Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T37: Focus Session: Semiconductor Qubits - Optically Addressed Dots and Impurities II |
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Sponsoring Units: GQI Chair: Danny Kim, HRL Laboratories, LLC Room: 212A |
Thursday, March 5, 2015 11:15AM - 11:27AM |
T37.00001: All-optical coherent control of energy transfer between a quantum dot and a cavity mode Tao Cai, Ranojoy Bose, Kaushik Choudhury, Glenn Solomon, Edo Waks Here we demonstrated all-optical coherent control of energy transfer in a quantum dot strongly coupled to a photonic crystal molecule at optical frequency. The photonic crystal molecule composes two photonic crystal cavities, supporting a pair of strongly coupled normal modes. One of the modes strongly couples with a quantum dot and the other induces a cavity enhanced a.c. stark shift to rapidly tune the quantum dot resonance on timescales much shorter than the vacuum Rabi period of the strongly coupled dot-cavity system. The quantum dot initially detunes from the cavity mode. By tuning the quantum dot onto resonance with the cavity mode on picosecond timescales, we achieved coherent transfer of energy between a quantum dot and the cavity mode through vacuum Rabi oscillation. We investigated the energy transfer as a function of stark laser power to confirm the coherence of the energy transfer process. We further demonstrated coherent control of light-matter states by implementing a Ramsey-type experiment. These results pave the path for achieving gigahertz controlled generation of quantum states of light and synthesis of photon wavefunctions using integrated semiconductor nano-photonics platform. [Preview Abstract] |
Thursday, March 5, 2015 11:27AM - 11:39AM |
T37.00002: Control of the cavity reflectivity using a single quantum dot spin Shuo Sun, Hyochul Kim, Glenn Solomon, Edo Waks The implementation of quantum network and distributive quantum information processing relies on interaction between stationary matter qubits and flying photons. The spin of a single electron or hole confined in a quantum dot is considered as promising matter qubit as it possesses microsecond coherence time and allows picosecond timescale control using optical pulses. The quantum dot spin can also interact with a photon by controlling the optical response of a strongly coupled cavity. Yet all the experimental demonstrations of the cavity spectrum control have used neutral dots. The spin-dependent cavity spectrum for a strongly coupled charged quantum dot and cavity system has not been reported. Here, we report an experimental realization of a spin-photon interface using a strongly coupled quantum dot and cavity system. We show large modulation of the cavity reflection spectrum by manipulating the spin states of the quantum dot. The spin-photon interface is crucial for realizing a quantum logic gate or generating hybrid entanglement between a quantum dot spin and a photon. Our results represent an important step towards semiconductor based quantum logic devices and on-chip quantum networks. [Preview Abstract] |
Thursday, March 5, 2015 11:39AM - 11:51AM |
T37.00003: Optical Control of Semiconductor Quantum Dot Spin Qubits with Microcavity Exciton-Polaritons Shruti Puri, Peter L. McMahon, Yoshihisa Yamamoto Topological surface codes demand the least stringent threshold conditions and are most promising for implementing large quantum algorithms. Based on the resource requirements to reach fault tolerance, we develop a hardware platform for large scale quantum computation with semiconductor quantum dot (QD) electron spin qubits. The current proposals for implementation of two-qubit gates and quantum non demolition (QND) readout in a QuDOS (Quantum Dots with Optically Controlled Spins) architecture suffer from large error rates. In our scheme, the optical manipulation of the QD spin qubits is carried out using their Coulomb exchange interaction with optically excited, spin-polarized, laterally confined quantum well (LcQW) exciton-polaritons. The small mass of polaritons protects them from interaction with their solid-state environment (phonons) and enables strong coupling between spin qubits separated by a few microns. Furthermore, the excitation manifold of the QD is well separated from that of the LcQW polaritons, preventing a spin-flip event during readout. We will outline schemes for implementing fast, high-fidelity, single qubit gate, two-qubit geometric phase gate and single-shot QND measurement and analyze important decoherence mechanisms. [Preview Abstract] |
