Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U26: Focus Session: Semiconductor Qubits - Impurity Complexes |
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Sponsoring Units: GQI Chair: Kai-Mei Fu, University of Washington Room: 328 |
Thursday, March 21, 2013 11:15AM - 11:51AM |
U26.00001: Single-atom spin qubits in silicon Invited Speaker: Andrew Dzurak Spin qubits in silicon are excellent candidates for scalable quantum information processing (QIP) due to their long coherence times and the enormous investment in silicon MOS technology. Here I discuss qubits based upon single phosphorus (P) dopant atoms in Si [1]. Projective readout of such qubits had proved challenging until single-shot measurement of a single donor electron spin was demonstrated [2] using a silicon single electron transistor (Si-SET) and the process of spin-to-charge conversion. The measurement gave readout fidelities \textgreater\ 90{\%} and spin lifetimes T$_{\mathrm{1e}}$ \textgreater\ 6 s [2], opening the path to demonstration of electron and nuclear spin qubits in silicon. Integrating an on-chip microwave transmission line enabled single-electron spin resonance (ESR) of the P donor electron. We used this to demonstrate Rabi oscillations of the electron spin qubit, while a Hahn echo sequence revealed electron spin coherence times T$_{\mathrm{2e}}$ \textgreater\ 0.2 ms [3]. This time is expected to become much longer in isotopically enriched $^{28}$Si devices. We also achieved single-shot readout of the $^{31}$P nuclear spin (with fidelity \textgreater\ 99.6{\%}) by monitoring the two hyperfine-split ESR lines of the P donor system. By applying (local) NMR pulses we demonstrated coherent control of the nuclear spin qubit, giving a coherence time T$_{\mathrm{2n}}$ \textgreater\ 60 ms. \\[4pt] [1] B.E. Kane, \textit{Nature} \textbf{393}, 133 (1998). \newline [2] A. Morello et al., \textit{Nature} \textbf{467}, 687 (2010). \newline [3] J.J. Pla et al., \textit{Nature} \textbf{489}, 541 (2012). [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U26.00002: Measurements of spin life time of an antimony-bound electron in silicon T.M. Lu, N.C. Bishop, L.A. Tracy, R. Blume-kohout, T. Pluym, J.R. Wendt, J. Dominguez, M.P. Lilly, M.S. Carroll We report our measurements of spin life time of an antimony-bound electron in silicon. The device is a double-top-gated silicon quantum dot with antimony atoms implanted near the quantum dot region. A donor charge transition is identified by observing a charge offset in the transport characteristics of the quantum dot. The tunnel rates on/off the donor are first characterized and a three-level pulse sequence is then used to measure the spin populations at different load-and-wait times in the presence of a fixed magnetic field. The spin life time is extracted from the exponential time dependence of the spin populations. A spin life time of 1.27 seconds is observed at B $=$ 3.25 T. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories Directed Research and Development Program. Sandia National Laboratories is a multi-program laboratory managed and 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. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U26.00003: Shuttling electrons on and off As donor atoms in silicon A.M. Tyryshkin, S.A. Lyon, C.C. Lo, R. Lo Nardo, J.J.L. Morton, S. Simmons, C.D. Weis, T. Schenkel, J. Bokor, J. Meijer, D. Rogalla Hybrid quantum devices where electron spins are used for state initialization, fast manipulation, long range entanglement and detection, while nuclear spins are used for long term storage promise revolutionary advantages. Here we report our first experiments using a silicon-based device that utilizes electron and nuclear spins of arsenic donors. The device is a large-area, parallel-plate capacitor fabricated on a silicon-on-insulator (SOI) wafer where the SOI layer is implanted with arsenic donors, and a back gate is formed in the silicon below the buried oxide by a high-energy boron implantation. The electrons can be controllably stripped from the donors and then reintroduced to the ionized donors by applying appropriate gate voltages. We use ensemble ESR experiments (X-band, magnetic field of 0.35 T) to track the occupancy of the donors during these operations. Pulsed ESR is used to characterize the spin state of the donor electrons and the effect of applied electric fields below the ionization threshold. The spin state of the arsenic nuclei, and the effect of electron removal and reintroduction on the nuclear state is expected to be observable in pulsed ENDOR experiments. The work is funded by LPS and NSF-MWN. