Quantum Networks

Entanglement purification/distillation is a quintessential task of a quantum network. Simple bipartite purification schemes use two or more noisy entangled pairs to probabilistically improve the fidelity of one pair by performing local operations and classical operations across the two parties sharing the pairs. However, these are asymptotically inefficient in the conversion of noisy input entanglement into higher fidelity output entanglement. Bennett et al. established the connection between one-way entanglement purification protocols and quantum error correction. In this talk, we will begin by reviewing this connection and discuss an equivalent protocol by Wilde et al. to purify Bell pairs. Then we will use the stabilizer formalism to generalize the protocol to GHZ states of any size. As part of this process, we will discuss a new insight on Pauli measurements on GHZ states. We will discuss the application of such protocols to quantum repeaters based on trapped ions. We will finally combine this protocol with a scheduling strategy for a linear chain of repeaters that determines the optimal policy between entanglement distillation and entanglement swapping at each repeater. The goal here is to maximize the total distillable entanglement from the final end-to-end pairs. If time permits, we will briefly discuss the concatenation of GKP codes with good quantum LDPC codes that has connections to GKP-based quantum repeater architectures.

Quantum communications have mostly been limited to terrestrial and fiber optic links. However, just as many of our classical communications occur in the wireless space, there are many reasons to develop free-space quantum communications. One of these is the ability to create links between moving platforms. To that end, we will discuss our recent efforts on quantum communication links between drones and moving vehicles, as well as our upcoming satellite project: Space Entanglement and Annealing QUantum Experiment (SEAQUE).

Quantum networks are required for distributing quantum states over long distances in order to overcome loss. A high-quality quantum network would enable new classes of secure communications protocols, the linking of remote quantum processors, and distributed entangled sensors. Such a high-quality network does not yet exist. The lack of efficient memories, transduction, and high-rate near-deterministic sources of entangled photons are some of the major obstacles that must be overcome. I'll talk about a new path-entanglement scheme that our team has been developing that we believe will enable truly scalable all-optical quantum networks. Using technologies that exist today, we are aiming to build longer range quantum links capable of distributing high-fidelity and low-loss entanglement over distances of hundreds of kilometers at high rates using multiple heralded repeater steps. We are currently working on our next-generation path-entangled sources, detectors, link-stabilization techniques, and optical memories that will allow us to build a metro-scale prototype quantum network that can be used to carry out quantum communications protocols that have a provable quantum advantage.