Researchers at the University of Science and Technology of China (USTC) have demonstrated efficient, low-noise quantum frequency conversion from 393 nanometers to the 1550-nanometer telecom band, a step toward linking trapped-ion quantum memories over long optical fibers. The work, by a team including Pan Jianwei and Zhang Qiang of USTC and Zheng Mingyang of the Jinan Institute of Quantum Technology, was published on October 13 in Physical Review Letters.
Trapped ions are one of the leading platforms for quantum networks because they offer long coherence times and high-fidelity control. However, many ions emit photons in the ultraviolet or violet range, which is absorbed quickly in optical fiber. The usual fix is quantum frequency conversion, or QFC, which shifts a photon to a telecom wavelength while preserving its quantum state. Until now, conversion from this band suffered from low efficiency and high noise, and entanglement between such ion nodes had been limited to distances of meters.
The team went back to the underlying physics of quasi-phase matching. It built a quantitative model of how phase disturbances caused by domain defects in the conversion crystal reduce efficiency, and verified it experimentally. It also found that, with a short-wavelength pump, the conversion process and the main noise source, spontaneous parametric down-conversion of the pump light, tune in opposite directions. The researchers used this to devise a method that strongly suppresses pump-induced noise.
The resulting 393-to-1550-nanometer QFC module reached an end-to-end efficiency of 28.8%, with pump noise suppressed to 35 counts per second. USTC says that is about 30 times more efficient than previous work, with noise lower by more than two orders of magnitude, setting a new record for this band.
The module has already been used in long-distance entanglement experiments with calcium-40 ion qubits, reaching a signal-to-noise ratio of 120 to 1. It supported demonstrations of long-lived, high-fidelity remote ion-ion entanglement and of device-independent quantum key distribution. The researchers say the work provides a key component for scalable quantum repeaters and wide-area quantum networks.
USTC doctoral student Yang Yi and Wang Bin, an associate researcher at the Jinan Institute of Quantum Technology, are co-first authors of the paper.
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