Figure 1: Optical initialisation and readout schemes envisioned to implement QIP protocols in molecular spin qubits, and examples of molecules going along with each scheme: (a) transition metal ion complexes, (b) rare-earth complexes, (c) organic molecules.
1. Pre-reading
Molecular systems that can feature electron and/or nuclear spin states together with optical transitions are one of the material platforms that can serve as optically addressable qubits. The attractiveness of molecular systems for quantum technologies relies on the fact that molecular structures of atomically defined nature can be obtained in endless diversity of chemical compositions. Moreover, properties of molecules can be tailored, aiding the design of optically addressable spin qubits and quantum sensors. In this review article the authors present the main results obtained to date in the emerging field of optically addressable spin-bearing molecules, envisioning that these systems could become a scalable building block of quantum hardware.
2. Background
The fundamental unit of a quantum information processing (QIP) architecture is the quantum bit or qubit, which can be realized taking advantage of a physical property—for example, the quantum nature of electronic or nuclear spin degrees of freedom—presenting two stable discrete states while being able to host arbitrary superposition states. To serve as a viable platform for practical applications, qubits should satisfy a set of basic criteria proposed by DiVincenzo. These include a long enough coherence time (T2)—lifetime of the superposition state—to allow for high-fidelity quantum gate operations; definitive initialization and readout mechanisms; and controllable interactions with other qubits. The reversible ability to convert flying qubits (photons) into stationary qubits (e.g. spin qubits), and vice versa, constitutes an extended criterion which is pertinent to not only quantum computation, but also communication. A spectrum of material platforms including superconducting circuits, colour-centres in semiconducting materials, quantum dots, trapped ions, rare-earth ions (REI) doped in inorganic host lattices and molecules, have been proposed and used to host qubits. Some systems offer advantages when a particular DiVincenzo criterion is considered but fall short in others, motivating the constant search for new material platforms that can eventually satisfy all the criteria. Molecular systems could offer solutions to some of the technological challenges as a remarkable intrinsic property of molecules is the fact that they can be prepared as identical copies and organized in a crystal lattice following self-assembly principles, offering enhanced design reproducibility and scalability potential with respect to the solid-state systems.
3. Innovative research
Light addressable molecular systems bearing electron and/or nuclear spin states suitable for implementing QIP protocols include organic molecules, transition metal (TM) complexes, and rare-earth ion (REI) complexes (Figure 2). In the manuscript, the main figures of merit of these molecular platforms to be eligible for future quantum devices are discussed. Pioneering results such as chemical tuning of optical and spin quantum coherence, efficient optical spin initialization and readout, intramolecular quantum teleportation, optical coherent storage, and enhanced optical addressing, are also highlighted.
Figure 2 : (a-d) Europium (Eu3+) complexes show narrow optical homogeneous linewidths and long nuclear spin relaxation times (e) Electronic levels of Eu3+. In the scheme, nuclear spin population polarization by optical pumping followed by spin relaxation causing population equilibrium is represented.
4. Applications and perspectives
While this is yet an emerging research field, landmark results have been obtained, setting the potential of these molecular systems and pointing at the possible directions for future developments. In all discussed platforms, the ability of chemistry to tune the molecular structure and/or the environment in a rationale and reproducible manner is harnessed to improve the qubit system properties. Quantum properties of molecules have been leveraged to demonstrate two-qubit gate operations, all-optical polarization and readout, as well as Rabi oscillations. In addition, some molecular systems, like those based on V3+/4+, Yb3+, and Er3+, offer optical addressing and readout in the NIR range combined with electron and nuclear spin degrees, making them ideal candidates for realizing photonic QIP architectures.
While the potential of all these molecular platforms for developing custom designed, reproducible, and scalable quantum hardware has been elucidated, important progress and developments are still required for these systems to meet the figures of merit achieved by more advanced QIP platforms, such as the already commercial superconducting qubits, or color centers in diamond. We are however confident that the required progress can be obtained, taking advantage of the large number of possible molecular structures, and the increasing understanding of the key design parameters toward achieving the desired quantum properties. In conclusion, the molecular road ahead opens a very exciting direction, in our opinion, worth embarking to develop qubit hosting platforms for QIP applications.
These research results are published online with the title “Spin-bearing molecules as optically addressable platforms for quantum technologies” in Nanophotonics.
The authors of this article are Senthil Kumar Kuppusamy, David Hunger, Mario Ruben, Philippe Goldner and Diana Serrano. The first and last author contributed equally to this work. Senthil Kumar Kuppusamy, David Hunger and Mario Ruben are affiliated to Institute for Quantum Materials and Technologies (IQMT), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany. In addition, David Hunger is affiliated to Physikalisches Institut, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany and Mario Ruben to Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany, as well as Centre Européen de Sciences Quantiques (CESQ), Institut de Science et d’Ingénierie Supramoléculaire (ISIS), Université de Strasbourg, Strasbourg, France. Philippe Goldner and Diana Serrano are affiliated to Chimie ParisTech, PSL University, CNRS, Institut de Recherche de Chimie Paris, Paris, France.
