Deterministic multi-qubit entanglement in a quantum network

Practical applications of quantum computers require millions of physical qubits. Superconducting qubits show great promise as a scalable approach to building practical quantum computers, where the state-of-the-art superconducting quantum processors have integrated about 100 qubits on a single chip so far. But limited by qubit size, wiring fanout, crosstalk, etc, it is quite challenging, if not impossible, to reach the qubit numbers for practical applications on a single chip. It is therefore timely to investigate a modular approach for scale-up, where multiple quantum processors are interconnected together by coherent links, forming a large-scale distributed quantum processor.

Superposition and entanglement are key resources that enable both quantum computing and quantum communication. The deterministic generation and distribution of entanglement in a scalable architecture is therefore a central requirement underpinning these technologies. The connection and deterministic entanglement of two superconducting qubits fabricated on separate chips have been demonstrated in several works recently. However, the deterministic generation and transmission of multi-qubit entanglement had not been demonstrated, primarily limited by low chip-to-chip state transfer fidelity (~80%).

In this talk, I will report a quantum network comprising two superconducting quantum nodes connected by a one-metre-long superconducting coaxial cable, where each node includes three interconnected superconducting qubits. By directly connecting the cable to one qubit in each node without going through any lossy components (e.g., SMA connector, circulator or printed circuit board trace), the channel coherence is significantly improved, and we can transfer quantum states between the nodes with a process fidelity of 91.1%, representing a two-fold improvement (in terms of infidelity) compared to the previous experiments using lossy components. We then prepare a three-qubit Greenberger–Horne–Zeilinger (GHZ) state (a maximally entangled state) in one node and deterministically transfer this state to the other node, with a transferred-state fidelity of 65.6%. We further use this system to deterministically generate a globally distributed two-node, six-qubit GHZ state with a state fidelity of 72.2%, by first entangling two qubits from the two nodes, then “amplifying” this two-qubit entanglement to a six-qubit entanglement using controlled-NOT gates. These GHZ state fidelities are clearly above the threshold of 1/2 for genuine multipartite entanglement, showing that this architecture can be used to coherently link together multiple superconducting quantum processors, providing a modular approach for building large-scale quantum computers.