We have recently developed a two-qubit gate which uses only semiconductor devices and is a basic building block for photonic quantum computing schemes.
Light is already used as a carrier of information over optical fibres. By encoding each bit of data onto a single photon, one can create a quantum bit, or “qubit”. A photon is the smallest unit, or ‘particle’ of light. By exploiting the quantum mechanical properties of light it will be possible to build an optical quantum computer – a machine which could drastically increase the speed of complicated calculations.
A photonic logic gate is essentially a switch, much like a set of points on a railway line, which can be used to control the route a photon takes through the gate. In the simplest case, the 2-qubit gate, one photon is travelling along the photonic “track”. If a second photon is present at a “control” channel it can essentially switch the points and re-route the photon to a second set of tracks. In practice our “photonic railway tracks” are made from semiconductor waveguides. Figure (a) shows a photograph of some waveguides on a photonic circuit board. Unlike a set of railway points, the photon re-routing is not a mechanical process. Instead we utilise a phenomenon called “two-photon interference” to control the path the photon takes through the waveguide.
In the same way that an electronic circuit board allows many different electronic devices to be connected together, we have used our waveguide circuit board together with one of our high-performance single photon sources to create our all-semiconductor two-qubit gate. Figures (b) and (c) are images of our pillar microcavity single photon sources, recorded using a scanning electron microscope.
The gate itself is quite straightforward and consists of a network of five couplers as shown in Figure (d). A photon arriving at each coupler can leave by either the top, or the bottom output port. The probability of this is controlled by the design of each individual coupler. When all five couplers are combined in this way, the result is a so-called “controlled-NOT gate”. The target (T) and control (C) photons arrive at the input ports on the left hand side of the circuit. If the control photon enters through port C0, the gate doesn’t re-route the target photon: If it entered through port T0, it will leave through port T0. If, however, the control photon enters through port C1, the gate will re-route the target photon from port T0 to port T1 and vice versa.
By combining this 2-qubit gate with 1-qubit gates, one has all the building blocks necessary to perform any photonic quantum operation.
[Technical] M.Pooley et al. “Controlled-NOT gate operating with single photons” Appl. Phys. Lett., vol 100, no 21, 211103 (2012)