Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L26: Spin Qubits  Control, Transport, Architecture and Decoherence 
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Sponsoring Units: GQI Chair: Mark Friesen, University of WisconsinMadison Room: D136 
Tuesday, March 16, 2010 2:30PM  2:42PM 
L26.00001: Fighting noise from the Si/SiO2 interface with optimal control Kevin Young, Dylan Gorman, K. Birgitta Whaley Electron donors in Si offer an attractive route to scalable quantum computation. When positioned close to the Si/SiO2 interface, however, donors are subject to strong decoherence due to, eg. coupling to trapped paramagnetic defects. These defects generate 1/f type dephasing noise. This system possesses a large asymmetry in the amplitude and phase relaxation times, T1 and T2, of the donor. We present a technique for the construction of numerically optimized pulse sequences designed to exploit this asymmetry to maximize the coherence of an arbitrary quantum state. Our results compare favorably to other dynamical decoupling procedures. [Preview Abstract] 
Tuesday, March 16, 2010 2:42PM  2:54PM 
L26.00002: Using a quantum dot spin bus to implement CNOT gates between remote qubits Jianjia Fei, Dong Zhou, Mark Friesen A spin bus is a chain of individual spins with strong, alwayson, static interactions (e.g., a linear array of singleelectron quantum dots).~ Here, we consider a spin bus coupled to multiple external qubits via the Heisenberg exchange interaction.~ Using both theoretical and numerical methods, we show that a continuous range of twoqubit gates can be constructed.~ In particular, we show that SWAP and squarerootSWAP gates can be achieved with high accuracy, when the couplings between the qubits and the bus are weak.~ In combination with singlequbit operations, we can then realize controlledNOT gates between remote qubits, as mediated by the bus.~ The spin bus therefore shows considerable potential for implementing universal quantum gates. [Preview Abstract] 
Tuesday, March 16, 2010 2:54PM  3:06PM 
L26.00003: Effects of randomness on a spin quantum data bus Sangchul Oh, Xuedong Hu A spin 1/2 Heisenberg chain with an odd number of spins could be used as a quantum data bus between spin qubits. Here we study how random exchange couplings and external magnetic fields affect the information transfer capability of such a spin bus. We find that for small external magnetic fields, the energy gap between the two lowest levels is robust against randomness in exchange couplings, while effective qubit couplings are more susceptible to such randomness. On the other hand, randomness in the external magnetic field could cause a change in the bus ground state. We also explore how randomness in a spin bus affects the fidelity of various bus gates. [Preview Abstract] 
Tuesday, March 16, 2010 3:06PM  3:18PM 
L26.00004: Quantum logic with cavityembedded quantum dots using global femtosecond pulses Jordan Kyriakidis, Catherine Holloway, Wayan Sudiarta There are several proposals utilizing quantum dots embedded in optical cavities as physical or logical qubits. The advantage of these systems is that distant qubits can be controllably coupled through virtual cavity modes. Typically, lasers are required to address individual dots in the cavity, which is exceedingly difficult. We present results of our work showing how this potential limitation can be overcome through design of global pulse shapes determined via genetic algorithms. Our results show fast entangling operations on distant qubits with global pulses even for arbitrarily closelyspaced energy levels. This level of quantum control has not yet been been demonstrated for multiple quantum dots embedded in cavities. Our scheme should be implementable with presentday experimental capabilities. [Preview Abstract] 
Tuesday, March 16, 2010 3:18PM  3:30PM 
L26.00005: Holonomic Quantum Computation with Electron Spins in Quantum Dots Massoud Borhani, Vitaly Golovach, Daniel Loss With the help of the spinorbit interaction, we propose a scheme to perform holonomic single qubit gates on the electron spin confined to a quantum dot. The manipulation is done in the absence (or presence) of an applied magnetic field. By adiabatic changing the position of the confinement potential, one can rotate the spin state of the electron around the Bloch sphere in semiconductor heterostructures. The dynamics of the system is equivalent to employing an effective nonAbelian gauge potential whose structure depends on the type of the spinorbit interaction. As an example, we find an analytic expression for the electron spin dynamics when the dot is moved around a circular path (with radius R) on the two dimensional electron gas (2DEG), and show that all single qubit gates can be realized by tuning the radius and orientation of the circular paths. Moreover, using the Heisenberg exchange interaction, we demonstrate how one can generate twoqubit gates by bringing two quantum dots near each other, yielding a scalable scheme to perform quantum computing on arbitrary N qubits. This proposal shows a way of realizing holonomic quantum computers in solidstate systems. [Preview Abstract] 
Tuesday, March 16, 2010 3:30PM  3:42PM 
