Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D36: Focus Session: Semiconductor Qubits: QED Interfaces & Multi-Qubit Coupling |
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Sponsoring Units: GQI Chair: Thaddeus Ladd, HRL Room: 703 |
Monday, March 3, 2014 2:30PM - 3:06PM |
D36.00001: Conversion of angular momentum from single photons to single electron spins in electrically controlled quantum dots Invited Speaker: Akira Oiwa Electrical controllability of gate-defined quantum dots (QDs) has brought significant developments in the coherent manipulation of electron spins and two-qubit gate operation toward scalable qubits for quantum computations. Moreover the suitability of gate-defined QDs to quantum information technologies would be considerably enhanced if spin states in the gate-defined QDs could couple to photon states coherently. Here we show that the photon polarization can couple to the spin degree of freedom in gate-defined GaAs QDs. Double QDs were fabricated in AlGaAs/GaAs quantum wells. By synchronizing a pulse laser irradiation with a charge sensing measurement we performed the real-time single photoelectron spin detection in the double QD. First we show that the resonant inter-dot tunneling can offer a robust detection scheme of the single photoelectrons trapped in the double QDs [1]. In the two-electron regime, the inter-dot tunneling of the photoelectrons strongly depends on the relative spin orientation (parallel or anti-parallel) of the two QDs. Therefore by combining the resonant inter-dot tunneling scheme with the Pauli spin effect, we have realized the nondestructive detection of single photoelectron spins. Finally, we demonstrate the angular momentum conversion from single photons to single electron spins in the double QD from the dependence of the detected spins on the incident photon polarization. \\[4pt] This work was done in collaboration with T. Fujita, K. Morimoto, G. Allison, M. Larsson, H. Kiyama, S. Teraoka, S. Haffouz, D. G. Austing, A. Ludwig, A. D. Wieck and S. Tarucha\\ [4pt] [1] T. Fujita et al, Phys. Rev. Lett. 110, 266803 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D36.00002: Entanglement Purification with the Exchange Interaction Adrian Auer, Guido Burkard Entanglement purification techniques provide means to create qubit pairs of arbitrary high fidelity with respect to a maximally entangled state, consuming a larger number of low fidelity pairs. So-called recurrence protocols act iteratively on two or more qubit pairs to produce one pair with higher fidelity, using local unitary operations, measurements, and communication of the measurement results. In this talk, we present a purification protocol that works with two input pairs and solely uses a single pulsed Heisenberg-type qubit interaction, therefore being especially suitable for spin qubits in tunnel-coupled quantum dots. In contrast to previously known protocols, we allow for asymmetric bilateral operations where the two communication parties operate differently on their qubits. In the most efficient version of our protocol, the local two-qubit interactions correspond to the $\sqrt{\textsc{swap}}$ gate and its inverse, which are the natural entangling gates generated from a Heisenberg-type interaction. Furthermore, we show how the same fidelity gain can be reached using $XY$-type interactions. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D36.00003: Driven nonlinear dynamics of two coupled exchange-only qubits Arijeet Pal, Emmanuel Rashba, Bertrand Halperin Exchange-only (RX) qubits are a promising candidate for the fundamental unit of a quantum computer. Recently, such a qubit has been experimentally realized and its complete two-axis control demonstrated in a system of exchange coupled triple dots [1, 2]. The next step is to establish the control of two such coupled RX qubits. We have explored the dynamical effects of two capacitively-coupled RX qubits. We formulate the Hamiltonian for two capacitively-coupled RX qubits constructed from six dots where they are arranged in different geometries. Under the conditions of resonant driving of one of the qubits, the other qubit serves as a detector of the coupling. When driven strongly even a modest strength of interaction can result in nonlinear effects and putatively make the control of two-qubit entanglement irregular. In this regard the different geometries give rise to substantially disparate responses which will be relevant for future experiments in these systems. 1. Self-Consistent Measurement and State Tomography of an Exchange-Only Spin Qubit, J. Medford et. al., Nature Nanotechnology 8, 654 (2013) 2. Quantum-Dot-Based Resonant Exchange Qubit, J. Medford et al., PRL 111, 050501 (2013) [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D36.00004: Theoretical Characterization of Nonlocal Two-qubit Operations for Electrostatically Coupled Singlet-Triplet Qubits Fernando Calderon, Jason Kestner Singlet-triplet qubits are an attractive candidate for implementing a quantum processor because of their scalability and fast control. In this system, entangling inter-qubit interactions can be performed via electrostatic coupling. It is an open question whether a single square pulse of the system's evolution operator can perform a maximally entangling operation or not. Using Makhlin's invariants [1], which characterize the nonlocal part of 2-qubit unitary transformations, and a geometric representation of those local invariants, we will give a description of the gates that can be directly generated by this particular Hamiltonian and its suitability for performing a maximally entangling gate. \\[4pt] [1] Y. Makhlin, ``Nonlocal properties of two-qubit gates and mixed states, and the optimization of quantum computations,'' Quantum Inf. Process., vol. 1, no. 4, 2002. