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Demonstration of an all-optical quantum controlled-NOT gate


The promise of tremendous computational power, coupled with the development of robust error-correcting schemes1, has fuelled extensive efforts2 to build a quantum computer. The requirements for realizing such a device are confounding: scalable quantum bits (two-level quantum systems, or qubits) that can be well isolated from the environment, but also initialized, measured and made to undergo controllable interactions to implement a universal set of quantum logic gates3. The usual set consists of single qubit rotations and a controlled-NOT (CNOT) gate, which flips the state of a target qubit conditional on the control qubit being in the state 1. Here we report an unambiguous experimental demonstration and comprehensive characterization of quantum CNOT operation in an optical system. We produce all four entangled Bell states as a function of only the input qubits' logical values, for a single operating condition of the gate. The gate is probabilistic (the qubits are destroyed upon failure), but with the addition of linear optical quantum non-demolition measurements, it is equivalent to the CNOT gate required for scalable all-optical quantum computation4.

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Figure 1: A schematic of the CNOT gate realized in this work.
Figure 2: Experimental demonstration of classical CNOT operation—operation in the logical basis.
Figure 3: Conditional coincidence fringes for non-orthogonal bases.
Figure 4: Density matrices for two highly entangled output states.


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We thank N. K. Langford for experimental work related to non-classical interference, T. B. Bell for work on the quantum state tomography system, and P. T. Cochrane, J. L. Dodd, A. Gilchrist, P. G. Kwiat, G. J. Milburn, W. J. Munro and M. A. Nielsen for discussions. This work was supported by the Australian government, the Australian Research Council, the US National Security Agency (NSA) and Advanced Research and Development Activity (ARDA) under the Army Research Office (ARO).

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Correspondence to J. L. O'Brien.

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O'Brien, J., Pryde, G., White, A. et al. Demonstration of an all-optical quantum controlled-NOT gate. Nature 426, 264–267 (2003).

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