Journal Article

Experimental superposition of orders of quantum gates

Brukner, Časlav, Araújo, Mateus, Dowd, Emma G., Procopio, Lorenzo M., Costa, Fabio, Moqanaki, Amir, Rozema, Lee A., Walther, Philip, Alonso Calafell, Irati, Hamel, Deny R.

2015

Overview

Quantum computers achieve a speed-up by placing quantum bits (qubits) in superpositions of different states. However, it has recently been appreciated that quantum mechanics also allows one to ‘superimpose different operations’. Furthermore, it has been shown that using a qubit to coherently control the gate order allows one to accomplish a task—determining if two gates commute or anti-commute—with fewer gate uses than any known quantum algorithm. Here we experimentally demonstrate this advantage, in a photonic context, using a second qubit to control the order in which two gates are applied to a first qubit. We create the required superposition of gate orders by using additional degrees of freedom of the photons encoding our qubits. The new resource we exploit can be interpreted as a superposition of causal orders, and could allow quantum algorithms to be implemented with an efficiency unlikely to be achieved on a fixed-gate-order quantum computer. It has been conjectured that not only states but also quantum operations can be placed in a superposition of causal order. Here, the authors use a qubit superpose the order in which two photonic gates are applied, which is shown to enable a more efficient detection of their commutation relations.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/766/400/482 / 639/766/483/2802 / 639/766/483/481 / Algorithms / Computers / Gates / Humanities and Social Sciences / multidisciplinary / Photonics / Photons / Quantum computers / Quantum mechanics / Quantum theory / Qubits (quantum computing) / Science / Science (multidisciplinary) / Superposition (mathematics)