Thursday, March 5, 2015 11:51AM - 12:27PM |
T37.00004: Explorations with a new qubit system: Exchange Interactions between Quantum Dot Spin Qubits and Quantum Well Excitons Invited Speaker: Peter McMahon In this talk I will present some of our recent work on constructing and optically investigating nanostructures consisting of quantum dots coupled to a nearby quantum well, all embedded in a planar microcavity. The overall goal of this line of work is to develop a platform in which long-range ($\sim 1$ micron) two-qubit interactions between quantum dots are possible, following the pioneering proposal of Piermarocchi, Chen, Sham, and Steel ({\it Phys. Rev. Lett.} {\bf 89} (16) 167402 [2002]). We have succeeded in demonstrating several fundamental aspects of this platform. We have realized a coupled quantum-dot--quantum-well system in a microcavity, and show that quantum dots in this system can be charged (allowing the storage of a spin qubit), and show that both the quantum dots and the quantum well retain favourable optical properties. Most importantly, we have fairly strong evidence suggesting that the operative mechanism of the theoretical proposals, the spin-dependent exchange interaction between a trapped electron in a quantum dot, and an exciton in the quantum well, is observable, and can be engineered to be of the magnitude required for the implementation of universal quantum gates and measurement operations. I will discuss these results, and highlight other recent (unrelated) work on site-controlled quantum dots, including with quantum dots in positioned nanowires. [Preview Abstract] |
Thursday, March 5, 2015 12:27PM - 12:39PM |
T37.00005: Proposed method of optical spin read-out in a quantum dot using the AC Stark effect Edward Flagg, Gary Lander, Cabot Zabriskie We propose a method to read-out the spin-state of a single electron trapped in a quantum dot via a cycling transition induced by the AC Stark effect. Optical spin initialization and manipulation are allowed by a magnetic field in the Voigt configuration, which modifies the polarization selection rules of the transitions. The lack of a cycling transition in the Voigt configuration, however, makes read-out of the spin-state very difficult. We show that cycling transitions can be made possible by a red-detuned, circularly-polarized laser, which modifies the spin eigenstates and polarization selection rules via the AC Stark effect. [Preview Abstract] |
Thursday, March 5, 2015 12:39PM - 12:51PM |
T37.00006: Ground state initialization in a doubly-charged, vertically-stacked InAs quantum dot molecule Aaron Ross, Colin Chow, Lu Sham, Allan Bracker, Daniel Gammon, Duncan Steel We report on the rapid optical initialization of a subset of the two-electron ground states of a self-assembled, vertically stacked InAs quantum dot molecule, where the states of the electron are approximately localized to separate quantum dots with very little spatial overlap. Four eigenstates, a singlet and three triplets (S,T$_{0}$,T$_{+\, }$, T$_{-})$, arise from the exchange coupling and are identified via bias-dependent photoluminescence measurements. The degeneracy of the triplet states is lifted using an in-plane magnetic field (Voigt geometry). This allows for the determination of the in-plane electron and hole g-factors using differential transmission measurements in the co-tunneling regime (to avoid optical pumping). Three of the four eigenstates (S,T$_{+\, }$, T$_{-})$ can then be initialized with high fidelity using continuous wave (CW) optical pumping. Optical transition degeneracies prohibit simple CW initialization of the T$_{0}$ state. Efforts towards near-unity initialization of the T$_{0}$ state via two-photon Raman transitions will be presented. This work represents the first step in demonstrating a two-qubit quantum register based on electron spins in self-assembled quantum dots. This work is supported by NSF, ARO, AFSOR, DARPA, and ONR. [Preview Abstract] |
Thursday, March 5, 2015 12:51PM - 1:03PM |
