Session W46: Invited Session: Silicon Spin Qubits: Relaxation and Decoherence

Single-shot readout of spin qubits in Si/SiGe quantum dots

Si/SiGe quantum dots are an attractive option for spin qubit development, because of the long coherence times for electron spins in silicon, arising from weak hyperfine interaction and low spin orbit coupling. I will present measurements of gate-defined single and double quantum dots formed in Si/SiGe semiconductor heterostuctures. Control of the gate voltages on these dots enables tuning of the tunnel coupling to the leads and to other dots. Careful tuning of these tunnel rates, in combination with fast, pulsed-gate manipulation and spin-to-charge conversion, allow spin state measurement using an integrated quantum point contact as a local charge detector. Single spin qubit readout relies on the Zeeman energy splitting from an external magnetic field for spin-to-charge conversion. Two-electron singlet-triplet qubits, on the other hand, can be measured by using Pauli spin blockade of tunneling between the dots to readout the qubit even at zero magnetic field. I will present real-time, single-shot readout measurements of both individual spin [1] and singlet-triplet qubits [2] in gated Si/SiGe quantum dots. Work performed in collaboration with J. R. Prance, Zhan Shi, B. J. Van Bael, Teck Seng Koh, D. E. Savage, M. G. Lagally, R. Joynt, L. R. Schreiber, L. M. K. Vandersypen, M. Friesen, S. N. Coppersmith, and M. A. Eriksson. \\[4pt] [1] C. B. Simmons et al. Physical Review Letters 106, 156804 (2011). \\[0pt] [2] J. R. Prance, et al., e-print: http://lanl.arxiv.org/abs/1110.6431   

Coherent manipulation of a Si/SiGe-based singlet-triplet qubit

Electrically defined silicon-based qubits are expected to show improved quantum memory characteristics in comparison to GaAs-based devices due to reduced hyperfine interactions with nuclear spins. Silicon-based qubit devices have proved more challenging to build than their GaAs-based counterparts, but recently several groups have reported substantial progress in single-qubit initialization, measurement, and coherent operation. We report [1] coherent control of electron spins in two coupled quantum dots in an undoped Si/SiGe heterostructure, forming two levels of a singlet-triplet qubit. We measure a nuclei-induced $T_{2}^{*}$ of 360 ns, an increase over similar measurements in GaAs-based quantum dots by nearly two orders of magnitude. We also describe the results from detailed modeling of our materials and devices that show this value for $T_{2}^{*}$ is consistent with theoretical expectations for our estimated dot sizes and a natural abundance of $^{29}$Si. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the United States Department of Defense or the U.S. Government. Approved for public release, distribution unlimited.\\[4pt] [1] B.~M. Maune et al., ``Coherent Singlet-Triplet Oscillations in a Silicon-based Double Quantum Dot,'' accepted by Nature.   

Simulating a solid state spin qubit in a spin bath

Powerful computational methods have been developed in recent years for understanding decoherence induced by environmental spins. Specifically, the cluster correlation expansion [Phys. Rev. B 78, 085315 (2008)] and adaptations [Phys. Rev. Lett. 105, 187602 (2010)] provide successive approximations that approach the solution to the full quantum mechanical problem for small and large spin baths with good efficiency. With these methods, we are able to study the nature of spin-bath decoherence in various regimes, for different types of qubits (e.g., donors or quantum dots) and for different types of spin baths (e.g., nuclei or electrons). Our quantitative analyses have implications for solid state spin qubit prospects and materials choices. Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Two-Spin Decoherence and Disentanglement in Semiconductor Nanostructures

A crucial issue in spin-based quantum information processing is spin coherence. Decoherence of a single electron spin confined in a quantum dot or to a donor ion has been studied extensively, with hyperfine interaction to the environmental nuclear spins being identified as the most important channel of spin decoherence [1]. Decoherence of two-spin-qubit states is inevitably affected by singe-spin decoherence. Moreover, for exchange-coupled spin qubits, there are new decoherence channels beyond those for single spins because of the Coulombic nature of the exchange interaction. Here we discuss a series of studies of two-spin decoherence mechanisms [2-6], including both known single-spin decoherence and relaxation channels due to nuclear spins and new channels based on electrostatic coupling. More specifically, we examine two-spin relaxation due to hyperfine interaction and phonon emission [2], spin-orbit interaction and phonon emission [3], nuclear spin dynamics mediated by electrons, charge noise [4,5], and electron-phonon interaction [6]. We also analyze the associated disentanglement of the two spins as the decoherence processes go on. Our results show that while nuclear spins affect two-spin states in a qualitatively similar manner as for single spin states, there are interesting new twists because of the weaker hyperfine interaction due to the electron orbital symmetry. On the other hand, the charge noise and phonon induced dephasing depends strongly on the electrical features of the nanostructure, and could pose a significant constraint on two-qubit gates and quantum computing schemes based on two-spin encoding. \\[4pt] [1] L. Cywinski, W. Witzel, and S. Das Sarma, Phys. Rev. B {\bf 79}, 245314 (2009). \newline [2] M. Borhani and X. Hu, Phys. Rev. B {\bf 82}, 241302R (2010). \newline [3] M. Borhani and X. Hu, arXiv:1110.2193. \newline [4] X. Hu and S. Das Sarma, Phys. Rev. Lett. {\bf 96}, 100501 (2006). \newline [5] D. Culcer, X. Hu, and S. Das Sarma, Appl. Phys. Lett. {\bf 95}, 073102 (2009). \newline [6] X. Hu, Phys. Rev. B {\bf 83}, 165322 (2011).