In recent years NV centers in diamond have emerged as one of the post promising candidates for solid-state implementations of quantum information and computing. NV centers are defects in the diamond lattice consisting of a substitutional nitrogen atom and a neighboring vacancy having trapped an additional electron. They present many unique features that make it particularly attractive for quantum information protocols. The spin of an individual NV center can be readout optically at room-temperature using standard confocal microscopy. It has among the longest coherence times ever reported in the solid-state even at room-temperature. NV centers are coupled to neighboring nuclear spins, offering the possibility of creating a quantum register based on nuclear spins. Finally, at low-temperatures, they have a narrow optical line (the zero-phonon line) promising for quantum communication protocols. All these unique properties gathered in a single system have resulted in an impressive number of experimental breakthroughs in the recent years. However, before NV centers can really form the basis for a scalable implementation of quantum information and communication protocols, one challenging issue absolutely needs to be solved : how to make NV centers communicate with each other in a quantum-coherent way. Indeed, if small-scale quantum registers of up to three entangled qubits have been demonstrated with one single NV center coupled to a few neighboring nuclei, it is clear that quantum information and communication protocols will require the possibility of entangling distant spins.
The objective of our consortium is to gather key research groups in Europe working in this field to join their effort towards this goal. One of the major strengths of our consortium is the authentic complementarity between the expertise of each group. In that sense, our project is truly interdisciplinary, involving world-leading experts in quantum optics and spins, in low-temperature condensed-matter physics, and in materials science, in strong interaction with one industrial partner and one theory group. This allows us to propose different highly innovative approaches to solve the crucial issue of NV-NV long-distance entanglement, all based on new ideas relying on existing technology.
I) Hybrid systems: Our first approach (WP1) is to couple NV centers to other types of quantum systems, in order to form hybrid quantum systems that will combine their strong points for quantum information processing. In addition to the goal of achieving long-distance entanglement between NV centers, these new quantum devices will also open a variety of new paths for quantum information. Two specific approaches will be investigated in our project : We will investigate a completely new scheme in order to achieve long-distance entanglement between individual NV centers. Our proposal consists in using a NV center mounted on an AFM tip as a quantum read/write head. More precisely, we will start with an array of implanted NV centers close enough to the diamond surface, fabricated by ion implantation. The mobile NV center will then be approached close enough to a given spin of the array to get entangled with it, then moved to another spin of the array to transfer to the latter the entanglement with the first spin.
QIP with NV spin ensembles combined with superconducting qubits
We will also investigate the coupling of NV centers ensembles to superconducting Josephson qubits. Superconducting qubits are electrical artificial atoms that ideally complement NV centers for quantum information processing: they are macroscopic quantum systems, fully tunable and very easy to couple between each other and to other systems, but their coherence time is relatively short. Coupling them to NV centers ensemble could result in a new architecture for a quantum processor in which the information is stored in the NV centers ensemble and processed via Josephson qubits. We will first realize a multi-mode quantum memory demonstrating that one superconducting qubit can be entangled with an ensemble of NV centers. The same qubit can then be used to transfer this entanglement to another ensemble of NV centers. Our consortium is very well suited to accomplish this ambitious project, as it includes two groups that are among the world leaders both in the field of superconducting qubits and in the field of quantum information processing with NV centers; in addition, a pioneering experiment demonstrating the coupling of NV centers to a superconducting resonator was recently achieved thanks to a collaboration between partners 1,2,3.
II) Optical properties: Another natural way of achieving long-distance entanglement between NV centers is to rely on optical photons; in that purpose our WP2 consists in studying the optical properties of NV centers at low temperatures. A first objective of this consists in investigating to what extent the optical techniques used to control electronic and nuclear spins at room-temperature can be applied at low-temperatures. The outcome of this study will have strong implications for the future development of the hybrid superconducting circuits studied in WP1. Another field in which NV centers could prove very fruitful is the realization of a quantum memory for optical quantum fields. Solid-state quantum memories have up to now mostly been realized with rare-earth ions in crystals, and rely on their long-lived spin degree of freedom in the ground state manipulated through spectral hole burning techniques. NV centers in diamond have as already discussed a very long-lived spin degree of freedom and as such are interesting potential candidates for quantum memories. We will in this WP investigate this possibility further. Note that the successful storage and retrieval of an optical quantum state of the field in an ensemble of NV centers could lead, combined with the techniques developed in WP1, towards the coherent conversion of an electromagnetic field between the microwave and the optical domain that would represent a major step for quantum information and communication.
III) Material science: In order to achieve these ambitious projects, we will need diamond samples of high purity and quality, with optimized specific properties. One strong point of our proposal is that our consortium also includes two groups that have a unique expertise in diamond processing and characterization as well as strong links with Element 6, the world leader in diamond growth. These groups will explicitly dedicate a part of their research to the optimization and realization of the best diamond samples possible in terms of purity, NV centers concentration, coherence time, and strain (WP3).