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Register a new userOn-chip Integration of Si/SiGe-based Quantum Dots and Switched-capacitor Circuits
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Solid-state qubits integrated on semiconductor substrates currently require at least one wire from every qubit to the control electronics, leading to a so-called wiring bottleneck for scaling. Demultiplexing via on-chip circuitry offers an effective strategy to overcome this bottleneck. In the case of gate-defined quantum dot arrays, specific static voltages need to be applied to many gates simultaneously to realize electron confinement. When a charge-locking structure is placed between the quantum device and the demultiplexer, the voltage can be maintained locally. In this study, we implement a switched-capacitor circuit for charge-locking and use it to float the plunger gate of a single quantum dot. Parallel plate capacitors, transistors and quantum dot devices are monolithically fabricated on a Si/SiGe-based substrate to avoid complex off-chip routing. We experimentally study the effects of the capacitor and transistor size on the voltage accuracy of the floating node. Furthermore, we demonstrate that the electrochemical potential of the quantum dot can follow a 100 Hz pulse signal while the dot is partially floating, which is essential for applying this strategy in qubit experiments.
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We demonstrate a reconfigurable quantum dot gate architecture that incorporates two interchangeable transport channels. One channel is used to form quantum dots and the other is used for charge sensing. The quantum dot transport channel can support either a single or a double quantum dot. We demonstrate few-electron occupation in a single quantum dot and extract charging energies as large as 6.6 meV. Magnetospectroscopy is used to measure valley splittings in the range of 35-70 microeV. By energizing two additional gates we form a few-electron double quantum dot and demonstrate tunable tunnel coupling at the (1,0) to (0,1) interdot charge transition.
Interactions between electrons can strongly affect the shape and functionality of multi-electron quantum dots. The resulting charge distributions can be localized, as in the case of Wigner molecules, with consequences for the energy spectrum and tunneling to states outside the dot. The situation is even more complicated for silicon dots, due to the interplay between valley, orbital, and interaction energy scales. Here, we study two-electron wavefunctions in electrostatically confined quantum dots formed in a SiGe/Si/SiGe quantum well at zero magnetic field, using a combination of tight-binding and full-configuration-interaction (FCI) methods, and taking into account atomic-scale disorder at the quantum well interface. We model dots based on recent qubit experiments, which straddle the boundary between strongly interacting and weakly interacting systems, and display a rich and diverse range of behaviors. Our calculations show that strong electron-electron interactions, induced by weak confinement, can significantly suppress the low-lying, singlet-triplet (ST) excitation energy. However, when the valley-orbit interactions caused by interfacial disorder are weak, the ST splitting can approach its noninteracting value, even when the electron-electron interactions are strong and Wigner-molecule behavior is observed. These results have important implications for the rational design and fabrication of quantum dot qubits with predictable properties.
The valley degree of freedom presents challenges and opportunities for silicon spin qubits. An important consideration for singlet-triplet states is the presence of two distinct triplets, comprised of valley vs. orbital excitations. Here we show that both of these triplets are present in the typical operating regime, but that only the valley-excited triplet offers intrinsic protection against charge noise. We further show that this protection arises naturally in dots with stronger confinement. These results reveal an inherent advantage for silicon-based multi-electron qubits.
We study theoretically the phonon-induced relaxation and decoherence of spin states of two electrons in a lateral double quantum dot in a SiGe/Si/SiGe heterostructure. We consider two types of singlet-triplet spin qubits and calculate their relaxation and decoherence times, in particular as a function of level hybridization, temperature, magnetic field, spin orbit interaction, and detuning between the quantum dots, using Bloch-Redfield theory. We show that the magnetic field gradient, which is usually applied to operate the spin qubit, may reduce the relaxation time by more than an order of magnitude. Using this insight, we identify an optimal regime where the magnetic field gradient does not affect the relaxation time significantly, and we propose regimes of longest decay times. We take into account the effects of one-phonon and two-phonon processes and suggest how our theory can be tested experimentally. The spin lifetimes we find here for Si-based quantum dots are significantly longer than the ones reported for their GaAs counterparts.
Electron spins in silicon quantum dots are promising qubits due to their long coherence times, scalable fabrication, and potential for all-electrical control. However, charge noise in the host semiconductor presents a major obstacle to achieving high-fidelity single- and two-qubit gates in these devices. In this work, we measure the charge-noise spectrum of a Si/SiGe singlet-triplet qubit over more than 13 decades in frequency using a combination of methods, including dynamically-decoupled exchange oscillations with up to 512 $pi$ pulses during the qubit evolution. The charge noise is colored across the entire frequency range of our measurements, although the spectral exponent changes with frequency. Moreover, the charge-noise spectrum inferred from conductance measurements of a proximal sensor quantum dot agrees with that inferred from coherent oscillations of the singlet-triplet qubit, suggesting that simple transport measurements can accurately characterize the charge noise over a wide frequency range in Si/SiGe quantum dots.
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