Quantum Memories make their way

Understanding the interaction between light and matter is of utmost importance for the advancement of qubit development for quantum information and the future quantum internet. Qubits can be used in various ways. One can construct flying qubits with photons, or matter qubits from crystals or gas atoms, used for quantum memories or repeaters.

Experts in this field, the research group led by ICREA Prof. at ICFO Hugues de Riedmatten has recently published in Physical Review Letters two studies in which they report an advancement in the development and improvement of quantum memories.

In the first study, ICFO researchers Kutlu Kutluer, Emanuele Distante, Bernado Casabone, Stefano Duranti, and Margherita Mazzera, led by ICREA Prof. at ICFO Hugues de Riedmatten, report on the first demonstration of the generation of entanglement in time between a photon and a collective spin excitation in a rare earth ion doped ensemble.

In their experiment, the team of researchers used a memory crystal and an interferometric filter crystal, both cooled down to 3.5K, to generate and measure entanglement between the photon and the crystal. The crystal showed to be capable of emitting a pair of entangled photons with an embedded quantum memory for one of the photons. They analyzed the quality of this entanglement by mapping the atomic excitation onto the photonic qubit and by using time-bin qubit analyzers implemented with the atomic frequency comb technique in the other doped crystal. With this approach, they were able to prove that the quality of the entanglement was enough to violate the Bell inequality, making the device setup suitable for quantum communication’s applications.

In the second study, ICFO researchers Alessandro Seri, Dario Lago-Rivera, Andreas Lenhard, and Margherita Mazzera, led by ICREA Prof. at ICFO Hugues de Riedmatten, in collaboration with Giacomo Corrielli and Roberto Osellame from CNR-IFN in Milan, report on the first demonstration of quantum storage of a frequency-multiplexed single photon into a laser-written waveguide integrated in the same crystal structure.

Using multimode quantum memories would lead to a significant improvement in the scaling of quantum networks since it would allow the reduction of entanglement distribution time between remote quantum nodes. Very few experiments have explored frequency multiplexing so far. Rare-earth doped crystals are well suited for this task, thanks to their static inhomogeneous broadening of the optical transition, allowing the creation of multiple quantum memories at different frequencies.

In order to test this, the team set up an experiment that consisted of two main parts: a photon-pair source an integrated quantum memory. For the photon-pair source, the researchers used a source based on cavity enhanced spontaneous down conversion to generate a heralded single photon pair compatible with the quantum memory, with a spectrum consisting of 15 discrete frequency bins. The photon, was then transmitted through a SM fiber, and then, by implementing a storage protocol based on the atomic frequency comb technique, the photon was stored into an integrated quantum memory based on a laser-written waveguide. The photon absorbed in the waveguide was mapped into a collective superposition of atomic excitations. To store the whole spectrum of frequency-multiplexed photons, they used electro-optical modulators, which allowed them to create 15 quantum memories at different frequencies, demonstrating this technique to be suitable for storing of frequency bin entanglement. Altogether, combining spectral and temporal multiplexing, the researchers were able to store 130 modes in the crystal.

The results of these studies open the door to the use of this technique and devices as multiplexed quantum repeaters and memories, as well as confirming them as ideal scenarios for research on high-dimensional entanglement in time and frequency between light and matter.