Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B1: Invited Session: Logical Spin Qubits for Quantum Computation |
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Sponsoring Units: DCMP Chair: Amir Yacoby, Harvard University Room: Ballroom I |
Monday, March 18, 2013 11:15AM - 11:51AM |
B1.00001: Entanglement of Singlet-Triplet Qubits Invited Speaker: Michael Shulman Spins in semiconductor quantum dots are promising candidates for the building blocks of a quantum information processor due to their potential for miniaturization and scalability. Singlet-triplet (S-T0) qubits, a certain type of spin qubit, store information in the joint spin state of two electrons. However, these qubits' weak interaction with the environment, which leads to their long coherence times, makes two-qubit operations challenging. We perform the first two-qubit operation between two S-T0qubits, exploiting the capacitive coupling between two adjacent qubits to generate a CPHASE gate. In order to combat low frequency noise we use a dynamically decoupled sequence that maintains the two-qubit coupling while decoupling each qubit from its fluctuating environment. Using state tomography we show that the two-qubit operation has the intended effect on the state of the qubits, and we provide definitive proof of entanglement by extracting a concurrence of 0.44 and a Bell state fidelity of 0.72. This two-qubit interaction lends itself to easily implemented improvements, which promise to generate higher fidelity entangled states that can be the basis for establishing a scalable architecture for quantum information processing. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B1.00002: The exchange-only spin qubit Invited Speaker: Charles Marcus |
Monday, March 18, 2013 12:27PM - 1:03PM |
B1.00003: Exchange-based CNOT gates for singlet-triplet qubits with spin orbit interaction. Invited Speaker: Jelena Klinovaja We propose a scheme for implementing the CNOT gate over qubits encoded in a pair of electron spins in a double quantum dot [1]. The scheme is based on exchange and spin-orbit interactions and on local gradients in Zeeman fields. We find that the optimal device geometry for this implementation involves effective magnetic fields that are parallel to the symmetry axis of the spin-orbit interaction. We show that the switching times for the CNOT gate can be as fast as a few nanoseconds for realistic parameter values in GaAs semiconductors. Guided by recent advances in surface codes, we also consider the perpendicular geometry. In this case, leakage errors due to spin-orbit interaction occur but can be suppressed in strong magnetic fields.\\[4pt] [1] J. Klinovaja, D. Stepanenko, B. I. Halperin, and D. Loss, Phys. Rev. B 86, 085423 (2012). [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:39PM |
B1.00004: Coherent Control and Manipulation of Three Spin States in a Triple Quantum Dot Invited Speaker: Andrew Sachrajda The triple quantum dot energy level spectrum is far more complex than its double quantum dot counterpart. As a result it is a challenge to cleanly manipulate only the two required qubit states without invoking more complex multi- state coherent evolution. In this talk I will describe experiments and modeling~of lateral triple quantum dot devices where by suitable device gate (i.e. energy level spectrum) tuning and pulse characteristics we were able to characterize and manipulate various three spin qubit species. In particular I will describe measurements where the Landau-Zener --St\"{u}ckelberg approach previously demonstrated in double dots is extended to three- interacting spin states permitting us to demonstrate phenomena such as pairwise exchange control. I will also~demonstrate how by tuning the experimental parameters one can~ controllably switch to coherent oscillations originating from alternative potentially~useful qubit states and how to distinguish them.\\[4pt] [1] ``Coherent Control of Three Spin States in a Triple Quantum Dot,'' L. Gaudreau et al. Nature Physics 89, 54-58 (2012)\\[0pt] [2] ``Coherent Exchange and Double Beam Splitter Oscillations in a Triple Quantum Dot,'' G.C.Aers et al. PRB 86 (2012) 045316\\[0pt] [3] ``Quantum Interference Between Three Two Spin States in a Double Quantum Dot,'' Studenikin et al. Phys. Rev. Lett. 108 (2012) 22608 [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 2:15PM |
B1.00005: Two single spin qubits with universal control and control of spin entanglement with exchange coupling Invited Speaker: Seigo Tarucha Single qubits and two-qubit gates are building blocks to prepare a universal set of logical operations. We use a micro-magnet technique to implement single spin qubits with individual quantum dots and two-qubit gates with inter-dot exchange coupling. I will talk about our recent experiments of a combined X-gate and exchange control to modulate and detect the degree of spin singlet coupling. The gate fidelity is restricted by X-gate operation time and fidelity. The X-gate is prepared by oscillating an electron inside a quantum dot with microwave (MW) in the presence of a micro-magnet induced field gradient. We have recently raised the MW power and optimized the magnet design to reduce the X-gate time $<$ 3 nsec with improved gate fidelity much shorter than the dephasing time. We have also developed a technique to use the micro-magnet induced inhomogeneous Zeeman field to make faste Z-gate and CPHASE. [Preview Abstract] |
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