Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D4: Spin Qubits in Quantum Dots |
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Sponsoring Units: GQI DCMP Chair: Jason Petta, Princeton University Room: 306/307 |
Monday, March 16, 2009 2:30PM - 3:06PM |
D4.00001: Singlet-triplet dynamics in double quantum dots probed with single-shot readout Invited Speaker: Christian Barthel We report single-shot readout of a two-electron spin qubit in a GaAs double quantum dot. The readout scheme allows repeated single quantum measurements with a readout fidelity above $90\%$, extracted from a simple model of the measurement outcome. In contrast to measurements on single spins, this scheme does not rely on tunneling of electrons out of the quantum dot or on high magnetic fields, which are incompatible with the operating requirements of the qubit. The spin state is mapped to a charge state, which is subsequently measured by a rapidly switched quantum point contact~(QPC). Single-shot readout is used to observe the precession of the electron spin qubit in the effective magnetic field due to the hyperfine interaction with the GaAs nuclei. All measurements are taken within the nuclear correlation time and the evolution of the nuclear spin bath is monitored continuously on a ms time scale. Finally the precession of the qubit is compared to the driven dynamics of the electron spin state at the resonance between singlet and a triplet with total spin one. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D4.00002: Locking electron spins into resonance by electron-nuclear feedback Invited Speaker: Katja Nowack All basic building blocks for spin-based quantum information processing using electron spins in GaAs quantum dots have recently been realized. Recent experiments have shown single-shot read-out of an individual spin [1], the implementation of the SWAP gate [2] and (magnetically induced) coherent single electron spin rotations [3]. However, the main drawback of using electron spins in a GaAs environment is the short spin coherence time, which is measured to be in the nanosecond range [2,4]. The source of this fast decoherence is the hyperfine interaction of the localized electron spin with the randomly fluctuating nuclear spins of the host lattice. The fluctuations of the nuclear spins have to be reduced to extend the electron spin coherence time. We therefore study the electron-nuclear spin interaction and use magnetically driven spin resonance to control the electron spin and indirectly manipulate the nuclear spins. We apply continuous microwave excitation to the electron spin and observe strong electron-nuclear feedback. One experimental signature of this feedback is the locking of the electron spin system into resonance with the microwaves. Once the electron spin is locked into resonance, this resonance condition remains fullfilled even when the external magnetic field or the microwave frequency is changed. This is due to dynamically build up nuclear polarizations (up to 500 mT) which generally counteract the external magnetic field. Locking of the electron spin system into resonance might indicate that the nuclear polarization exhibits stable configurations where fluctuations of the nuclear distribution are reduced [5]. \\[4pt] References \\[0pt] [1] J. M. Elzerman et al. , \textit{Nature} \textbf{430}, 431 (2004) \\[0pt] [2]. J. R. Petta et al., \textit{Science} \textbf{309}, 2180 (2005). \\[0pt] [3] F. H. L. Koppens et al., \textit{Nature }\textbf{442}, 766 (2006). \\[0pt] [4] F. H. L. Koppens et al., \textit{Phys. Rev. Lett.} \textbf{100}, 236802 (2008). \\[0pt] [5] J. Danon and Yu. V. Nazarov, \textit{private communication}. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D4.00003: Decoherence mechanisms for electron and hole spins in quantum dots Invited Speaker: Bill Coish One major obstacle to the realization of electron-spin qubits is decoherence with a random environment. While relaxation ($T_1$) processes are dominated in these systems by spin-orbit coupling and phonon emission, much faster dephasing processes are determined by coupling to an uncontrolled environment of nuclear spins. I will review work on electron-spin decoherence due to nuclear spins [1] and how to control this decoherence through a sequence of measurements performed on the nuclear-spin system [2,3]. This talk will then focus on coherence properties of \emph{hole}, rather than electron spins. Remarkably, in contrast to statements frequently made in the literature, we have found that the coupling of hole spins to nuclei can be appreciable [4] (comparable to that for electrons). However, in a two-dimensional quantum dot, the hole-nuclear spin coupling takes on an Ising-like form, which may allow for substantially longer coherence times than for electron spins.\\[4pt] [1] W. A. Coish, J. Fischer and D. Loss, Phys. Rev. B 77, 125329 (2008)\\[0pt] [2] D. Klauser, W. A. Coish, and D. Loss, Phys. Rev. B 73, 205302 (2006)\\[0pt] [3] D. Klauser, W. A. Coish, and D. Loss, Phys. Rev. B 78, 205301 (2008)\\[0pt] [4] J. Fischer, W. A. Coish, D. V. Bulaev, and D. Loss, Phys. Rev. B 78, 155329 (2008) [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:54PM |
D4.00004: Experiments on hole spins in quantum dots Invited Speaker: Richard Warburton A single electron in a nano-sized quantum dot is so strongly quantized that the interaction with the phonons is highly suppressed. This leads to potentially long spin dephasing times, ideal for applications in quantum information. However, the hyperfine interaction, the interaction of the electron spin with the spins of the host nuclei, leads to a rapid loss of spin coherence and this presently represents the largest stumbling block in quantum dot spin physics. An alternative to an electron spin is a hole spin. The p-like atomic part of the hole wave function conveniently goes to zero at the locations of the atomic nuclei, removing the contact part of the hyperfine interaction. Could it be the case that a heavy hole spin is more coherent than an electron spin in a self-assembled quantum dot? Optical experiments will be presented on single hole spins, exploring the spin relaxation time with an optical pumping experiment and the spin dephasing time with coherent population trapping. The results are very encouraging: both the hole spin T1 and T2* times are surprisingly large. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:30PM |
D4.00005: Dynamic nuclear polarization with single electron spins Invited Speaker: Jacob Taylor Hyperfine interactions limit electron spin coherence times in GaAs quantum dots. By separating a spin singlet state on a chip, we measure an ensemble averaged spin dephasing time $T_2^*$ of 10 ns, limited by the contact hyperfine interaction with the GaAs host nuclei\footnote{J. R. Petta \textit{et al.}, Science $\bf{309}$, 2180 (2005).}. We use electrical control of the exchange interaction to drive coherent spin rotations. Exchange driven spin rotations are used to implement a ``singlet-triplet spin echo'' pulse sequence, which leads to a spin coherence time, $T_2$, exceeding 1 microsecond. We show that nuclear spins can be polarized by controlling two-electron spin states near the anti-crossing of the singlet (S) and triplet ($T_+$). An initialized S state is cyclically brought into resonance with the $T_+$ state, where hyperfine fields drive rapid rotations between S and $T_+$, ``flipping'' an electron spin and ``flopping'' a nuclear spin\footnote{J. R. Petta, J. M. Taylor \textit{et al.}, Phys. Rev. Lett. $\bf{100}$, 067601 (2008).}. The resulting Overhauser field approaches 80 mT, in agreement with a simple rate-equation model. A self-limiting pulse sequence is developed that allows the steady-state nuclear polarization to be set using a gate voltage. [Preview Abstract] |
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