Diamond has many facets: it is not only a fascinating gem, but also a useful tool for physicists that might well change our lives in the near future.
The second quantum revolution aims at exploiting the peculiar properties of quantum physics to develop breakthrough devices that are promised to have a huge impact on our daily life in areas such as health, security, computing or sensing. For example, by processing data encoded in the quantum states of matter, supercomputers with unprecedented calculation power could be developed to solve complex mathematical problems in an instant. Improved medical imaging using quantum sensors might also help in the earlier diagnosis of diseases such as cancer. Currently, there is a strong incentive from the European Union to support research activities in this area through the Quantum Flagship, which was initiated in 2018 (see: https://qt.eu/).
To this end, new systems are needed, in particular solid-state materials in which the quantum properties of specific atoms, impurities or defects can be manipulated on demand and read out optically. By harnessing the light emitted or absorbed by specific optical or spin transitions, information encoded by photons could indeed be stored and retrieved on demand. Besides, any external perturbation, even extremely weak, will strongly impact the coherence or strength of the transitions, thus unleashing the ability to detect it with an exceptional sensitivity. These principles are at the basis of quantum memories and magnetic or electric sensors that are set to revolutionise the next generation of electronic devices.
Diamond as a quantum platform
Standard off-the-shelf materials cannot be used and the extreme requirements of these most demanding applications require that they are specially designed and engineered. Several material platforms are being considered, including, for example, point defects in silicon carbide (SiC), phosphorous donors in silicon (Si), rare-earth emitters in oxide crystals, and so on. Nevertheless, special attention has been dedicated over the past decade to colour centres in diamond and in particular to the so called Nitrogen-Vacancy (NV) in which two carbon atoms from the diamond crystal are replaced by one nitrogen atom (see Fig. 1a). This centre is a single emitter of red photons at 637nm that presents outstanding physical and optical properties.
One essential figure of merit for quantum systems is the so-called ‘coherence time’ (T2) that represents the time during which the spin or optical levels can be coherently manipulated. For NVs, it can reach up to several milliseconds which, although it may appear short, leaves plenty of time to perform calculations or store information. Indeed, T2 is among the longest for a solid-state system at room temperature and it can be further extended by using dedicated decoupling schemes to limit the interaction of the spins with the environmental magnetic noise. It is therefore of no surprise that these defects and other impurities in diamond are thoroughly investigated for sensing and communications.
Synthesising quantum-grade diamond films
Nevertheless, obtaining exceptional quantum properties requires dedicated material synthesis. Fortunately, the Plasma Assisted Chemical Vapour Deposition (PACVD) technique is now fairly mature. At the Laboratory of Sciences and Material Process engineering (LSPM-France) we have accumulated more than 20 years’ experience in optimising reactors and growth conditions to grow some of the highest purity and quality single crystal diamonds and nanoparticles (see Fig. 1b).
In this process, H2 and CH4 mixtures are heated up to extreme temperatures (3,000K) in a micro-wave cavity reactor allowing carbon precursors to build-up homoepitaxially on a diamond seed while atomic hydrogen in the plasma ensures that the diamond phase is kinetically stabilised. The films can be isotopically purified by controlling the gas environment to exclude any 13C atoms that are naturally present to a concentration of about 1.1%. This further reduces the spin bath arising from the nuclear spin of these atoms and pushes further the coherence properties. This engineered and purified crystalline material is thus a highly suitable platform for physicists to explore quantum applications.
Creating optimised colour centres
NV centres can be introduced in this matrix either by the direct implantation of N+ ions accelerated to a few tens of keV or by direct in situ doping. The latter is achieved by adding a low amount (typically a few tens of ppm) of N2 or N2O to the gas phase. A small fraction of this nitrogen will get incorporated into the growing layer as NV centres (see Fig. 1c).
In general, the environment and thus the properties of such grown-in NVs are better controlled than in ex-situ doped colour centres.
To further optimise the performance, growth can also be performed on substrates other than the standard (100) crystal plane. Indeed, while NVs incorporated on a (100) growing surface have a random orientation along the four possible (111) axis, NVs grown into (111) or (113) crystal planes can be preferentially oriented by up to 100% along a specific direction. Having oriented NVs is particularly useful since the spin state polarisation required for sensing or information processing usually involves aligning them along a particular magnetic field or laser beam.
Towards nanodiamonds containing other colour centres
The potential of the CVD technique is, however, not limited to the production of NVs embedded in a high quality bulk diamond matrix. Indeed, to promote the coupling of the emitted light into a cavity or to facilitate their integration into a practical device, diamonds with nanoscale dimensions are desirable.
At LSPM, we have recently optimised the batch production of such nanodiamonds with good structural and optical properties despite their small dimensions (see Fig. 2). In addition, we have been able to intentionally introduce other interesting defects, such as the silicon-vacancy (SiV) or germanium-vacancy (GeV) centres. While their coherence properties are not as good as that of NVs, these centres possess a higher photostability and most of their emission occurs at a central wavelength of 737nm and 603nm respectively (see Fig. 2). They are thus currently considered for photonic and bio applications, for which they have advantageous properties.
A key enabling technology
Over the years, the CVD technique has thus emerged as a key enabling technology that provides an unprecedented control over the type, concentration, and properties of colour centres embedded in high quality diamond films and nanoparticles. Given the intense activity on diamond research for quantum technologies, it could be made more democratic than expected in the near future, and we might very soon all have a tiny piece of specially engineered CVD diamond in our items and portable
This research has been supported by the European Community’s H2020 Framework Programme (H2020 – FETLAG – 2018-2020) under Grant Agreement n° 820394 (ASTERIQS) and by the French Agence Nationale de la Recherche through the project ASPEN (ANR-17-ASTR-0020-03) and the project MICROSENS (ANR-18-QUAN-0008-02)**