- 5 July 2005 -

Hewlett Packard goes opto quantum computing

Hewlett Packard scientists said that they had developed a strategy for designing a quantum computer composed of switches of light beams. The new strategy for designing a quantum computer was outlined in an article researchers at Hewlett-Packard published in the May issue of The New Journal of Physics.

The paper - written by Drs Bill Munro and Tim Spiller of Hewlett-Packard Laboratories in Bristol, England, with Professor Kae Nemoto of the National Institute of Informatics in Tokyo propose an approach that generates interactions between photons by using so-called weak optical
nonlinearities and intense laser fields. The result is the creation of two-photon gates, the basic building blocks of a quantum computer.

Normally, photons, the basic components of light, do not easily interact or ‘talk’ to each other.
That is why multiple light signals carrying different information can be sent along a thin optical
fibre without interfering with each other. But for quantum information processing and communication, it is vital that photons do interact when called upon to do so. The photons are
the qubits – the basic information bits – in this model of a quantum computer.

Why use an optical quantum system rather than solid state? Dr Spiller points out that light can be used for both quantum computing and quantum communication at the same time, which would not be the case with a solid-state system, where “static” quantum information would have to be mapped onto light to communicate it.

This means that the approach is suitable for distributed quantum computing, so that small but useful clusters of qubits can be physically separated – even at different sites – but linked together for computation.

The new approach uses weak nonlinearities and strong laser pulses to generate the interaction between the two individual photons. The laser pulse acts as an intermediary between the photons, first ‘talking’ to one, then the other, so that the two photons become entangled.

In quantum processing, generally attempting to check on the state of entangled qubits leads to the collapse of the information they carry. But with the HP-NII team’s approach, only the information in the laser pulse collapses; the qubit photons become entangled through this collapse.

Dr Spiller describes single photons – in fact any kind of qubit – as “precious” and points out that optical quantum computing systems that have previously been proposed would need hundreds of them to operate at all. And most of those photons would be wasted. The HP-NII system operates with single photons and wastes none. This makes it much more practical and efficient for quantum information and communication because today, single photons are hard to generate.

At the heart of the system is a single-photon detector – an innovation proposed by the HP Labs team – that is also used as a single-photon source. This is used to generate photons on demand. The scheme is reliable because the communication between separated quantum processing sites can be mediated by robust laser pulses rather than fragile single photon qubits.

Dr Munro said: “Our approach provides the fundamental building blocks for quantum computation, including highly efficient non-absorbing single-photon detectors, two-qubit parity detectors, near deterministic CNOT gates and more. All these elements are essential quantum information processing devices.” The approach is open for experimentalists to test.

HP is assembling a research program at its laboratories in Palo Alto, Calif., to build a working prototype based on the paper.

Source: http://www.physorg.com/news4924.html