Towards long-distance quantum networks with superconducting processors and optical links

  1. Sourabh Kumar,
  2. Nikolai Lauk,
  3. and Christoph Simon
We design a quantum repeater architecture, necessary for long distance quantum networks, using the recently proposed microwave cat state qubits, formed and manipulated via interaction
between a superconducting nonlinear element and a microwave cavity. These qubits are especially attractive for repeaters because in addition to serving as excellent computational units with deterministic gate operations, they also have coherence times long enough to deal with the unavoidable propagation delays. Since microwave photons are too low in energy to be able to carry quantum information over long distances, as an intermediate step, we expand on a recently proposed microwave to optical transduction protocol using excited states of a rare-earth ion (Er3+) doped crystal. To enhance the entanglement distribution rate, we propose to use spectral multiplexing by employing an array of cavities at each node. We compare our achievable rates with direct transmission and with a popular ensemble-based repeater approach and show that ours could be higher in appropriate regimes, even in the presence of realistic imperfections and noise, while maintaining reasonably high fidelities of the final state. In the short term, our work could be directly useful for secure quantum communication, whereas in the long term, we can envision a large scale distributed quantum computing network built on our architecture.

Electron Spin Coherences in Rare-Earth Optically Excited States for Microwave to Optical Quantum Transducers

  1. Sacha Welinski,
  2. Philip J. T. Woodburn,
  3. Nikolai Lauk,
  4. Rufus L. Cone,
  5. Christoph Simon,
  6. Philippe Goldner,
  7. and Charles W. Thiel
Efficient and reversible optical to microwave coherent transducers are required to enable entanglement transfer between superconducting qubits and light for quantum networks. Rare-earth-doped
crystals that possess narrow optical and spin transitions are a promising way to implement these devices. Current approaches use ground-state electron spin transitions that have coherence lifetimes (T2) often limited by spin flip-flop processes and/or spectral diffusion, even at very low temperatures. Here, we investigate spin coherence in an optically excited state of an Er3+:Y2SiO5 crystal at temperatures from 1.6 to 3.5 K and under a weak 8.7 mT magnetic field. Spin coherence and population lifetimes of up to 1.6 μs and 1.2 ms, respectively, are measured by 2- and 3-pulse optically-detected spin echo experiments. Analysis of the dephasing processes suggest that ms T2 can be reached at lower temperatures for the excited-state spins, whereas ground-state spin states could be limited to a few μs due to resonant interactions with the other Er3+ spins in the lattice (spin diffusion). We propose a quantum transducer scheme with the potential for close to unit efficiency that exploits the specific advantages offered by the spin states of optically excited electronic energy levels.