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U26.00004: Electronic structure of sub-surface Boron acceptors in silicon for potential qubits Rajib Rahman, Jan Mol, Gerhard Klimeck, Sven Rogge Single acceptors in silicon are investigated as potential qubits. Due to the p-type nature of the valence band (VB), the acceptor states are less susceptible to the hyperfine interaction of the neighboring nuclear spins. The presence of a stronger spin-orbit coupling in the VB also enables the possibility of an all-electric qubit control. Whereas donor qubits exhibit exchange oscillation with separation distance due to conduction band valleys, Boron acceptors are expected to have smoother exchange curves. We investigate the electronic structure of single Boron acceptors in silicon in the presence of electric field, strain, magnetic field, and interfaces. Bulk Boron acceptors have a four-fold degenerate ground state 45 meV above the VB with angular momentum states of 3/2 and 1/2. An interface splits this manifold into Kramer's doublets. Application of E and B fields allow several possibilities for forming a two-level qubit driven by an ac electric field. We compare calculations from atomistic tight-binding theory to scanning tunneling microscope (STM) measurements and k.p calculations. The tight-binding method captures additional wavefunction symmetries due to the crystal that help to explain the STM measurements. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U26.00005: Interface-split Kramers doublets for acceptor-based qubits in silicon Jan Mol, Joseph Salfi, Rajib Rahman, Sven Rogge Single dopants in silicon form a particular attractive platform for hosting spin quantum bits (qubits). The effective spin-3/2 states of acceptor-bound holes in silicon can be used to store bits of quantum information for several $\mu$s. Strong coupling of spin and momentum in the silicon valence band allows for rapid electrical manipulation of the hole spin. Acceptors in silicon have a four-fold degenerate ground-state, reflecting character of the top of the valence band. Symmetry breaking, by an electric field, strain or confinement, lifts this degeneracy, resulting in two Kramers doublets. The states within these isolated Kramers doublets are protected against decoherence by time reversal symmetry and form the working levels of a hole spin qubit. Here we investigate the effect of the presence of an interface on the ground-state energy splitting of individual sub-surface acceptors, as a function of dopant depth, by means of low temperature scanning tunneling spectroscopy. The depth of individual acceptors is determined by probing the Coulomb potential of the ionized acceptor nuclei. Resonant tunneling through the localized acceptor states provides a direct measure of the excited state spectrum of single dopants. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U26.00006: Towards isolating a single impurity-bound hole Russell Barbour, Todd Karin, Kai-Mei Fu, Yoshiro Hirayama, Arne Ludwig, Andreas Wieck Single acceptor-bound holes embedded in III-V semiconductor quantum wells could provide an ideal qubit system for scalable quantum information processing and quantum computation. This system combines strong homogenous optical transitions and millisecond long spin coherence times in a fabrication ready material (GaAs). However, single acceptor-bound excitons (A$^{0}$X) have yet to be optically isolated even in the purest bulk GaAs samples. This is primarily due to the high acceptor density (10$^{14}$ cm$^{-3}$) and exceptional optical homogeneity. We propose using stimulated emission depletion microscopy (STED) to increase our optical resolution far beyond the diffraction limit in order to spatially isolate a single acceptor-bound exciton. We report the first demonstration of stimulated emission of acceptor-bound excitons at 4.2K. We resonantly excite the A$^{0}$1s-A$^{0}$X transition and apply a second laser with high power (P$=$10mW) resonant with the 2s two-hole transition (THT). We observe a 30 percent reduction in the 1s PL intensity when the STED laser is resonant with the THT's. We will present our two-laser spectroscopy work that explores this coherent system and discuss our progress towards isolating a single acceptor-bound exciton using STED microscopy. