The experimental device uses beryllium ions to store qubits in the way they spin while the laser-pulse quantum gates perform simple logic operations on the qubits. The trick to making a quantum logic gate is in designing a series of laser pulses that manipulate the beryllium ions in a way that processes information. Another laser then reads off the results of the calculations.
At the heart of the device is a gold-patterned aluminium wafer containing a tiny electromagnetic trap some 200 micrometres across, into which the team placed four ions – two of magnesium and two of beryllium. The magnesium ions act as “refrigerants”, removing unwanted vibrations from the ion chain and so keeping the device stable.
There are an infinite number of possible two-qubit operations, so the team chose a random selection of 160 to demonstrate the universality of the processor. Each operation involves hitting the two qubits with 31 distinct quantum gates encoded into the laser pulses. The majority were single-qubit gates, and so the pulse needed to interact with just one ion, but a small number were two-qubit gates requiring the pulse to “talk” to both ions.
By controlling the voltage on the gold electrodes surrounding the trap, the team can couple the ions when single-qubit gates are needed and couple them again for two-qubit operations.
Each gate is more than 90 per cent accurate, but when you stack them together the total figure falls to 79 per cent or so for a given operation. The fidelity needs to increase to around 99.99 per cent before it could be a useful component of a quantum computer. That could be done by improving the stability of the laser and reducing the errors from optical hardware.
The universal quantum computer is a device capable of simulating any physical system and represents a major goal for the field of quantum information science. In the context of quantum information, ‘universal’ refers to the ability to carry out arbitrary unitary transformations in the system’s computational space. Combining arbitrary single-quantum-bit (qubit) gates with an entangling two-qubit gate provides a set of gates capable of achieving universal control of any number of qubits provided that these gates can be carried out repeatedly and between arbitrary pairs of qubits. Although gate sets have been demonstrated in several technologies, they have so far been tailored towards specific tasks, forming a small subset of all unitary operators. Here we demonstrate a quantum processor that can be programmed with 15 classical inputs to realize arbitrary unitary transformations on two qubits, which are stored in trapped atomic ions. Using quantum state and process tomography, we characterize the fidelity of our implementation for 160 randomly chosen operations. This universal control is equivalent to simulating any pairwise interaction between spin-1/2 systems. A programmable multiqubit register could form a core component of a large-scale quantum processor, and the methods used here are suitable for such a device.