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  • 1
    Publication Date: 2014-11-14
    Description: Topology, with its abstract mathematical constructs, often manifests itself in physics and has a pivotal role in our understanding of natural phenomena. Notably, the discovery of topological phases in condensed-matter systems has changed the modern conception of phases of matter. The global nature of topological ordering, however, makes direct experimental probing an outstanding challenge. Present experimental tools are mainly indirect and, as a result, are inadequate for studying the topology of physical systems at a fundamental level. Here we employ the exquisite control afforded by state-of-the-art superconducting quantum circuits to investigate topological properties of various quantum systems. The essence of our approach is to infer geometric curvature by measuring the deflection of quantum trajectories in the curved space of the Hamiltonian. Topological properties are then revealed by integrating the curvature over closed surfaces, a quantum analogue of the Gauss-Bonnet theorem. We benchmark our technique by investigating basic topological concepts of the historically important Haldane model after mapping the momentum space of this condensed-matter model to the parameter space of a single-qubit Hamiltonian. In addition to constructing the topological phase diagram, we are able to visualize the microscopic spin texture of the associated states and their evolution across a topological phase transition. Going beyond non-interacting systems, we demonstrate the power of our method by studying topology in an interacting quantum system. This required a new qubit architecture that allows for simultaneous control over every term in a two-qubit Hamiltonian. By exploring the parameter space of this Hamiltonian, we discover the emergence of an interaction-induced topological phase. Our work establishes a powerful, generalizable experimental platform to study topological phenomena in quantum systems.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Roushan, P -- Neill, C -- Chen, Yu -- Kolodrubetz, M -- Quintana, C -- Leung, N -- Fang, M -- Barends, R -- Campbell, B -- Chen, Z -- Chiaro, B -- Dunsworth, A -- Jeffrey, E -- Kelly, J -- Megrant, A -- Mutus, J -- O'Malley, P J J -- Sank, D -- Vainsencher, A -- Wenner, J -- White, T -- Polkovnikov, A -- Cleland, A N -- Martinis, J M -- England -- Nature. 2014 Nov 13;515(7526):241-4. doi: 10.1038/nature13891.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Physics, University of California, Santa Barbara, California 93106-9530, USA. ; Department of Physics, Boston University, Boston, Massachusetts 02215, USA. ; 1] Department of Physics, University of California, Santa Barbara, California 93106-9530, USA [2] Google Inc., Santa Barbara, California 93117, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/25391961" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
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  • 2
    Publication Date: 2014-04-25
    Description: A quantum computer can solve hard problems, such as prime factoring, database searching and quantum simulation, at the cost of needing to protect fragile quantum states from error. Quantum error correction provides this protection by distributing a logical state among many physical quantum bits (qubits) by means of quantum entanglement. Superconductivity is a useful phenomenon in this regard, because it allows the construction of large quantum circuits and is compatible with microfabrication. For superconducting qubits, the surface code approach to quantum computing is a natural choice for error correction, because it uses only nearest-neighbour coupling and rapidly cycled entangling gates. The gate fidelity requirements are modest: the per-step fidelity threshold is only about 99 per cent. Here we demonstrate a universal set of logic gates in a superconducting multi-qubit processor, achieving an average single-qubit gate fidelity of 99.92 per cent and a two-qubit gate fidelity of up to 99.4 per cent. This places Josephson quantum computing at the fault-tolerance threshold for surface code error correction. Our quantum processor is a first step towards the surface code, using five qubits arranged in a linear array with nearest-neighbour coupling. As a further demonstration, we construct a five-qubit Greenberger-Horne-Zeilinger state using the complete circuit and full set of gates. The results demonstrate that Josephson quantum computing is a high-fidelity technology, with a clear path to scaling up to large-scale, fault-tolerant quantum circuits.