Quantum Computing Continues to Move Forward

A newly published study looks at recent advances in quantum measurements, coherent control, and the generation of entangled states, while describing some of the challenges that remain ahead for quantum computing and other applications.

New tech­nolo­gies that exploit quan­tum behav­ior for com­put­ing and other appli­ca­tions are closer than ever to being real­ized due to recent advances, accord­ing to a review arti­cle pub­lished this week in the jour­nal 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.”

Reference: “Quan­tum Spin­tron­ics: Engi­neer­ing and Manip­u­lat­ing Atom-Like Spins in Semi­con­duc­tors” by David D. Awschalom, Lee C. Bassett, Andrew S. Dzurak, Evelyn L. Hu and Jason R. Petta, 8 March 2013, Sci­ence.
DOI: 10.1126/science.1231364

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