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.

Coherent storage of microwave excitations in rare-earth nuclear spins

  1. Gary Wolfowicz,
  2. Hannes Maier-Flaig,
  3. Robert Marino,
  4. Alban Ferrier,
  5. Hervé Vezin,
  6. John J.L. Morton,
  7. and Philippe Goldner
Interfacing between various elements of a computer – from memory to processors to long range communication – will be as critical for quantum computers as it is for classical
computers today. Paramagnetic rare earth doped crystals, such as Nd3+:Y2SiO5 (YSO), are excellent candidates for such a quantum interface: they are known to exhibit long optical coherence lifetimes (for communication via optical photons), possess a nuclear spin (memory) and have in addition an electron spin that can offer hybrid coupling with superconducting qubits (processing). Here we study two of these three elements, demonstrating coherent storage and retrieval between electron and 145Nd nuclear spin states in Nd3+:YSO. We find nuclear spin coherence times can reach 9 ms at ≈5 K, about two orders of magnitude longer than the electron spin coherence, while quantum state and process tomography of the storage/retrieval operation reveal an average state fidelity of 0.86. The times and fidelities are expected to further improve at lower temperatures and with more homogeneous radio-frequency excitation.