Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U28: Focus Session: Semiconductor Qubit Approaches II |
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Sponsoring Units: DCMP Chair: Malcolm Carroll, Sandia National Laboratories Room: Morial Convention Center 220 |
Thursday, March 13, 2008 8:00AM - 8:12AM |
U28.00001: Radio-frequency single-electron transistor coupled to few-electron double quantum dot Feng Pan, Joel Stettenheim, Mustafa Bal, Weiwei Xue, Zhongqing Ji, Alexander Rimberg, L.N. Pfeiffer, K.W. West The radio frequency single-electron transistor (rf-SET) has been shown to be an ultra fast and highly sensitive electrometer, and also has been used as a qubit readout device operated close to the quantum noise limit [1]. The interplay between the rf-SET electrometer and a two-level system offers an interesting system for study. Here we report our progress on investigating rf-SETs capacitively coupled to few-electron double quantum dots (DQDs). We fabricate lateral-defined DQDs from an AlGaAs/GaAs heterostructure and the rf-SET from superconducting aluminum embedded in a tank circuit. The sensitivity and bandwidth of on-chip rf-SET electrometer can be used to probe DQD operated in the few-electron regime. Alternatively, the DQD can be used as high-frequency quantum noise detector to probe SET operation in the subgap region [2,3]. Recent experimental results will be discussed. [1] M. H. Devoret and R. J. Schoelkopf, Nature, \textbf{406}, 1039 (2000). [2] R. Aguado and L. P. Kouwenhoven, Phys. Rev. Lett., \textbf{84}, 1986 (2000). [3] O. Naaman and J. Aumentado, Phys. Rev. Lett. \textbf{98}, 227001 (2007). [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U28.00002: Decoherence of coupled spin qubits due to charge fluctuations Guy Ramon, Xuedong Hu One of the significant advantages of using the spin of quantum dot electrons as a qubit rather than their charge is their relative insulation from the environment. A number of recent works have utilized two-spin singlet and unpolarized triplet states in biased configuration to encode a logical qubit, which offer better control and coherence properties as compared with single spin states. When spin states are exchange coupled, however, they are potentially vulnerable to environmental fluctuations affecting charge qubits, since exchange coupling is electrostatic in nature. Here we carry out a quantitative calculation of the coupling between a biased two-spin qubit and a nearby charge fluctuator represented by a two-level-system (TLS), utilizing a multipole expansion up to and including the Quadrupole-Quadrupole order. The resulting coupling terms are used in a master equation formalism to study the dynamics of the open system that is formed by the spontaneous emission of the TLS coupled to the vacuum. We are thus able to provide a reliable estimate of the decoherence effects during various gate operations on the spin qubit as a function of the geometry and other characteristics of the system. Possible ways to alleviate the sensitivity of coupled spin qubits to charge fluctuations are also discussed. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U28.00003: Hyperfine-mediated gate-driven electron spin resonance Edward Laird, Christian Barthel, Emmanuel Rashba, Charles Marcus, Micah Hanson, Art Gossard An all-electrical spin resonance effect in a GaAs few-electron double quantum dot is investigated experimentally and theoretically. The magnetic field dependence and absence of associated Rabi oscillations are consistent with a novel hyperfine mechanism. The resonant frequency is sensitive to the instantaneous hyperfine effective field, and the effect can be used to detect and create sizable nuclear polarizations. A device incorporating a micromagnet exhibits a magnetic field difference between dots, allowing electrons in either dot to be addressed selectively. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U28.00004: Cluster Techniques to Study Spin Decoherence in a Spin Bath Invited Speaker: Noisy nuclear spin environments in many solid state materials pose a serious threat to the feasibility of solid-state spin quantum computation where localized electron spins, as qubits, may interact with millions of lattice nuclei [1, 2, 3, 4]. Such nuclear induced decoherence may be partially reduced through the application of a strong magnetic field that suppresses electro-nuclear flip-flops (due to a large mismatch of their gyromagnetic ratios). However, even in the limit where electro-nuclear flip-flops are completely suppressed, dephasing decoherence, known as spectral diffusion, occurs as a result of fluctuations of the nuclear field that is caused by dipolar (or other) interactions among the nuclear bath spins. While a direct approach to this problem is impossible due to the intractable Hilbert space of many interacting spins, we have devised a cluster method to formally solve this problem. Direct application of perturbation theories are futile due to the large size of the bath. Perturbation methods become effective, however, in the cluster expansion framework. These techniques will be discussed and qubit decoherence calculation results will be shown, including effects of dynamical decoupling pulse sequences [5, 6] that prolong qubit coherence. [1] W. M. Witzel, Rogerio de Sousa, S. Das Sarma, Phys. Rev. B 72, 161306(R) (2005). [2] W. M. Witzel, S. Das Sarma, Phys. Rev. B 74, 035322 (2006). [3] W. M. Witzel, S. Das Sarma, Phys. Rev. Lett. 98, 077601 (2007). [4] W. M. Witzel, Xuedong Hu, S. Das Sarma, Phys. Rev. B 76, 035212 (2007). [5] W. M. Witzel, S. Das Sarma, arXiv:0707.1037. [6] B. Lee, W. M. Witzel, S. Das Sarma, arXiv:0710.1416. