Unprecedented processing power using quantum mechanics
Information processing underpins many key technologies which have been propelled by advances in digital computing. However, conventional computers do not fully take advantage of quantum mechanics which ultimately governs the properties of these devices. Quantum computing promises to transform the way we process, transmit and interpret information and will enable us to solve some of the most complex problems in the universe.
Classical computers encode information in bits which are usually represented by their logic symbols ‘0’ and ‘1’. Meanwhile, quantum computers employ quantum bits, or qubits, which harnesses quantum effects such as superposition and entanglement to represent information in more intricate ways. Superposition allows qubits to take on both the values 0 and 1 at the same time. While entanglement allows correlations between different qubits, such that the state of one qubit can depend on another even if they are separated far apart. Taking advantage of these effects lies at the heart of quantum information processing.
In our lab, we encode qubits onto single photons (light particles) as they are low-noise physical systems which remain isolated from its environment allowing them to preserve their quantum effects. Photons, which inherently travel at the speed of light, serve as effective information carriers for long range transmission taking advantage of fibre optic networks and free-space links using satellites. In addition, photons have many physical degrees of freedom which can be precisely controlled.
Quantum information processing, and ultimately to implement the functions of a quantum computer, requires controlling the states of many qubits. To do this, qubits are fed into quantum circuits which are built up from logic gates that perform specific tasks to manipulate the state of qubit. By chaining together arrays of these gates, more complex operations can be performed on many qubits at once. However, this comes at a cost of large resource overheads associated with implementing each gate. An ongoing challenge is realising large quantum circuits in a scalable and reliable way.
A quantum Fredkin gate
Here we have experimentally implemented the Fredkin gate using a technique which implements a quantum circuit directly without chaining together multiple gates. The Fredkin (or controlled-SWAP) gate is a three qubit logic gate that exchanges the information of two target qubits at the output of the gate whenever the control qubit is in the logical-1 state. The control qubit can be prepared in a quantum superposition to generate entanglement using the gate. This entanglement can be used as a resource in quantum algorithms, secure communication and metrology tasks.
QOIL's quantum Fredkin gate
QOIL's quantum Fredkin (controlled-SWAP) gate
Our quantum Fredkin gate consists of two spontaneous parametric down-conversion sources, each producing a pair of polarisation entangled photons which are fed into an arrangement of three interferometers.