One of the greatest challenges in modern science is the realisation of quantum computers which, as their scale increases, will allow enhanced performance of tasks across many areas of quantum information processing. Quantum logic gates play a vital role in realising these applications by carrying out the elementary operations on the qubits; a key aim is minimising the resources needed to build these gates into useful circuits. While the salient features of a quantum computer have been shown in proof-of-principle experiments, e.g., single- and two-qubit gates, difficulties in scaling quantum systems to encode and manipulate multiple qubits has hindered demonstrations of more complex operations. This is exemplified by the classical Fredkin (or controlled-SWAP) gate  for which, despite many theoretical proposals [2,3] relying on concatenating multiple two-qubit gates, a quantum analogue has yet to be realised.
Here, by directly adding control to a two-qubit SWAP unitary , we use photonic qubit logic to report the first experimental demonstration of a quantum Fredkin gate . Our scheme uses linear optics and improves on the overall probability of success by an order of magnitude over previous proposals [2,3]. This optical approach allows us to add control an arbitrary black-box unitary which is otherwise forbidden in the standard circuit model . Additionally, the action of our gate exhibits quantum coherence allowing the generation of the highest fidelity three-photon GHZ states to date.
The quantum Fredkin gate has many applications in quantum computing, quantum measurements  and cryptography [8,9]. Using our scheme, we apply the Fredkin gate to the task of direct measurements of the purity and state overlap of a quantum system  without recourse to quantum state tomography.
A large vertical electric field can be used to linearly change the fine-structure splitting of a single InGaAs/GaAs quantum
dot by over 100 μeV. In each single dot an avoided crossing is observed, where the magnitude of the splitting reaches
a minimum value. We confirm in experiment that polarization-entangled photon pair emission occurs from quantum dots
tuned in this manner.
One of the criteria for the physical implementation of quantum computing is that it must be possible to manipulate
individual qubits to prepare them in the same well-defined state. In schemes involving linear optics
and single photons from self-assembled quantum dots, this requirement can be intractable as quantum dots form
with a distribution of emission energies. We have designed a structure based on InAs quantum dots embedded
at the center of an AlGaAs/GaAs/AlGaAs quantum well which facilitates carrier confinement under application
of electric fields up to |F| = 500 kVcm
-1. This is an order of magnitude greater than previous reports. Through
the quantum-confined Stark effect, individual dots can be tuned by up to 25meV, three orders of magnitude
greater than their linewidth. This unprecedented tuning range allows many quantum dots to be tuned to the
same energy with ease, enabling us to carry out two-photon interference measurements using remote quantum
dots. Furthermore, we demonstrate scalability by performing the interference experiment with three pairs of
quantum dots. By post-selecting events where two photons arrive simultaneously at a beamsplitter, we find that
the interference visibility is limited only by the timing resolution of our system. We also performed measurements
as a function of energy detuning which showed that maximum visibility can only be achieved when the
dots are degenerate in energy. This opens up the possibility of transferring quantum information between remote
A single-photon source capable of emitting indistinguishable photons is a key element in schemes for scalable
quantum information processing with linear optics. Whilst several groups have reported such sources, up until
now an electrically driven source capable of making these protocols technologically viable has yet to be reported.
We present the first demonstration of an electrically driven single-photon source emitting indistinguishable
photons. Our sample consists of a layer of InAs/GaAs quantum dots embedded in the intrinsic region of a
p-i-n microcavity diode. We test the indistinguishability of consecutive photons by carrying out a Hong-Ou-
Mandel-type two-photon interference experiment whereby two identical photons arriving simultaneously at two
input ports of a 50:50 beamsplitter exit together. The device was operated under two modes, continuous and
pulsed current injection. In the former case, we measured a coherence time of up to 400 ps at low pump current
- the longest reported under these excitation conditions. A two-photon interference visibility was measured,
limited only by the timing resolution of our detection system and further suggesting a 100% overlap of photon
wavepackets at the output beamsplitter. In the case of pulsed injection, we employed a two-pulse voltage sequence
which allowed us to carry out temporal filtering of photons which had undergone dephasing. The characteristic
Hong-Ou-Mandel "dip" was measured resulting in a visibility of 64 ± 4%.