Researchers Use Multiplexing to Improve Quantum Memory Capacity

Researchers have built a quantum memory-enabled source of spatially structured nonclassical light, based on the principle of wavevector multiplexing. The high-capacity memory using laser-cooled atoms can store up to 665 quantum states of light simultaneously. According to researchers, their system has a larger capacity than any other existing quantum memory. The results of the research could lead to ways to further increase the capacity of quantum memory. Simultaneous processing of many qubits, key to efficient quantum parallel computation, could open up new possibilities in imaging and in communications.

This is a cooled and trapped cloud of cold atoms used to realize the quantum memory protocol. The atoms reside in the center of the vacuum chamber, around which the magnetic coils necessary to trap the atoms are visible. The blue color is caused by two near-infrared lasers illuminating the atoms and driving a two-photon transition, which results in spontaneous emission of visible blue light. Courtesy of FUW, Mateusz Mazelanik.

Researchers at the University of Warsaw built the quantum memory using spatial multiplexing aided by a single-photon resolved camera. Their setup contained a magneto-optical trap (MOT), where a group of rubidium atoms inside a glass vacuum chamber was trapped and cooled by lasers in the presence of a magnetic field to about 20 microkelvins.

The memory light-atoms interface was based on off-resonant light scattering. In the write-in process, the cloud of atoms was illuminated by a laser beam, resulting in photon scattering. Each scattered photon was emitted in a random direction and registered on the single-photon sensitive camera.

The information about the scattered photons was stored inside the atomic ensemble in the form of collective excitations — spin-waves that could be retrieved on demand as another group of photons.

By measuring correlations between emission angles of photons created during the write-in and read-out processes, the team was able to confirm that the memory was quantum, and that the light-state that had been generated could not be explained by classical optics.

The current prototype quantum memory from the University of Warsaw team requires two optical tables, and uses nine lasers and three control computers.

The quantum information about all the stored photons resides in a single cloud of cold atoms. Each atom is engaged in the storage of each photon, making the memory resilient to quantum decoherence. Researchers confirmed this resilient state by observing quantum interference of two distinct excitations, differing by just one quantum number.

Article authors: (first row, from left) M. Parniak, M. Mazelanik, M. Dabrowski and M. Lipka (second row, from left) A. Leszczyski and W. Wasilewski, along with the setup of cold-atom quantum memory. Two optical tables with elements used to construct the prototype setup are visible. Courtesy of FUW, Mateusz Mazelanik.

The team believes that its proposed protocol utilizing multimode memory along with the single-photon camera could facilitate generation of multi-photon states. Such states will be required for quantum-enhanced sensing technologies and as an input to photonic quantum circuits.

“This will allow even more complex manipulations of the atomic state, finally to prepare quantum states of light with accurately controlled parameters,” said professor Wojciech Wasilewski.

The research was published in Nature Communications (doi:10.1038/s41467-017-02366-7).