Coherent feedback control of a single qubit in diamond

Ronald Hanson

Ashkan Balouchi, Kurt A. Jacobs

Abstract We compare the performance of continuous coherent feedback, implemented using an ideal single-qubit controller, to that of continuous measurement-based feedback (MBF) for the task of controlling the state of a single qubit. We take as the basic dynamical resource the ability to couple the system to a quadrature of a traveling-wave field (for example, a transmission line) via a system observable, and the fundamental limitation is the maximum rate that is available for this coupling. We focus on the question of the best achievable control given ideal controllers. To obtain a fair comparison we acknowledge that the amplification involved in measurement-based control allows the controller to use macroscopic fields to apply feedback forces to the system, so it is natural to allow these feedback forces to be much larger than the mesoscopic coupling to the transmission line that mediates both the measurement for measurement-based control and the coupling to the mesoscopic controller for coherent control. Interestingly our numerical results indicate that under the above platform for comparison, coherent feedback can achieve exactly the performance of MBF given ideal controllers. We also discuss two control mechanisms available to coherent feedback.

Yasunobu Nakamura

Yoshiki Kashiwamura, Naoki Yamamoto

To control a quantum system, recently the coherent feedback method has attracted wide attention thanks to its several advantages over the conventional measurement-based feedback method. However, for nonlinear quantum systems, no systematic guiding principle for designing a coherent feedback controller has been known, while for the measurement case one can employ the so-called quantum non-demolition (QND) feedback method as a systematic strategy. In this paper, we develop a coherent feedback counterpart of this QND-based control strategy. The effectiveness of this proposal is evaluated in some control problems; qubit stabilization and single photon production.

A. A. Wood, Emmanuel Lilette, Yaakov Y. Fein +7 more

Amy Rouillard, Tanita Permaul, Sandeep K. Goyal +1 more

We propose a class of preparation schemes for orbital angular momentum and polarization qubits carried by single photons or classical states of light based on coherent feedback control by an ancillary degree of freedom of light. The preparation methods use linear optics and include the transcription of an arbitrary polarization state onto a two-level OAM system (swap) for arbitrary OAM values ± ℓ within a light beam, i.e., without a spatial interferometer. The preparations can be carried out with unit efficiency independent from the potentially unknown initial state of the system. The swap scheme also allows us to implement arbitrary unitary gates on OAM qubits (± ℓ ) by reducing them to polarization gates. In addition, we show how to translate measurement-based qubit control channels into coherent feedback schemes for optical implementation.