Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A17: Focus Session: Photons and Quantum Dots |
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Sponsoring Units: GQI Chair: Thaddeus Ladd, Stanford University Room: 318 |
Monday, March 16, 2009 8:00AM - 8:36AM |
A17.00001: Excited-State Spectroscopy and Control of Single Spins in Diamond Invited Speaker: Nitrogen Vacancy (NV) defect centers in diamond are a promising system for spin-based applications in quantum information and communication at room temperature. Using a combination of optical microscopy and spin resonance, the spin of individual NV centers can be initialized, manipulated and read out. These techniques have been used to study the long room temperature spin coherence times of NV centers as well as their interactions with nearby electrical and nuclear spins. There remain significant challenges, however, both in understanding the physics of these defects as well as the development of technologies based on their quantum properties. In particular, knowledge of the detailed structure of the orbital excited-state, which continues to be an active research area, is critical to ultra-fast quantum control schemes. Here we present recent experiments using single-spin resonant spectroscopy of the excited-state of an NV center at room temperature.\footnote{G. D. Fuchs, V. V. Dobrovitski, R. Hanson, A. Batra, C. D. Weis, T. Schenkel, and D. D. Awschalom, \emph{Phys. Rev. Lett} \textbf{101}, 117601 (2008).} We observe these spin levels over a broad range of magnetic fields allowing for a direct measurement of the zero-field splitting, g-factor and transverse anisotropy splitting. The latter of these is nearly zero in the ground-state spin levels, but plays an important role in the excited-state. In addition, we find strong hyperfine coupling between the nitrogen nuclear spin and the NV electronic spin in the excited-state. These findings will be discussed in the context of quantum control of single and coupled spins in diamond. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A17.00002: Time-resolved measurements of single electron spins using continuous wave lasers Patrick Irvin, Yanjun Ma, Jeremy Levy, Jesse Berezovsky, David D. Awschalom Applications such as spin-based quantum computing require that the dynamics of single spins are monitored. Single spins produce a small signal and measurement is further complicated by the background from the large number of neighboring spins. We have developed a time-resolved Kerr rotation technique that uses continuous wave lasers. This technique is able to resonantly address a particular spin. Furthermore, it provides an efficient means of data collection that allows for more signal averaging. Finally, we decrease the background and increase the light-matter interaction by utilizing a solid-immersion lens. We will describe our recent efforts to measure single spins in GaAs/AlGaAs fluctuation-type quantum dots. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A17.00003: A photonic cluster state machine gun Terry Rudolph, Netanel Lindner, Sophia Economou A method is developed to convert certain single photon sources into devices capable of emitting large strings of photonic cluster state in a controlled and pulsed ``on demand'' manner. Such sources greatly alleviate the resources required to achieve linear optical quantum computation. Standard spin errors, such as dephasing, are shown to affect only 1 or 2 of the emitted photons at a time. This allows for the use of standard fault tolerance techniques. Using realistic parameters for current quantum dot sources, we conclude high entangled-photon emission rates are achievable, with Pauli-error rates less than 0.2{\%}. For quantum dot sources the method has the added advantage of circumventing the problematic issue of obtaining identical photons from independent, non-identical quantum dots. By using recently controlled-phase gates between two spins in neighboring quantum dots, a two-dimensional cluster can be generated. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A17.00004: Complete quantum control of a single quantum dot spin using ultrafast optical pulses David Press, Thaddeus Ladd, Bingyang Zhang, Yoshihisa Yamamoto We demonstrate a complete set of ultrafast all-optical single-qubit operations on a single electron spin in a quantum dot [\textit{Nature} \textbf{456}, 218 (2008)].~ First, the spin is initialized by optical pumping into a pure spin-state with 92{\%} fidelity.~ Next, a single-qubit gate is implemented by rotating the spin about any arbitrary axis using a sequence of two ultrafast optical pulses separated by a time delay.