Quantum fluctuations of the vacuum are both a surprising and fundamental phenomenon of nature. Understood as virtual photons flitting in and out of existence, they still have a veryreal impact, \emph{e.g.}, in the Casimir effects and the lifetimes of atoms. Engineering vacuum fluctuations is therefore becoming increasingly important to emerging technologies. Here, we shape vacuum fluctuations using a „mirror“, creating regions in space where they are suppressed. As we then effectively move an artificial atom in and out of these regions, measuring the atomic lifetime tells us the strength of the fluctuations. The weakest fluctuation strength we observe is 0.02 quanta, a factor of 50 below what would be expected without the mirror, demonstrating that we can hide the atom from the vacuum.
We demonstrate the active suppression of transmon qubit dephasing induced by dispersive measurement, using parametric amplification and analog feedback. By real-time processing of thehomodyne record, the feedback controller reverts the stochastic quantum phase kick imparted by the measurement on the qubit. The feedback operation matches a model of quantum trajectories with measurement efficiency η~≈0.5, consistent with the result obtained by postselection. We overcome the bandwidth limitations of the amplification chain by numerically optimizing the signal processing in the feedback loop and provide a theoretical model explaining the optimization result.
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 present a scheme for dissipative preparation of an entangled steady state of two superconducting qubits in a circuit QED setup. Combining resonator photon loss, a dissipative processalready present in the setup, with an effective two-photon microwave drive, we engineer an effective decay mechanism which prepares a maximally entangled state of the two qubits. This state is then maintained as the steady state of the driven, dissipative evolution. The performance of the dissipative state preparation protocol is studied analytically and verified numerically. In view of the experimental implementation of the presented scheme we investigate the effects of potential experimental imperfections and show that our scheme is robust to small deviations in the parameters. We find that high fidelities with the target state can be achieved both with state-of-the-art 3D, as well as with the more commonly used 2D transmons. The promising results of our study thus open a route for the demonstration of an entangled steady state in circuit QED.
We realize indirect partial measurement of a transmon qubit in circuit
quantum electrodynamics by interaction with an ancilla qubit and projective
ancilla measurement with a dedicatedreadout resonator. Accurate control of the
interaction and ancilla measurement basis allows tailoring the measurement
strength and operator. The tradeoff between measurement strength and qubit
back-action is characterized through the distortion of a qubit Rabi oscillation
imposed by ancilla measurement in different bases. Combining partial and
projective qubit measurements, we provide the solid-state demonstration of the
correspondence between a non-classical weak value and the violation of a
Leggett-Garg inequality.
We analyze the backaction of homodyne detection and photodetection on
superconducting qubits in circuit quantum electrodynamics. Although both
measurement schemes give rise to backactionin the form of stochastic phase
rotations, which leads to dephasing, we show that this can be perfectly undone
provided that the measurement signal is fully accounted for. This result
improves upon that of Phys. Rev. A, 82, 012329 (2010), showing that the method
suggested can be made to realize a perfect two-qubit parity measurement. We
propose a benchmarking experiment on a single qubit to demonstrate the method
using homodyne detection. By analyzing the limited measurement efficiency of
the detector and bandwidth of the amplifier, we show that the parameter values
necessary to see the effect are within the limits of existing technology.