Demonstration of controlledNOT quantum gates on a pair of superconducting quantum bits
Abstract
Quantum computation requires quantum logic gates that use the interaction within pairs of quantum bits (qubits) to perform conditional operations. Superconducting qubits may offer an attractive route towards scalable quantum computing. In previous experiments on coupled superconducting qubits, conditional gate behaviour and entanglement were demonstrated. Here we demonstrate selective execution of the complete set of four different controlledNOT (CNOT) quantum logic gates, by applying microwave pulses of appropriate frequency to a single pair of coupled flux qubits. All twoqubit computational basis states and their superpositions are used as input, while two independent singleshot SQUID detectors measure the output state, including qubitqubit correlations. We determined the gate's truth table by directly measuring the state transfer amplitudes and by acquiring the relevant quantum phase shift using a Ramseylike interference experiment. The four conditional gates result from the symmetry of the qubits in the pair: either qubit can assume the role of control or target, and the gate action can be conditioned on either the 0state or the 1state. These gates are now sufficiently characterized to be used in quantum algorithms, and together form an efficient set of versatile building blocks.
 Publication:

Nature
 Pub Date:
 June 2007
 DOI:
 10.1038/nature05896
 Bibcode:
 2007Natur.447..836P