Session S29: Designing High Fidelity Gates for Spin Qubits

Live

The prospect of building quantum circuits using advanced semiconductor manufacturing positions quantum dots as an attractive platform for quantum information processing. Extensive studies on various materials have led to demonstrations of two-qubit logic in gallium arsenide, silicon, and germanium. However, interconnecting larger numbers of qubits in semiconductor devices has remained an outstanding challenge. Here, I will present our group efforts based on silicon and germanium quantum dots and wills show the realization of a two-dimensional four-qubit quantum processor based on hole spins in germanium quantum dots. Qubit logic is implemented all-electrically and the exchange interaction can be pulsed to freely program one-qubit, two-qubit, three-qubit, and four-qubit operations, resulting in a compact and high-connectivity circuit. A quantum logic circuit that generates a four-qubit Greenberger-Horne-Zeilinger state is executed and coherent evolution is obtained by incorporating dynamical decoupling. I will furthermore discuss opportunities and challenges for quantum dot qubits, such as operation at comparatively high temperatures as a step toward integrated quantum circuits.   

Live

Spin qubits in silicon quantum dots are a promising platform for quantum computation due to long decoherence times and fast operations. Demonstrations of single qubit gates show fidelities up to 99.9% [1] and two-qubit gates show fidelities of 92-98% [1,2]. These two-qubit gate implementations are not robust against low-frequency charge noise, which couples in via the exchange interaction. Furthermore, the limited bandwidth of the signals, diabatic errors, and microwave-crosstalk are additional error sources which have to be considered.

We show that advanced pulse shaping techniques commonly used for superconducting qubits [3] can be used to significantly improve the fidelities and the quality factor of gate operations. Furthermore, we propose a simple yet effective scheme for a decoupled version of a driven exchange gate [4] that is resilient against low-frequency charge noise. We use analytic calculations and numerical simulations under realistic conditions to verify gate fidelities greater than 99.9%, which allow for fault-tolerant gates.

[1] Huang et al., Nature (London) 569, 532 (2019)
[2] Xu et al., Phys. Rev. X 9, 021011 (2019)
[3] Motzoi et al., Phys. Rev. Lett. 103, 110501 (2009)
[4] Sigillito et al., npj Quantum Information 5 (1), 1 (2019)   

Live

The ability to transport electrons across large distances will improve greatly the scalability of quantum computing systems by paving the way for fault-tolerant architectures that has a lower required number of physical qubits while allowing for long distance interactions between qubits. To this end, we demonstrated the ability to transport a single spin qubit within a double quantum dot in silicon, reporting a 99.97% polarisation transfer fidelity and a 99.4% average coherent transfer fidelity. This experimental feat also opens up questions from a theoretical perspective on the sources of error and what it could mean for further work.
In this talk, I will highlight these achievements and discuss the theoretical aspects of coherent spin transport in the context of these experimental results. We believe that these results are instrumental in demonstrating the scalability of silicon-based spin qubits.   

Live

Solid state spin qubits are promising candidates for the realization of a quantum computer due to their long coherence times and easy electrical manipulation. However, spin-spin interactions, which are needed for entangling gates, have only limited range as they generally rely on tunneling between neighboring quantum dots. This severely constrains scalability. Here, we study a setup where an extension of the tunneling range is obtained by using a superconductor as a quantum mediator. We analyze the impact of spin-orbit (SO) coupling, external magnetic fields, and the geometry of the superconductor. We show that while SO scattering in the superconducting bulk and the addition of an external magnetic field decrease the strength of the exchange interaction, the geometry of the superconducting link offers a lot of room to optimize the interaction range, with gains of over an order of magnitude from a 2D film to a quasi-1D strip. We estimate that for superconductors with weak SO coupling (e.g., aluminum) exchange rates of up to 100 MHz over a micron-scale range can be achieved with this setup in the presence of magnetic fields of the order of 100 mT.   

Live

Spin qubits are one of the promising candidates to build a large-scale solid-state quantum computer. These qubits are given by the spin state of a single electron confined in a quantum dot. Spin qubits are attractive because they can be operated with relatively high fidelity [1,2] and are compatible with the current CMOS industry standards [3]. In this presentation we will show the first silicon qubit system that achieves universal control beyond 2 qubits [4,5]. We will also discuss the challenges of operating a multi-qubit system, including crosstalk phenomena when driving multiple qubits at the same time, novel ways of entangling qubits and the engineering efforts involved.

[1] Yoneda, Jun, et al., Nature nanotechnology (2018)
[2] Huang, W., et al. , Nature (2019)
[3] Vandersypen, L. M. K., et al., NPJ Quantum Information (2017)
[4] Watson, T. F., et al., Nature (2018)
[5] Zajac, D. M., et al., Science (2018)   

Live

The operation of spin-based quantum processors requires the ability to perform simultaneous and fast measurements in qubit arrays, while overcoming challenges like mitigating crosstalk, tuning in large parameter spaces, and calibrating gate-voltage pulses.
We present the simultaneous coherent manipulation and readout of a two-by-two array of singlet-triplet qubits in GaAs in a geometry where all four qubits can in principle be coupled to a multielectron dot located at the center of the array. Using frequency-multiplexed radio-frequency reflectometry to read out four independent charge sensors, each capacitively coupled to one qubit, we monitor via simultaneous single-shot measurements the fluctuating Overhauser gradients within all four qubits and further induce simultaneous coherent exchange rotations within all qubits. Finally, we demonstrate coherent spin-exchange rotations between one qubit and the multielectron dot, suggesting its use as a coherent coupler that may provide on-demand “any-to-any” qubit-qubit connectivities within the array.   

