Loss resilience of driven-dissipative remote entanglement in chiral waveguide quantum electrodynamics

  1. Abdullah Irfan,
  2. Mingxing Yao,
  3. Andrew Lingenfelter,
  4. Xi Cao,
  5. Aashish A. Clerk,
  6. and Wolfgang Pfaff
Establishing limits of entanglement in open quantum systems is a problem of fundamental interest, with strong implications for applications in quantum information science. Here, we
study limits of entanglement stabilization between remote qubits. We theoretically investigate the loss resilience of driven-dissipative entanglement between remote qubits coupled to a chiral waveguide. We find that by coupling a pair of storage qubits to the two driven qubits, the steady state can be tailored such that the storage qubits show a degree of entanglement that is higher than what can be achieved with only two driven qubits coupled to the waveguide. By reducing the degree of entanglement of the driven qubits, we show that the entanglement between the storage qubits becomes more resilient to waveguide loss. Our analytical and numerical results offer insights into how waveguide loss limits the degree of entanglement in this driven-dissipative system, and offers important guidance for remote entanglement stabilization in the laboratory, for example using superconducting circuits.

Fast superconducting qubit control with sub-harmonic drives

  1. Mingkang Xia,
  2. Chao Zhou,
  3. Chenxu Liu,
  4. Param Patel,
  5. Xi Cao,
  6. Pinlei Lu,
  7. Boris Mesits,
  8. Maria Mucci,
  9. David Gorski,
  10. David Pekker,
  11. and Michael Hatridge
Increasing the fidelity of single-qubit gates requires a combination of faster pulses and increased qubit coherence. However, with resonant qubit drive via a capacitively coupled port,
these two objectives are mutually contradictory, as higher qubit quality factor requires a weaker coupling, necessitating longer pulses for the same applied power. Increasing drive power, on the other hand, can heat the qubit’s environment and degrade coherence. In this work, by using the inherent non-linearity of the transmon qubit, we circumvent this issue by introducing a new parametric driving scheme to perform single-qubit control. Specifically, we achieve rapid gate speed by pumping the transmon’s native Kerr term at approximately one third of the qubit’s resonant frequency. Given that transmons typically operate within a fairly narrow range of anharmonicity, this technique is applicable to all transmons. In both theory and experiment, we show that the Rabi rate of the process is proportional to applied drive amplitude cubed, allowing for rapid gate speed with only modest increases in applied power. In addition, we demonstrate that filtering can be used to protect the qubit’s coherence while performing rapid gates, and present theoretical calculations indicating that decay due to multi-photon losses, even in very strongly coupled drive lines, will not limit qubit lifetime. We demonstrate π/2 pulses as short as tens of nanoseconds with fidelity as high as 99.7\%, limited by the modest coherence of our transmon. We also present calculations indicating that this technique could reduce cryostat heating for fast gates, a vital requirement for large-scale quantum computers.

A modular quantum computer based on a quantum state router

  1. Chao Zhou,
  2. Pinlei Lu,
  3. Matthieu Praquin,
  4. Tzu-Chiao Chien,
  5. Ryan Kaufman,
  6. Xi Cao,
  7. Mingkang Xia,
  8. Roger Mong,
  9. Wolfgang Pfaff,
  10. David Pekker,
  11. and Michael Hatridge
In this work, we present the design of a superconducting, microwave quantum state router which can realize all-to-all couplings among four quantum modules. Each module consists of a
single transmon, readout mode, and communication mode coupled to the router. The router design centers on a parametrically driven, Josephson-junction based three-wave mixing element which generates photon exchange among the modules‘ communication modes. We first demonstrate SWAP operations among the four communication modes, with an average full-SWAP time of 760 ns and average inter-module gate fidelity of 0.97, limited by our modes‘ coherences. We also demonstrate photon transfer and pairwise entanglement between the modules‘ qubits, and parallel operation of simultaneous SWAP gates across the router. These results can readily be extended to faster and higher fidelity router operations, as well as scaled to support larger networks of quantum modules.