The ability to detect the presence of a single, travelling photon without destroying it has been a long standing project in optics and is fundamental for applications in quantum informationand measurement. The realization of such a detector has been complicated by the fact that photon- photon interactions are very weak at optical frequencies. At microwave frequencies, very strong photon-photon interactions have been demonstrated. Here however, the single-photon detector has been elusive due to the low energy of the microwave photon. In this article, we present a realistic proposal for quantum nondemolition measurements of a single propagating microwave photon. The detector design is built on a of chain of artificial atoms connected through circulators which break time-reversal symmetry, making both signal and probe photons propagate in one direction only. Our analysis is based on the theory of cascaded quantum systems and quantum trajectories which takes the full dynamics of the atom-field interaction into account. We show that a signal-to-noise ratio above one can be realized with current state of the art microwave technology.
We describe a multi-mode quantum memory for propagating microwave photons
that combines a solid-state spin ensemble resonantly coupled to a frequency
tunable single-mode microwave cavity.We first show that high efficiency
mapping of the quantum state transported by a free photon to the spin ensemble
is possible both for strong and weak coupling between the cavity mode and the
spin ensemble. We also show that even in the weak coupling limit unit
efficiency and faithful retrieval can be obtained through time reversal
inhomogeneous dephasing based on spin echo techniques. This is possible
provided that the cavity containing the spin ensemble and the transmission line
are impedance matched. We finally discuss the prospects for an experimental
implementation using a rare-earth doped crystal coupled to a superconducting
resonator.
We investigate quantum correlations in microwave radiation produced by the
dynamical Casimir effect in a superconducting waveguide terminated and
modulated by a superconducting quantuminterference device. We apply
nonclassicality tests and evaluate the entanglement for the predicted field
states. For realistic circuit parameters, including thermal background noise,
the results indicate that the produced radiation can be strictly nonclassical
and can have a measurable amount of intermode entanglement. If measured
experimentally, these nonclassicalilty indicators could give further evidence
of the quantum nature of the dynamical Casimir radiation in these circuits.