arXiv 1503.089552015-03-31quant-ph

D波机器的长时间相干量子行为

Long-time coherent quantum behaviour of the D-Wave machine

D. Drakova, G. Doyen

摘要：

大量的实验已经证明了D-Wave量子计算机长时间运行的量子行为。据报道，单个通量量子比特的退相干时间为纳秒量级，这比在秒的时间尺度上进行计算所需的时间要短得多。在我们的贡献中，我们研究了一个模型的四个量子比特与一个声子耦合和（可选）的环境粒子的高密度的状态，称为gravonons。计算表明，当没有引力子存在时，量子比特中的电流在某个时候翻转，绝热演化停止。含时波泛函成为许多激发态的不可校正的叠加。结果表明，当考虑引力子纠缠时，可以有效地抑制电流翻转，并允许继续绝热演化。然而，这种绝热演化是高维时空中的相干演化，不能理解为四维时空中薛定谔含时方程的解。与薛定谔的时间发展相比，演化速度大大减慢，但仍然是绝热的。我们的模型的性质正确地反映了实验发现的行为的D波机和解释的因素$10^8$的差异之间的退相干时间和量子计算时间。观察和我们的解释是在类比的$10^8$差异因子发现，当比较实验结果的吸附质量子扩散速率与薛定谔的时间依赖方程的预测，这也可以解决在一个模型中的耦合到引力子。

英文摘要：

Extensive experiments have demonstrated quantum behaviour in the long-time operation of the D-Wave quantum computer. The decoherence time of a single flux qubit is reported to be on the order of nanoseconds, which is much shorter than the time required to carry out a computation on the timescale of seconds. In our contribution we investigate a model of four qubits with one qubit coupled to a phonon and (optionally) to environmental particles of high density of states, called gravonons. The calculations indicate that when no gravonons are present, the current in the qubit is flipped at some time and adiabatic evolution is discontinued. The time dependent wave functional becomes a non-correctable superposition of many excited states. The results demonstrate the possibility of effectively suppressing the current flip and allowing for continued adiabatic evolution when the entanglement to gravonons is included. This adiabatic evolution is, however, a coherent evolution in high dimensional spacetime and cannot be understood as a solution of Schrödinger's time dependent equation in four dimensional spacetime. Compared to Schrödinger's time development, the evolution is considerably slowed down, though still adiabatic. The properties of our model reflect correctly the experimentally found behaviour of the D-Wave machine and explain the factor of $10^8$ discrepancy between decoherence time and quantum computation time. The observation and our explanation are in analogy to the $10^8$ discrepancy factor found, when comparing experimental results on adsorbate quantum diffusion rate with predictions of Schrödinger's time dependent equation, which can also be resolved in a model with the coupling to gravonons included.

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