ｇ因子制御された量子ドットにおける単一光子から単一電子スピンへの量子状態転写

2014 Fiscal Year Annual Research Report

• Project/Area Number: 13F03322
• Research Institution: The University of Tokyo
• Principal Investigator: 樽茶 清悟 東京大学, 工学(系)研究科(研究院), 教授 (40302799)
• Co-Investigator(Kenkyū-buntansha): LARSSON Marcus 東京大学, 工学(系)研究科(研究院), 外国人特別研究員
• Project Period (FY): 2013-04-01 – 2016-03-31
• Keywords: Quantum information / Quantum coherence / Quantum dots / Quantum optics / Semiconductors / Nano physics / Lasers / Condensed matter physics

Outline of Annual Research Achievements

The aim of our research is to show that quantum information stored in the polarization state of a single photons can be coherently transfered to the spin state of single electrons confined to a laterally defined quantum dot. To achieve this, we have fabricated double quantum dots in specifically engineered GaAs/AlGaAs heterostructures. The GaAs/AlGaAs heterostructures are supplied to us by Prof. Andreas Wieck in Bochum, Germany. The devices include a charge sensor, which allow us to read out single optically excited electrons in the quantum dots in real time. We have successfully observed optical excitation of electrons in theses devices by using a pulsed titanium-sapphire laser and the integrated charge sensor. Crucially, we have been able to resolve both the heavy hole and light hole band optical transitions in our measurements by tuning the laser wavelength appropriately. We measured an excitation efficiency of 0.7% per photon for the heavy hole band and 0.3% per photon for the light hole band. Furthermore, we have performed so called electron spin resonance (ESR) in our devices by employing very fast electrical gate pulses. The ESR measurements allowed us to extract the effective electron g-factor, g* = 0.27, in our quantum dot device. Both the verification of light hole excitation as well as performing ESR are important steps to show coherent transfer.

Current Status of Research Progress

3: Progress in research has been slightly delayed.

Reason

Progress has been slightly delayed due to sample related issues. We have successfully observed optical excitations in our devices, however, the efficiency of the light hole excitation is low, around 0.3% of the photons that hits the quantum dot result in an electron excition and subsequent detection event in the charge sensor. This low efficiency makes measurements difficult and time consuming. We believe that the low efficiency is related to a misalignment of the quantum dot to an on chip aperture that allows us to selectively excite electrons in only one of the dots. Since the quantum dots and aperture are very small, around 400 nm in diameter, the alignment has to be very good. If the aperture is misaligned to the quantum dot due to sample fabrication errors a smaller percentage of the photons from the laser will hit the quantum dot thereby reducing the efficiency. We are now in the process of measuring new samples that we believe will have a better alignment and therefor a higher efficiency.

Strategy for Future Research Activity

Following the successful observation of light hole excitation, the next step in our research plan is to show that we can adequately lift the light hole state degeneracy by applying an in-plane magnetic field. This will allow us to excite electrons from only one of the light hole state, a requirement which is necessary for coherent transfer. Partial verification of coherent transfer can then be achieved by measuring the spin state of the optically excited electron for different polarizations of the incident photons. Spin read out is performed in the Pauli spin blockade double quantum dot configuration. Furthermore, in order to fully verify that the state transfer is coherent we will combine optical excitation of the Zeeman-split light hole state with the ESR technique which we have demonstrated. By performing ESR coherent spin rotation onto the measurement basis within the decoherence time of the electron spin we will be able to map the photon polarization states on the Poincare sphere onto the spin states on the Bloch sphere.

Research Products

• [Journal Article] Single photoelectron detection after selective excitation of electron heavy-hole and electron light-hole pairs in double quantum dots (2014)
  • Author(s): K. Morimoto, T. Fujita, G. Allison, S. Teraoka, M. Larsson, H. Kiyama, S. Haffouz, D. G. Austing, A. Ludwig, A. D. Wieck, A. Oiwa, and S. Tarucha
  • Journal Title: PHYSICAL REVIEW Volume: B 90 Pages: 085306 1-5
  • DOI: 10.1103/PhysRevB.90.085306
  • Peer Reviewed
• [Journal Article] Tuning the electrically evaluated electron Land´e g factor in GaAs quantum dots and quantum wells of different well widths (2014)
  • Author(s): G. Allison, T. Fujita, K. Morimoto, S. Teraoka, M. Larsson, H. Kiyama, A. Oiwa, S. Haffouz, D. G. Austing, A. Ludwig, A. D. Wieck, and S. Tarucha
  • Journal Title: PHYSICAL REVIEW Volume: B 90 Pages: 235310 1-5
  • DOI: 10.1103/PhysRevB.90.235310
  • Peer Reviewed / Acknowledgement Compliant
• [Presentation] Towards coherent transfer of quantum states from single photons to single spins in double quantum dots (2015)
  • Author(s): M. Larsson
  • Organizer: QSNS
  • Place of Presentation: 宮城蔵王ロイヤルホテル, Miyagi, Japan
  • Year and Date: 2015-02-20
• [Presentation] Detection efficieny of optically excited electrons in double quantum dots based on GaAs/AlGaAs quantum wells (2014)
  • Author(s): M. Larsson, K. Morimoto, T. Fujita, G. Allison, S. Teraoka, H. Kiyama, D. G. Austing, A. Ludwig, A. D. Wieck, A. Oiwa, S. Tarucha
  • Organizer: ICPS
  • Place of Presentation: Austin Convention Center, Austin, USA
  • Year and Date: 2014-08-14