University of Basel  > Condensed Matter Theory

Oliver Gywat

 
Address:  Department of Physics
Broida Hall
University of California
Santa Barbara, CA 93106-9530
USA
e-mail:  [email protected]
phone (office):  ++1-805-893-5010 
fax:  ++1-805-893-6132



Research Interests
Single nitrogen-vacancy centers in diamond: Coherent manipulation and spin-spin interactions

Optical and magnetic properties of semiconductor nanostructures

Entanglement transfer from electron spins to photons

Quantum information processing in solid-state structures

Cavity quantum electrodynamics with solid-state structures


Publications
Spins in Optically Active Quantum Dots: Concepts and Methods
O. Gywat, H. J. Krenner, J. Berezovsky
Wiley-VCH (2009) (ISBN: 978-3-527-62899-5)

Filling a gap in the literature, this up-to-date introduction to the field provides an overview of current experimental techniques, basic theoretical concepts, and sample fabrication methods. Following an introduction, this monograph deals with optically active quantum dots and their integration into electro-optical devices, before looking at the theory of quantum confined states and quantum dots interacting with the radiation field. Final chapters cover spin-spin interaction in quantum dots as well as spin and charge states, showing how to use single spins for break-through quantum computation. A conclusion and outlook round off the volume. The result is a primer providing the essential basic knowledge necessary for young researchers entering the field, as well as semiconductor and theoretical physicists, PhD students in physics and material sciences, electrical engineers and materials scientists.


Coherent Dynamics of a Single Spin Interacting with an Adjustable Spin Bath
R. Hanson, V. V. Dobrovitski, A. E. Feiguin, O. Gywat, D. D. Awschalom
Science 320, 352 (2008) ; published online 13 March 2008 (Science DOI: 10.1126/science.1155400)

Phase coherence is a fundamental concept in quantum mechanics. Understanding the loss of coherence is paramount for future quantum information processing. We studied the coherent dynamics of a single central spin (a nitrogen-vacancy center) coupled to a bath of spins (nitrogen impurities) in diamond. Our experiments show that both the internal interactions of the bath and the coupling between the central spin and the bath can be tuned in situ, allowing access to regimes with surprisingly different behavior. The observed dynamics are well explained by analytics and numerical simulations, leading to valuable insight into the loss of coherence in spin systems. These measurements demonstrate that spins in diamond provide an excellent test bed for models and protocols in quantum information.


Initialization and read-out of spins in coupled core–shell quantum dots
J. Berezovsky, O. Gywat, F. Meier, D. Battaglia, X. Peng, and D. D. Awschalom,
Nature Physics 2, 831 (2006); published online 12 November 2006 (doi:10.1038/nphys458).

In the field of quantum information science, semiconductor quantum dots are of significant interest for their ability to confine a single electron for use as a qubit. However, to realize the potential offered by quantum information processing, it is necessary to couple two or more qubits. In contrast to coupling individual quantum dots, we demonstrate the integration of two coupled electronic states within a single quantum dot heterostructure. These chemically-synthesized nanocrystals, known as quantum dot quantum wells (QDQWs), are comprised of concentric layers of different semiconducting materials. We investigate carrier and spin dynamics in these structures using transient absorption and time-resolved Faraday rotation measurements. By tuning the excitation and probe energies, we find that we can selectively initialize and read out spins in different coupled states within the QDQW. These results open a pathway for engineering coupled qubits within a single nanostructure.


Non-destructive optical measurements of a single electron spin in a quantum dot
J. Berezovsky, M. H. Mikkelsen, O. Gywat, N. G. Stoltz, L. A. Coldren, D. D. Awschalom,
Science 314, 1916 (2006) ; published online 9 November 2006 (10.1126/science.1133862).

Kerr rotation measurements on a single electron spin confined in a charge-tunable semiconductor quantum dot demonstrate a means to directly probe the spin off-resonance, thus minimally disturbing the system. Energy-resolved magneto-optical spectra reveal information about the optically-oriented spin polarization and the transverse spin lifetime of the electron as a function of the charging of the dot. These results represent progress towards the manipulation and coupling of single spins and photons for quantum information processing.


Room-temperature manipulation and decoherence of a single spin in diamond
R. Hanson, O. Gywat, D. D. Awschalom,
Phys. Rev. B 74, 161203 (2006); see also quant-ph/0608233.

We report on room-temperature coherent manipulation of the spin of a single nitrogen-vacancy center in diamond and a study of its coherence as a function of magnetic field. We use magnetic resonance to induce Rabi nutations, and apply a Hahn spin echo to remove the effect of low-frequency dephasing. A sharp rise in the decoherence rate is observed at magnetic fields where the nitrogen-vacancy center spin couples resonantly to substitutional nitrogen spins via the magnetic dipolar coupling. Finally, we find evidence that away from these energy resonances spin flips of nitrogen electrons are the main source of decoherence.


