Quantum computing moves forward

A sil­i­con chip lev­i­tates indi­vid­ual atoms used in quan­tum infor­ma­tion pro­cess­ing. Photo: Curt Suplee and Emily Edwards, Joint Quan­tum Insti­tute and Uni­ver­sity of Mary­land. Credit: Science.

These advances could enable the cre­ation of immensely pow­er­ful com­put­ers as well as other appli­ca­tions, such as highly sen­si­tive detec­tors capa­ble of prob­ing bio­log­i­cal sys­tems. "We are really excited about the pos­si­bil­i­ties of new semi­con­duc­tor mate­ri­als and new exper­i­men­tal sys­tems that have become avail­able in the last decade," said Jason Petta, one of the authors of the report and an asso­ciate pro­fes­sor of physics at Prince­ton University.

Petta co-authored the arti­cle with David Awschalom of the Uni­ver­sity of Chicago, Lee Bas­set of the Uni­ver­sity of California-Santa Bar­bara, Andrew Dzu­rak of the Uni­ver­sity of New South Wales and Eve­lyn Hu of Har­vard University.

Two sig­nif­i­cant break­throughs are enabling this for­ward progress, Petta said in an inter­view. The first is the abil­ity to con­trol quan­tum units of infor­ma­tion, known as quan­tum bits, at room tem­per­a­ture. Until recently, tem­per­a­tures near absolute zero were required, but new diamond-based mate­ri­als allow spin qubits to be oper­ated on a table top, at room tem­per­a­ture. Diamond-based sen­sors could be used to image sin­gle mol­e­cules, as demon­strated ear­lier this year by Awschalom and researchers at Stan­ford Uni­ver­sity and IBM Research (Sci­ence, 2013).

The sec­ond big devel­op­ment is the abil­ity to con­trol these quan­tum bits, or qubits, for sev­eral sec­onds before they lapse into clas­si­cal behav­ior, a feat achieved by Dzurak's team (Nature, 2010) as well as Prince­ton researchers led by Stephen Lyon, pro­fes­sor of elec­tri­cal engi­neer­ing (Nature Mate­ri­als, 2012). The devel­op­ment of highly pure forms of sil­i­con, the same mate­r­ial used in today's clas­si­cal com­put­ers, has enabled researchers to con­trol a quan­tum mechan­i­cal prop­erty known as "spin." At Prince­ton, Lyon and his team demon­strated the con­trol of spin in bil­lions of elec­trons, a state known as coher­ence, for sev­eral sec­onds by using highly pure silicon-28.

Quantum-based tech­nolo­gies exploit the phys­i­cal rules that gov­ern very small par­ti­cles -- such as atoms and elec­trons -- rather than the clas­si­cal physics evi­dent in every­day life. New tech­nolo­gies based on "spin­tron­ics" rather than elec­tron charge, as is cur­rently used, would be much more pow­er­ful than cur­rent technologies.

In quantum-based sys­tems, the direc­tion of the spin (either up or down) serves as the basic unit of infor­ma­tion, which is anal­o­gous to the 0 or 1 bit in a clas­si­cal com­put­ing sys­tem. Unlike our clas­si­cal world, an elec­tron spin can assume both a 0 and 1 at the same time, a feat called entan­gle­ment, which greatly enhances the abil­ity to do computations.

A remain­ing chal­lenge is to find ways to trans­mit quan­tum infor­ma­tion over long dis­tances. Petta is explor­ing how to do this with col­lab­o­ra­tor Andrew Houck, asso­ciate pro­fes­sor of elec­tri­cal engi­neer­ing at Prince­ton. Last fall in the jour­nal Nature, the team pub­lished a study demon­strat­ing the cou­pling of a spin qubit to a par­ti­cle of light, known as a pho­ton, which acts as a shut­tle for the quan­tum information.

Yet another remain­ing hur­dle is to scale up the num­ber of qubits from a hand­ful to hun­dreds, accord­ing to the researchers. Sin­gle quan­tum bits have been made using a vari­ety of mate­ri­als, includ­ing elec­tronic and nuclear spins, as well as superconductors.

Some of the most excit­ing appli­ca­tions are in new sens­ing and imag­ing tech­nolo­gies rather than in com­put­ing, said Petta. "Most peo­ple agree that build­ing a real quan­tum com­puter that can fac­tor large num­bers is still a long ways out," he said. "How­ever, there has been a change in the way we think about quan­tum mechan­ics -- now we are think­ing about quantum-enabled tech­nolo­gies, such as using a spin qubit as a sen­si­tive mag­netic field detec­tor to probe bio­log­i­cal systems."