High-fidelity software-defined quantum logic on a superconducting qudit

  1. Xian Wu,
  2. S.L. Tomarken,
  3. N. Anders Petersson,
  4. L.A. Martinez,
  5. Yaniv J. Rosen,
  6. and Jonathan L DuBois
Nearly all modern solid-state quantum processors approach quantum computation with a set of discrete qubit operations (gates) that can achieve universal quantum control with only a
handful of primitive gates. In principle, this approach is highly flexible, allowing full control over the qubits‘ Hilbert space without necessitating the development of specific control protocols for each application. However, current error rates on quantum hardware place harsh limits on the number of primitive gates that can be concatenated together (with compounding error rates) and remain viable. Here, we report our efforts at implementing a software-defined 0↔2 SWAP gate that does not rely on a primitive gate set and achieves an average gate fidelity of 99.4%. Our work represents an alternative, fully generalizable route towards achieving nontrivial quantum control through the use of optimal control techniques. We describe our procedure for computing optimal control solutions, calibrating the quantum and classical hardware chain, and characterizing the fidelity of the optimal control gate.

Active protection of a superconducting qubit with an interferometric Josephson isolator

  1. Baleegh Abdo,
  2. Nicholas T. Bronn,
  3. Oblesh Jinka,
  4. Salvatore Olivadese,
  5. Antonio D. Corcoles,
  6. Vivekananda P. Adiga,
  7. Markus Brink,
  8. Russell E. Lake,
  9. Xian Wu,
  10. David P. Pappas,
  11. and Jerry M. Chow
Nonreciprocal microwave devices play several critical roles in high-fidelity, quantum-nondemolition (QND) measurement schemes. They separate input from output, impose unidirectional
routing of readout signals, and protect the quantum systems from unwanted noise originated by the output chain. However, state-of-the-art, cryogenic circulators and isolators are disadvantageous in scalable superconducting quantum processors because they use magnetic materials and strong magnetic fields. Here, we realize an active isolator formed by coupling two nondegenerate Josephson mixers in an interferometric scheme. Nonreciprocity is generated by applying a phase gradient between the same-frequency pumps feeding the Josephson mixers, which play the role of the magnetic field in a Faraday medium. To demonstrate the applicability of this Josephson-based isolator for quantum measurements, we incorporate it into the output line of a superconducting qubit, coupled to a fast resonator and a Purcell filter. We also utilize a wideband, superconducting directional coupler for coupling the readout signals into and out of the qubit-resonator system and a quantum-limited Josephson amplifier for boosting the readout fidelity. By using this novel quantum setup, we demonstrate fast, high-fidelity, QND measurements of the qubit while providing more than 20 dB of protection against amplified noise reflected off the Josephson amplifier.

Kinetic Inductance Traveling Wave Amplifiers For Multiplexed Qubit Readout

  1. Leonardo Ranzani,
  2. Mustafa Bal,
  3. Kin Chung Fong,
  4. Guilhem Ribeill,
  5. Xian Wu,
  6. Junling Long,
  7. Hsiang-Sheng Ku,
  8. Robert P. Erickson,
  9. David Pappas,
  10. and Thomas A. Ohki
We describe a kinetic inductance traveling-wave (KIT) amplifier suitable for superconducting quantum information measurements and characterize its wideband scattering and noise properties.
We use mechanical microwave switches to calibrate the four amplifier scattering parameters up to the device input and output connectors at the dilution refrigerator base temperature and a tunable temperature load to characterize the amplifier noise. Finally, we demonstrate the high fidelity simultaneous dispersive readout of two superconducting transmon qubits. The KIT amplifier provides low-noise amplification of both readout tones with readout fidelities in excess of 89% and negligible effect on qubit lifetime and coherence.

High Coherence Plane Breaking Packaging for Superconducting Qubits

  1. Nicholas T. Bronn,
  2. Vivekananda P. Adiga,
  3. Salvatore B. Olivadese,
  4. Xian Wu,
  5. Jerry M. Chow,
  6. and David P. Pappas
We demonstrate a pogo pin package for a superconducting quantum processor specifically designed with a nontrivial layout topology (e.g., a center qubit that cannot be accessed from
the sides of the chip). Two experiments on two nominally identical superconducting quantum processors in pogo packages, which use commercially available parts and require modest machining tolerances, are performed at low temperature (10 mK) in a dilution refrigerator and both found to behave comparably to processors in standard planar packages with wirebonds where control and readout signals come in from the edges. Single- and two-qubit gate errors are also characterized via randomized benchmarking. More detailed crosstalk measurements indicate levels of crosstalk less than -40 dB at the qubit frequencies, opening the possibility of integration with extensible qubit architectures.

Low-noise kinetic inductance traveling-wave amplifier using three-wave mixing

  1. Michael R. Vissers,
  2. Robert P. Erickson,
  3. Hsiang-Sheng Ku,
  4. Leila Vale,
  5. Xian Wu,
  6. Gene Hilton,
  7. and David P. Pappas
We have fabricated a wide-bandwidth, high dynamic range, low-noise cryogenic amplifier based on a superconducting kinetic inductance traveling-wave device. The device was made from
NbTiN and consisted of a long, coplanar waveguide on a silicon chip. By adding a DC current and an RF pump tone we are able to generate parametric amplification using three-wave mixing. The devices exhibit gain of more than 15 dB across an instantaneous bandwidth from 4 to 8 GHz. The total usable gain bandwidth, including both sides of the signal-idler gain region, is more than 6 GHz. The noise referred to the input of the devices approaches the quantum limit, with less than 1 photon excess noise. Compared to similarly constructed four-wave mixing amplifiers, these devices operate with the RF pump at ∼20 dB lower power and at frequencies far from the signal. This will permit easier integration into large scale qubit and detector applications.