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Demonstration of controlled-NOT quantum gates on a pair of superconducting quantum bits


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.

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Figure 1: Coupled-qubits set-up.
Figure 2: Operation of the coupled-qubits device.
Figure 3: 1 C -controlled gate operation.
Figure 4: Truth table amplitudes of the CNOT quantum gate.
Figure 5: Quantum phase induced after n = 2–6 CNOT operations.


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We thank L. M. K. Vandersypen and S. Lloyd for discussions, and R. N. Schouten and C. M. Huizinga for technical assistance. This work was supported by the Dutch organization for Fundamental Research on Matter (FOM), the EU projects SQUBIT2 and EuroSQIP, and the Dutch National Initiative on Nano Science and Technology, NanoNed.

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Correspondence to J. H. Plantenberg or J. E. Mooij.

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Supplementary Information 1

This file contains Supplementary Notes and Supplementary Figures 1-2 with legends. The Supplementary Information presents a more detailed analysis of the controlled rotations that were performed on the four-level system. Furthermore, experimental data are presented for 0C- and 1C-controlled operations and the detector calibration through conditional spectroscopy is explained. (PDF 1677 kb)

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Plantenberg, J., de Groot, P., Harmans, C. et al. Demonstration of controlled-NOT quantum gates on a pair of superconducting quantum bits. Nature 447, 836–839 (2007).

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