Realization of a Universal Quantum Gate Set for Itinerant Microwave Photons

  1. Kevin Reuer,
  2. Jean-Claude Besse,
  3. Lucien Wernli,
  4. Paul Magnard,
  5. Philipp Kurpiers,
  6. Graham J. Norris,
  7. Andreas Wallraff,
  8. and Christopher Eichler
Deterministic photon-photon gates enable the controlled generation of entanglement between mobile carriers of quantum information. Such gates have thus far been exclusively realized
in the optical domain and by relying on post-selection. Here, we present a non-post-selected, deterministic, photon-photon gate in the microwave frequency range realized using superconducting circuits. We emit photonic qubits from a source chip and route those qubits to a gate chip with which we realize a universal gate set by combining controlled absorption and re-emission with single-qubit gates and qubit-photon controlled-phase gates. We measure quantum process fidelities of 75% for single- and of 57% for two-qubit gates, limited mainly by radiation loss and decoherence. This universal gate set has a wide range of potential applications in superconducting quantum networks.

Microwave Quantum Link between Superconducting Circuits Housed in Spatially Separated Cryogenic Systems

  1. Paul Magnard,
  2. Simon Storz,
  3. Philipp Kurpiers,
  4. Josua Schär,
  5. Fabian Marxer,
  6. Janis Luetolf,
  7. Jean-Claude Besse,
  8. Mihai Gabureac,
  9. Kevin Reuer,
  10. Abdulkadir Akin,
  11. Baptiste Royer,
  12. Alexandre Blais,
  13. and Andreas Wallraff
Superconducting circuits are a strong contender for realizing quantum computing systems, and are also successfully used to study quantum optics and hybrid quantum systems. However,
their cryogenic operation temperatures and the current lack of coherence-preserving microwave-to-optical conversion solutions have hindered the realization of superconducting quantum networks either spanning different cryogenics systems or larger distances. Here, we report the successful operation of a cryogenic waveguide coherently linking transmon qubits located in two dilution refrigerators separated by a physical distance of five meters. We transfer qubit states and generate entanglement on-demand with average transfer and target state fidelities of 85.8 % and 79.5 %, respectively, between the two nodes of this elementary network. Cryogenic microwave links do provide an opportunity to scale up systems for quantum computing and create local area quantum communication networks over length scales of at least tens of meters.

Realizing a Deterministic Source of Multipartite-Entangled Photonic Qubits

  1. Jean-Claude Besse,
  2. Kevin Reuer,
  3. Michele C. Collodo,
  4. Arne Wulff,
  5. Lucien Wernli,
  6. Adrian Copetudo,
  7. Daniel Malz,
  8. Paul Magnard,
  9. Abdulkadir Akin,
  10. Mihai Gabureac,
  11. Graham J. Norris,
  12. J. Ignacio Cirac,
  13. Andreas Wallraff,
  14. and Christopher Eichler
Sources of entangled electromagnetic radiation are a cornerstone in quantum information processing and offer unique opportunities for the study of quantum many-body physics in a controlled
experimental setting. While multi-mode entangled states of radiation have been generated in various platforms, all previous experiments are either probabilistic or restricted to generate specific types of states with a moderate entanglement length. Here, we demonstrate the fully deterministic generation of purely photonic entangled states such as the cluster, GHZ, and W state by sequentially emitting microwave photons from a controlled auxiliary system into a waveguide. We tomographically reconstruct the entire quantum many-body state for up to N=4 photonic modes and infer the quantum state for even larger N from process tomography. We estimate that localizable entanglement persists over a distance of approximately ten photonic qubits, outperforming any previous deterministic scheme.