1. Dipartimento di Fisica and Istituto Nazionale di Fisica della Materia, Università "La Sapienza", 00185 Roma, Italy
2. Research Center for Quantum Information, Institute of Physics, Slovak Academy of Sciences, Dúbravská cesta 9, 842 28 Bratislava, Slovakia
3. Department of Mathematical Physics, National University of Ireland, Maynooth, Ireland

Correspondence to: F. De Martini¹ Correspondence and requests for materials should be addressed to F.D.M. (e-mail: Email: francesco.demartini@uniroma1.it).

Abstract

In classical computation, a 'bit' of information can be flipped (that is, changed in value from zero to one and vice versa) using a logical NOT gate; but the quantum analogue of this process is much more complicated. A quantum bit (qubit) can exist simultaneously in a superposition of two logical states with complex amplitudes, and it is impossible^{1, 2, 3} to find a universal transformation that would flip the original superposed state into a perpendicular state for all values of the amplitudes. But although perfect flipping of a qubit prepared in an arbitrary state (a universal NOT operation) is prohibited by the rules of quantum mechanics, there exists an optimal approximation² to this procedure. Here we report the experimental realization of a universal quantum machine⁴ that performs the best possible approximation to the universal NOT transformation. The system adopted was an optical parametric amplifier of entangled photon states, which also enabled us to investigate universal quantum cloning.