~ Finally, the spin is measured by detecting single-photon photoluminescence.~ As a manifestation of controlling the spin with optical pulses, we demonstrate six complete Rabi oscillations between the two spin states, and a complete set of Ramsey interference fringes.~ The fidelity of our $\pi $/2- and $\pi $-rotations exceed 90{\%}.~ The single-qubit gate is completed in 38 ps, potentially allowing for approximately 10$^{5}$ operations within the qubit's expected microsecond coherence time, and quantum information processing with clock speeds exceeding 10 GHz. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A17.00005: All-optical coherent control and spin-echo of electron spins bound to neutral donors in GaAs Susan Clark, Kai-Mei Fu, Qiang Zhang, Thaddeus Ladd, Colin Stanley, H.C. Holland, Yoshihisa Yamamoto Electron spins bound to neutral donors in GaAs are promising systems for quantum information processes. These electron spins form three-level Lambda-type systems that can be manipulated quickly by ultrafast light pulses and have potentially long storage times, making them natural candidates for quantum information manipulation and storage. Unlike quantum dots, they are extremely homogenous, making multi-qubit interactions and entanglement more accessible. Here, we report on our efforts to coherently control these electron spins using fast pulses and an all-optical spin-echo technique. Using three, off-resonant, small-angle (pi/3), ultrafast (2 ps) pulses, we have demonstrated that the spins exhibit an echo signal indicating T2 coherence times much longer than the previously measured 1 ns T2* bulk dephasing time. The visibility of the echo signal for different pulse delays gives us insights into the T2 decoherence time and decoherence processes in this system. Currently, we are measuring coherences as long as 4 microseconds. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A17.00006: Time-resolved luminescence of hierarchically self-assembled GaAs/AlGaAs quantum dots Botao Zhang, Armando Rastelli, Oliver Schmidt, Jeremy Levy, David Snoke, Albert Heberle Hierarchically self-assembled GaAs/AlGaAs quantum dots are promising building blocks for quantum information processing and novel lasers because they combine the tight confinement of InGaAs dots with the size homogeneity and a shorter emission wavelength of the GaAs/AlGaAs system at which many photodetectors are especially sensitive. So far, the emission dynamics of these structures has been unexplored. With a streak camera connected to a confocal microscope, we have measured the luminescence dynamics after direct optical picosecond excitation into the quantum dot states at a sample temperature of 10 K. Ensembles of high-density quantum dots (30 dots/$\mu $m$^{2})$ with well-separated transitions give information on state filling as well as intra- and interband relaxation. Single quantum dots on low-density samples (0.5 dots/$\mu $m$^{2})$ with microelectronvolt emission line widths reveal furthermore the time scale of biexciton formation and decay, as well as coherent effects. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A17.00007: Highly-reduced Fine-structure splitting in InAs/InP quantum dots offering efficient on-demand 1.55 $\mu$m entangled photon emitter Lixin He, M. Gong, C-F Li, G-C Guo, A. Zunger There has been intense recent interest in finding efficient entangled photon sources, including the demonstration of generation of ``event-ready'' entangled photon pairs via a biexciton cascade process using an (In,Ga)As/GaAs quantum dot(QD). However, a genuine finite energy difference between photons with different polarizations, known as the fine structure splitting (FSS), can destroy the entanglement of the photon pairs. To achieve entanglement from (In,Ga)As/GaAs QD, it was, indeed, necessary to Cherry-pick a sample with extremely small FSS from a large number of samples, or to apply strong in-plane magnetic field. Furthermore, the emission wavelength of (In,Ga)As/GaAs QD (880 - 950 nm) is mismatched with the 1.55 $\mu$m needed for communications using the optical fibers. Using theoretical modeling of the fundamental causes of FSS in QDs, we predict that the intrinsic FSS of InAs/InP QDs is an order of magnitude smaller than that of InAs/GaAs dots, and better yet, their excitonic gap matches the 1.55 $\mu$m fiber optic wavelength,therefore offer efficient on-demand entangled photon emitters for long distance quantum communication. