Matt Reagor

Simulating nonlinear optical processes on a superconducting quantum device

  1. Yuan Shi,
  2. Bram Evert,
  3. Amy F. Brown,
  4. Vinay Tripathi,
  5. Eyob A. Sete,
  6. Vasily Geyko,
  7. Yujin Cho,
  8. Jonathan L DuBois,
  9. Daniel Lidar,
  10. Ilon Joseph,
  11. and Matt Reagor
Simulating plasma physics on quantum computers is difficult, because most problems of interest are nonlinear, but quantum computers are not naturally suitable for nonlinear operations.
In weakly nonlinear regimes, plasma problems can be modeled as wave-wave interactions. In this paper, we develop a quantization approach to convert nonlinear wave-wave interaction problems to Hamiltonian simulation problems. We demonstrate our approach using two qubits on a superconducting device. Unlike a photonic device, a superconducting device does not naturally have the desired interactions in its native Hamiltonian. Nevertheless, Hamiltonian simulations can still be performed by decomposing required unitary operations into native gates. To improve experimental results, we employ a range of error mitigation techniques. Apart from readout error mitigation, we use randomized compilation to transform undiagnosed coherent errors into well-behaved stochastic Pauli channels. Moreover, to compensate for stochastic noise, we rescale exponentially decaying probability amplitudes using rates measured from cycle benchmarking. We carefully consider how different choices of product-formula algorithms affect the overall error and show how a trade-off can be made to best utilize limited quantum resources. This study provides a point example of how plasma problems may be solved on near-term quantum computing platforms.
arxiv pdf

Discovery of Nb hydride precipitates in superconducting qubits

  1. Jaeyel Lee,
  2. Zuhawn Sung,
  3. Akshay A. Murthy,
  4. Matt Reagor,
  5. Anna Grassellino,
  6. and Alexander Romanenko
We report the first evidence of the formation of niobium hydrides within niobium films on silicon substrates in superconducting qubits fabricated at Rigetti Computing. We combine complementary
techniques including room and cryogenic temperature atomic scale high-resolution and scanning transmission electron microscopy (HR-TEM and STEM), atomic force microscopy (AFM), and the time-of-flight secondary ion mass spectroscopy (TOF-SIMS) to reveal the existence of the niobium hydride precipitates directly in the Rigetti chip areas. Electron diffraction and high-resolution transmission electron microscopy (HR-TEM) analyses are performed at room and cryogenic temperatures (~106 K) on superconducting qubit niobium film areas, and reveal the formation of three types of Nb hydride domains with different crystalline orientations and atomic structures. There is also variation in their size and morphology from small (~5 nm) irregular shape domains within the Nb grains to large (~10-100 nm) Nb grains fully converted to niobium hydride. As niobium hydrides are non-superconducting and can easily change in size and location upon different cooldowns to cryogenic temperatures, our findings highlight a new previously unknown source of decoherence in superconducting qubits, contributing to both quasiparticle and two-level system (TLS) losses, and offering a potential explanation for qubit performance changes upon cooldowns. A pathway to mitigate the formation of the Nb hydrides for superconducting qubit applications is also discussed.
arxiv pdf