In conventional electronics, a NOT gate inverts the value of a bit of information from 1 to 0 or from 0 to 1. This works because ordinary bits can only have a value of 1 or 0. Physicists have long believed that such binary information could also be stored in certain two-state quantum systems, such as the horizontal and vertical polarization states of photons, or the spin-up and spin-down states of electrons.

But unlike conventional bits, quantum bits – or qubits – can exist in a superposition of the two states. This makes it harder to invert the value of a qubit and limits the efficiency – or ‘fidelity’ – of a quantum NOT gate to 2/3.

De Martini’s team used polarized photons as qubits in their set-up, which was based on a crystal of barium borate with nonlinear optical properties. The researchers fired an ultraviolet photon into this crystal, and the photon split into two longer-wavelength photons by a process known as down-conversion. Some of these pairs of photons are ‘entangled’, which means that a measurement of the polarization of one photon reveals the polarization of the other one.

One of these entangled photons travelled to a detector, which measured its polarization, while a mirror reflected its partner back into the crystal. When this reflected photon emerged from the crystal, a second detector measured its polarization.

After repeating this process several hundred times, De Martini and co-workers found that the polarizations of the output photons were opposite to those of the input photons 63.0% of the time, compared with the maximum theoretical value of 66.7% - or 2/3. The researchers checked the ‘universality’ of their device by repeating the process using input photons with a range of different polarizations.

Although this demonstration of a quantum NOT gate is an important step in the field of quantum computation, it is not certain that optical methods would be used in a real quantum computer. But it is likely that such techniques would be used in quantum cryptography, in which encoded optical signals would be transmitted over long distances.