Design and Construction of a Three-Node Quantum Network

Abstract

Quantum computers have wide-ranging potential applications, many of which will require thousands or even millions of quantum bits to be useful. Current state-of-the-art universal quantum computers, on the other hand, contain only several tens of qubits, and scaling to larger system sizes remains one of the primary challenges. Among current quantum computing platforms, trapped ions are a leading hardware option. One proposal for scaling such systems consists of a modular architecture.

The architecture consists of multiple nodes, each with an ion trap containing a communication qubit (138Ba+) and a memory qubit (171Yb+). The communication qubit is responsible for generating photons that link the remote nodes together via entanglement swapping while the memory qubits are used for storing information and performing local computations. We report progress towards demonstration of the remote entanglement of two barium ions. The creation of this link is a probabilistic process and fails much more often than it succeeds. The success rate does not impact the fidelity of the resulting entangled state but imposes significant constraints on the utility of this protocol. We examine the current limitations on both the fidelity of the resulting entangled state and the success probability.

In addition to the two-node experiment, we have designed and built a new ion trap system that should yield much higher photon collection rates. This design represents a significant shift from previous systems because of the inclusion of optical elements inside the vacuum chamber and their resulting proximity to the ions. We incorporate two objective lenses with a numerical aperture of 0.8, each of which can collect twice as much light as the objectives used for the remote entanglement experiment. We present preliminary results characterizing the performance of this system and discuss how it could be incorporated into a three-node network, which has not yet been demonstrated using trapped ions.