Live

Spin qubits are a promising architecture for quantum computers thanks to their long coherence time and compatibility with industrial fabrication techniques. However, qubits characterization and initialization in a desired configuration is a time-consuming process. Such challenges are slowing down progress in this field of research, especially when studying multi-qubit systems. To address those problems, a fast and modular measurement platform for spin qubits, using Field Programmable Gate Arrays (FPGAs), is presented. We also introduce a “Park and Fly” technique, implemented on this platform, to measure the stability diagram of quantum dots. By doing so, it is possible to achieve high voltage amplitudes with high resolution by combining DC and AC signals. A “Park” DC signal is set to measure a specific part of a stability diagram while a “Fly” AC signal is allowing high resolution sweeping in the region around the “Park” signal. As a proof of concept, this technique is tested on a quantum dot simulator implemented directly inside an FPGA. By combining this approach with the modularity of the platform, efficient and simultaneous characterization of multiple qubit devices is feasible.   

Live

Integrated on-chip control logic is an enabling technique on the path towards high-throughput qubit characterization and large-scale quantum processors in the solid state. We present the fabrication and measurement of a multiplexed array that addresses sixteen Si/SiGe quantum devices. On-chip field-effect transistors allow an array of Hall bars fabricated using electron-beam lithography to be measured with a tenfold reduction in electrical interconnects. We report transport measurements of the Hall bars at 2 Kelvin and discuss variations in threshold voltage, electron mobility, and percolation threshold density across chip.   

Live

Spin qubits in gate-defined quantum dots in silicon are one of the most promising platforms for large-scale quantum computers. To implement deep-circuit quantum algorithms such as quantum error correction, long coherence times and fast operations are needed. In electric dipole spin resonance (EDSR), where electric signals are converted to magnetic signals, the anharmonicity of confinement potential becomes crucial in strong driving regime [1]. Utilizing two spin qubits in a Si/SiGe heterostructure, we have found a crosstalk effect which behaves as an amplitude modulation and a frequency modulation in the Rabi oscillation experiments. This effect is observed routinely but not well understood [2, 3]. Supported by a theoretical model, we provide evidence that the crosstalk is induced by the nonlinear confinement when both qubits are being strongly driven simultaneously. Furthermore, we study the impact of crosstalk on the control fidelities andcrosstalk mitigation by Hamiltonian engineering.
[1] P. Scarlino, et al. PRL, 115, 106802 (2015).
[2] T. F. Watson, et al. Nature, 555, 633–637 (2018).
[3] K. Takeda, et al. npj Quantum Information 4(1), 1-6 (2018).   

Live

In this work we discuss the device advantages and challenges of pitch scaling for spin quantum array designs for Si fin based and buried Si/ SiGe channel based technologies. We show with modeling and data how the scaling of qubit parameters important for quantum computing depends on the interplay between device electrostatics, disorder, and quantum confinement. We discuss a need for a virtual compensation in scaled designs, and propose schemes for its implementation. We will also present implications of scaling on a two-qubit-gate fidelity. We discuss that considered template designs and dimensions are achievable in the environment of high volume semiconductor manufacturing.   

Live

Recent experiments on semiconductor quantum dots have demonstrated the ability to utilize a large quantum dot to mediate superexchange interactions and generate entanglement between distant spins. This opens up a possible mechanism for selectively coupling pairs of remote spins in a larger network of quantum dots. We describe our theoretical efforts to understand the controllability of superexchange interactions in these systems. We focus on a triple-dot system arranged in linear and triangular geometries and use configuration interaction calculations to investigate the interplay of superexchange and nearest-neighbor exchange interactions. We show that superexchange processes strongly enhance the range of the net spin-spin coupling as the dots approach a linear configuration. Furthermore, we show that the strength of the exchange interaction depends sensitively on the number of electrons in the mediator. Our results can be used as a guide to assist further experimental efforts towards scaling up to larger, two-dimensional quantum dot arrays.   

Live

Electron spin qubits in silicon quantum dots hold promise for scalable quantum information processing. Recently, micromagnets have been employed to achieve high-fidelity spin manipulation and strong spin-photon coupling, which paves the way for the future scale-up of spin qubits. However, experiments on spin relaxation in silicon with micromagnets show a magnetic field dependence clearly different from previous theoretical and experimental results without micromagnets. We reveal that a synthetic spin-orbit field from micromagnets leads to such signatures in spin relaxation, where the interference effect plays a critical role. Furthermore, we show an enhancement of the electric dipole spin resonance and spin-photon coupling in a synthetic spin-orbit field as a result of the interference effect and the strong mixing at the spin-valley hotspot. The results enable novel schemes for fast quantum gates with potential applications in semiconducting quantum computing.   

Live

Alternative approaches to gate based double-quantum dot spin-qubit quantum operations by measurements has been investigated by combining exchange-interaction with fast qubit measurements. These approaches are evaluated with respect their implications for spin-based quantum computing by their gate speed and fidelity, as well as robustness to spin-state leakage, and applications to calibration. Dependance on of the number of physical qubits and compatibility with stabiliser codes are also considered.