantum squeezing 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 into another.
In the case of a laser, the team found that if quantum fluctuations in the coupler were squeezed, it could improve the precision, or the timing of oscillations, in the outgoing laser beam, even as noise in the laser’s power would increase as a result. “When you find some quantum mechanical limit, there’s always some question of how malleable is that limit?” Sudhir says. “Is it really a hard stop, or is there still some juice you can extract by manipulating some quantum mechanics? In this case, we find that there is, which is a result that applies to a huge class of oscillators.”
