Single Photon Emission

One of the most promising technological applications for colour centres in diamond is single photon sources. These are needed for quantum information processing which looks set to be an important technology in the future [95].

Many different types of single photon sources are currently being investigated. These include quantum dots and a variety of defect centres in diamond. A critical advantage of diamond is that it can be used at room temperature whereas quantum dots require cryogenic cooling.

One of the most promising competitors are quantum dots in InGaAs but only one dot in 1000 works [96]. The multitude of diamond systems being investigated include chromium [97], xenon [98], nickel [99] and oxygen related [100] defect centres. The negative nitrogen vacancy centre has also been used [95], however, it also requires cryogenic cooling for emission of indistinguishable photons due to the high fraction of photons which are emitted in the phonon side band. The brightest centre seen to date was the negative silicon vacancy [7].

Diamond is a favourable material to utilise because of the ease of creating defects and the defect structural stability. Diamond has a low magnetic noise environment which means the defect spins are accessible. Diamond is also a very stable material which means that small particles can be made from it. As mentioned above it can be used at room temperature which is a huge advantage. The defects within the material are photostable [84].

The main rival to the superiority of the negative silicon vacancy centre is the nitrogen vacancy, the most studied defect in diamond. The two defects are stoi-

means there is a smaller electron dipole and therefore photons do not interact with the centres as strongly as for the nitrogen vacancy. The silicon vacancy has a small Huang-Rhys factor, the number which describes vibronic coupling, of S = 0.08 [84, 102].

The nitrogen vacancy has a major disadvantage, due to its wide ZPL, which is the reason so many different systems are being investigated. The maximum output so far seen from nitrogen vacancy is two million counts per second compared to negative silicon vacancy which has been demonstrated to have six million [95]. Some methods have been investigated for improving the emission at the ZPL. These include using nano-cavities which are small structures which enhance emis- sion at a particular frequency [92] and using nano-pillars of diamond which would help getting light in and out.

Another advantage to negative silicon vacancy is that the ZPL is emitted in a region where there is little background fluorescence from intrinsic diamond. This is also true of the neutral silicon vacancy [84]. This does not reduce the importance of having very high purity diamond to reduce noise. The ZPL of the negative silicon vacancy can be doubled using non-linear optics. It would then be at 1480 nm which has been demonstrated already [103]. This is within the region of current optical communication, 1310 nm to 1550 nm, which means this could be used in current systems. The ZPL from the neutral silicon vacancy, at 946 nm is even better for communications as without doubling it can be used with some of the optical fibres now used, without too much attenuation.

The lifetime of the excited state of the negative silicon vacancy is 1 to 5 ns, compared to 10 to 20 ns for the nitrogen vacancy [74, 95]. The silicon vacancy is thus more useful as photons will be emitted quicker. The shorter lifetime results in a higher emission rate.

It seems pretty certain that these systems are going to be important in the future but there are many obstacles to overcome. There has been proof of principle but more work needs to be done [84, 95, 104]. Nanodiamonds have much potential. They are better both due to the lack of internal reflection and lack of background fluorescence. Silicon vacancy containing nanodiamonds have been made on iridium using CVD by Neu et al. [84]. These nanodiamonds showed no background fluorescence, were stable, had linewidths which were smaller than normal, had 88% emission in the ZPL and in 2016 were the brightest emitter found so far in diamond [105]. Nanodiamonds also have longer relaxation times [106]. Work has also been done on decreasing the size of these nanoparticles by Vlasov et al. [104]. This is important for biological applications such as probing cells. Theoretically these silicon vacancy containing nanodiamonds are stable down to 1.6 nm though this is a size which which is currently impossible to make in the lab. However Vlasov et al. managed to test very small nanodiamonds which had come to Earth in a crater of presolar origin. These contained silicon vacancy and were stable although they did not perform as well as previous silicon vacancy nanodiamond samples. They showed blinking which reduced their efficiency.

There are a number of developments which are still being made. The most impor- tant include: suppressing multiphonon events, improving efficiency and stability and ensuring the centres are maintenance free. The silicon vacancy centre appears to be a good option for these quantum applications but more work needs to be done.