L26.00006: Pulse Sequences for ExchangeBased Quantum Computation Nick Bonesteel, Robert Cipri, Daniel Zeuch Switching on and off, or pulsing, the isotropic exchange interaction between pairs of spin1/2 particles (e.g. electrons in an array of quantum dots) is universal for quantum computation, provided the logical qubits of the computer are suitably encoded. A specific scheme for carrying out such exchangebased quantum computation was provided by DiVincenzo et al.\footnote{D. DiVincenzo et al., Nature {\bf 408}, 339 (2000).} who, through numerical minimization of a cost function, found a sequence of 19 pulses that carry out a CNOT (up to single qubit rotations) on two logical qubits. In this scheme, the logical qubits are encoded using triplets of spin1/2 particles with total spin 1/2, and so the total spin of any two qubits can be either 0 or 1. One limitation of the pulse sequence found in Ref. 2 is that it works only if this total spin is 1 (a requirement which can, in principle, be met by initialing the computer in a magnetic field). We present a new class of pulse sequences which carry out twoqubit gates on logical qubits using the same encoding as Ref. 2 but which work regardless of the value of this total spin. These new sequences, consisting of approximately 50 pulses, are obtained analytically, without the need for numerical minimization. [Preview Abstract] 
Tuesday, March 16, 2010 3:42PM  3:54PM 
L26.00007: A low noise exchange gate in double quantum dots Erik Nielsen, Malcolm Carroll, Richard Muller Minimizing the effects of noise is a central challenge to the creation of solidstate singlettriplet double quantum dot (DQD) quantum bits (qubits). Charge noise, electronics error or inhomogeneous fields have all separately been addressed with different approaches. The demand for qubit operations robust to the combination of all noise sources places simultaneous requirements, however, that are not clearly compatible. We investigate the feasibility of achieving an exchange gate in a DQD system that is more robust to multiple sources of noise such as slight error around the applied bias point due to electronics or charge noise combined with external inhomogeneous Bfield effects, addressed with dynamically coupled gates. A full configuration interaction (CI) method is used to compute the exchange energy as a function of dot shape and detuning voltage in order to identify the more robust operations. In particular the CI calculation provides significantly better accuracy for the (2,0) configuration of the DQD system, which is a potentially important low noise operating regime. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DEAC0494AL85000. [Preview Abstract] 
Tuesday, March 16, 2010 3:54PM  4:06PM 
L26.00008: Optimal electric control for twospin single and twoqubit operations in gated double quantum dots Guy Ramon, Xuedong Hu We consider gate operations on qubits encoded in the singlet and unpolarized triplet states of pairs of spins localized in biased double quantum dots. Assuming a capacitively coupled pair of double dots, the Coulomb couplings between the two qubits are calculated by considering their charge distributions within a multipole expansion. For single qubit gates our proposed architecture builds on the recently demonstrated nuclear magnetic field gradient between the dots (for $X$rotations) and the effective exchange energy that takes into account the presence of the second qubit (for $Z$rotations). Our calculated entangling coupling between the two qubits demonstrates six ordersofmagnitude tunability with bias, allowing for efficient twoqubit gates. Our analysis highlights the distinct effects of the control qubit and fluctuating charge environment on the performance of the target qubit, where the former contributes to gate errors and the latter to spin dephasing. We are thus able to propose an optimal qubit design and working points for single and two qubit operations, as well as optimal idle positions. [Preview Abstract] 
Tuesday, March 16, 2010 4:06PM  4:18PM 
L26.00009: A fully connected qubit network model for quantum information processing applications Mark Coffey We describe a fully connected spin network model for quantum information processing applications. This scalable network in the case of spin 1/2 has recently been realized in the laboratory, using Josephson phase qubits, and other solidstate implementations are likely. We have recently jointly developed a rigorous protocol for producing the important maximally entangled generalized GHZ states for this implementation [1]. (GHZ states generalize the well known Bell states for two qubits.) An exact solution for the eigenstructure of a certain subspace of partial uniform superpositions enables the protocol to be detailed for an arbitrary number of qubits. An overview of this work and a short description of other quantum information processing applications of the spin network will be given. Joint work with Andrei Galiautdinov and Ron Deiotte. [1] A. Galiautdinov, M. W. Coffey, and R. Deiotte, arXiv:0907.2225v2 (2009); to appear in Phys. Rev. A. [Preview Abstract] 
Tuesday, March 16, 2010 4:18PM  4:30PM 
L26.00010: Decoherence in double quantum qubits due to coupling with the electromagnetic environment Diego Valente, Frank Wilhelm, Eduardo Mucciolo Quantum dots are strong candidates for the physical realization of quantum computers due to their underlying semiconductor technology. Double quantum dot (DQD) setups can be used as qubits by manipulation of either the charge or spin degree of freedom of the excess electron inside the dots. These manipulations in turn make them vulnerable to coupling to environmental degrees of freedom, causing undesired decoherence effects that can potentially lead to errors in the quantum computation. The environment can be modeled as a bosonic bath, where the bath is the electromagnetic environment or phonons, for instance. In this work we study dissipative effects in lateral DQD systems due to electromagnetic fluctuations in the gate voltages that feed the dots. In the context of the fluctuationdissipation theorem we model the noise source as a frequency dependent impedance and make use of effective circuit models to estimate decoherence parameters such as the quality ($Q$) factor of quantum oscillations and the energy ($T_1$) and phase ($T_2$) relaxation times in the system. We discuss the dependence of the $Q$ factor with respect to physical parameters such as temperature and the capacitive coupling between the electrodes. We also comment on the effects of electromagnetic fluctuations induced decoherence in the context of spin qubits. [Preview Abstract] 
