Scientists have achieved, for the first time, on-demand storage of photonic qubits in an integrated solid-state quantum memory.
The research team from the Key Laboratory of Quantum Information of the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences has described the new study in the Physical Review Letters journal.
Quantum memory forms the core device in the development of large-scale quantum networks. Quantum memory-based quantum hard drives or quantum repeaters can effectively prevent the photon loss in the channel, thereby increasing the working distance of quantum networks.
For on-demand storage, it is essential to identify the storage time once the photon is absorbed by the quantum memory, which is crucial for quantum networks. But all integrated solid-state quantum memories developed thus far are based on the atomic frequency comb (AFC) scheme, which involves a preset storage time.
The researchers achieved on-demand storage by using a modified quantum memory scheme—the Stark-modulated AFC scheme. They used the Stark effect to control the evolution of the rare-earth ions in real time by introducing two electrical pulses to regulate the storage time of the quantum memory.
First, a femtosecond laser micromachining (FLM) system was used by the team to create optical waveguides on the surface of a europium-doped yttrium silicate crystal. Then, two on-chip electrodes were placed on both sides of the optical waveguides to enable manipulation of the storage time in real-time by using a transistor-transistor logic (TTL)-compatible voltage.
The optical waveguide’s insertion loss was less than 1 dB, which is the best-ever value achieved for integrated solid-state quantum memories.
With such integrated solid-state quantum memory, the researchers demonstrated on-demand storage of time-bin qubits with a storage fidelity of 99.3% ± 0.2%. This outcome is close to the optimal storage fidelity realized in bulk crystals (99.9%, PRL108, 190505), which was also described in 2012 by the same research team. Such a high fidelity suggests the reliability of the integrated quantum memory.
This study is highly crucial for the development of large-capacity quantum memory and building quantum networks.
Liu, C., et al. (2021) On-Demand Quantum Storage of Photonic Qubits in an On-Chip Waveguide. Physical Review Letters. doi.org/10.1103/PhysRevLett.125.260504.