Nature447, 836-839 (14 June 2007) | doi:10.1038/nature05896; Received 21 February 2007; Accepted 26 April 2007
Demonstration of controlled-NOT quantum gates on a pair of superconducting quantum bits
J. H. Plantenberg1, P. C. de Groot1, C. J. P. M. Harmans1 & J. E. Mooij1
Kavli Institute of NanoScience, Delft University of Technology, PO Box 5046, 2600 GA, Delft, The Netherlands
Correspondence to: J. H. Plantenberg1J. E. Mooij1 Correspondence and requests for materials should be addressed to J.H.P. (Email: j.h.plantenberg@tudelft.nl) and J.E.M. (Email: j.e.mooij@tudelft.nl).
Quantum computation requires quantum logic gates that use the interaction within pairs of quantum bits (qubits) to perform conditional operations1. Superconducting qubits may offer an attractive route towards scalable quantum computing. In previous experiments on coupled superconducting qubits, conditional gate behaviour2 and entanglement3 were demonstrated. Here we demonstrate selective execution of the complete set of four different controlled-NOT (CNOT) quantum logic gates, by applying microwave pulses of appropriate frequency to a single pair of coupled flux qubits. All two-qubit computational basis states and their superpositions are used as input, while two independent single-shot SQUID detectors measure the output state, including qubit–qubit 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 Ramsey-like 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 0-state or the 1-state. These gates are now sufficiently characterized to be used in quantum algorithms, and together form an efficient set of versatile building blocks.
