Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session Z40: Solid State Quantum Computing II |
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Sponsoring Units: TGQI DCMP Chair: Mark Gyure, HRL Laboratories Room: Baltimore Convention Center 343 |
Friday, March 17, 2006 11:15AM - 11:27AM |
Z40.00001: Double quantum dot coupled to a superconducting single-electron transistor for measurement of back action Madhu Thalakulam, Joel Stettenheim, A. J. Rimberg, L.N. Pfeiffer, K. W. West The superconducting single electron transistor SET is a highly sensitive electrometer operating near the quantum limit. The back action current noise of the SET has been a topic of interest ever since the SET was proposed as a readout device for charge or spin based qubits [1]. We study the back-action noise of a superconducting SET by means of an electrostatically coupled double quantum dot (DQD) system. Inelastic current through the DQD is sensitive to the spectral density of voltage fluctuation in its electromagnetic environment [2,3]. By properly choosing the dot size and inter-dot tunnel barrier, one can cause inelastic processes to dominate the transport. A measurement of the inelastic current through the double dot system can then be used to calculate the spectral density of quantum noise associated with the RF-SET and hence its back-action. We have fabricated samples consisting of a DQD formed in a GaAs/AlGaAs heterostructure and strongly coupled to an Al/AlO$_x$/Al SET$\@$. Recent results of measurements on such devices will be discussed. [1] M.H. Devoret and R.J. Schoelkopf, Nature, {\bf 406}, 1039(2000). [2]T. Fujisawa et al., Science, {\bf 282}, 932 (1998). [3]R. Aguado and L. P. Kouwenhoven, Phys. Rev. Lett., {\bf 84}, 1986(2000). [Preview Abstract] |
Friday, March 17, 2006 11:27AM - 11:39AM |
Z40.00002: Quantum Capacitance for Quantum Computation C.M. Wilson, T. Duty, F. Persson, M. Sandberg, G. Johansson, L. Tornberg, J. Bylander, P. Delsing We present measurements of superconducting quantum bit (qubit) circuits utilizing the quantum capacitance (QC) of a single cooper-pair box (SCB). The QC is essentially the curvature of the SCB energy bands near the charge degeneracy point, and has recently been measured by our group and others. The curvature arises from the avoided level crossing induced by the Josephson coupling of the SCB. The QC can be much larger than the geometric capacitance, and changes sign between the ground and excited states. We present a qubit with integrated readout that embeds a SCB in a resonant circuit and detects changes in the QC as changes in the phase of a reflected microwave signal. We have calculated that this readout method is strictly quantum limited independent of the quality factor (Q) of the the resonator. This allows great flexibility in the design and optimization of the readout. Calculations show that the method should be able to achieve single-shot discrimination of the qubit state under realistic experimental conditions. We also present preliminary measurements of coupled qubit circuits, with both fixed coupling and a variable coupling scheme based on the ground-state QC. [Preview Abstract] |
Friday, March 17, 2006 11:39AM - 11:51AM |
Z40.00003: Manipulation and readout of deep-submicron Nb-trilayer-based persistent-current qubits David M. Berns, Sergio O. Valenzuela, William D. Oliver, Terry P. Orlando Lithographically patterned persistent-current (PC) qubits are promising candidates for realizing a large-scale quantum computer. While challenging to fabricate in a trilayer technology, deep-submicron Josephson junctions (JJs) are required to realize large qubit tunnel-couplings and allow improved immunity to dielectric-induced decoherence. Here, we present recent results on the measurement and characterization of PC qubits designed with deep-submicron JJs and fabricated with Nb-Al/AlOx-Nb trilayers. [Preview Abstract] |
Friday, March 17, 2006 11:51AM - 12:03PM |
Z40.00004: Macroscopic Quantum Coherence in a Multi-Level Nb Persistent-Current Qubit Yang Yu, W. D. Oliver, J. C. Lee, K.K. Berggren, L.S. Levitov, T.P. Orlando We drove a niobium persistent-current qubit with strong microwaves and observed single-, two-, and three-photon transitions between its macroscopic quantum states. A multi- level energy-band diagram was extracted by mapping the frequency of the induced transitions as a function of applied magnetic flux to the qubit, and the anti-crossing caused by the superposition between the third and fourth excited states were directly measured. The energy relaxation time T1 between two states connected by multi-photon transitions ranged from 30 to 100 ms. In addition, three-photon coherent temporal oscillations between the ground state and fourth excited state were observed with a decoherence time of approximately 50 ns. [Preview Abstract] |
Friday, March 17, 2006 12:03PM - 12:15PM |