Tuesday, March 16, 2010 4:30PM  4:42PM 
L26.00011: Finitefrequency shot noise as a spinrelaxation probe in quantum dots Farzad Qassemi Maloomeh, William A. Coish, Frank K. Wilhelm, Joakim Bergli Long spinrelaxation times are an important prerequisite for spinbased quantum information processing. However, conventional pulsedgate techniques for measuring spin relaxation in a quantum dot operate only at large energy splitting. An alternative is to measure a transient effective charge $e^*$ [1], or equivalently, the zerofrequency noise. However, multilevel systems often exhibit several decay rates due to distinct physical mechanisms, where a more refined approach is necessary. We have formulated a theory of the frequencydependent current noise through a multilevel system in the dynamical channel blockade regime, including the effects of multiple relaxation processes. This theory gives a onetoone correspondence between the form of the frequencydependent Fano factor and the relevant relaxation rates and can therefore be used to determine these rates through a measurement of the current noise. We have applied it to the case of a quantumdot spin diode (or spin valve) and to a double quantum dot in the Pauli spin blockade regime.\\[4pt] [1] F. Qassemi, W. A. Coish and F. K. Wilhelm, Phys. Rev. Lett. \textbf{102}, 176806 (2009) [Preview Abstract] 
Tuesday, March 16, 2010 4:42PM  4:54PM 
L26.00012: Extended Hubbard model simulations of chargequbit circuits: from idealism to realism Zahra ShaterzadehYazdi, Barry C. Sanders Charge qubits are promising quantum logical elements for performing quantum computation or as intermediate states to prepare and read other qubit realizations such as spin or flux. Instead of idealizing the charge qubits at the outset and using standard quantum circuit theory, we use the extended Hubbard model as a firstprinciples model of charge qubit dynamics and model idealized proposals for chargequbit circuits using this secondquantized description with short and mediumrange interactions. In particular we study how one and twoqubit gates would perform for realistic systems, and we apply our theory to teleportation of a single charge qubit in a threequbit system. We also discuss how to incorporate phonon noise into the model. [Preview Abstract] 

L26.00013: ABSTRACT WITHDRAWN 
Tuesday, March 16, 2010 5:06PM  5:18PM 
L26.00014: Spatial Wavefunction Switched (SWS) FieldEffect Transistors: Computing Using More Than Few Electrons Faquir Jain, Evan Heller, John Chandy Spatial Wavefunction Switched (SWS) FieldEffect Transistors (FETs), comprising two or more verticallystacked asymmetric coupledquantum wells (QWs), function as having multiple inversion channels, where the spatial location of the carrier ensemble wavefunction determines the state of the device [1]; e.g. electrons in well W2 (01), in W1 (10), in both (11), in neither (00). Carriers can be transferred within a FET vertically from one channel to the other or laterally to the channels of adjacent SWSFET devices by the manipulation of the gate voltages (V$_{g})$. This vertical and lateral manipulation of carrier location enables processing of 2 or more bits simultaneously, which results in reduced power, delay and device count. The wavefunction transfer to an upper well has been experimentally verified in an InGaAs SWS device. SWSFET structures have been simulated to realize 3 bits or 8 states in three QW channels. Additional states can be implemented in SWSFETs configured as quantum dots (QDs). SWSQD structures, processing more than few electrons than Coulomb blockade devices [2], provide an alternate path to quasiquantum computing. 1. F. Jain and E. Heller, Am. Phys. Soc. Proc., March 20, 2009 (Y288). 2. N. Shaji \textit{et al.}, Nature Physics, 4, p.540, 2008. [Preview Abstract] 
Tuesday, March 16, 2010 5:18PM  5:30PM 
L26.00015: Stark effect for Si:Li spin qubits Luke Pendo, Erin Handberg, Vadim Smelyanskiy, Andre Petukhov We study the effect of a static electric field on lithium donors in silicon. Our treatment is based on a variational procedure utilizing a large set of basis functions similar to those used by Faulkner [1]. We take into account the valleyorbit splitting, arbitrary external stress and magnetic field as well as spinorbit interaction. We believe that our variational method captures the effects of higher energy excited states as well as nonlinear Efield contributions to the energy level splitting and the electrical dipole moment. The anisotropy of the effective mass in a single valley leads to an anisotropy of the quadratic Stark susceptibility. This anisotropy could be used to manipulate and control Si:Li spin qubits because it causes a nontrivial interplay of the Stark and Zeeman effects due to the unique inverted electronic structure of the Li donor in Si. [1] R.A. Faulkner, Phys. Rev. 184, 713 (1969). [Preview Abstract] 
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