Dynamics of Coupled Qubits Interacting with an Off-Resonant Cavity
Oliver Gywat, Florian Meier, Daniel Loss, D. D. Awschalom,
Phys. Rev. B 73, 125336 (2006); see also cond-mat/0511592.

We study a model for a pair of qubits which interact with a single off-resonant cavity mode and, in addition, exhibit a direct inter-qubit coupling. Possible realizations for such a system include coupled superconducting qubits in a line resonator as well as exciton states or electron spin states of quantum dots in a cavity. The emergent dynamical phenomena are strongly dependent on the relative energy scales of the inter-qubit coupling strength, the coupling strength between qubits and cavity mode, and the cavity mode detuning. We show that the cavity mode dispersion enables a measurement of the state of the coupled-qubit system in the perturbative regime. We discuss the effect of the direct inter-qubit interaction on a cavity-mediated two-qubit gate. Further, we show that for asymmetric coupling of the two qubits to the cavity, the direct inter-qubit coupling can be controlled optically via the ac Stark effect.


Electron-photon interaction in quantum dots : spin and entanglement
Oliver Gywat
PhD thesis, University of Basel (2005) html pdf

Recipes for spin-based quantum computing
Veronica Cerletti, Bill Coish, Oliver Gywat, Daniel Loss,
Nanotechnology 16, R27 (2005), see also cond-mat/0412028.

Technological growth in the electronics industry has historically been measured by the number of transistors that can be crammed onto a single microchip. Unfortunately, all good things must come to an end; spectacular growth in the number of transistors on a chip requires spectacular reduction of the transistor size. For electrons in semiconductors, the laws of quantum mechanics take over at the nanometre scale, and the conventional wisdom for progress (transistor cramming) must be abandoned. This realization has stimulated extensive research on ways to exploit the spin (in addition to the orbital) degree of freedom of the electron, giving birth to the field of spintronics. Perhaps the most ambitious goal of spintronics is to realize complete control over the quantum mechanical nature of the relevant spins. This prospect has motivated a race to design and build a spintronic device capable of complete control over its quantum mechanical state, and ultimately, performing computations: a quantum computer.
In this tutorial we summarize past and very recent developments which point the way to spin-based quantum computing in the solid-state. After introducing a set of basic requirements for any quantum computer proposal, we offer a brief summary of some of the many theoretical proposals for solid-state quantum computers. We then focus on the Loss-DiVincenzo proposal for quantum computing with the spins of electrons confined to quantum dots. There are many obstacles to building such a quantum device. We address these, and survey recent theoretical, and then experimental progress in the field. To conclude the tutorial, we list some as-yet unrealized experiments, which would be crucial for the development of a quantum-dot quantum computer.


Entanglement transfer from electron spins to photons in spin light-emitting diodes containing quantum dots
Veronica Cerletti, Oliver Gywat, Daniel Loss,
Phys. Rev. B 72, 115316 (2005), see also cond-mat/0411235.

We show that electron recombination using positively charged excitons in single quantum dots provides an efficient method to transfer entanglement from electron spins onto photon polarizations. We propose a scheme for the production of entangled four-photon states of GHZ type. From the GHZ state, two fully entangled photons can be obtained by a measurement of two photons in the linear polarization basis, even for quantum dots with observable fine structure splitting for neutral excitons and significant exciton spin decoherence. Because of the interplay of quantum mechanical selection rules and interference, maximally entangled electron pairs are converted into maximally entangled photon pairs with unity fidelity for a continuous set of observation directions. We describe the dynamics of the conversion process using a master-equation approach and show that the implementation of our scheme is feasible with current experimental techniques.


Probing Single-Electron Spin Decoherence in Quantum Dots using Charged Excitons
Oliver Gywat, Hans-Andreas Engel, Daniel Loss,
J. Superconductivity 18, 175 (2005); see also cond-mat/0408451.

We propose to use optical detection of magnetic resonance (ODMR) to measure the decoherence time T2 of a single electron spin in a semiconductor quantum dot. The electron is in one of the spin 1/2 states and a circularly polarized laser can only create an optical excitation for one of the electron spin states due to Pauli blocking. An applied electron spin resonance (ESR) field leads to Rabi spin flips and thus to a modulation of the photoluminescence or, alternatively, of the photocurrent. This allows one to measure the ESR linewidth and the coherent Rabi oscillations, from which the electron spin decoherence can be determined. We study different possible schemes for such an ODMR setup, including cw or pulsed laser excitation.


