Lu Dawei, Li Hang, Trottier Denis-Alexandre, Li Jun, Brodutch Aharon, Krismanich Anthony P, Ghavami Ahmad, Dmitrienko Gary I, Long Guilu, Baugh Jonathan, Laflamme Raymond
Institute for Quantum Computing and Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, China.
Phys Rev Lett. 2015 Apr 10;114(14):140505. doi: 10.1103/PhysRevLett.114.140505. Epub 2015 Apr 8.
One of the major experimental achievements in the past decades is the ability to control quantum systems to high levels of precision. To quantify the level of control we need to characterize the dynamical evolution. Full characterization via quantum process tomography is impractical and often unnecessary. For most practical purposes, it is enough to estimate more general quantities such as the average fidelity. Here we use a unitary 2-design and twirling protocol for efficiently estimating the average fidelity of Clifford gates, to certify a 7-qubit entangling gate in a nuclear magnetic resonance quantum processor. Compared with more than 10^{8} experiments required by full process tomography, we conducted 1656 experiments to satisfy a statistical confidence level of 99%. The average fidelity of this Clifford gate in experiment is 55.1%, and rises to at least 87.5% if the signal's decay due to decoherence is taken into account. The entire protocol of certifying Clifford gates is efficient and scalable, and can easily be extended to any general quantum information processor with minor modifications.
过去几十年的主要实验成果之一是能够将量子系统控制到高精度水平。为了量化控制水平，我们需要描述动力学演化。通过量子过程层析成像进行完全表征是不切实际的，而且通常也没有必要。对于大多数实际目的，估计更一般的量，如平均保真度，就足够了。在这里，我们使用酉2 - 设计和旋转协议来有效估计克利福德门的平均保真度，以在核磁共振量子处理器中验证一个7量子比特纠缠门。与完全过程层析成像所需的超过10^8次实验相比，我们进行了1656次实验以满足99%的统计置信水平。该克利福德门在实验中的平均保真度为55.1%，如果考虑到由于退相干导致的信号衰减，该保真度至少可提高到87.5%。验证克利福德门的整个协议高效且可扩展，只需进行少量修改就能轻松扩展到任何通用量子信息处理器。
