Diamond Defects Could Form the Basis of Future Quantum Communication Networks

A thorough review details how diamond colour defects represent a leading candidate for constructing the nodes of future large-scale quantum networks, with potential to revolutionise quantum communication and distributed computing. Ayan Majumder and colleagues at Indian Institute of Technology Bombay, in collaboration with Northwestern University, highlight the favourable optical properties and extended spin coherence times exhibited by these defects. Recent advancements in integrating diamond nanophotonics with photonic integrated circuits are also presented, paving the way towards more efficient and scalable quantum architectures. The review consolidates current progress and critically assesses both the fundamental and experimental key hurdles remaining in the realisation of metropolitan-scale quantum networks.

Diamond nanophotonics and photonic integration for scalable quantum networks

Advances in the heterogeneous integration of diamond nanophotonic structures with photonic integrated circuits have resulted in more efficient systems suited for scalable quantum processor architectures. This thorough review discusses the optical and spin properties of these systems, recent progress in the building blocks of quantum networks, demonstrations of metropolitan-scale quantum networks, and the challenges associated with these systems at both the fundamental and experimental levels, along with potential solutions. The realization of large-scale quantum networks constitutes a major milestone in quantum technologies, with the potential to enable major applications in secure quantum communication, distributed quantum computing, precision sensing, and advanced metrology.

Strong quantum nodes, combining efficient optical interfaces with long-lived and controllable qubits for information processing and storage, are required to achieve such networks. Figure 1 illustrates a large-scale quantum network featuring solid-state, optically active spin-qubit-based nodes. Colour defect centres in diamond have emerged as leading candidates for implementing these quantum nodes, offering a unique combination of favourable optical properties and exceptionally long spin-coherence times that enables reliable generation, storage, and processing of quantum information.

Recent advances in diamond nanophotonics have led to sharp improvements in photon collection efficiency and light-matter interaction strength, paving the way toward integrated and scalable quantum processors and network nodes. Substantial progress has been achieved over the past decade, ranging from proof-of-principle laboratory experiments to demonstrations of entanglement distribution over metropolitan-scale fibre networks. Despite these remarkable advances, several key challenges remain, including limited photon indistinguishability, low entanglement generation rate, moderate entanglement fidelity, scaling-up the memory qubits, and the integration of diamond-based quantum emitters with scalable nanophotonic and photonic integrated circuit platforms.

Addressing these challenges is essential for the development of fault-tolerant, scalable quantum networks with high entanglement-generation rates. Photon-detection-based entanglement generation represents one promising route to interconnecting stationary qubits in large-scale networks. Each node comprises quantum memories alongside quantum information processing qubits that support fast, efficient, and high-fidelity single- and two-qubit gate operations.

At least one qubit per node must be optically interfaced with a shared photonic channel to enable connectivity with other nodes. Through this design, quantum nodes can process, store, and transmit quantum information across the network, with multiple physical qubits allowing implementation of quantum error correction, leading to fault-tolerant logical qubits. The three principal remote-entanglement protocols, detection-in-midpoint, sender-receiver, and source-in-midpoint, are discussed.

In the detection-in-midpoint scheme, both nodes generate stationary qubit-photon entanglement, directing emitted photons to a central station for measurement in the Bell basis, projecting the two remote stationary qubits into an entangled state. The sender-receiver topology involves one node creating stationary qubit-photon entanglement and transmitting the photon to the second node, where interaction with another stationary qubit and photon detection establishes entanglement. Using an entangled photon-pair source at a central station, the source-in-midpoint approach distributes photons to distant nodes where reflection of a photonic qubit by an optical-cavity coupled stationary qubit system, followed by photon measurements, projects the remote stationary qubits into an entangled state.

To realise a suitable material platform for quantum nodes in large-scale quantum networks, stationary qubits serving as information-processing and memory qubits are required. The memory qubits must be capable of storing quantum states during entanglement generation, requiring coherence times exceeding the entanglement generation time. Each node should support the storage of multiple entangled states and allow high-fidelity operations between them, enabling multiqubit protocols such as quantum error correction.

Table I summarize and compares the physical characteristics of various solid-state quantum systems from the perspective of their suitability as quantum nodes. Key node-level performance metrics, including storage capabilities and optical properties, influence quantum network performance. These encompass the number of qubits and their interaction properties, operating temperature, and the coherence time of communication and memory qubits. Diamond colour defects are promising candidates for building network nodes due to their excellent optical properties, fast spin-qubit control, and long spin coherence times.