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U28.00005: Spin-Dependent Scattering off Neutral Donors in Silicon Field-Effect Transistors C.C. Lo, J. Bokor, T. Schenkel, J. He, A.M. Tyryshkin, S.A. Lyon One promising route towards single donor spin readout for donor qubits in silicon is by detecting spin-dependent scattering of conduction electrons by the neutral donors. We use accumulation-mode field-effect transistors formed in isotopically enriched silicon to study this effect. Only small ensembles of donor spins are present in our devices, ruling out bolometric effects caused by bulk donors. Spin-dependent scattering was detected using electrically detected magnetic resonance (EDMR) where spectra show resonant changes in the source-drain voltage for conduction electrons and electrons bound to donors. The utilization of spin-dependent scattering for the readout of donor spin-states in silicon based quantum computers will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U28.00006: Time-Reversal Symmetry and Electron Spin Relaxation of Lithium Donors in Silicon V.N. Smelyanskiy, A.G. Petukhov, A.M. Tyryshkin, S.A. Lyon We report theoretical and experimental studies of longitudinal electron spin relaxation time of interstitial shallow Li donors in Si. Ground state of a donor electron has a unique ten-fold (near) degeneracy which is only slightly lifted by stresses, magnetic field and spin-orbital interaction. Despite this degeneracy, we predict, on the basis of the time-reversal symmetry and weakness of the umklapp phonon processes, an extraordinary long relaxation times for lithium donor electron spin for the temperatures below 0.3 K. Strong temperature dependence of the spin relaxation time is due to activation-type processes with several activation exponents. Experimentally observed traces of magnetization reversal and longitudinal spin relaxation times at $T=$ 2.1~K and $T=4.5$~K are in remarkably close agreement with the theory. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U28.00007: Spin relaxation of exchange-coupled donors in silicon A.M. Tyryshkin, S. Shankar, S.A. Lyon The exchange interaction (J-coupling) between electron spins provides a natural way to accomplish two-qubit operations in a spin-based, solid-state quantum processor. The J-coupling, because of its electrostatic (Coulombic) nature, is susceptible to charge noise in the environment, and hence turning on the J- coupling may cause fast decoherence of the interacting spin qubits. To clarify the effect of J-coupling on spin coherence, we performed spin relaxation measurements for exchange-coupled donors in dimers and trimers randomly formed in bulk-doped natural silicon and isotopically-purified $^{28}$Si. The longitudinal relaxation time, T$_1$, for donors in exchange- coupled dimers is found to be identical to that of isolated donors at temperatures 8-15K, solely determined by a two-phonon Orbach mechanism. The transverse relaxation time, T$_2 $, for dimers is even longer than that of isolated donors because of the lower density of the dimers in our samples and thus substantially reduced dipole-dipole interactions. In natural silicon containing 5\% $^{29}$Si magnetic nuclei, an additional decoherence results from the nuclear-induced spectral diffusion. The spectral diffusion decoherence of J-coupled dimers and trimers is also identical to that of isolated donors. We conclude that J-coupling does not induce any additional decoherence in bulk donors in Si. However, the situation may change for donor dimers placed closer to the surface where more charge noise is expected. Supported by LPS/ARO. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U28.00008: Solid state quantum memory using the $^{31}$P nuclear spin J.J.L. Morton, A.M. Tyryshkin, S. Shankar, A. Ardavan, T. Schenkel, J.W. Ager, S.A. Lyon Nuclear spins benefit from long coherence times compared to electron spins, but are slow to manipulate and suffer from weak thermal polarisation. A powerful model for quantum computation is thus one in which electron spins are used for processing and readout while nuclear spins are used for storage. Here we demonstrate the coherent transfer of an electron spin superposition to the nuclear spin using a combination of microwave and radiofrequency pulses applied to $^{31}$P donors in an isotopically pure $^{28}$Si crystal. The state is left in the nuclear spin on a time scale long compared with the electron T$_2$ and then coherently transferred back to the electron spin, thus demonstrating the $^{31}$P nuclear spin as a solid-state quantum memory. The transfer fidelity is about 84$\%$ each way, attributed to imperfect rotations which could be corrected using composite pulses [JJL Morton et al. Phys Rev Lett 95, 200501 (2005)]. Varying the time for which the state is stored in the nuclear spin permits the direct measurement of the nuclear spin T$_2$, which we have studied in the range 6.5 to 10~K. