Squeezing via feedback

We present the quantum theory of optical cavity feedback mediated by homodyne detection, with an arbitrary time delay. We apply this theory to a system with nonclassical dynamics, a sub-Poissonian pumped laser. By using the feedback to phase lock the laser it is possible to produce output light which exhibits perfect quadrature squeezing on resonance, rather than just sub-Poissonian intensity statistics. However, we also show that feedback mediated by homodyne detection (or any other extracavity measurement) cannot produce nonclassical light unless the cavity dynamics can do so without feedback. Furthermore, in systems which already exhibit squeezing, such feedback can only degrade the squeezing in the output. With feedback mediated by an intracavity measurement, these theorems do not apply. We show that an (admittedly unrealistic) intracavity quantum nondemolition quadrature measurement allows arbitrary squeezing to be produced by controlling the amplitude of a coherent driving field

Quantum-Noise Reduction in a Driven Cavity with Feedback

We show that amplitude-squeezed states may be produced by driving a feedback-controlled cavity with a coherent input signal. The feedback controls the transmissivity of one output from the cavity and is essentially equivalent to nonlinear absorption. The cavity effectively acts as a nonlinear reflector. Hence, amplitude-squeezed states with arbitrarily strong coherent intensities can be obtained

Creating number states in the micromaser using feedback

We use the quantum theory of feedback developed by Wiseman and Milburn [Phys. Rev. Lett. 70, 548 (1993)] and Wiseman [Phys. Rev. A 49, 2133 (1994)] to investigate the photon-number noise properties of the micromaser with direct detection feedback. We find that the feedback can significantly reduce the amount of noise in the photon number. Under the right conditions the feedback locks the systems onto a number state. As opposed to other schemes in the past [P. Meystre, Opt. Lett. 12, 669 (1987); J. Krause, M. O. Scully, and H. Walther, Phys. Rev. A 36, 4547 (1987)], we can fix the number states to which the system evolves. We also simulate the micromaser using the quantum-trajectories method and show that these results agree with the quantum theory of feedback. We show that the noise of quantum island states [P. Bogar, J. A. Bergou, and M. Hillary, Phys. Rev. A 50, 754 (1994)] can be significantly reduced by the feedback

The Transplanted Appropriate Adult Scheme in China

Borrowed from England and Wales, the Chinese Appropriate Adult Scheme involves a dynamic of selective adaptation. This article analyses two salient features of the appropriate adult scheme within the Chinese context, in comparison with its counterpart in England and Wales: its complementarity of the juvenile's parent, and the passive role that appropriate adults play during pretrial interrogations. Drawing upon empirical evidence, the article argues that the transplanted Chinese appropriate adult scheme has failed to oversee the legality of interrogations, nor does it provide adequate safeguards for juvenile suspects. The concept of vulnerability that lies at the heart of the appropriate adult safeguard in England and Wales appears to be lost in translation. Rather than providing a safeguard for juveniles at their most vulnerable, the appropriate adult is more concerned with indulging the needs of the interrogators in China