Recent advances integrating diamond nanophotonic structures with photonic integrated circuits have made these systems more efficient and well-suited for scalable quantum processor architectures. Maintaining quantum states during entanglement generation requires stationary qubits to have coherence times exceeding the generation duration, alongside support for high-fidelity operations for multiqubit protocols. Consequently, operating temperature is vital for maintaining communication-qubit coherence, while achievable storage time is directly determined by the coherence time (T2) of the memory qubits.

The Debye-Waller factor and the linewidth of the ZPL transition govern the number of indistinguishable photons and their spectral indistinguishability in ZPL emission, respectively. Optical coherence is characterised by the coherence time τ2, governed by the excited-state lifetime τ1 and pure dephasing τ ∗ 2, with the lifetime τ1 setting an upper bound τ2 ≤2τ1, where the optical linewidth reaches its minimum ∆ν = 1/2πτ1. Quantum efficiency, defined as the ratio of radiative decay to all decay channels, also plays an important role in spin-photon interface. Collectively, these parameters critically affect the entanglement generation rate, state transfer efficiency, and overall fidelity of quantum networking protocols. TABLE I provide a comparison of key physical properties of representative solid-state quantum platforms relevant to quantum-node applications, detailing ancilla qubits, operating temperature (K), spin coherence time (T2), optical transition wavelength (nm), Debye-Waller factor, linewidth, lifetime (ns), and quantum efficiency for systems including NV-centre (diamond), SiV-centre (diamond), GeV-centre (diamond), SnV-centre (diamond), SiC Divacancy, VSi (SiC), Quantum dots (TMDCs), InAs/GaAs quantum dots, and RE crystals (Er3+:Y2SiO5, Pr3+:Y2SiO5, Eu3+:Y2SiO5, Yb3+:YVO).

Diamond nanophotonics enable high-speed metropolitan quantum entanglement distribution

Entanglement distribution rates, a critical metric for quantum networking, have now reached 10 kilohertz over metropolitan-scale fibre networks, a substantial improvement over previous limitations. This threshold crossing enables quantum key distribution and distributed quantum computing protocols previously hampered by slow connection speeds; practical, secure communication demands rates exceeding 1kHz. Diamond colour defects are proving central to this progress, offering both the extended spin coherence times necessary for quantum memory and the optical interfaces required for photon-based entanglement generation. Recent integration of diamond nanophotonic structures with photonic integrated circuits further enhances system efficiency and scalability, addressing a key obstacle to building larger, more complex quantum networks.

Light collection efficiency improved five-fold compared to earlier designs following the successful integration of diamond nanophotonic structures with photonic integrated circuits. Furthermore, the exceptional coherence properties of diamond defects extend to nuclear spins, with carbon-13 nuclear spins maintaining information for over 6 minutes at 4 Kelvin, offering long-term quantum memory potential. Single-qubit gate fidelities have reached 99.995 percent, and two-qubit gates exceed 97 percent, demonstrating precise control over these quantum systems; however, these figures represent performance under cryogenic conditions.

Balancing entanglement rate and fidelity in diamond-based quantum networks

Establishing metropolitan-scale quantum networks promises breakthroughs in secure communication and distributed computing, yet realising these ambitions demands strong quantum nodes. Diamond colour defects currently stand out as leading candidates, offering a compelling combination of long coherence times and efficient optical interfaces. A persistent tension exists, however, between achieving high entanglement generation rates and maintaining the fidelity of those entangled states; scaling up the number of qubits while preserving coherence proves exceptionally difficult.

Diamond colour defects remain uniquely positioned to form the backbone of future quantum networks due to their favourable properties. Improvements in materials science and device fabrication will inevitably address these scaling challenges. Progress, even incremental, unlocks potential applications in secure communication and distributed computing, justifying continued investigation and refinement of these promising quantum nodes. Recent integration of these diamond structures with photonic integrated circuits, tiny chips that manipulate light, improves the efficiency with which quantum signals can be transmitted and processed, a crucial step towards scalability. Demonstrations of entanglement distribution over metropolitan-scale fibre networks confirm the potential of this technology to extend quantum communication beyond the laboratory environment; further research must address fundamental and experimental challenges to fully realise these networks.

The research demonstrates that diamond colour defects can maintain quantum information for over six minutes at 4 Kelvin, and achieve single-qubit gate fidelities of 99.995 percent and two-qubit gates exceeding 97 percent. This is significant because it highlights the potential of these defects as strong quantum nodes for building metropolitan-scale quantum networks. These networks promise advances in secure communication and distributed computing, and researchers are currently working to address challenges related to scaling up these systems and maintaining coherence. The integration of diamond structures with photonic integrated circuits represents a key step towards achieving scalable quantum processing architectures.