Long wavelength single photon sources and dotonic molecules

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics


If a quantum computer can be realised over the next couple of decades it would adversely affect secure communications which are vital in the financial, military and diplomatic sectors, since current encryption methods will be vulnerable to hacking. Totally secure systems, based on quantum mechanics and in particular the properties of single photons, have been investigated for some years and a few working systems demonstrated. These systems operate at wavelengths which took advantage of available single photon detectors but unfortunately coincide with moderately high absorption in conventional optical fibres. If such quantum key distribution systems based on single photon sources are to be commercially successful then the operating wavelength needs to move to the 1300nm or 1500nm range. This is certainly possible using quantum dots as the photon emitter and recent advances in the quality of single photon detectors at these wavelengths means that detection of the single photons should no longer be an impediment. A major problem is efficient extraction of the single photons produced by the dot. Placing the dot in a photonic crystal cavity (PhC) structure overcomes this difficulty allowing only photons that coincide with an optical mode of the PhC to be emitted along the normal to the surface of the device. This is far from trivial since the dot must be located at the centre of the PhC (to within a few nanometers) and the dot photons must be nearly identical in wavelength to the mode of the cavity or at least be able to be tuned to the same wavelength. Here we use methods familiar to the electronics industry to identify the dot position with metal film markers and use these to fabricate the PhC by drilling a particular pattern of holes in the sample. When this is done correctly the dot and cavity are said to be strongly coupled, meaning that a photon emitted by the dot will always be emitted in the normal direction and can subsequently be collected by an optical fibre.
Single photon sources are also crucial components of quantum information networks. Here photons are used like electrons in electrical circuits and are guided along waveguides (which can easily be incorporated into the PhC structures mentioned above) and used to perform logic operations. There are many possible schemes for such networks but in this proposal we hope to demonstrate some basic manipulations of photons using only a few PhC cavities connected by a waveguide and strongly coupled to one or perhaps two dots. This would represent the very first steps towards a possible photonics network. Drawing an analogy with electrical circuits again this would represent the very early stages of an integrated circuit.
Description There have been two main findings arising from this project. In the first we showed that by adjusting the hole separation near two coupled photonic crystal cavities we could introduce an optical well which enabled us to control the coupling and splitting between two cavities with unprecedented precision, allowing for the possibility of using this technique to engineer coupled photonic molecules using single quantum dots in the future. The second key finding involved using a photoresist to define an optical cavity in a waveguide structure. This has the advantage of being able to pick an individual quantum dot in the correct spatial position, with the correct spectral profile such that it could couple strongly to the induced cavity mode. Work is ongoing following the end of the grant to improve the Q-factor obtained, and initial results are extremely promising. We have now managed to produce photonic molecules using this technique and a paper on this has appeared in Optics Express. The output was also disseminated in a major international confrerencei n one oral and one invited paper in Chengdu in China in 2018.
Exploitation Route Photonic engineers may well be able to use the findings to design new single photon emitters where the yield for producing strong coupling is enhanced hugely compared to the common method which relies on chance to produce such coupling.
Sectors Digital/Communication/Information Technologies (including Software),Electronics

Description Collaboration with Hitachi Cambridge Laboratories on design and production of photonic crystals 
Organisation Hitachi Cambridge Laboratory
Country United Kingdom 
Sector Private 
PI Contribution We make the optical measurements on the photonics crystals.
Collaborator Contribution Hitachi produce the crystals and pay for their production.
Impact All publications which use Hitachi expertise have the authors and institution credited.
Description Produced an animation for use on the Oxford Sparks public engagement website on single photon sources 
Form Of Engagement Activity Engagement focused website, blog or social media channel
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Schools
Results and Impact The Oxford Sparks website hosts animations on science generated by researchers in Oxford. This animation deals with single photon sources and is aimed at informing the general public and schoolchildren about the research field and also refers to quantum computing. There are school materials for use by teachers available which supplement the animation and add impact through education.
Year(s) Of Engagement Activity 2016
URL http://www.oxfordsparks.ox.ac.uk/content/our-media