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U26.00007: Ultrafast coherent optical control of a single diamond spin L.C. Bassett, F.J. Heremans, D.D. Awschalom, G. Burkard As an optically addressable solid-state electronic spin, the nitrogen-vacancy (NV) center in diamond has great promise for applications in quantum information science and metrology. At temperatures below $\approx 10$ K, the NV center's optical fine structure facilitates coherent coupling between the electronic spin and light, providing the means for all-optical spin control and other applications in quantum optics. Here, using ultrafast optical pump-probe techniques, we investigate the interplay of orbital, vibrational, and spin dynamics on timescales ranging from femtoseconds to nanoseconds. These techniques provide a flexible and powerful probe of orbital dynamics in the NV center's optically excited state, and enable optical spin control with sub-picosecond resolution. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U26.00008: All-optical quantum dynamical control of an NV center spin in diamond B.B. Buckley, C.G. Yale, D.J. Christle, F.J. Heremans, L.C. Bassett, D.D. Awschalom, G. Burkard The nitrogen-vacancy (NV) center in diamond has emerged as a promising optically addressable qubit candidate, but optical methods are usually used only for spin initialization and readout through the defect's spin-dependent intersystem crossing (ISC) transition. Quantum dynamical control typically requires the application of microwave magnetic fields, limiting possible applications. Here, we demonstrate an all-optical method for unitary, arbitrary-axis spin control of single NV spins below 10 K based on stimulated Raman transitions\footnote{C. G. Yale*, B. B. Buckley*, D. J. Christle, G. Burkard, F. J. Heremans, L. C. Bassett, and D. D. Awschalom (submitted)}. Using our recently-demonstrated arbitrary-basis spin initialization and readout, we perform time-domain spin coherence measurements on single NV center spins solely with optical pulses. These techniques enable individual addressing of proximal NV center spins and could be used to probe other previously-inaccessible defect spin systems without ISC spin addressability. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U26.00009: Experimental control of a nuclear spin quantum register in diamond with decoherence-protected gates Tim Hugo Taminiau, Toeno van der Sar, V. V. Dobrovitski, Ronald Hanson Nuclear spins are one of the most promising candidates for long-lived quantum bits that store and process quantum information. Individual nuclear spins in diamond have been addressed using the nearby electron spin of a nitrogen vacancy center. However, the relatively fast decoherence of the electron spin limits coherent control to the nearest, strongly coupled, nuclear spins. Here, we employ decoherence-protected gates [1] to access individual spins embedded in a bath of nuclear spins that are weakly coupled to an electron spin [2]. We demonstrate the initialization, control and readout of the nuclear spins and discuss our recent progress in implementing two-qubit entangling operations between nuclear spins. These results greatly extend the number of available quantum bits in diamond and provide a way towards tomography with single nuclear spin sensitivity even in decohering environments. [1] T. van der Sar et al., Nature 484, 82 (2012). [2] T. H. Taminiau et al., Phys. Rev. Lett. 109, 137602 (2012). [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U26.00010: Entanglement by measurement and Bell inequality violation with spins in diamond Wolfgang Pfaff, Tim Taminiau, Lucio Robledo, Hannes Bernien, Matthew Markham, Daniel Twitchen, Ronald Hanson Single spins in diamond have emerged as a promising platform for quantum information processing in the solid state. In particular, individual nuclear spins coupled to nitrogen-vacancy (NV) centers have been recognized as excellent candidates for solid state qubits, because they combine outstanding stability, excellent control by spin resonance techniques, and high-fidelity optical initialization and readout provided by the NV center. Here we report the achievement of a milestone towards quantum computation with spins: The creation of high quality quantum entanglement between two nuclear spins in diamond. Such entanglement is an important resource for quantum computation and lies at the heart of many key quantum protocols, such as