Quantum Optics Seminar: Bruno Goes, University of Grenoble Alpes

Exploring quantum measurements in light-matter interfaces

In this talk I want to discuss the main results of Refs. [1, 2]. In Ref. [1], we show that anomalous weak values and the Wigner function’s negativity, occur when analyzing the energetics of a single-qubit gate generated by a resonant coherent field traveling in a waveguide. The build-up of correlations between the qubit and the field is responsible for bounds on the gate fidelity, but also for a nontrivial energy balance recently observed in a superconducting setup. In the experimental scheme, the field is continuously monitored through heterodyne detection and then postselected over the outcomes of a final qubit’s measurement [3]. The postselected data can be interpreted as the field’s weak values and can show anomalous values in the variation of the field’s energy. We model the joint system dynamics with a collision model, gaining access to the qubit-field entangled state at any time. We find an analytical expression of the quasiprobability distribution of the postselected heterodyne signal, i.e., the conditional Husimi-Q function. The latter grants access to all the field’s weak values: we use it to obtain that of the field’s energy change and display its anomalous behavior. Finally, we derive the field’s conditional Wigner function and show that anomalous weak values and Wigner function negativities arise for the same values of the gate’s angle.

In Ref. [2], we consider a quantum interface made of a spin coupled to a one-dimensional atom, and study its potential for energy-efficient entanglement generation and readout. We show that quantum superpositions of zero and single-photon states outperform coherent pulses of light, producing more entanglement with the same energy. Entanglement is generally distributed over the polarisation and the temporal degrees of freedom, except for quasi-monochromatic pulses whose shape is preserved by light-matter interaction. The energetic advantage provided by quantum pulses over coherent ones is maintained when information on the spin state is extracted at the classical level. The proposed schemes are robust against imperfections in state-of-the-art semi-conducting devices.

[1] Maria Maffei, Cyril Elouard, Bruno O. Goes, Benjamin Huard, Andrew N. Jordan, and AlexiaAuffèves. Anomalous energy exchanges and wigner-function negativities in a single-qubit gate. Phys. Rev. A, 107:023710, Feb 2023. [2] Maria Maffei, Bruno O. Goes, Stephen C. Wein, Andrew N. Jordan, Loïc Lanco, and Alexia Auffèves. Energy-efficient entanglement generation and readout in a spin-photon interface, May 2022. arXiv:2205.09623 [quant-ph].