Browsing by Author "Niskanen, Antti O."

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    Control of quantum evolution and Josephson junction circuits
    (VTT Technical Research Centre of Finland, 2004-11-26) Niskanen, Antti O.; Department of Engineering Physics and Mathematics; Teknillisen fysiikan ja matematiikan osasto
    Ever since Peter Shor's ground-breaking discovery in 1994 of an algorithm capable of factoring large integers on a quantum-mechanical computer exponentially faster than using any known classical method, research on quantum computing has boomed. Quantum information – a unique mixture of computer science, physics and mathematics – has developed into a new branch of information theory. On the experimental side, physicists from many different disciplines including atomic, solid-state and low-temperature physics, as well as optics, are striving today towards a practical quantum computer. All the candidate quantum bit (qubit) technologies have one thing in common: They rely on the controlled time-evolution of a closed quantum system, a seemingly paradoxical task. In this Thesis the temporal control of quantum systems is studied. The topics included can be divided into two according to the type of temporal evolution; geometrical or dynamical. Geometrical realization-independent methods for quantum computing are studied first. Then the study is extended into dynamical quantum computing and the so-called Josephson charge-qubit register is considered as a test bench. Finally, a spin-off application of the geometrical evolution of a Josephson junction system is studied, i.e. Cooper pair pumping. A novel Cooper pair pump, the Cooper pair "sluice", is introduced. The work on quantum computing reported in this Thesis is theoretical while the Cooper pair "sluice" is studied both theoretically and experimentally. Numerical simulations, both sequential and parallel, are used extensively throughout the Thesis. The experiments were carried out under cryogenic mK conditions and the sample fabrication was done using e-beam nanolithography. Because the execution time of a quantum algorithm is always limited by the inevitable process of decoherence, it is important to utilize any measure available for accelerating quantum computations. It is found that practical quantum algorithms could greatly benefit from classical computer-aided optimization. Moreover, it is found that even a modest demonstrator of a full quantum algorithm using Josephson charge qubits is just barely realizable within present-day coherence times. However, the experimental part of this Thesis shows clear evidence of the functioning of the "sluice". While the worldwide effort of improving the coherence properties of qubits is underway, the "sluice" could well find practical use, e.g., in metrology in the foreseeable future.
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    Evidence of Cooper-pair pumping with combined flux and voltage control
    (American Physical Society (APS), 2005) Niskanen, Antti O.; Kivioja, Jani M.; Seppä, Heikki; Pekola, Jukka P.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of Science
    We have experimentally demonstrated pumping of Cooper pairs in a single-island mesoscopic structure. The island was connected to leads through SQUID (superconducting quantum interference device) loops. Synchronized flux and voltage signals were applied whereby the Josephson energies of the SQUIDs and the gate charge were tuned adiabatically. From the current-voltage characteristics one can see that the pumped current increases in 1e steps which is due to quasiparticle poisoning on the measurement time scale, but we argue that the transport of charge is due to Cooper pairs.
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    Evidence of Cooper-pair pumping with combined flux and voltage control
    (2005-01-26) Niskanen, Antti O.; Kivioja, Jani; Seppä, Heikki; Pekola, Jukka; Department of Applied Physics
    We have experimentally demonstrated pumping of Cooper pairs in a single-island mesoscopic structure. The island was connected to leads through SQUID (superconducting quantum interference device) loops. Synchronized flux and voltage signals were applied whereby the Josephson energies of the SQUIDs and the gate charge were tuned adiabatically. From the current-voltage characteristics one can see that the pumped current increases in 1e steps which is due to quasiparticle poisoning on the measurement time scale, but we argue that the transport of charge is due to Cooper pairs.
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    Fast and accurate single-island charge pump: implementation
    (2003-10-24) Niskanen, Antti O.; Pekola, Jukka; Seppä, Heikki; Department of Applied Physics
    We introduce a Cooper pair “sluice” for the implementation of a frequency-locked current source. The device consists of two mesoscopic SQUIDs and of a single superconducting island with a gate. We demonstrate theoretically that it is possible to obtain a current as high as 0.1 nA at better than ppm accuracy via periodically modulating both the gate charge and the effective Josephson coupling. We find that the device is tolerant against background charge noise and operates well even in a dissipative environment. The effect of the imperfect suppression of the Josephson coupling and the finite operating frequency are also investigated.
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    Fast and Accurate Single-Island Charge Pump: Implementation of a Cooper Pair Pump
    (American Physical Society (APS), 2003) Niskanen, Antti O.; Pekola, Jukka P.; Seppä, Heikki; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of Science
    We introduce a Cooper pair “sluice” for the implementation of a frequency-locked current source. The device consists of two mesoscopic SQUIDs and of a single superconducting island with a gate. We demonstrate theoretically that it is possible to obtain a current as high as 0.1 nA at better than ppm accuracy via periodically modulating both the gate charge and the effective Josephson coupling. We find that the device is tolerant against background charge noise and operates well even in a dissipative environment. The effect of the imperfect suppression of the Josephson coupling and the finite operating frequency are also investigated.
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    Holonomic Quantum Computing
    (2002) Niskanen, Antti O.; Teknillisen fysiikan ja matematiikan osasto; Teknillinen korkeakoulu; Helsinki University of Technology; Salomaa, Martti M.
    Kvanttitietokoneista on muodostunut intensiivisen tutkimuksen kohde viimeisen vuosikymmenen aikana. Shorin kokonaislukujen tekijöihin jakoon tarkoitetun polynomisessa ajassa toimivan algoritmin keksimisestä 1995 lähtien eri alojen fyysikot ovat alkaneet pohtia kvanttitietokoneen rakentamisen mahdollisuutta. Käytännöllisen kvanttilaskennan vahvuus olisi tiettyjen eksponentiaalisesti vaikeiden ongelmien ratkaisu polynomisessa ajassa. Toimivan kvanttitietokoneen tulisi olla riittävän eristetty kvanttisysteemi, mutta sen kytkentää ympäristöön tulisi silti voida kontrolloida hallitusti. Toimivaa kvanttitietokonetta ei ole vielä rakennettu. Tässä diplomityössä tutkitaan ehdotusta niin kutsutusta holonomisesta kvanttilaskennasta. Holonomisessa kvanttilaskennassa kvanttisysteemin annetaan kehittyä adiabaattisesti samalla kun kontrolliparametreja säädetään siten, että parametriavaruuteen syntyy suljettuja polkuja. Jokainen polku vastaa loogista kvanttioperaatiota. Nämä operaatiot ovat luonteeltaan täysin geometrisia. Tämän vuoksi holonomisen kvanttilaskennan uskotaan sietävän hyvin ympäristön aiheuttamaa dekoherenssia. Tätä skenaariota tutkitaan sekä analyttisesti että numeerisesti käyttäen apuna realisaatiosta riippumatonta mallia. Sopivien polkujen löytämiseksi kehitetään menetelmä, jota testataan käyttäen Fortan 90 -ohjelmia. Tällä numeerisella menetelmällä ratkaistaan monidimensioinen optimointitehtävä, jossa optimoitava funktio on erittäin epätasainen. Lisäksi pohditaan holonomisen kvanttilaskennan kokeellista insinööritoteutusta käyttäen suprajohtavia nanorakenteita ja Josephsonin liitoksia. Kvanttitietokoneesta esitetään mallisuunnitelma. Erityisesti yksittäisen kvanttibitin suunnitelma saattaisi olla kokeellisesti toteutettavissa lähitulevaisuudessa. Myös kvanttibittien kytkemiseksi ehdotetaan mahdollista ratkaisua.