Z40.00005: Electronic control and readout of qubit states in Si:Li-based quantum computing system V.V. Osipov, V.N. Smelyanskiy, A.G. Petukhov In our previous work (V. N. Smelyanskiy {\it et al.} Phys. Rev. B {\bf 72}, 081304 (2005)) we predicted a gigantically long lifetime of the first excited state of an interstitial lithium donor in silicon. The nature of this effect roots in the anomalous level structure of the 1{\it s} Li manifold under external stress. Namely, the coupling between the lowest two states of the opposite parity is very weak and occurs via intervalley phonon transitions only. We proposed to use these states under the controlled ac and dc stress to process quantum information. In this work we consider some practical aspects of the proposed scheme such as formation of heavily doped semiconductor electrodes for electrical control of the qubit states and single-qubit readout by means of the resonant tunneling stimulated by polarized infrared radiation. We propose a proof-of-the principle experiment on photo-stimulated time-dependent resonant tunneling in a $\delta$-doped layer of Li donors in Si placed between two $n^+$ Si electrodes. The effect will be characterized by a high sensitivity of the signal to the polarization of photons and by long-term relaxation of the resonant tunneling photocurrent. [Preview Abstract] |
Friday, March 17, 2006 12:15PM - 12:27PM |
Z40.00006: Single electron tunneling in a controllable electromagnetic environment Z. Ji, W. Xue, A.J. Rimberg, L.N. Pfeiffer, K.W. West Real-time counting of single electrons is the most fundamental means of measuring current [1]. Direct observation of single electron tunneling oscillations requires embedding a tunnel barrier in a high-impedance electromagnetic environment. Beginning with a two dimensional electron gas in a GaAs heterostructure we first etch a narrow mesa to serve as a conducting channel. We fabricate two staggered arrays of quantum point contacts (QPCs) across the mesa to serve as ballistic resistors controlled by tuning the QPC gate voltage. An additional QPC placed between the arrays serves as the tunnel barrier and a nearby radio-frequency single electron transistor (RF-SET) serves as an electrometer. We have fabricated several such samples. Typically the conductance G versus gate voltage of such an array of 10 QPCs shows plateau-like structures at fractions of the conductance quantum, $G_0=2e^2/h$. The first plateau, below which the conductance drops rapidly to zero, is the preferred working point corresponding to one open channel in each QPC. When the arrays are at their working points and the central barrier is formed, the samples show a large gap in their I-V characteristics corresponding to dynamical Coulomb blockade. Recent measurements of such samples will be presented, and the use of the RF-SET to directly observe single electron tunneling will be discussed. [1] J. Bylander, T. Duty and P. Delsing, Nature {\bf 434}, 361 (2005). [Preview Abstract] |
Friday, March 17, 2006 12:27PM - 12:39PM |
Z40.00007: Calculated tunneling rates for single electron charging events in Vertical ``Enhancement Mode'' quantum dot devices. Richard Ross, Mark Gyure, Chris Anderson We report on calculations of tunneling rates associated with single electron charging events in vertical ``enhancement mode'' quantum dot device structures. These devices consist of two vertically stacked quantum well layers. A pair of surface depletion gates define a Quantum Point Contact (QPC) and a single localized enhancement gate creates a quantum dot (QD) in the upper quantum well. Single electron charging events in this device occur via tunneling between the vertically separated QD and QPC states. Tunneling rates are computed using Fermi's Golden Rule based on numerical eigenstates derived from fully 3-dimensional self-consistent Poisson-Schroedinger calculations. The effects of coulomb interaction on the quantum dot states and hence tunneling rates will be considered. Additionally, a comparison of these numerical results with experimental estimates of tunneling rates derived from random telegraph signals will be presented. [Preview Abstract] |
Friday, March 17, 2006 12:39PM - 12:51PM |
Z40.00008: Observation of Sequential Single Electron Charging in Vertical ``Enhancement Mode'' Quantum Dot Devices Edward Croke, Geoffrey Simms, Mark Gyure The vast majority of devices currently being explored for quantum information processing with semiconductor quantum dots rely on multiple surface gates operating in reverse bias (depletion mode) to constrict a buried 2D electron gas to the few electron regime. Although successful in demonstrating one and two qubit operations, they are unlikely to scale to large arrays that are needed to perform practical quantum information processing. In this talk, we present the first experimental results from a new type of ``enhancement mode'' device that requires only a single gate electrode operating in forward bias to create a few electron quantum dot. We observe random telegraph signals in several well defined regions of gate voltage, indicative of single electrons tunneling between states of the quantum dot and the readout channel. The absence of these signatures below a particular gate voltage suggests that these quantum dots are in the few electron regime and can be easily depleted to contain only one electron. [Preview Abstract] |
Friday, March 17, 2006 12:51PM - 1:03PM |