T37.00007: Nuclear Spin Locking and Extended Two-Electron Spin Decoherence Time in an InAs Quantum Dot Molecule Colin Chow, Aaron Ross, Duncan Steel, L. J. Sham, Allan Bracker, Daniel Gammon The spin eigenstates for two electrons confined in a self-assembled InAs quantum dot molecule (QDM) consist of the spin singlet state, $S$, with \textbf{\textit{J}}$=$ 0 and the triplet states $T_{\mathrm{-}}$, $T_{\mathrm{0}}$ and $T_{\mathrm{+}}$, with \textbf{\textit{J}}$=$ 1. When a transverse magnetic field (Voigt geometry) is applied, the two-electron system can be initialized to the different states with appropriate laser excitation. Under the excitation of a weak probe laser, non-Lorentzian lineshapes are obtained when the system is initialized to either $T_{\mathrm{-}}$ or $T_{\mathrm{+}}$, where $T_{\mathrm{-}}$ results in a ``resonance locking'' lineshape while $T_{\mathrm{+}}$ gives a ``resonance avoiding '' lineshape: two different manifestations of hysteresis showing the importance of memory in the system. These observations signify dynamic nuclear spin polarization (DNSP) arising from a feedback mechanism involving hyperfine interaction between lattice nuclei and delocalized electron spins, and Overhauser shift due to nuclear spin polarization. Using pump configurations that generate coherent population trapping, the isolation of the electron spin from the optical excitation shows the stabilization of the nuclear spin ensemble. The dark-state lineshape measures the lengthened electron spin decoherence time, from 1 ns to 1 $\mu $s. Our detailed spectra highlight the potential of QDM for realizing a two-qubit gate. [Preview Abstract] |
Thursday, March 5, 2015 1:03PM - 1:15PM |
T37.00008: Quantum Dot Molecule Polaritons and a Voltage-Tunable Vacuum Rabi Splitting Patrick Vora, Allan Bracker, Samuel Carter, Mijin Kim, Chul Soo Kim, Sophia Economou, Daniel Gammon InAs quantum dots (QDs) are a popular system for realizing quantum information protocols and studying cavity-QED. An additional class of optical transitions can be accessed by using quantum dot molecules (QDMs): a pair of tunnel-coupled QDs. Recombination can occur within one of the QDs (intradot) or across the tunnel barrier (interdot). Interdot transitions are typically weaker due to reduced wavefunction overlap. Recently, our team embedded a QDM within a GaAs photonic crystal cavity and demonstrated photonic enhancement of a singlet-triplet qubit. Here, we realize a strongly-coupled cavity-QDM system and demonstrate cavity-QED effects inaccessible in single QDs. These include the first observation of molecular polaritons in InAs QDs and a voltage-tunable vacuum Rabi splitting (2$g)$. The tunable vacuum Rabi splitting can only occur in QDMs and provides an advantage as $g$ is typically fixed post-fabrication. This flexibility could be useful for optical signal processing schemes that exploit the anharmonicity of the Jaynes-Cummings ladder. [Preview Abstract] |
Thursday, March 5, 2015 1:15PM - 1:27PM |
T37.00009: Screening nuclear field fluctuations to generate highly indistinguishable photons from negatively charged self-assembled InGaAs quantum dots Ralph Malein, Ted Santana, Joanna Zajac, Pierre Petroff, Brian Gerardot Quantum dots (QDs) can generate highly coherent and indistinguishable single photons. However, a ground-state electron spin interacts with a QD's nuclear spins to create an effective Overhauser field ($\delta B_n$) of $\sim$30mT. We probe this interaction using resonance fluorescence. We observe the effect of $\delta B_n$ in high resolution (27 MHz) spectroscopy of the elastic and inelastic scattered photons, and characterize the effect of $\delta B_n$ on photon indistinguishability by monitoring the visibility of two-photon interference. With no external magnetic field ($B_z=0$), $\delta B_n$ effectively splits the ground state, and at low Rabi frequencies we observe two broad ($\Gamma=200$MHz) peaks equally spaced by $\sim$100MHz from the central elastic peak. The ratio of elastic to inelastic photons in the spectra gives a dephasing time $T_2=0.52~T_1=406$ps, far from the transform limit. With an external field $B_z>\delta B_n$, we can successfully screen the fluctuating nuclear field. For $B_z=300$mT, nearly all photons in the spectrum are elastically scattered and we extract $T_2=1.94~T_1=1512$ps. This transform limited linewidth enables us to demonstrate very high visibility two-photon interference. These results point towards robust generation of indistinguishable photons. [Preview Abstract] |
Thursday, March 5, 2015 1:27PM - 1:39PM |
