A quantum repeater is a key element in a scalable quantum network. Component devices such as quantum memories and quantum frequency converters are essential to the realization of a quantum repeater. We leverage three different types of state-of-the-art qubit systems that are being developed at Argonne as quantum nodes to demonstrate a quantum network repeater (see below): trapped ytterbium (Yb) atom arrays, erbium ions (Er) in solids, and superconducting quantum devices. This heterogeneous architecture addresses a key scalability requirement by allowing for remote quantum entanglement and communication between different kinds of quantum information carriers in both the optical and microwave regimes.
Each of the three qubit systems can generate either photon-photon or photon-spin entanglement to serve as a quantum node for entanglement swapping and entanglement-based quantum communications. We will use the Yb atom array system as the key quantum repeater station; this will provide both long-lived quantum memories based on the atom spin states and the necessary two-qubit gates between local atoms for implementing the error mitigation schemes of Thrust 2. Below, we provide detailed descriptions of the three distinct qubit platforms and clarify the role of each component device.
Ytterbium Atoms
Neutral atoms trapped in optical tweezer arrays have become a compelling candidate for realizing key functions of a quantum repeater, such as high-fidelity, multiqubit entanglement distribution. Yb atoms are of particular interest because their long-lived, optically excited 3P0 “clock” state couples to several transitions in the telecom band (1390, 1480, and 1540 nm). These transitions enable the generation of atom-photon entangled states where the emission time (or “time bin” of a telecom photon) is entangled with the atomic state. We recently demonstrated this atom-photon entanglement using the 1390 nm 3P0 to 3D1 transition in Yb.
Erbium Ions
Erbium ions boast long spin and optical coherence times and exhibit a natural telecom-band optical transition near the 1520–1530 nm range. These properties render them promising candidates for quantum memory applications. Our work aims to leverage recent advancements in the nanophotonic integration of Er to establish a telecom-wavelength solid-state quantum memory node in ARQNET. Integration with nanophotonic cavities provides compelling advantages by significantly enhancing the single-photon emission rate via the Purcell enhancement and greatly increasing the scalability of erbium-based devices through CMOS-compatible nanofabrication.
Superconducting Devices
A scalable quantum communication network should be able to not only transmit quantum information carried by optical photons but also interface with solid-state qubits working at the microwave frequencies. At the microwave quantum node, we will leverage GHz qubit devices based on superconducting transmons and/or single electron-onsolid neon (eNe) qubits to create and receive microwave quantum communication signals and integrated quantum transducers to generate M-O photon entanglement, hence achieving remote entanglement between a microwave qubit and a distant Yb atom or Er ion.
Optical Quantum Frequency Converters
Quantum frequency converters (QFCs) provide the important capability to convert an input mode of light from one frequency to another while preserving its quantum state. A QFC was proposed and demonstrated in the 1990s and has since been recognized as a key component for emerging quantum networks. In our heterogeneous network, different quantum nodes emit photons at different wavelengths (1390, 1520, 1550 nm for Yb, Er, superconducting, respectively). Thus, we will develop high-efficiency QFC devices for converting single photons emitted from different qubit systems to a matched wavelength of 745.7 nm, enabling two-photon interference and Bell state measurements to herald remote entanglement.