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D36.00005: Long-range, low-noise gates for dopant and quantum dot spin qubits V. Srinivasa, H. Xu, J. Medford, J. M. Taylor Coupling spins by exchange interactions provides a rapid, tunable method of entanglement generation. However, this necessarily occurs only at short distances, and often incurs susceptibility to charge noise. To address these challenges, we consider two approaches. First, we investigate the coupling of two qubits localized on spatially separated impurity atoms or quantum dots. We show that a third multi-electron, multi-level quantum dot can mediate an exchange interaction between the qubits that is tunable via gate voltage control of level splittings and tunneling amplitudes. This approach suggests an experimentally accessible method for coupling donor electron spins in silicon via a hybrid impurity-dot system. Second, we discuss the resonant exchange (RX) qubit, defined within a triple quantum dot in the three-electron regime. Electric field control of the dipole moment of the RX qubit at microwave frequencies enables single-qubit and two-qubit gates that are protected against low-frequency charge noise. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D36.00006: Coherently driven double-quantum dot at finite bias: Analogy with lasers and beyond Manas Kulkarni, Ovidiu Cotlet, Yinyu Liu, Karl Petersson, George Stehlik, Jason Petta, Hakan Tureci Hybrid circuit-QED systems consisting of a double-quantum dot (DQD) coupled to a microwave resonator provide a unique platform to explore non-equilibrium impurity physics with coupled light-matter systems. We present a theoretical and experimental study of photonic and electronic transport properties of such a system. We obtain a Hamiltonian and the Liouvillian super-operators considering systematically both the presence of phonons and the effect of leads at finite voltage bias. We subsequently derive analytical expressions for transmission, phase response, photon number and nonequilibrium steady state electron current and show that the system realizes an unconventional version of a single-atom laser. Our analytical results are compared to numerically exact ones establishing regimes of validity of various analytical models. Finally, we compare our findings to experimental measurements. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D36.00007: Coupling InSb quantum dots to a superconducting microwave resonator Maja Cassidy, Jakob Kammhuber, Diana Car, Sebastien Plissard, Erik Bakkers, Leo DiCarlo, Leo Kouwenhoven We present measurements of a superconducting half-wave resonator coupled to two InSb nanowire quantum dots. Precise nanowire alignment at the electric field antinodes at opposite ends of the microwave cavity allows for a maximal electric field along the wire axis, without compromising the intrinsic quality factor of the cavity. This architecture may be useful for reaching the strong coupling limit between a single spin and a microwave photon, paving the way to on-chip coupling of single spins for quantum information processing. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D36.00008: Out of Equilibrium Charge dynamics in a cQED Architecture Jeremie Viennot, Matthieu Delbecq, Matthieu Dartiailh, Audrey Cottet, Takis Kontos In the context of circuit quantum electrodynamics, recent developments made it possible to build hybrid circuits [1], including many types of quantum dots. The versatility of these systems allows us to explore several directions, from quantum information engineering to many-body physics, all in a circuit QED architecture. I will present some of the experiments of our group where a carbon nanotube-based double quantum dot is coupled to a microwave cavity. We demonstrate an efficient electron confinement in this carbon nanotube, allowing us to bring the system at resonance with the cavity and used it as a charge Qbit. We characterise the response of this circuit out of equilibrium, either at finite bias or large microwave power [2]. We are also able to perform microwave spectroscopy of the device. Combined with ferromagnet interface exchange Zeeman fields, such a control should enable us to go towards spin-photon coupling and spin qubit experiments for circuit QED [3]. References : [1] M.R. Delbecq et al. Nature Comm., 4, 1400 (2013). [2] J.J. Viennot et al., arXiv:1310.4363 [3] A. Cottet et al., Phys. Rev. Lett. 105, 160502 (2010). [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D36.00009: Ultra-low power microwave manipulation of electron spin ensembles A.J. Sigillito, H. Malissa, A.M. Tyryshkin, S.A. Lyon Superconducting coplanar waveguide (CPW) resonators are a promising alternative to standard dielectric resonators for many electron spin resonance experiments. Their high sensitivity and low power requirements make them particularly well suited to applications where the sample volume is small and when microwave heating is a concern. Experiments utilizing rectangular pulses are possible with a peak microwave power of less than 1uW for 400ns pi-rotations, and under 100 uW of peak power for 40ns pi-rotations. However, CPW resonators have an inherently inhomogeneous microwave magnetic field (B$_{\mathrm{1}})$. Therefore, to uniformly rotate all spins in a sample, adiabatic microwave pulses must be used. Here we report on the use of such pulses to correct B$_{\mathrm{1}}$ inhomogeneities spanning an order of magnitude. We also present data indicating single shot sensitivity to 1x10$^{\mathrm{7}}$ phosphorus donors in isotopically enriched $^{\mathrm{28}}$Si at 1.7K. These show that superconducting CPW resonators are fully compatible with experiments requiring rapid manipulation of spins in dilution refrigerators. