New quantum gate realised
    

 A new important building block for a future quantum computer has been realised by physicists at the Institute for Experimental Physics in Innsbruck and the Institute for Quantum Optics and Quantum Information (IQOQI): a gate acting on three quantum bits, the so-called Toffoli gate, as has been reported in Physical Review Letters.

Quantum mechanical laws allow quantum computers to process information significantly faster and more efficiently than normal computers. Even the most demanding algorithms can be realised in just a few steps. The basic building blocks for quantum computers are gates acting on one or more quantum bits (qubits). Already single qubit operations and one two-qubit gate allow for basic experiments in the world of quantum physics. Innsbruck scientists around Rainer Blatt impressively demonstrated this during the recent years: In 2005, they were able to deterministically teleport quantum information from one atom to another. Last year, they were the first ones to deterministically generate entanglement between noninteracting quantum bits (entanglement swapping).

Gate acting on three qubits

In principle, every quantum algorithm can be realised by using only single- and two-qubit gates. This approach, however, is unfavourable for more complex tasks and would rapidly reach the limits of current implementations. Hence, physicists worldwide strive for the efficient realisation of multi-qubit gates. The scientists in Innsbruck succeeded in implementing a three-qubit gate acting on three trapped calcium ions representing the qubits. The target qubit of the so-called Toffoli gate will only be switched when both control qubits are set to „1“, while in all other cases the target qubit will not be changed.

Illustration: A calcium ion may be in the state S or D. This is equivalent to a quantum bit being in 0 or 1. For a given input state, the figure shows the output state after the Toffoli gate is applied. For almost all input states, nothing happens, while the final two states SSD and SSS (both control qubits are in state S) are being changed.

Important step towards a quantum computer

This novel gate does not only augment the set of available quantum gates, but also raises the achievable efficiency. Thomas Monz, junior scientist at the experiment, explains:„A Toffoli gate based on the conventional approach would require a sequence of six controlled switch operations. In comparison, our approach is three times faster while operating at a reduced error rate.“ Applications of the Toffoli gate lie within quantum error correction or quantum mechanical prime factorization. Thus, it represents an important component of a future quantum computer.

We are financially supported by Universität Innsbruck,  Österreichische Akademie der Wissenschaften,  Fonds zur Förderung der wissenschaftlichen Forschung (FWF) within the program "Control and Measurement of Coherent Quantum Systems", the European networks "SCALA", "CONQUEST", as well as Institut für Quanteninformation GmbH and ARO.

Links

• "Realization of the quantum Toffoli gate with trapped ions"
  T. Monz, K. Kim, W. Hänsel, M. Riebe, A. S. Villar, P. Schindler, M. Chwalla, M. Hennrich, and R. Blatt,
  Physical Review Letters 102, 040501 (2009), arXiv:0804.0082.