Quantum repeaters with multiple quantum memories provide high throughput, low latency, and high fidelity quantum state (qubit) transfer over long distances. However, conventional quantum repeater protocols require full connections among the multiple quantum memories in a repeater node, which is technically challenging. Here, we propose a quantum repeater protocol based on backward propagating photon emission and absorption, where the quantum memories are multiplexed in the time-domain to speed up a single transmission channel without requiring full connectivity, thereby drastically facilitating physical implementation. Although the protocol is described with nitrogen-vacancy (NV) centers in diamond, it is applicable to various physical systems and opens up the possibility of high-speed high-fidelity quantum networks for distributed quantum computation and the quantum Internet.

We study the relationship between the levitation gap change and losses in a superconducting magnetic bearing (SMB) system. We develop a polarization modulator bearing for a cosmic microwave background (CMB) polarization experiment. The heat generated by the polarization modulator needs to be kept very low. Therefore, it is essential to model the effect of the levitation gap change of a rotor due to gravity on the rotational loss. We have constructed a prototype small experimental platform of an axial-flux SMB with an array of high-temperature superconductors, YBCO, and a permanent magnet ring. In this system, the weight of the rotor can be set. We measured the effect of the change in the levitation gap of the rotor due to gravity on the loss. When the levitation force was 5.5 N and 16.0 N, the losses generated in the rotor were 9.1 × 10−3 W and 1.6 × 10−2 W, respectively. It was confirmed that the loss increases with the increase in the levitation force of the SMB.

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