Scientists build miniature quantum memory element that can be mass produced

Researchers at the University of Basel have built a quantum memory element based on atoms in a tiny glass cell. In the future, such quantum memories could be mass-produced on a wafer.

It is difficult to imagine our lives without networks like the Internet or mobile phone networks. In the future, similar networks are planned for quantum technologies, which will enable the transmission of messages without eavesdropping using quantum cryptography and allow quantum computers to be connected together.

Like their conventional counterparts, these quantum networks require memory elements in which information can be temporarily stored and routed as needed. A team of researchers from the University of Basel led by Professor Philipp Treutlein has developed such a memory element, which can be microfabricated and therefore suitable for mass production. Their results were published in Physical Examination Letters.

Photon storage in glass cells

Light particles are particularly suited to transmitting quantum information. Photons can be used to send quantum information through fiber optic cables, to satellites, or into a quantum memory element. There, the quantum mechanical state of the photons must be stored as precisely as possible and, after a certain time, converted back into photons.

Two years ago, Basel researchers demonstrated that this worked well using rubidium atoms in a glass cell. “However, this glass cell was made by hand and measured several centimeters,” explains postdoctoral researcher Dr. Roberto Mottola. “To be suitable for everyday use, these cells need to be smaller and able to be produced in large numbers.”

This is precisely what Treutlein and his collaborators managed to achieve. To use a much smaller cell measuring just a few millimeters, obtained through the mass production of atomic clocks, they had to develop a few tricks. In order to have enough rubidium atoms for quantum storage despite the small size of the cell, they had to heat the cell to 100° centigrade to increase the vapor pressure.

Additionally, they exposed the atoms to a magnetic field of 1 Tesla, more than 10,000 times stronger than Earth’s magnetic field. This changed the atomic energy levels in a way that facilitated quantum storage of photons using an additional laser beam. This method allowed researchers to store photons for around 100 nanoseconds. The free photons would have traveled 30 meters during this time.

A thousand quantum memories on a single wafer

“We have thus constructed for the first time a miniature quantum memory for photons, around 1,000 copies of which can be produced in parallel on a single wafer,” explains Treutlein.

In the current experiment, storage was demonstrated using highly attenuated laser pulses, but in the near future Treutlein, together with CSEM Neuchâtel, also wants to store single photons in miniature cells. Furthermore, the format of the glass cells must still be optimized, in order to store the photons for as long as possible while preserving their quantum states.