reservoir enGinEering for quAntum entanglement in the micRowavE Domain – GEARED

Entanglement is the key resource for quantum information processing (quantum simulation, computation and communication). Entangled states are however difficult to generate and sustain as interaction with a noisy environment typically leads to rapid loss of their unique quantum properties. We propose to investigate different complementary approaches to master the entanglement of microwave photons coupled to quantum superconducting circuits. These microwave photons benefit from the enormous advantage over optical ones that they can couple very strongly to non-dissipative nonlinear systems. Moreover the coherence time of superconducting quantum bits has increased by 5 orders of magnitude in 15 years, reaching now 0.1ms. Finally, microwaves are well-controlled using commercial electronics, and are deep in the quantum regime when cooled at dilution refrigerator temperatures.

In this context of quantum superconducting circuits, the entanglement has been generated between a few quantum degrees of freedom, while exploring various types of local interactions. Here, we aim at exploring two new directions for such entanglement generation. The first direction consists of achieving many-body entangled states. The second one will be to generate non-local entanglement between spatially separated quantum systems that are isolated from each other. In this aim, the present proposal GEARED will address several key issues:

-We will build a superconducting two-level system connected to an engineered environment based on high impedance transmission lines, that implements Kondo physics for photons propagating in the system. Here, the entanglement between the two-level system and the degrees of freedom of the electromagnetic environment is at the origin of a Kondo effect for photons. This experiment implements a quantum simulator for Kondo physics.

-We will design, fabricate and operate a quantum node based on a microwave cavity coupled using suitable non-linear Josephson elements. This device can be operated both as a quantum memory, catching and releasing the quantum state of a microwave photon, and as an entanglement generator, distributing entanglement between two distinct memories. Theses unique properties will then be exploited for achieving the teleportation of photonic quantum states and remote state preparation.

-Using the simple situation of a Josephson junction embedded in a high impedance environment, we will demonstrate bright sources of non-classical states of the electromagnetic field, of entangled photonic states, and of single photons. We will characterize the entanglement of these quantum states.

-We will investigate how, by coupling two distant superconducting qubits to non-classically correlated noise sources, one can maintain the coherence of entangled states. We will follow an autonomous stabilization approach: by coupling the quantum system to a reservoir with an engineered Hamiltonian, we direct and evacuate the entropy, induced by the coupling to other noisy degrees of freedom, through the intentionally coupled reservoir.

These problems will require a significant synergy between the partners for solving conceptual challenges, such as engineering suitable environments and resource-efficient entanglement measurements, or technical ones, such as conceiving environments with characteristic impedance approaching the resistance quantum. We are convinced that this proposal opens new avenues within the field of circuit quantum electrodynamics.