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A17.00008: Estimation of extrinsic detection efficiency using intrinsic detection sensitivity of the commercial single photon detector Kiyotaka Hammura, Xiulai Xu, Frederic Brossard, David Williams The detection efficiency ($DE$) of the commercial single-photon-receiver based on InGaAs gate-mode avalanche photodiode is estimated using the detection sensitivity ($DS$). Instalment of a digital-blanking-system (DBS) to reduce dark current makes the difference between $DS$, which is an efficiency of the detector during its open-gate/active state, and the total/overall detection efficiency ($DE$). By numerical simulations, it is found that the average number of light-pulses, blanked by DBS, following a registered pulse is 0.333. $DS$ is estimated at 0.216, which can be used for estimating $DE$ for an arbitrary photon arriving rate and a gating frequency of the receiver. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A17.00009: Quantum key distribution with an unknown and untrusted source Yi Zhao, Bing Qi, Hoi-Kwong Lo The security of a standard bi-directional ``plug \& play'' quantum key distribution (QKD) system has been an open question for a long time. This is mainly because its source is equivalently controlled by an eavesdropper, which means the source is unknown and untrusted. Qualitative discussion on this subject has been made previously. In this paper, we present the first quantitative security analysis on a general class of QKD protocols whose sources are unknown and untrusted. The securities of standard BB84 protocol, weak+vacuum decoy state protocol, and one-decoy decoy state protocol, with unknown and untrusted sources are rigorously proved. We derive rigorous lower bounds to the secure key generation rates of the above three protocols. Our numerical simulation results show that QKD with an untrusted source gives a key generation rate that is close to that with a trusted source. Our work is published in [1]. \\[4pt] [1] Y. Zhao, B. Qi, and H.-K. Lo, Phys. Rev. A, 77:052327 (2008). [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A17.00010: Spectral study of type-0/type-I spontaneous parametric down-conversion in a PPKTP waveguide Jun Chen, Aaron Pearlman, Alexander Ling, Alan Migdall, Jingyun Fan Compared with their bulk-crystal counterparts, SPDC in second-order ($\chi ^{(2)})$ nonlinear optical waveguides has been used to generate correlated photons that are naturally emitted into a single spatial mode in a collinear geometry, easing the effort in efficient photon collection and leading the potential to make chip-scale devices for quantum-information-processing applications. Here towards building chip-scale devices for quantum-information-processing applications, we performed the first spectral characterization of correlated two-photon, and single-photon emission for both type-0 and type-I spontaneous parametric down-conversion (SPDC) in a periodically-poled KTiOPO$_{4}$ (PPKTP) waveguide. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A17.00011: Deterministic generation of entangled photon pairs from a semiconductor quantum dot Andreas Muller, Wei Fang, John Lawall, Glenn Solomon Optical tuning based on the AC Stark effect is used to cancel the fine-structure splitting in a single self-assembled InAs quantum dot. Under this condition, polarization anisotropy vanishes, and photon pairs emitted from the biexcitonic radiative cascade become polarization-entangled. Entanglement is verified by well-known criteria applied to the two-photon density matrix that was reconstructed experimentally via quantum state tomography. Our approach uses a planar optical microcavity for efficient background laser discrimination, and yields triggered polarization-entangled photons deterministically. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A17.00012: Indistinguishable photons from independent semiconductor single-photon devices Thaddeus Ladd, Kaoru Sanaka, Alexander Pawlis, Klaus Lischka, Yoshihisa Yamamoto We demonstrate quantum interference between single photons generated by the radiative decay processes of excitons that are bound to isolated fluorine donor impurities in ZnSe/ZnMgSe quantum-well nanostructures. Single photon generation is confirmed by auto-correlation experiments, and indistinguishability of single photons from independent devices is confirmed via a Hong-Ou-Mandel dip. These results indicate that donor impurities in appropriately engineered semiconductor structures can portray atom-like homogeneity and coherence properties, potentially enabling scalable technologies for future large-scale optical quantum computers and quantum communication networks. [Preview Abstract] |
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