T37.00010: Optical Hyperpolarization of Donor Electron Spins in Silicon Using a Widely-Tunable DBR Laser Brendon Rose, Gary Wolfowicz, Alexei Tyryshkin, Michael Thewalt, Kohei Itoh, John Morton, Stephen Lyon We report on measurements of isotopically enriched silicon samples (45 ppm Si-29) with very low donor densities (1e12-1e14 cm$^{-3})$. Pushing the donor density limit necessitates optimizing the experimental sensitivity and enhancing spin polarization. Donor spin polarization greater than Boltzmann (hyperpolarization) can be established by optically exciting the no-phonon bound exciton transitions followed by Auger recombination. We established significant donor spin hyperpolarization with a distributed Bragg reflector laser, tunable across all donors including P-31 and Bi-209. For phosphorus doped silicon we observed combined electron-nuclear spin polarizations of 100{\%}-200{\%} depending on donor density. For bismuth donors at a clock transition (B $=$ 80.6 mT), we observed about 50{\%} spin polarization, and 500{\%} away from the clock transition. This increase in spin polarization allows for single shot measurement of low density samples (4e12 P/cm$^{3}$. and 1e14 Bi/cm$^{3})$. [Preview Abstract] |
Thursday, March 5, 2015 1:39PM - 1:51PM |
T37.00011: Hybrid optical-electrical detection of donor electron spins with bound excitons in silicon C. C. Lo, M. Urdampilleta, P. Ross, M. F. Gonzalez-Zalba, J. Mansir, S. A. Lyon, M. L. W. Thewalt, J. J. L. Morton Electrical detection of spin resonance is a powerful technique for understanding the dynamics of spins in semiconductors. For electrons bound to shallow donors in silicon, thus far it has been demonstrated by coupling donors to spin readout partners, such as paramagnetic defects or conduction electrons, which fundamentally limit the donor coherence times. Here we demonstrate electrical detection of donor bound excitons in a silicon device, and show that the spin-selective bound exciton transitions can be exploited for the electrical detection of coherent spin manipulation of isolated donors. We use this method to measure electron spin Rabi oscillations, and we are able to obtain long intrinsic electron spin coherence times, limited only by the donor concentration. Furthermore, we address critical issues for adopting such a hybrid optical-electrical detection scheme for single spin detection in silicon nanodevices, laying the foundations for realizing a versatile readout method for single spin readout with relaxed magnetic field and temperature requirements compared with spin-dependent tunneling. [arXiv:1411.1324] [Preview Abstract] |
Thursday, March 5, 2015 1:51PM - 2:03PM |
T37.00012: Nearly degenerate light- and heavy-hole trions bound to isoelectronic centers Gabriel Ethier-Majcher, Philippe St-Jean, Sebastien Francoeur Many optical quantum control schemes of spin qubits in semiconductors rely on the existence of trion states. In this work, we investigate the fine structure of negative trions bound to isoelectronic centers formed from a pair of nitrogen isovalent impurities in GaAs, which represent interesting candidates for optical quantum information processing. Using polarization resolved microluminescence, we find that the fine structure is composed of two unpolarized lines, characteristic of light- and heavy-hole trion states, evolving into two quadruplets under a longitudinal magnetic field. The availability of both light- and heavy-hole states on the same trion could lead to new powerful optical quantum control schemes where both spin initialization and single-shot readout could be conveniently realized. [Preview Abstract] |
Thursday, March 5, 2015 2:03PM - 2:15PM |
T37.00013: Initialization of a hole spin bound to an isoelectronic center Philippe St-Jean, Gabriel Ethier-Majcher, Sebastien Francoeur Hole spins are promising candidates for solid-state qubits because they interact weakly with nuclear spins, leading to long relaxation and coherence times. In this work, we demonstrate the ability to optically initialize a single hole spin bound to an isoelectronic center, which is an atomic defect formed from a small number of isovalent impurities in a semiconductor host. Using time-resolved magneto-photoluminescence of a positive trion bound to a Te dyad in ZnSe, we measured the degree of polarization of the emission under various conditions of excitation and magnetic field. Under non-resonant excitation, the trion emission is partially polarized and becomes completely unpolarized under a longitudinal magnetic field. In contrast, resonant excitation of the heavy-hole valence band of the ZnSe host leads to highly polarized emission, implying that the hole has been initialized in a known spin state. [Preview Abstract] |
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