We are in the midst of a second quantum revolution based on harnessing quantum information, superposition, entanglement, and measurement for new tasks in quantum science and technology. Quantum computers have the potential to solve such problems that classical computers cannot address, quantum states allow for secure communications whose security is guaranteed by the laws of physics, and quantum sensing opens up new frontiers in resolution and sensitivity. These technologies are all built on controllable quantum systems with quantum states that can be used as fiducial quantum bit (qubit) states. Implementations of qubits include single atoms or ions trapped in vacuum or in a crystal lattice, superconducting circuits, single photons emitted from quantum dots, and single photons/spins associated with point defects in semiconductors [1,2]. A very important parameter of qubits is the coherence time that should be significantly longer than the operation time in the quantum information processing. In practice, quantum states are fragile and may be readily disturbed by external noise such as electromagnetic waves. This phenomenon can be turned to an advantage when qubits are applied for noise spectroscopy to sense very weak fields to enable quantum sensing.

Although the first intermediate-scale quantum computers are now being built and developed in industry, the winning platform is far from being established. Similarly, there exist commercial single photon sources, but none that are connected to quantum memories that would allow for scalable quantum networks, motivating immense activity in discovering and developing new platforms that could form the basis of a large-scale quantum network.

For quantum sensors, the leading candidate is the complex of a vacancy and a nearby nitrogen atom (NV center) in diamond [3–5]. These NV centers serve as optically addressable spin qubits with exceptional room-temperature coherence and sensitivity to magnetic and electric fields, strain, and temperature. Beyond their utility as standalone sensors, NV centers have recently enabled key advances in entanglement-based quantum science, such as the creation of entanglement between distant NV centers using spin-photon entanglement and heralded entanglement swapping protocols. These capabilities are crucial for quantum repeaters and distributed quantum networks, and recent demonstrations of Bell inequality violations with loophole-free configurations using NV centers have showcased their potential as foundational building blocks in future quantum communication infrastructures.

NV centers also play a central role in entanglement-enhanced metrology, where quantum correlations between multiple spin or photon states can surpass the standard quantum limit. Experiments have demonstrated the use of entangled NV ensembles and quantum error correction techniques to extend the coherence and sensitivity of magnetometers, opening the path toward practical quantum-enhanced devices for biology, geology, and materials science.

Solid-state defect qubits such as the NV center in diamond can offer quantum memories (e.g., electronic or nuclear spins), spin-photon interfaces (quantum communication), and ready integration into solid-state devices. Furthermore, realizing these qubits in technologically mature semiconductors such as silicon or silicon carbide is a promising way to interconnect semiconductor and quantum technologies in a single materials platform. In particular, silicon carbide hosts a family of color centers with NV-like properties but offers compatibility with existing semiconductor manufacturing infrastructure. Entanglement between defect qubits in SiC has also been pursued, with progress toward optically coherent emitters and spin control at telecom wavelengths, which is advantageous for long-distance fiber-based quantum communication.

However, scaling up defect qubits in three-dimensional materials is extremely challenging due to fabrication variability, spectral inhomogeneity, and integration difficulties. On the other hand, defect creation in two-dimensional (2D) materials or the synthesis of identical spin-active molecules in host matrices of spin-less molecules appear promising. These spin systems are of high interest for realizing defect qubit quantum computing platforms and quantum sensors [6,7].

The discovery of single photon sources and optically detected magnetic resonance centers in the two-dimensional (2D) material hexagonal boron nitride (hBN) [6–8] has triggered intense research. hBN is host to several point defects that exhibit spin-dependent optical transitions at room temperature. This positions hBN-based systems as contenders for on-chip quantum light sources and sensors. Current efforts focus on identifying the atomic structure of these defect states, improving their optical coherence, and integrating them into photonic devices.

In parallel, the exploration of alternative qubit platforms based on heavy elements such as rare-earth ions (e.g., Er, Yb, Eu) in crystals has gained momentum. These systems offer long-lived optical and spin coherence and naturally emit at telecom wavelengths, making them prime candidates for quantum memories and interfaces in networked quantum systems. Additionally, spin-orbit coupled heavy-atom defects can offer robust control schemes and intrinsic protection against decoherence.

An exciting frontier lies in the field of molecular spin qubits, where synthetic chemistry provides an unprecedented degree of control over qubit properties, including symmetry, spin-orbit coupling, and host environment. Recent progress in engineering single-molecule magnets, lanthanide-based complexes, and open-shell transition metal complexes has demonstrated long coherence times and the potential for integrating molecular qubits with photonic, electronic, and superconducting quantum devices. Molecular platforms also enable the development of scalable qubit arrays with atomically precise architectures, a prospect that merges bottom-up materials design with quantum device engineering.

References

[1] Gang Zhang, Yuan Cheng, Jyh-Pin Chou, and Adam Gali, Applied Physics Reviews 7, 031308 (2020). DOI: 10.1063/5.0006075

[2] Gary Wolfowicz, F. Joseph Heremans, Christopher P. Anderson, Shun Kanai, Hosung Seo, Adam Gali, Giulia Galli & David D. Awschalom, Nature Reviews Materials 6, 906 (2021). DOI: 10.1038/s41578-021-00306-y

[3] L. Childress, M. V. Gurudev Dutt, J. M. Taylor, A. S. Zibrov, F. Jelezko, J. Wrachtrup, P. R. Hemmer, M. D. Lukin, Science 314, 281 (2006). DOI: 10.1126/science.1131871

[4] Marcus W. Doherty, Neil B. Manson, Paul Delaney, Fedor Jelezko, Jörg Wrachtrup, Lloyd C. L. Hollenberg, Physics Reports 528, 1 (2013). DOI: 10.1016/j.physrep.2013.02.001

[5] Adam Gali, Nanophotonics 8, 1907 (2019). DOI: 10.1515/nanoph-2019-0154

[6] Toan Trong Tran, Kerem Bray, Michael J. Ford, Milos Toth and Igor Aharonovich, Nature Nanotechnology 11, 37 (2016). DOI: 10.1038/nnano.2015.242

[7] Andreas Gottscholl, Mehran Kianinia, Victor Soltamov, Sergei Orlinskii, Georgy Mamin, Carlo Bradac, Christian Kasper, Klaus Krambrock, Andreas Sperlich, Milos Toth, Igor Aharonovich and Vladimir Dyakonov, Nature Materials 19, 540 (2020). DOI: 10.1038/s41563-020-0619-6

[8] Nathan Chejanovsky, Amlan Mukherjee, Jianpei Geng, Yu-Chen Chen, Youngwook Kim, Andrej Denisenko, Amit Finkler, Takashi Taniguchi, Kenji Watanabe, Durga Bhaktavatsala Rao Dasari, Philipp Auburger, Adam Gali, Jurgen H. Smet, and Jörg Wrachtrup, Nature Materials 20, 1079-1084 (2021). DOI: 10.1038/s41563-021-00979-4