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Barends, R -- Kelly, J -- Megrant, A -- Veitia, A -- Sank, D -- Jeffrey, E -- White, T C -- Mutus, J -- Fowler, A G -- Campbell, B -- Chen, Y -- Chen, Z -- Chiaro, B -- Dunsworth, A -- Neill, C -- O'Malley, P -- Roushan, P -- Vainsencher, A -- Wenner, J -- Korotkov, A N -- Cleland, A N -- Martinis, John M -- England -- Nature. 2014 Apr 24;508(7497):500-3. doi: 10.1038/nature13171.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉1] Department of Physics, University of California, Santa Barbara, California 93106, USA [2]. ; Department of Physics, University of California, Santa Barbara, California 93106, USA. ; Department of Electrical Engineering, University of California, Riverside, California 92521, USA. ; 1] Department of Physics, University of California, Santa Barbara, California 93106, USA [2] Centre for Quantum Computation and Communication Technology, School of Physics, The University of Melbourne, Victoria 3010, Australia.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/24759412" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
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  • 3
    Publication Date: 2015-03-06
    Description: Quantum computing becomes viable when a quantum state can be protected from environment-induced error. If quantum bits (qubits) are sufficiently reliable, errors are sparse and quantum error correction (QEC) is capable of identifying and correcting them. Adding more qubits improves the preservation of states by guaranteeing that increasingly larger clusters of errors will not cause logical failure-a key requirement for large-scale systems. Using QEC to extend the qubit lifetime remains one of the outstanding experimental challenges in quantum computing. Here we report the protection of classical states from environmental bit-flip errors and demonstrate the suppression of these errors with increasing system size. We use a linear array of nine qubits, which is a natural step towards the two-dimensional surface code QEC scheme, and track errors as they occur by repeatedly performing projective quantum non-demolition parity measurements. Relative to a single physical qubit, we reduce the failure rate in retrieving an input state by a factor of 2.7 when using five of our nine qubits and by a factor of 8.5 when using all nine qubits after eight cycles. Additionally, we tomographically verify preservation of the non-classical Greenberger-Horne-Zeilinger state. The successful suppression of environment-induced errors will motivate further research into the many challenges associated with building a large-scale superconducting quantum computer.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Kelly, J -- Barends, R -- Fowler, A G -- Megrant, A -- Jeffrey, E -- White, T C -- Sank, D -- Mutus, J Y -- Campbell, B -- Chen, Yu -- Chen, Z -- Chiaro, B -- Dunsworth, A -- Hoi, I-C -- Neill, C -- O'Malley, P J J -- Quintana, C -- Roushan, P -- Vainsencher, A -- Wenner, J -- Cleland, A N -- Martinis, John M -- England -- Nature. 2015 Mar 5;519(7541):66-9. doi: 10.1038/nature14270.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Physics, University of California, Santa Barbara, California 93106, USA. ; 1] Department of Physics, University of California, Santa Barbara, California 93106, USA [2] Centre for Quantum Computation and Communication Technology, School of Physics, The University of Melbourne, Victoria 3010, Australia. ; 1] Department of Physics, University of California, Santa Barbara, California 93106, USA [2] Department of Materials, University of California, Santa Barbara, California 93106, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/25739628" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
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  • 4
    Publication Date: 2018-04-13
    Description: A key step toward demonstrating a quantum system that can address difficult problems in physics and chemistry will be performing a computation beyond the capabilities of any classical computer, thus achieving so-called quantum supremacy. In this study, we used nine superconducting qubits to demonstrate a promising path toward quantum supremacy. By individually tuning the qubit parameters, we were able to generate thousands of distinct Hamiltonian evolutions and probe the output probabilities. The measured probabilities obey a universal distribution, consistent with uniformly sampling the full Hilbert space. As the number of qubits increases, the system continues to explore the exponentially growing number of states. Extending these results to a system of 50 qubits has the potential to address scientific questions that are beyond the capabilities of any classical computer.
    Keywords: Physics
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
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