Long wavelength single photon sources and dotonic molecules

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.

There are a growing number of researchers in top academic and national institutions in the US, Japan and Europe investigating light matter interactions, in particular the coupling of quantum dots to optical cavities. A major technological goal is the possibility of achieving totally secure quantum cryptographic systems based on single photon sources. To date progress in this area has been limited to relatively short wavelengths ~900 nm to take advantage of Si-based single photon detectors and commercially funded laboratories such as Toshiba in the UK, ID Quantique in Switzerland and MagiQ in the US, as well as national laboratories such as NPL and NIST have reported significant successes. However such systems would have much wider scope and appeal if the emitted single photons could be collected and guided by standard optical fibre. Already one of the main obstacles, the lack of high efficiency single photon detectors, is being addressed and superconducting NbN and InGaAs/InP avalanche detectors are commercially available and will no doubt improve with time making our project even timelier. For example, NIST are currently assessing the potential of WSi detectors and claim an ~80% quantum efficiency for telecoms wavelength photons. We have agreed to send some of our best samples to them for testing so that the work is world leading in terms of the single photon source and detection (this will also set the bar for other detector manufacturers). The interest in coupled cavity systems is gaining momentum and whilst there have been reports of cavity networks, especially in silicon, little has yet been published for GaAs networks coupled to QDs and certainly nothing to our knowledge at long wavelengths.
The project already has two commercial collaborators, Hitachi and NPL, with quite distinct interests in this work, both of whom are willing to dedicate significant resources to the project. For instance Hitachi are pushing ahead with research in the priority areas of spintronics, quantum information processing and cavity quantum electrodynamics, with a view towards the production of the next generation of photonic devices, while NPL have strong interests in absolute photometric metrology and triggered entangled photons (although entangled photon sources are not specifically included as part of this project we already have plans to pursue this holy grail of quantum information as well). Thus the possible long-term economic benefits of the work are strong.
We also plan to advertise our successes through attendance at workshops organised by UK research councils to disseminate our work and at appropriate conferences such as CLEO and Photonics West, which bring together fundamental science and technology development.
Research into InAs/GaAs quantum dots has been an active area for more than two decades. Initially the major thrust was to produce low threshold, temperature independent lasers for telecoms applications using high densities of dots. QD lasers have now reached commercial maturity (QDLaser Inc.) and the academic focus is now on the properties of single dots, their interaction with light and control of carrier spin. The UK community is very active in this area with excellent work carried out in several universities (Sheffield, Cambridge, Oxford, Nottingham, Imperial and Bristol) as well as companies such as Toshiba and Hitachi. The interest is reflected in the very successful QD one-day meetings (previously held at all the above institutions), the annual III-V Semiconductors meeting in Sheffield and the international QD meeting in 2010 in Nottingham (both PIs have been involved in the organisation of these meetings). Nevertheless we feel that there is scope for a focussed one/two day meeting to take stock of the UK experimental and theoretical effort in this area and will propose this as a topic to be supported by the Semiconductor and Quantum Electronics groups of the IoP for 2013.