Zhihui Wang

Defining and detecting quantum speedup

  1. Troels F. Rønnow,
  2. Zhihui Wang,
  3. Joshua Job,
  4. Sergio Boixo,
  5. Sergei V. Isakov,
  6. David Wecker,
  7. John M. Martinis,
  8. Daniel A. Lidar,
  9. and Matthias Troyer
The development of small-scale digital and analog quantum devices raises the question of how to fairly assess and compare the computational power of classical and quantum devices, and
of how to detect quantum speedup. Here we show how to define and measure quantum speedup in various scenarios, and how to avoid pitfalls that might mask or fake quantum speedup. We illustrate our discussion with data from a randomized benchmark test on a D-Wave Two device with up to 503 qubits. Comparing the performance of the device on random spin glass instances with limited precision to simulated classical and quantum annealers, we find no evidence of quantum speedup when the entire data set is considered, and obtain inconclusive results when comparing subsets of instances on an instance-by-instance basis. Our results for one particular benchmark do not rule out the possibility of speedup for other classes of problems and illustrate that quantum speedup is elusive and can depend on the question posed.
arxiv pdf

Quantum annealing with more than one hundred qubits

  1. Sergio Boixo,
  2. Troels F. Rønnow,
  3. Sergei V. Isakov,
  4. Zhihui Wang,
  5. David Wecker,
  6. Daniel A. Lidar,
  7. John M. Martinis,
  8. and Matthias Troyer
At a time when quantum effects start to pose limits to further miniaturisation of devices and the exponential performance increase due to Moore’s law, quantum technology is maturing
to the point where quantum devices, such as quantum communication systems, quantum random number generators and quantum simulators, may be built with powers exceeding the performance of classical computers. A quantum annealer, in particular, finds solutions to hard optimisation problems by evolving a known initial configuration towards the ground state of a Hamiltonian that encodes an optimisation problem. Here, we present results from experiments on a 108 qubit D-Wave One device based on superconducting flux qubits. The correlations between the device and a simulated quantum annealer demonstrate that the device performs quantum annealing: unlike classical thermal annealing it exhibits a bimodal separation of hard and easy problems, with small-gap avoided level crossings characterizing the hard problems. To assess the computational power of the quantum annealer we compare it to optimised classical algorithms. We discuss how quantum speedup could be detected on devices scaled to a larger number of qubits where the limits of classical algorithms are reached.
arxiv pdf