Quantum dark states lead to an advantage in noise reduction

Although atomic clocks are already the most accurate timekeeping devices in the universe, physicists are working hard to further improve their accuracy. One way is to take advantage of the spin states in the clock atoms.

Spin-squeezing states are entangled states in which the particles of a system conspire to cancel their internal quantum noise. These states, therefore, offer excellent opportunities for quantum-enhanced metrology because they allow for more precise measurements. Nevertheless, it is difficult to generate and maintain spin states in desired optical transitions with little external noise.

A special way to create a spin state, or squeeze, by placing clock atoms together Optical cavity, a set of mirrors where light can bounce back and forth many times. In a cavity, atoms can coordinate their photon emission and emit much brighter light than any single atom alone, a phenomenon known as superradiance. Depending on how superradiance is used, it can cause confusion, or alternatively, it can disrupt the intended quantum state.

In a previous study co-authored by JILA and NIST Fellows, Anna Maria Ray and James Thompson, the researchers discovered that multi-level atoms (with more than two internal energy states) super Radiant emissions provide unique opportunities to use to cancel each other’s emissions and stay in the dark.

Now, that’s reported in a pair of new papers published in Physical examination letters And Physical examination A, Ray and his team discovered a way to not only create dark states in a cavity, but more importantly, to squeeze those states. Their findings could open up remarkable opportunities for creating entangled clocks, which could push the frontier of quantum metrology in exciting ways.

Spin in the dark on a super radiant roller coaster

For several years, Ray and his team have studied the possibility of harnessing superradiance by creating dark states inside the cavity. Because dark states are unique configurations where normal light emission pathways are disrupted, these states do not emit light. Ray and his team have shown that dark states can be realized when atoms prepared in certain initial states are placed inside a cavity.

Created in this way, quantum states can remain unaffected by the effects of superradiance or light emission in the cavity. Atoms can still emit light outside the cavity, but at a speed much slower than superradiance.

Former JILA postdoctoral researcher Asier Piñeiro Orioli, who was the lead researcher on the earlier study with Thomson, and also contributed to the two recently published studies, has developed a simple way to understand the emergence of the dark state in a cavity. Found the method they call. A great ability.

“We can think of superradiant potential as a roller coaster where atoms ride. They collectively emit light as they roll down the hill, but when they reach a valley they stop,” says Ray. can be trapped. In the valleys, the atoms take on a dark form. Describe and stop the emission of light into the cavity.”

In their previous work with Thomson, the JILA researchers found that dark states must be at least slightly entangled.

“The question we set out to address in two new works is whether they can be both dark and highly entangled,” explains first author Bhuvnesh Sundar, a former postdoctoral researcher at JILA. “Interestingly, we found not only that the answer is yes, but that these types of squeezed states are quite straightforward to produce.”

Creating highly entangled dark states

In the new study, the researchers explored two possible ways to produce atoms in highly entangled spin-squeezed states. One method was to irradiate atoms with a laser to energize them above their ground state and then place them at special points at the superradiant potential, also known as saddle points. At the saddle points, the researchers turn off the laser and let the atoms relax in the cavity, and interestingly, the atoms reshape their noise distribution and become much more squeezed.

“Saddle points are valleys where the potential is zero curvature and zero slope at the same time,” explained Ray. “These are special points because atoms are dark but on the verge of being unstable and therefore have a tendency to shape their noise distribution.”

Another proposed method involved the transition of superradiant states to dark states. Here, the team also found other special points where the atoms are close to special “bright” points—not in a roller coaster valley, but at points of zero spin—where the interplay between the superradiance and an external laser creates spin squeezing. Is. .

“Clearly, the spin extracts generated at these bright spots can then be transferred to a dark state where, after proper alignment, we can turn off the laser and preserve the extracts,” Sundar said. added.

This transition works by first driving the atoms into a valley of superradiant potential and then coherently aligning the squeezing directions using lasers with the proper polarization (or directions of light oscillations), which The squeezed states are protected by superradiance.

The transition of the squeezed states to dark states not only preserved the low-noise properties of the squeezed states but also ensured their survival in the absence of external laser excitation, which is important for practical applications in quantum metrology. is an element.

While the study was published in Physical examination letters Using only one polarization of laser light to squeeze the spin, to generate two squeezing modes, Physical examination A The paper took this simulation further by using both polarizations of the laser light, resulting in four spin-squeezed modes (two modes for each polarization).

Piñeiro Orioli says, “In these two papers, we considered multi-level atoms with several internal levels, and having many internal levels is more difficult to simulate than two levels, which are often studied in the literature.” . So, we developed a set of tools to solve these multilevel systems. We developed a formula to calculate the perturbation arising from the initial state.”

The results of these studies may have far-reaching implications. Atomic clocks. By overcoming the limits of superradiance through the generation of dark entangled states, physicists either use atoms as memory to store entangled states (allowing retrieval of information from those states) or entangled state into a clock or interferometer set up for quantum – Improved measurements.