Molecular spintronics: Coherent spin transfer in coupled quantum dots
Florian Meier, Veronica Cerletti, Oliver Gywat, Daniel Loss, D. D. Awschalom,
Phys. Rev. B 69, 195315 (2004), see also cond-mat/0401397.

Time-resolved Faraday rotation has recently demonstrated coherent transfer of electron spin between quantum dots coupled by conjugated molecules. Using a transfer Hamiltonian ansatz for the coupled quantum dots, we calculate the Faraday rotation signal as a function of the probe frequency in a pump-probe setup using neutral quantum dots. Additionally, we study the signal of one spin-polarized excess electron in the coupled dots. We show that, in both cases, the Faraday rotation angle is determined by the spin transfer probabilities and the Heisenberg spin exchange energy. By comparison of our results with experimental data, we find that the transfer matrix element for electrons in the conduction band is of order 0.08 eV and the spin transfer probabilities are of order 10%.


Optical Detection of Single-Electron Spin Decoherence in a Quantum Dot
Oliver Gywat, Hans-Andreas Engel, Daniel Loss, R. J. Epstein, F. Mendoza, D. D. Awschalom
Phys. Rev. B 69, 205303 (2004), see also cond-mat/0307669

We propose a method based on optically detected magnetic resonance (ODMR) to measure the decoherence time T2 of a single electron spin in a semiconductor quantum dot. The electron spin resonance (ESR) of a single excess electron on a quantum dot is probed by circularly polarized laser excitation. The photoluminescence is modulated due to the ESR which enables the measurement of electron spin decoherence. We study different possible schemes for such an ODMR setup.


Quantum computation and the production of entangled photons using coupled quantum dots
Oliver Gywat, Guido Burkard, Daniel Loss
Superlattices and Microstructures 31, 127 (2002). Special issue on quantum dots for quantum computing.

We review recent theoretical progress on the use of electron spins as qubits in coupled semiconductor quantum dots for quantum information processing. We discuss the spin exchange mechanism and its microscopic origin in both laterally and vertically tunnel-coupled quantum dots and explain how it can be used to implement the quantum XOR gate which, in combination with single spin rotations, allows to perform arbitrary quantum computations. In addition to their functionality as a quantum gate, coupled quantum dots can act as a source for photon pairs in entangled polarization states which are useful for quantum communication. We describe a mechanism for the production of such entangled photon pairs via a biexciton state in tunnel-coupled quantum dots.


Biexcitons in coupled quantum dots as a source of entangled photons
Oliver Gywat, Guido Burkard, Daniel Loss
Phys. Rev. B 65, 205329 (2002), see also cond-mat/0109223

We study biexcitonic states in two tunnel-coupled semiconductor quantum dots and show that such systems provide the possibility to produce polarization-entangled photons or spin-entangled electrons that are spatially separated at production. We distinguish between the various spin configurations and calculate the low-energy biexciton spectrum using the Heitler-London approximation as a function of magnetic and electric fields. The oscillator strengths for the biexciton recombination involving the sequential emission of two photons are calculated. The entanglement of the photon polarizations resulting from the spin configuration in the biexciton states is quantified as a function of the photon emission angles.



Invited Talks
Single spins in diamond: Polarization, readout, and coherent control
  • APS March meeting, Denver CO, March 5-9, 2007.


    • Entanglement Transfer from Electron Spins to Photons and Dynamics of Coupled Qubits Interacting with an Off-Resonant Cavity
  • KITP Program Spintronics, Kavli Institute of Theoretical Physics, University of California at Santa Barbara, April 27 2006.


    • Probing Single-Electron Spin Decoherence in Quantum Dots using Charged Excitons
  • Laser Seminar of the Institute of Quantum Electronics, ETH, Zurich, January 10, 2005.


    • Entanglement transfer from electron spins to photons
  • Imamoglu group Seminar, ETH, Zurich, December 3, 2004.


    • Entangled photons from tunnel-coupled double quantum dots
  • Bouwmeester group Seminar, University of California, Santa Barbara, November 5, 2004.


    • Optical Detection of Single-Electron Spin Decoherence in a Quantum Dot
  • Seminar of the California Nanosystems Institute, University of California, Santa Barbara, July 26, 2004.
  • Yamamoto group Seminar, Stanford University, July 16, 2004.
  • Seminar of the Institut de Photonique et Electronique Quantiques, EPF Lausanne, April 19, 2004.
  • Seminar über spezielle Fragen der Halbleiterphysik, LMU Munich, November 24, 2003.
  • Condensed matter theory seminar, University of Basel, November 20, 2003.


    • Biexcitons in coupled quantum dots as a source of entangled photons
  • Condensed matter theory seminar, University of Basel, February 13, 2002.


