Quantum physics: Coherence control

Harrison Ball, Trung Nguyen, Philip H. W. Leong +1 more

As more and more small-scale quantum-coherent devices are realized, we next must learn to organize them efficiently into large-scale computers. The authors use the physics of quantum control to engineer efficient, scalable physical-layer controllers at the interface between conventional and quantum systems. They demonstrate an ``assembly language'' based on Walsh functions that unifies classical hardware constraints and capabilities, the underlying needs of the quantum-coherent devices, and the mathematics of quantum algorithms and control.

A. J. Shaiju, Ian R. Petersen

A recently emerging approach to the feedback control of linear quantum systems involves the use of a controller which itself is a quantum linear system. This approach to quantum feedback control, referred to as coherent quantum feedback control, has the advantage that it does not destroy quantum information, is fast, and has the potential for efficient implementation. An important issue which arises both in the synthesis of linear coherent quantum controllers and in the modeling of linear quantum systems, is the issue of physical realizability. This issue relates to the property of whether a given set of linear quantum stochastic differential equations corresponds to a physical quantum system satisfying the laws of quantum mechanics. Under suitable assumptions, the paper shows that the question of physical realizability is equivalent to a frequency domain (J,J) -unitary condition. This is important in controller synthesis since it is the transfer function matrix of the controller which determines the closed loop system behavior.

José Tito Mendonça, Hugo Terças

W. Pötz, Ulrich Hohenester, Jaroslav Fabian

Byoung Seung HAM

Quantum coherence control is reinvestigated for a new physical insight in quantum nonlinear optics and applied for a wavelength-convertible quantum memory in a solid ensemble whose spin states are inhomogeneously broadened. Unlike typical atomic media whose spin decays are homogeneous, a spin inhomogeneously broadened solid ensemble requires a counter-intuitive quantum coherence control to avoid spontaneous emission-caused quantum noises. Such a quantum coherence control in a solid ensemble satisfying both near perfect retrieval efficiency and ultralong photon storage offers a solid framework to quantum repeaters, scalable qubit generations, quantum cryptography, and highly sensitive magnetometry. Here, the basic physics of the counter-intuitive quantum coherence control is presented not only for a fundamental understanding of collective ensemble phase control but also for a coherence conversion mechanism between optical and spin states involving Raman rephasing.