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Silicon Nanoelectronics Workshop (SNW)

Controlling single qubit gate fidelity in double quantum dots

J. Mosakowski, T. Ferrus, D. A. Williams, E. T. Owen, M. C. Dean, C. H. W. Barnes

Published Jun 1, 2014• 0 citations
Publisher

Abstract

Quantum computation offers ways of solving problems which are unreachable for classical computers [1]. Double quantum dots are of great interest as candidates for implementing solid state quantum bits (qubits) because of the long electron coherence times. The use of geometrically isolated structures (with the detectors coupled capacitively) improves the electrical isolation of the dots, however, it makes the detection more challenging [2].

DOI
10.1109/snw.2014.7348583

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Quantum dots are microscopic structures that can trap single electrons for study and manipulation. Such devices are of interest as potential logic elements in a future quantum computer in which electron spins are used as quantum bits. The problem is that the spins on the nuclei of the device material itself can interfere with the electron spins. In his Perspective, [DiVincenzo][1] discusses results reported in the same issue by [ Petta et al. ][2] in which a double quantum dot structure was used to avoid the effect of the background spins. By swapping the quantum spin states back and forth between the dots, the researchers were able to greatly increase the quantum coherence time of the stored states. The results show that fully electric (as opposed to magnetic) control of the spin states offers the possibility of using such double dots in a quantum computer. [1]: http://www.sciencemag.org/cgi/content/full/309/5744/2173 [2]: http://www.sciencemag.org/cgi/content/short/309/5744/2180
Hole spin qubits in semiconductor quantum dots (QDs) are promising candidates for quantum information processing due to their weak hyperfine coupling to nuclear spins, and to the strong spin-orbit coupling, which allows for rapid operation time. We propose a coherent control on two heavy-hole spin qubits in a double QD by a fast adiabatic driving protocol, which helps to achieve higher fidelities than other experimentally commonly used protocols as linear ramping, $\ensuremath{\pi}$ pulses or Landau-Zener passages. Using fast quasiadiabatic driving via spin-orbit coupling, it is possible to reduce charge noise significantly for qubit manipulation and achieve high robustness for the qubit initialization. We also implement one- and two-qubit gates, in particular, not, cnot, and swaplike gates, of hole spins in a double QD achieving fidelities above $99\mathrm{%}$, exhibiting the capability of hole spins to implement universal gates for quantum computing.
We study the two-qubit controlled-NOT gate operating on qubits encoded in the spin state of a pair of electrons in a double quantum dot. We assume that the electrons can tunnel between the two quantum dots encoding a single qubit, while tunneling between the quantum dots that belong to different qubits is forbidden. Therefore, the two qubits interact exclusively through the direct Coulomb repulsion of the electrons. We find that entangling two-qubit gates can be performed by the electrical biasing of quantum dots and/or tuning of the tunneling matrix elements between the quantum dots within the qubits. The entangling interaction can be controlled by tuning the bias through the resonance between the singly occupied and doubly occupied singlet ground states of a double quantum dot.
Reliable quantum information processing requires high-fidelity universal manipulation of quantum systems within the characteristic coherence times. Non-adiabatic holonomic quantum computation offers a promising approach to implement fast, universal, and robust quantum logic gates particularly useful in nano-fabricated solid-state architectures, which typically have short coherence times. Here, we propose an experimentally feasible scheme to realize high-speed universal geometric quantum gates in nano-engineered pseudo-spin charge qubits. We use a system of three coupled quantum dots containing a single electron, where two computational states of a double quantum dot charge qubit interact through an intermediate quantum dot. The additional degree of freedom introduced into the qubit makes it possible to create a geometric model system, which allows robust and efficient single-qubit rotations through careful control of the inter-dot tunneling parameters. We demonstrate that a capacitive coupling between two charge qubits permits a family of non-adiabatic holonomic controlled two-qubit entangling gates, and thus provides a promising procedure to maintain entanglement in charge qubits and a pathway toward fault-tolerant universal quantum computation. We estimate the feasibility of the proposed structure by analyzing the gate fidelities to some extent.
Spins in semiconductor quantum dots are a candidate for cryogenic quantum processors due to their exceptionally long coherence times. One major challenge to scaling quantum dot spin qubits is the dense wiring requirements, making it difficult to envision fabricating large arrays of nearest-neighbor-coupled qubits necessary for error correction. We describe a method to solve this problem by spacing the qubits out using high-impedance superconducting resonators with a 2D grid unit cell area of 0.16 mm20.16~\text{mm}^20.16 mm2 using 3D integration. To prove the viability of this approach, we demonstrate 3D integration of a high-impedance TiN resonator coupled to a double quantum dot in a Si/SiGe heterostructure. Using the resonator as a dispersive gate sensor, we tune the device down to the single electron regime with an SNR = 5.36 limited by the resonator-dot capacitance. Characterization of the dot and resonator systems shows such integration can be done while maintaining low charge noise metrics for the quantum dots and with improved loaded quality factors for the superconducting resonator (QL=2.14×104Q_L = 2.14 \times 10^4QL​=2.14×104), allowing for high-sensitivity charge detection and the potential for high fidelity 2-qubit gates. This work paves the way for 2D quantum dot qubit arrays with cavity mediated interactions.