    Contributed Talks
    Imaging and manipulating single spins in diamond
  • CNID Program Review Meeting, University of California at Santa Barbara, May 30, 2007


    • Dynamics of Coupled Qubits Interacting with an Off-Resonant Cavity
  • APS March meeting, Baltimore MD, March 13-17, 2006.


    • Optical Detection of Single-Electron Spin Decoherence in a Quantum Dot
  • International Workshop on Solid State Based Quantum Information Processing QIP2004, Herrsching, Bavaria, September 17, 2004.
  • DPG-Frühjahrstagung, Universität Regensburg, March 12, 2004. Contributed talk HL 50.1.
  • Jahrestagung der Schweizerischen Physikalischen Gesellschaft, Uni Neuchatel, March 3, 2004.

    Biexcitons in coupled quantum dots as a source of entangled photons
  • DPG-Frühjahrstagung, Universität Regensburg, March 12, 2002. Contributed talk HL 15.1.
  • Jahrestagung der Schweizerischen Physikalischen Gesellschaft, EPF Lausanne, March 1, 2002.
    •


    Posters
    Dynamics of Coupled Qubits Interacting with an Off-Resonant Cavity
  • Gordon Research Conference on Quantum Information Science (QIS2006), Il Ciocco, Barga, Italy, May 7 - 12, 2006.


    • Molecular spintronics: Coherent spin transfer in coupled quantum dots
  • DARPA SpinS Spintronics PI Review meeting, San Francisco, California, October 25 -28, 2004.
  • Annual meeting of Spintronics RTN, Technical University of Budapest, Hungary, October 4 - 7 2004.


    • Optical Detection of Single-Electron Spin Decoherence in a Quantum Dot
  • Monte Verità Summer School on Semiconductor Quantum Dots, Ascona, Switzerland, September 5 - 10, 2004.
  • The 3rd International Conference on Physics and Applications of Spin-Related Phenomena in Semiconductors, Santa Barbara, California, USA, July 21 - 23, 2004.
  • Latsis Symposium 2004, Quantum Optics for Communication and Computing, EPF Lausanne, March 1 - 3, 2004.
  • 13th International Winterschool on New Developments in Solid State Physics, Mauterndorf, Province of Salzburg, Austria, February 15 - 20, 2004.
  • The International Conference on Solid State Quantum Information Processing, Amsterdam, December 15 - 18, 2003.


    • Biexcitons in Coupled Quantum Dots as a Source of Entangled Photons
  • Frühjahrstagung des Arbeitskreises Festkörperphysik bei der DPG, TU Dresden, March 24 - 28, 2003. Poster HL 49.68.
  • Jahrestagung der Schweizerischen Physikalischen Gesellschaft, Pharmazentrum Basel, March 20 - 21, 2003.
  • DARPA SpinS Spintronics Workshop, Delray Beach Marriott, Florida, September 30 - October 4, 2002.



    • Scientific Visits
    KITP Program Spintronics
    Kavli Institute of Theoretical Physics, University of California at Santa Barbara, March 13, 2006 - June 23, 2006.

    Monte Verità Summer School on Semiconductor Quantum Dots: Physics and Devices
    Centro Stefano Franscini, Monte Verità, Ascona, Switzerland, September 5 - 10, 2004.

    13th International Winterschool on New Developments in Solid State Physics
    Low-dimensional systems. Mauterndorf, Province of Salzburg, Austria, February 15 - 20, 2004.

    Group of Prof. David D. Awschalom
    Center for Spintronics and Quantum Computation, University of California, Santa Barbara. June 27 - July 9, 2003 and November 1 - 12, 2004.

    International School on Quantum Computation and Information
    ISQCI 2002, Instituto Superior Técnico, Lisbon, Portugal, September 2 - 7, 2002.


    Teaching
    University (as teaching assistant)
  • Summer term 04: Exercises in Advanced Quantum Mechanics [Prof. Loss]
  • Winter term 03/04: Exercises in Physics I for students of Pharmacy [Prof. Oelhafen]
  • Summer term 03: Exercises in Physics II for students of Biology [Prof. Güntherodt/Prof. Hug]
  • Winter term 02/03: Exercises in Theoretical Mechanics [Prof. Thielemann]
  • Summer term 02: Exercises in Physics II for students of Chemistry and Earth Sciences [Prof. Oelhafen]
  • Winter term 01/02: Exercises in Physics III (Introduction to Quantum Mechanics) [Prof. Trautmann]


    • Gymnasium Oberstufe (High School)
  • August 2000 - July 2001: teacher of Physics and Applied Mathematics (PAM) at the Freies Gymnasium Basel.