The universal quantum computer is a device that could simulate any physical system and represents a major goal for the field of quantum information science. Algorithms performed on such a device are predicted to offer significant gains for some important computational tasks. In the context of quantum information, "universality" refers to the ability to perform arbitrary unitary transformations in the system's Hilbert space. The combination of arbitrary single-quantum-bit (qubit) gates with an entangling two-qubit gate is a universal set for computation in any size Hilbert space, provided these can be performed repeatedly and between arbitrary pairs of qubits. Though universal gate sets have been demonstrated in several technologies, they have as yet been tailored toward specific tasks, forming an infinitesimal group in the space of unitary operators. Here we demonstrate a programmable quantum computer that realises arbitrary unitary transformations on two qubits, which are stored in trapped atomic ions. Using quantum state and process tomography, we characterise the fidelity of our implementation for a large number of randomly chosen operations. This universal control is equivalent to simulating any pairwise interaction between spin-1/2 systems. A programmable multi-qubit register could form a core component of a larger quantum processor, and the methods used here are suitable for such a device.
Citation: Nature Physics
Pub Type: Journals
arbitrary unitary transformation, quantum information science, quantum register, trapped ions, universal quantum computer