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U28.00009: Detection of low energy single ion impacts in silicon transistors Christoph Weis, Arunabh Batra, Stefano Cabrini, Jeffrey Bokor, Cheuk Lo, Thomas Schenkel We report a technique for single ion doping of field effect transistors through monitoring of changes in the source-drain currents at room temperature [1]. Implant apertures are formed in the interlayer dielectrics and gate electrodes of planar, micro-scale transistors by electron beam assisted etching. Device currents increase due to the generation of positively charged defects in gate oxides when ions (121Sb12+, 14+, Xe6+; 50 to 70 keV) impinge into channel regions. Implant damage is repaired by rapid thermal annealing, enabling iterative cycles of device doping and electrical characterization. We discuss integration of single ion doping for the development of silicon based quantum computer structures with donor electron and nuclear spin qubits. [1] A. Batra, et al., Appl. Phys. Lett. 91, 193502 (2007) [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U28.00010: Real-time decoherence of hyperfine-coupled electrons in quantum dots Jordan Kyriakidis, Jean-Marc Samson We approach the study of the electron spin decoherence due to the Fermi contact hyperfine interaction with the density matrix formalism of quantum relaxation. We consider an s-type electron in the ground state of a quantum dot interacting with a thermal distribution of nuclear spins. We directly compute the time dependence of the reduced density matrix by solving the system of integro-differential equations resulting from the Liouville equation at the Born (but not Markov) approximation. We show how the spin precession can, under certain circumstances, slow down and even reverse its rotation sense. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U28.00011: Coupling of Mechanical Modes and Shot Noise in a Radio Frequency Quantum Point Contact J. Stettenheim, Feng Pan, Z. Ji, Mustafa Bal, W.W. Xue, Madhu Thalakulam, L.N. Pfeiffer, K.W. West, A.J. Rimberg Interesting interactions exist between the electrical and mechanical degrees of freedom in GaAs quantum nanostructures due to the piezoelectric nature of the substrate. Here, we report measurements in radio-frequency quantum point contacts (RF-QPCs) in which the photon assisted shot noise (PASN) in the vicinity of the carrier wave shows strong frequency dependence correlated with sample dimensions. In particular we find that the rf drive excites surface acoustic waves (SAWs) of frequency $f\approx v_{s}/2l, $where $v_{s}$ = 3010 m/s is the speed of sound in GaAs and $l$ is a length of the device or substrate chip. As a result by varying the sample dimensions, we can manipulate the shot noise spectrum so that the resulting noise is excluded from a chosen bandwidth. Implications for use of the RF-QPC as a fast charge detector will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U28.00012: Intrinsic noise measurement of an ultra-sensitive radio-frequency single electron transistor W.W. Xue, Z. Ji, Feng Pan, A.J. Rimberg The radio-frequency single electron transistor (rf-SET) has been the focus of intense interest since its invention in 1998[1]. Using cryogenic ultra-thin film evaporation techniques [2] and an improved on-chip superconducting matching network [3], we have consistently fabricated rf-SETs with charge sensitivity of 1.7--5$\mu e/\sqrt{\mathrm{Hz}}$ and uncoupled energy sensitivity 1.1--5$\hbar$. Using our 1GHz resonant circuit, intrinsic noise in the SET arising from a dc voltage bias was measured in the white noise limit. We measured the offset charge dependence of the intrinsic noise in the vicinity of the Josephson-quasiparticle and double Josephson-quasiparticle transport cycles. In regions for which the offset charge and resistance noise are strongly suppressed, we can determine the SET shot noise in the sup-gap regime. We discuss the effects of correlations between charge carriers on the measured Fano factor. [1] R.J.Schoelkopf et al., Science 280,1238 (1998); [2] N.A.Court et al., Cond-mat 0706.4150 (2007); [3] W.W.Xue et al., Appl.Phys.Lett. 91, 093511 (2007). [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U28.00013: Coherent electron-phonon states in suspended quantum dots: decoherence and dissipation effects Luis G.C. Rego The dynamics of coherent electron-phonon (el-ph) states is investigated for a suspended quantum dot structure. Exact quantum dynamics calculations reveal that electron and phonons (comprising a thermal bath) couple quantum mechanically to perform coherent oscillations with periods in the range of tens of nanoseconds, despite the finite temperature of the phonon bath. Mechanical energy dissipation due to clamping loss is taken into account in the calculations. Although the lifetime of the coupled el-ph states decreases with the temperature, well defined Rabi oscillations are obtained for temperatures up to 100 mK. The dynamics of the coupled electron-phonon state is susceptible to various forms of external control. For instance, a weak external magnetic field can be used to control the dynamics of the system, by decoupling the electron from the phonon bath. The results cast light upon the underlying physics of a yet unexplored system that could be suitable for novel quantum device applications. [Preview Abstract] |
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