Quantum feedback

Quantum feedback or Quantum feedback control is a class of methods to prepare and manipulate a quantum system in which that system's quantum state or trajectory is used to evolve the system towards some desired outcome. Just as in the classical case, feedback occurs when outputs from the system are used as inputs that control the dynamics (e.g. by controlling the Hamiltonian of the system). The feedback signal is typically filtered or processed in a classical way, which is often described as measurement based feedback. However, quantum feedback also allows the possibility of maintaining the quantum coherence of the output as the signal is processed (via unitary evolution), which has no classical analogue.^[1][2]

A feedback loop where all outputs of a process are available as causal inputs to that process

Notes

1. Lloyd, Seth (14 July 2000). "Coherent quantum feedback". Physical Review A 62 (2). doi:10.1103/PhysRevA.62.022108.
2. Nelson, Richard J.; Weinstein, Yaakov; Cory, David; Lloyd, Seth (2 October 2000). "Experimental Demonstration of Fully Coherent Quantum Feedback". Physical Review Letters 85 (14): 3045–3048. doi:10.1103/PhysRevLett.85.3045.

References

• H. M. Wiseman and G. J. Milburn, Quantum Measurement and Control (Cambridge University Press, 2009).
• H. Wiseman and G. J. Milburn, "Quantum theory of optical feedback via homodyne detection," Phys. Rev. Lett. 70, 548–551 (1993) .
• H. Wiseman, "Quantum theory of continuous feedback," Phys. Rev. A 49, 2133–2150 (1994) .
• H. M. Wiseman and G. J. Milburn, "Quantum theory of field-quadrature measurements," Phys. Rev. A 47, 642 (1993) .
• H. M. Wiseman and G. J. Milburn, "Interpretation of quantum jump and diffusion processes illustrated on the Bloch sphere," Phys. Rev. A 47, 1652–1666 (1993) .
• H. Wiseman and G. Milburn, "Squeezing via feedback," Phys. Rev. A 49, 1350–1366 (1994) .
• H. M. Wiseman, "Quantum trajectories and quantum measurement theory," Quantum Semiclassical Opt. J. Eur. Opt. Soc. Part B 8, 205–222 (1996) .
• S. Lloyd, "Coherent quantum feedback," Phys. Rev. A 62, (2000) .
• R. J. Nelson, Y. Weinstein, D. Cory, and S. Lloyd, "Experimental demonstration of fully coherent quantum feedback," Phys. Rev. Lett. 85, 3045–3048 (2000) .