Stabler clocks could be used to quantify quantum phenomena, such as dark matter.
Stable oscillations are necessary for the keeping of time practise. An individual pendulum swing in a grandfather clock indicates one second. Much smaller time intervals are indicated by the vibrations of a quartz crystal in digital timepieces. Furthermore, in the world’s most advanced timekeeping devices, atomic clocks, the oscillations of a laser beam cause atoms to vibrate at a rate of 9.2 billion times per second. The timing of today’s financial markets, GPS systems, and satellite communications was determined by these tiniest, most reliable divisions of time.
The noise level of a clock is dependent on its surroundings. A pendulum’s swing can become unbalanced by a slight wind. Furthermore, heat can cause an atomic clock’s atoms to stop oscillating. Removing these kinds of outside influences can increase a clock’s accuracy. but only to a certain extent.
Quantum Boundaries in Chronology
According to a recent study from MIT, quantum mechanical phenomena could still affect the stability of oscillators such as clocks and laser beams, even if all external noise is removed. Quantum noise would ultimately be the limit on oscillator precision.
However, theoretically, it is possible to overcome this quantum limit. Researchers have demonstrated in their work that it is possible to enhance an oscillator’s stability beyond its quantum limit by adjusting, or “squeezing,” the states that give rise to quantum noise.
According to MIT assistant professor of mechanical engineering Vivishek Sudhir, “what we’ve shown is, there’s actually 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.” Then, we’ve demonstrated that it is possible to overcome this quantum mechanical shaking. However, it takes more ingenuity than simply removing the object from its surroundings. You must experiment with the actual quantum states.
Future Technologies and Experimental Applications
The group is developing an experimental test for their hypothesis. The scientists predict super-quantum precision in the tuning of clocks, lasers, and other oscillators if they can show that they can influence the quantum states in an oscillating system. Then, these devices might be used to monitor minuscule time variations, such those caused by a single qubit fluctuating in a quantum computer or a dark matter particle darting between detectors.
Over the next few years, we hope to demonstrate multiple examples of lasers with quantum-enhanced timekeeping capabilities,” says graduate student Hudson Loughlin of MIT’s Department of Physics. “We anticipate that the theoretical advances we have made recently and the experiments we have planned will improve our basic ability to keep accurate time and pave the way for new and revolutionary technologies.”
An open-access paper by Loughlin and Sudhir that was published in the journal Nature Communications provides specifics about their findings.
First, the laser, an optical oscillator that emits a beam of highly synchronised photons, was examined by researchers investigating oscillator stability. The namesake of the laser, light amplification by stimulated emission of radiation, was created by physicists Arthur Schawlow and Charles Townes, and they are primarily credited with its development.
The “lasing medium,” which is a group of atoms typically imbedded in glass or crystals, is the focal point of a laser’s construction. A flash tube encircling the lasing medium of the first lasers caused the energy of the atoms’ electrons to increase. A photon of radiation is released by the electrons when they relax back to their lower energy. In order to stimulate additional electrons and create more photons, the photon that is emitted is reflected back into the atoms by two mirrors at either end of the lasing medium. The lasing medium and one mirror work together as a “amplifier” to increase photon production, and the second mirror, which is partially transmissive, serves as a “coupler” to extract some photons as a concentrated laser light beam.
Schawlow and Townes proposed the theory that quantum noise should be the limit to a laser’s stability ever since the laser was invented. Since then, several researchers have used laser microscopy to test their theory. They demonstrated, via extremely precise calculations, that the stability of the oscillations produced by the laser might in fact be limited by subtle, quantum interactions between its atoms and photons.
According to Sudhir, “this work involved incredibly delicate and detailed calculations, so the limit was understood, but only for a specific kind of laser.” “We aimed to comprehend lasers and a broad range of oscillators, and to greatly simplify this.”
Applying the “Squeeze”
The researchers tried to make the issue simpler rather than concentrate on the actual details of a laser.
According to Sudhir, “an electrical engineer creates an oscillator by taking an amplifier and feeding the amplifier’s output into its own input.” Like a snake consuming its own tail, that is. That’s a very freeing way of thinking. You are not need to understand every detail of a laser. Rather, you have an abstract image that represents all oscillators, not only lasers.
The group created a simplified diagram of an oscillator that resembles a laser in their study. Their model consists of a coupler (like a partially reflective mirror), an amplifier (like the atoms in a laser), and a delay line (like the time it takes for light to travel between a laser’s mirrors).
Next, the scientists calculated the locations of quantum noise in the system and recorded the physics equations that characterise its behaviour.
“We can identify where quantum fluctuations enter the system by abstracting this problem to a simple oscillator, and they enter in two places: the amplifier and the coupler that enables us to extract a signal from the oscillator,” adds Loughlin. “We can determine the quantum limit on the stability of that oscillator if we know those two things.”
According to Sudhir, researchers can determine the quantum limit in their own oscillators by utilising the formulas they present in their paper.
Additionally, the group demonstrated that if quantum noise in one of the two sources could be “squeezed,” this quantum restriction might be bypassed. The concept of “quantum squeezing” describes how one component of a system might minimise quantum fluctuations while proportionally increasing fluctuations in another aspect. Squeezing air from one area of a balloon into another produces a similar effect.
The researchers discovered that in the case of a laser, even while noise in the laser’s output would grow, squeezing quantum fluctuations in the coupler might enhance the accuracy, or the timing of oscillations, in the outgoing laser beam.
“The question of how malleable a quantum mechanical limit is always arises when one finds one.” Says Sudhir. Is there really no more to be gained by tinkering with quantum mechanics, or is this a hard stop? We discover in this instance that there is, a finding that holds true for a broad class of oscillators.
Citation: Hudson A. Loughlin and Vivishek Sudhir, “Quantum noise and its evasion in feedback oscillators,” Nature Communications, November 4, 2023.