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D36.00010: Extreme Harmonic Generation in an InAs Spin-Orbit Qubit J. Stehlik, M.D. Schroer, M.Z. Maialle, M.H. Degani, J.R. Petta Strong spin-orbit materials have shown great promise in the field of quantum computation. Unlike conventional semiconductor materials, fast all-electrical control is achieved through electric dipole spin resonance (EDSR). In this work we explore EDSR in an InAs nanowire spin-orbit qubit. We observe signs of harmonic generation where spin flips occur at the resonance condition $n h f = g \mu_{\rm B} B$, where $f$ is the applied frequency, $B$ is the magnetic field, $g$ is the $g$-factor and $n$ is an integer. Near the interdot charge transition we observe harmonics up to $n$ = 8, indicating extreme harmonic generation. At far detuning we only observe the $n=1$ resonance. Further, we find odd/even structure in the harmonic response: odd harmonics result in an increase in the leakage current while even harmonics result in its suppression. Finally we observe oscillations in the resonant current as a function of detuning. The striking detuning dependence suggests that the harmonics may be caused by Landau-Zener transitions occurring due to the anti-crossing between the differing charge states. Numerical simulations of the system are qualitatively consistent with this picture. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D36.00011: Minimal-excitation states for electron quantum optics using levitons Preden Roulleau, Thibaut Jullien, Julie Dubois, Fabien Portier, Patrice Roche, Antonella Cavanna, Yong Jin, Werner Wegscheider, D. Christian Glattli The on-demand generation of pure quantum excitations is important for the operation of quantum systems, but it is particularly difficult for a system of fermions. This is because any perturbation affects all states below the Fermi energy, resulting in a complex superposition of particle and hole excitations. However, it was predicted nearly 20 years ago that a Lorentzian time-dependent potential with quantized flux generates a minimal excitation with only one particle and no hole. Here we report that such quasiparticles (hereafter termed levitons) can be generated on demand in a conductor by applying voltage pulses to a contact. Partitioning the excitations with an electronic beam splitter generates a current noise that we use to measure their number. Minimal-excitation states are observed for Lorentzian pulses, whereas for other pulse shapes there are significant contributions from holes. Further identification of levitons is provided in the energy domain with shot-noise spectroscopy, and in the time domain with electronic Hong--Ou--Mandel noise correlations. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D36.00012: Observation of dark states in a superconductor diamond quantum hybrid system Xiaobo Zhu, Yuichiro Matsuzaki, Robert Amsuss, Kosuke Kakuyanagi, Takaaki Shimo-Oka, Norikazu Mizuochi, Kae Nemoto, William J. Munro, Kouichi Semba, Shiro Saito We observed a remarkably sharp resonance ($\sim$ 1 MHz) at 2.878 GHz in the spectrum of flux qubit NV-diamond hybrid quantum system under zero external magnetic field. This width is much narrower than that of both the flux-qubit and spin-ensemble. We show this resonance is evidence of a collective dark state in the ensemble which is coherently driven by the superposition of clockwise and counter-clockwise macroscopic persistent super-currents owing in the flux qubit. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D36.00013: Correlations in Charge Transfer and Photon Emission by a Double Quantum Dot Connected to High Quality Resonator Canran Xu, Maxim Vavilov We analyze the full counting statistics of charge transfer and photon emission by a double quantum dot (DQD) coupled to a high-quality microwave resonator by electric dipole interaction. We show that at the resonant condition between the energy splitting of the DQD and the photon energy in the resonator, charge and photon statistics exhibits both a sub-Poissonian distribution and antibunching. In the ideal case, when the system decoherence stems only from photodetection, the photon noise is reduced below one-half of the noise for the Poisson distribution and is consistent with current noise. Our analysis justifies that sub-Poissonian photon noise occurs when the cross-correlation between emitted photons and electrons is strong. We demonstrate that Josephson junction based photomultipliers can be used to experimentally assess statistics of emitted photons. [Preview Abstract] |
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