teleportation and error correction. We show that we can produce entangled states of high fidelity using a projective quantum measurement. Our technique is non-destructive, and thus leaves the quantum information that is required for further computation unharmed. This enables us to demonstrate the violation of Bell's inequality for the first time with spins in the solid state. Reference: Pfaff et al., Nature Physics, doi:10.1038/nphys2444 (2012). [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U26.00011: Pulsed ESR of photo-polarized NV centers in diamond at X-band magnetic fields Brendon Rose, Alexei Tyryshkin, Stephen Lyon, Christoph Weis, Thomas Schenkel Recently nitrogen-vacancy (NV) color centers in diamond have become the focus of many studies aimed towards their use as quantum bits (qubits) in quantum computing applications and as precision magnetic field sensors in scanned imaging applications. The NVs have a ground triplet state (S$=$1) with ZFS of 2.88 GHz. It has been previously shown that optical excitation, when shining green light at low magnetic fields (below 100 G), polarizes spins preferentially into the T$_{\mathrm{0}}$ state. Here we will report an X-band pulsed ESR measurement and demonstrate that the optical spin polarization is more complex at higher magnetic fields (3400 G) and can lead to preferential spin polarization into T$_{\mathrm{+}}$ and T$_{\mathrm{-}}$ states, instead of T$_{\mathrm{0}}$. This effect can be understood from a simple one electron spin Hamiltonian and depends mainly on the relative orientation of the ZFS and external magnetic field. In addition, we observe strong ESEEM effects originating from the central nitrogen nucleus which are most prominent when measuring the T$_{\mathrm{0}}$ to T$_{\mathrm{-}}$ transition and when the field is along the ZFS. From the orientation dependence of ESEEM we are able to accurately determine the nitrogen hyperfine and nuclear quadrupole tensors. Spin coherence of 0.8 ms is seen at 10 K, limited by 1 percent of magnetic $^{\mathrm{13}}$C nuclei in our natural diamond sample. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U26.00012: Polytype control of spin qubits in silicon carbide A.L. Falk, B.B. Buckley, G. Calusine, W.F. Koehl, A. Politi, D.D. Awschalom, V.V. Dobrovitski, C.A. Zorman, P. X.-L. Feng The search for coherently addressable spin states in technologically important materials is a promising direction for solid-state quantum information science. Silicon carbide, a particularly suitable target, is not a single material but a collection of about 250 known polytypes, each with its own set of physical properties and technological applications. We show that in spite of these differences, the 4H-, 6H-, and 3C-SiC polytypes all exhibit optically addressable spins with long coherence times [1]. These results include room temperature spins in all three polytypes and suggest a new method for tuning quantum states using crystal polymorphism. Long spin coherence times allow us to use double electron-electron resonance to measure magnetic dipole interactions between spin ensembles in inequivalent lattice sites of the same crystal. Since such inequivalent spin have distinct optical and spin transition energies, these interactions could lead to dipole-coupled networks of separately addressable spins.\\[4pt] [1] A. Falk et al., submitted [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U26.00013: Defects as qubits in 3C and 4H polymorphs of SiC Luke Gordon, Audrius Alkauskas, WIlliam F. Koehl, Anderson Janotti, David D. Awschalom, Chris G. Van de Walle Using hybrid functional calculations we study defects in SiC that can serve as qubits for quantum computing. We investigate the divacancy in 4H- and 3C-SiC and the N-V center in 3C-SiC, in which the N impurity replacing a C atom is sitting next to a Si vacancy. The calculated excitation and emission energies of the divacancy in 4H-SiC are in excellent agreement with the available experimental data. Most importantly, we predict that the neutral divacancy and the negatively charged NV center in 3C-SiC have all the required characteristics to serve as qubits; in addition, both defects are stable in n-type 3C-SiC, which is in principle easy to fabricate. We calculate luminescence lineshapes and Huang-Rhys factors for these defects in 4H and 3C-SiC, and compare with experimental photoluminescence spectra. [Preview Abstract] |
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