Microwave quantum tokens
in electronic and nuclear spin ensembles

The core objective of this collaborative project is the realization, investigation and demonstration of quantum tokens (Q-tokens) in the microwave or GHz frequency range in the form of quantum keys stored in quantum memories based on spin ensembles. In general, scalable and long-living quantum memories represent one of the missing elements needed to build local microwave-based quantum networks. Quantum tokens in the form of propagating squeezed states represent a particularly important use case for such quantum networks.

To achieve our ambitious goals, we identify the following work items which correspond to individual sub-projects within WMI:

1. Experimental realization, characterization and optimization of Q-token generation using time and frequency multiplexing techniques.

2. Experimental realization, characterization and improvement of rare-earth spin-based microwave quantum memory including storage and retrieval of generated Q-tokens encoded in frequency domain.

3. Experimental realization, characterization and improvement of microwave quantum memory based on phosphorous donors in silicon including storage and retrieval of generated Q-tokens encoded in time-domain.

Generation of Q-Tokens in the GHz-frequency range can be achieved by exploiting displaced squeezed states. The latter are generated with the help of superconducting Josephson parametric amplifiers. The actual quantum memories are realized with spin ensembles possessing transition frequencies in the GHz range, represented by two approaches: the storage of keys in (i) the rare earth ion system ¹⁶⁷Er:Y₂SiO₅, and (ii) in the isotopically-purified ²⁸Si:P donor system coupled to a microwave resonator. These different spin systems have mutually exclusive advantages, and therefore, both will have useful applications in certain quantum communication scenarios.

Achieving our key goals will require a close coordination between the sub-projects by combining those into final joint experiments towards successful storage of microwave Q-Tokens in quantum spin memories.