No abstract

Experimental microwave teleportation brings quantum local area networks within reach by exceeding the no-cloning limit.

For a quantum system controlled by an external field, time-optimal control is referred to as the shortest time duration control that can still permit maximizing an objective function J, which is especially a desirable goal for engineering quantum dynamics against decoherence effects. However, since rigorously finding a timeoptimal control is usually very difficult, and in many circumstances the control is only required to be sufficiently short and precise, one can design algorithms seeking such suboptimal control solutions for much reduced computational effort. In this paper, we propose an iterative algorithm for finding near time-optimal control in a high level-set (i.e., the set of controls that achieves the same value of J) that can be arbitrarily close to the global optima. The algorithm proceeds seeking to decrease the time duration T with the value of J remains invariant, until J leaves level-set value; the deviation of J due to numerical errors is corrected by gradient climbing that brings the search back to the level-set J value. Since the level-set is very close to the maximum value of J, the resulting control solution is nearly time-optimal with manageable precision. Numerical examples demonstrate the effectiveness and general applicability of the algorithm.

The low-noise amplification of weak microwave signals is crucial for countless protocols in quantum information processing. Quantum mechanics sets an ultimate lower limit of half a photon to the added input noise for phase-preserving amplification of narrowband signals, also known as the standard quantum limit (SQL). This limit, which is equivalent to a maximum quantum efficiency of 0.5, can be overcome by employing nondegenerate parametric amplification of broadband signals. We show that, in principle, a maximum quantum efficiency of unity can be reached. Experimentally, we find a quantum efficiency of 0.69 ± 0.02, well beyond the SQL, by employing a flux-driven Josephson parametric amplifier and broadband thermal signals. We expect that our results allow for fundamental improvements in the detection of ultraweak microwave signals.