A high on-off ratio beamsplitter interaction for gates on bosonically encoded qubits

  1. Benjamin J. Chapman,
  2. Stijn J. de Graaf,
  3. Sophia H. Xue,
  4. Yaxing Zhang,
  5. James Teoh,
  6. Jacob C. Curtis,
  7. Takahiro Tsunoda,
  8. Alec Eickbusch,
  9. Alexander P. Read,
  10. Akshay Koottandavida,
  11. Shantanu O. Mundhada,
  12. Luigi Frunzio,
  13. M. H. Devoret,
  14. S. M. Girvin,
  15. and R. J. Schoelkopf
Encoding a qubit in a high quality superconducting microwave cavity offers the opportunity to perform the first layer of error correction in a single device, but presents a challenge:
how can quantum oscillators be controlled while introducing a minimal number of additional error channels? We focus on the two-qubit portion of this control problem by using a 3-wave mixing coupling element to engineer a programmable beamsplitter interaction between two bosonic modes separated by more than an octave in frequency, without introducing major additional sources of decoherence. Combining this with single-oscillator control provided by a dispersively coupled transmon provides a framework for quantum control of multiple encoded qubits. The beamsplitter interaction gbs is fast relative to the timescale of oscillator decoherence, enabling over 103 beamsplitter operations per coherence time, and approaching the typical rate of the dispersive coupling χ used for individual oscillator control. Further, the programmable coupling is engineered without adding unwanted interactions between the oscillators, as evidenced by the high on-off ratio of the operations, which can exceed 105. We then introduce a new protocol to realize a hybrid controlled-SWAP operation in the regime gbs≈χ, in which a transmon provides the control bit for the SWAP of two bosonic modes. Finally, we use this gate in a SWAP test to project a pair of bosonic qubits into a Bell state with measurement-corrected fidelity of 95.5%±0.2%.

Error-detected state transfer and entanglement in a superconducting quantum network

  1. Luke D Burkhart,
  2. James Teoh,
  3. Yaxing Zhang,
  4. Christopher J Axline,
  5. Luigi Frunzio,
  6. M.H. Devoret,
  7. Liang Jiang,
  8. S.M. Girvin,
  9. and R. J. Schoelkopf
Modular networks are a promising paradigm for increasingly complex quantum devices based on the ability to transfer qubits and generate entanglement between modules. These tasks require
a low-loss, high-speed intermodule link that enables extensible network connectivity. Satisfying these demands simultaneously remains an outstanding goal for long-range optical quantum networks as well as modular superconducting processors within a single cryostat. We demonstrate communication and entanglement in a superconducting network with a microwave-actuated beamsplitter transformation between two bosonic qubits, which are housed in separate modules and joined by a demountable coaxial bus resonator. We transfer a qubit in a multi-photon encoding and track photon loss events to improve the fidelity, making it as high as in a single-photon encoding. Furthermore, generating entanglement with two-photon interference and postselection against loss errors produces a Bell state with success probability 79% and fidelity 0.94, halving the error obtained with a single photon. These capabilities demonstrate several promising methods for faithful operations between modules, including novel possibilities for resource-efficient direct gates.