Storage Life Extended for Room-Temperature Qubits

JOEL WILLIAMS, ASSOCIATE EDITOR
joel.williams@photonics.com

Researchers at the University of Copenhagen have demonstrated a method for storing qubits at room temperature for a duration that is hundreds of times longer than has been achieved with previous approaches.

The method takes a new approach to the quantum measurements taken within a quantum chip, which is coated with paraffin to remove the need for large, often more expensive cooling systems. The research team, led by Eugene Polzik, previously worked with paraffin for room-temperature experiments such as quantum measurements and teleportation — as have other groups.

In earlier approaches to quantum memory, quantum states were encoded in the amplitude and phase of light transmitted/emitted by the atoms. The amplitude and phase of the light were measured continuously by homodyne detection.

“This made possible quantum memory and teleportation with the so-called continuous variables, for example, coherent or squeezed states,” Polzik said.

The current experiment instead uses “click-type” photon counters. “This brings about a new challenge — the atomic motion,” Polzik said.

In the past when an atomic cloud was used to store a single photon, it was done with immobilized atoms in solid state at liquid helium temperature or in a laser-cooled gas of atoms, he said.

When room-temperature atoms were used, the atoms moved around within the chip at speeds of 200 m/s and collided with the walls of the chip every microsecond. To mitigate this, the team employed a paraffin coating, a waxy substance used to soften the collisions of the atoms. Without the coating, the collisions would ruin the atoms’ quantum state of spin.

“This leads them to emit photons that are very different from each other. But we need them to be exactly the same in order to use them for safe communication in the future,” Polzik said.

For the coating to work, it must be kept within 20 to 50 °C. The specific temperature within that range depends on the optimal density of the atomic vapor. If the temperature is too high, the coating will melt and lose its protective qualities, Polzik said.

In previous experiments with atoms at room temperature, the memory was limited to the time it takes an atom to move in and out from the laser beam — about a microsecond.

“The breakthrough that allowed us to overcome this time limit comes from the so-called motional averaging, which in the quantum mechanical language is the fundamental ‘erasing which way information’ principle,” Polzik said.

The “which way information” refers to the quantum eraser experiment, which establishes that when action is taken to determine which of two paths a photon has passed through, the photon cannot interfere with itself. Quantum interference is therefore conditioned on the absence of “which way information.”

“In our setting, the ‘which way information’ corresponds to the statement that when a heralding photon is detected, it is emitted by a certain spatial configuration of atoms,” Polzik said. “It is as if the click of the photodetector makes ‘a snapshot’ of the positions of atoms at the time of the click. If we wish to retrieve the photon from the atomic memory after some time, this retrieval will be very inefficient because the atoms will change their spatial configuration and the collective enhancement will be lost.”

A solution involved an existing principle, and a filtering cavity.

“To overcome this ‘which way’ or ‘which atom information,’ we use the principle of motional averaging: The heralding photon is delayed by a narrow band filtering cavity,” Polzik said. “The delay time (tens of microseconds) is, on average, much longer than the time it takes the atoms to go in and out of the laser beam. In this fashion all atoms contribute to the photon’s detection in the same way, hence the single photon is stored in all atoms together, as the protocol requires for collective enhancement of memory.”

Polzik said that the technology will need further development to find practical application; the memory time would need to be increased by about a factor of 10.

“Right now we produce the qubits of light at a low rate — one photon per second, while cooled systems can produce millions in the same amount of time,” Polzik said. “But we believe there are important advantages to this new technology, and that we can overcome this challenge.”

The research was published in Nature Communications (www.doi.org/10.1038/s41467-021-24033-8).