Z40.00009: Analysis of Random Telegraph Signals from Vertical ``Enhancement Mode" Qunatum Dot Devices Mark Gyure, Geoffrey Simms, Richard Ross, Edward Croke Clear signatures of single electron tunneling in vertical ``enhancement mode'' semiconductor quantum dot devices have recently been observed. These devices hold great promise for future scalability of semiconductor-based quantum information processing because they require only a single gate electrode to create the quantum dot. In this talk, we present a detailed analysis of the random telegraph signals observed in these devices as various N to N+1 electron transitions are swept through by varying the gate voltage and magnetic field. This analysis allows us to extract a variety of information about the transitions and further supports our conclusion that electrons are tunneling between the gate-induced quantum dot and the readout channel below. Results of self-consistent Schrodinger-Poisson simulations of these devices are presented that also support this interpretation of the experimental results. [Preview Abstract] |
Friday, March 17, 2006 1:03PM - 1:15PM |
Z40.00010: MOSFET-like single electron transistor built in pure silicon G.M. Jones, B.H. Hu, C.H. Yang, M.J. Yang Solid state implementations of qubits offer the advantage of being scalable, and, in particular, those based on semiconductors can be integrated by existing technologies. The two Zeeman states of an electron spin in a quantum dot (QD) provide a promising candidate for a qubit, and lateral quantum dots provide the best opportunity for scaling. Spins in lateral QDs in the GaAs/AlGaAs single electron transistors (SETs) have been intensively investigated. In contrast, Si provides a number of advantages, including long spin coherence time, large g-factor, and small spin-orbit coupling effect. However, isolation of a single electron in a Si QD has not yet been achieved. We will report a fabrication technique that utilizes the established MOSFET concept on highly resistive Si substrates in order to minimize the potential disorder resulting from impurities. In our approach, 2D (or 1D) electrons are induced by a top gate, which laterally overlaps with the ion-implanted source/drain, but vertically separated by SiO2. Several side gates buried in the SiO2 help define the tunneling barriers and control the number of electrons in the island. We will discuss the operating principle, computer simulation, and experimental results that confirm the validity of the design concept. [Preview Abstract] |
Friday, March 17, 2006 1:15PM - 1:27PM |
Z40.00011: Proposal to stabilize and detect half-quantum vortices in strontium ruthenate thin films: Non-Abelian braiding statistics of vortex matter in a ${p_x}+i{p_y}$ superconductor Sumanta Tewari, Sankar Das Sarma, Chetan Nayak We propose a simple way to stabilize half-quantum vortices in superconducting strontium ruthenate, assuming the order parameter is of chiral $p_x + ip_y$ symmetry, as is suggested by recent experiments. The method, first given by Salomaa and Volovik in the context of Helium-3, is very naturally suited for strontium ruthenate, which has a layered, quasi-two-dimensional, perovskite crystal structure. We propose possible experiments to detect their non-abelian braiding statistics. These experiments are of potential importance for topological quantum computation. [Preview Abstract] |
Friday, March 17, 2006 1:27PM - 1:39PM |
Z40.00012: Measurement errors for phase qubits Qin Zhang, Abraham Kofman, Alexander Korotkov We analyze error mechanisms in measurement of superconducting phase qubits, including measurement cross-talk for two coupled phase qubits and effect of nonadiabaticity during the measurement pulse. Each qubit is represented by a fictitious particle moving in an asymmetric double-well potential. A measurement, e.g., of the state $|10\rangle$ perturbs the second qubit which may result in a wrong measurement result $|11 \rangle$. In the study of this cross-talk the first qubit is described classically, since it is highly excited, whereas the second qubit can be treated either classically or quantum- mechanically. We obtain conditions for minimizing the cross- talk. We also study the nonadiabatic errors for different shapes and durations of the measurement pulse and discuss optimal conditions for fast and reliable measurements. [Preview Abstract] |
Friday, March 17, 2006 1:39PM - 1:51PM |
Z40.00013: Entanglement distillation by adiabatic passage in coupled quantum dots Jaroslav Fabian, Ulrich Hohenester Adiabatic passage of two correlated electrons in three coupled quantum dots is shown to provide a robust and controlled way of distilling, transporting and detecting spin entanglement, as well as of measuring the rate of spin disentanglement. Employing tunable interdot coupling the scheme creates, from an unentangled two-electron state, a superposition of spatially separated singlet and triplet states. A single measurement of a dot population (charge) collapses the wave function to either of these states, realizing entanglement to charge conversion. The scheme is robust, with the efficiency close to 100\%, for a large range of realistic spectral parameters. [Preview Abstract] |
Friday, March 17, 2006 1:51PM - 2:03PM |
Z40.00014: Detecting the squeezing and entanglement of nanomechanical modes: a practical scheme L. Tian, S. M. Carr Nanomechanical systems are promising candidates for realizing the continuous variable protocols of quantum information processing. The detection of the squeezing and entanglement of nanomechanical modes is a crucial step towards such applications. Here, we show that by coupling a nanomechanical mode with another continuous variable mode -- a superconducting phase variable, the squeezing and entanglement can be observed within current experimental techniques. [Preview Abstract] |
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