More stable clocks could measure quantum phenomena, including the presence of dark matter. The practice of timing relies on stable oscillations. In a grandfather clock, the length of a second is marked by one swing of the pendulum. In a digital watch, the vibrations of the quartz crystal mark a much smaller fraction of time. In the atomic clock, the world's most advanced timing device, the oscillation of the laser beam stimulates atoms to vibrate at a frequency of 9.2 billion times per second. These smallest, most stable time segments provide precise timing for today's satellite communications, global positioning systems, and financial markets.
Clocks, lasers and other oscillators can be tuned to ultra-quantum precision, allowing researchers to track infinitesimal differences in time, according to a new MIT study. Image source: MIT News
The stability of a clock depends on the noise of its environment. A slight breeze can throw the pendulum's swing out of sync. Heat can also disrupt the oscillations of atoms in atomic clocks. Eliminating these environmental effects can improve the accuracy of the clock. But that's all.
A new MIT study finds that even if all noise from the outside world is eliminated, the stability of clocks, laser beams and other oscillators is still susceptible to quantum mechanical effects. The accuracy of the oscillator will ultimately be limited by quantum noise.
But in theory, there is a way to break this quantum limit. In their study, the researchers also showed that by manipulating or "squeezing" the states that cause quantum noise, the oscillator's stability can be improved or even pushed beyond its quantum limits.
Vivishek Sudhir, an assistant professor in the MIT Department of Mechanical Engineering, said: "What we have shown is that there is actually a limit to the stability of oscillators such as lasers and clocks, and this limit is not only It's set by the environment they're in, and by the fact that quantum mechanics forces them to wobble around. And then we've shown that you even have a way around the wobble of quantum mechanics. But you have to be smarter, not just It's to isolate it from the environment and have to play with the quantum state itself." The
research team is now conducting experimental tests of their theory. If they can show that they can manipulate quantum states in oscillating systems, the researchers envision clocks, lasers and other oscillators could be tuned to ultra-quantum precision. These systems can then be used to track infinitesimal differences in time, such as the fluctuations of individual qubits in a quantum computer, or the presence of dark matter particles flickering between detectors.
"We plan to demonstrate several lasers with quantum-enhanced timing capabilities in the next few years," said Hudson Loughlin, a graduate student in MIT's Department of Physics. "We hope that our recent theoretical developments and upcoming The experiments will advance our fundamental ability to keep precise time and enable new revolutionary 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 the oscillators, the researchers first looked at lasers - an optical oscillator that produces a wave-like beam of highly synchronized photons. The invention of the laser is largely attributed to physicists Arthur Schawlow and Charles Townes. The design of the
laser is centered around the "luminescent medium," which is a collection of atoms, usually embedded in a glass or crystal. In the earliest lasers, a flash tube surrounding a luminescent medium stimulated electrons in atoms to jump in energy. When the electron relaxes back to a lower energy, some radiation is emitted in the form of photons. Two mirrors at opposite ends of the illuminating medium reflect the emitted photons back into the atoms, exciting more electrons and producing more photons. One of the mirrors acts as an "amplifier" together with the laser medium, promoting the production of photons, while the second mirror is partially transmissive and acts as a "coupler", extracting some of the photons to form a concentrated laser beam.
Since the invention of the laser, Schawlow and Townes proposed the hypothesis that the stability of lasers should be limited by quantum noise.Others have since tested their hypothesis by simulating the microscopic features of laser light. Through very specific calculations, they showed that imperceptible quantum interactions between laser photons and atoms do indeed limit the stability of their oscillations.
Sudhir points out: "But this work had to involve extremely detailed and subtle calculations in order to understand this limitation, but only for specific kinds of lasers. We hope to greatly simplify this process to understand lasers and various oscillators." "
'Applying pressure'
Rather than focusing on the intricate physics of lasers, the research team worked to simplify the problem.
" Sudhir explains: "When electrical engineers think about building an oscillator, they take an amplifier and feed the amplifier's output into their own input. It's like a snake eating its own tail. This is an extremely free way of thinking. You don’t need to know the ins and outs of lasers. In its place is an abstract picture, not just of lasers but of all oscillators. "
In their study, the team drew a simplified diagram of something like a laser oscillator. Their model consists of an amplifier (like the atoms of the laser), a delay line (like the light propagating between the laser mirrors). time required) and a coupler (such as a partial mirror).
The research team then wrote down the physical equations that describe the system's behavior and performed calculations to understand where in the system quantum noise would appear.
"via By abstracting the problem into a simple oscillator, we can pinpoint where quantum fluctuations enter the system, and they come from two places: the amplifier and the coupler that allows us to get the signal from the oscillator," Loughlin said. "If. "We know these two things, and we know what the quantum limit of the stability of this oscillator is".
Scientists can use the equations they laid out in their study to calculate the quantum limit of their own oscillator. What's more, The team demonstrated that it is possible to overcome this quantum limit if quantum noise from one of the two signal sources can be "squeezed". Quantum squeezing occurs at the expense of proportionally increasing quantum fluctuations in one aspect of the system. It is minimized. The effect is similar to squeezing air in a balloon from one part to another.
In lasers, the team found that if the quantum fluctuations in the coupler are squeezed, they can improve the accuracy of the output laser beam or The oscillation time, even though the noise in the laser power increases as a result.
"When you find some kind of quantum mechanical limit, there's always the question: How malleable is this limit? Sudhir said. "Is it really a hard limit, or can you still extract some juice by manipulating quantum mechanics?" In this case we find that there is, a result that holds for a large class of oscillators. "
