Quantum computation has moved another step closer with the first demonstration of a quantum NOT gate. Although it is impossible to build perfect logic gates for quantum bits of information, a team led by Francesco De Martini of the University of Rome “La Sapienza” and INFM in Italy has achieved almost the maximum theoretical fidelity with its device (F De Martini et al 2002 Nature 419 815). The development follows the near-perfect cloning of quantum bits by physicists at the University of Oxford earlier this year.
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.
