Seminar w/ Carlos Gonzalez-Ballestero

Quantum operations on light and matter in waveguide QED

Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, E-28049 Madrid, Spain

Waveguides have arisen as excellent candidates for photonic quantum circuitry, since they allow for an enhanced light-matter interaction thanks to their field confinement, and also for a high degree of control through their built-in input/output ports. While cavity QED theory strongly relies on formalisms such as the Master Equation, where the photonic degrees of freedom are traced out, the full potential of waveguides cannot be theoretically estimated unless such variables are explicitly accounted for. Among different proposals for a proper theoretical description of these systems, a Bethe Ansatz method was developed a few years ago [1] and successfully employed to describe few-photon scattering problems.

In the first part of our work, we extend the above method to the study of state of the art qubit-qubit entanglement generation protocols in waveguide setups [2]. Dark states are shown to arise naturally as Fabry-Perot resonances between the qubits, demonstrating the extreme sensitivity of the protocol to external parameters and, additionally, the possible appearance of disadvantageous non-Markovian effects. We continue by proposing an alternative scheme based on the recently demonstrated chiral waveguide-emitter couplings [3,4]. This new protocol is able to generate a very robust transient entangled state, therefore being in principle more attainable from an experimental point of view.

The above methods were based on the so-called dissipative entanglement generation protocols. In the second part of our work, we design a different scheme to generate, manipulate, and detect two-qubit entanglement by means of guided photons instead. First, we show how a single-photon can generate transient entangled states, and how a second photon can be employed to manipulate the time profile of the generated entanglement by exploiting the mechanisms of sudden death and revival. Finally, we demonstrate the possibility of detecting such entangled state in the scattering output of a single-photon probe. With this, we design a strategy to manipulate and detect qubit-qubit entanglement which in principle allows a much higher degree of control trough the waveguide ports.

Finally, we introduce a four-port device in which chiral waveguide-emitter couplings allow for the design of non-reciprocal few-photon devices based on quantum interference. We show the possibility of efficient single-photon routing and, for a two-photon input, we demonstrate how a transistor-like device can be achieved. This widens the range of applicability of chiral waveguide-emitter couplings and demonstrates the potential and versatility of wavegide QED systems.