Abstract Establishing low-error and fast detection methods for qubit readout is crucial for efficient quantum error correction. Here, we test neural networks to classify a collection of single-shot spin detection events, which are the readout signal of our qubit measurements. This readout signal contains a stochastic peak, for which a Bayesian inference filter including Gaussian noise is theoretically optimal. Hence, we benchmark our neural networks trained by various strategies versus this latter algorithm. Training of the network with 10 6 experimentally recorded single-shot readout traces does not improve the post-processing performance. A network trained by synthetically generated measurement traces performs similar in terms of the detection error and the post-processing speed compared to the Bayesian inference filter. This neural network turns out to be more robust to fluctuations in the signal offset, length and delay as well as in the signal-to-noise ratio. Notably, we find an increase of 7% in the visibility of the Rabi oscillation when we employ a network trained by synthetic readout traces combined with measured signal noise of our setup. Our contribution thus represents an example of the beneficial role which software and hardware implementation of neural networks may play in scalable spin qubit processor architectures.
Coherent coupling between distant qubits is needed for many scalable quantum computing schemes. In quantum dot systems, one proposal for long-distance coupling is to coherently transfer electron spins across a chip in a moving dot potential. Here, we use simulations to study challenges for spin shuttling in Si/SiGe heterostructures caused by the valley degree of freedom. We show that for devices with valley splitting dominated by alloy disorder, one can expect to encounter pockets of low valley splitting, given a long-enough shuttling path. At such locations, intervalley tunneling leads to dephasing of the spin wave function, substantially reducing the shuttling fidelity. We show how to mitigate this problem by modifying the heterostructure composition, or by varying the vertical electric field, the shuttling velocity, the shape and size of the dot, or the shuttling path. We further show that combinations of these strategies can reduce the shuttling infidelity by several orders of magnitude, putting shuttling fidelities sufficient for error correction within reach. Published by the American Physical Society 2024
We investigate the lifetime of two-electron spin states in a few-electron Si/SiGe double dot. At the transition between the (1,1) and (0,2) charge occupations, Pauli spin blockade provides a readout mechanism for the spin state. We use the statistics of repeated single-shot measurements to extract the lifetimes of multiple states simultaneously. When the magnetic field is zero, we find that all three triplet states have equal lifetimes, as expected, and this time is ~10 ms. When the field is nonzero, the T(0) lifetime is unchanged, whereas the T- lifetime increases monotonically with the field, reaching 3 sec at 1 T.
The lattice strain induced by metallic electrodes can impair the functionality of advanced quantum devices operating with electron or hole spins. Here we investigate the deformation induced by CMOS-manufactured titanium nitride electrodes on the lattice of a buried, 10 nm-thick Si/SiGe Quantum Well by means of nanobeam Scanning X-ray Diffraction Microscopy. We were able to measure TiN electrode-induced local modulations of the strain tensor components in the range of $2 - 8 \times 10^{-4}$ with ~60 nm lateral resolution. We have evaluated that these strain fluctuations are reflected into local modulations of the potential of the conduction band minimum larger than 2 meV, which is close to the orbital energy of an electrostatic quantum dot. We observe that the sign of the strain modulations at a given depth of the quantum well layer depends on the lateral dimensions of the electrodes. Since our work explores the impact of device geometry on the strain-induced energy landscape, it enables further optimization of the design of scaled CMOS-processed quantum devices.
We introduce ZnSe/(Zn, Mg)Se as a new promising material system for advanced electrical devices. By combination of MBE grown and n-type chlorine doped ZnSe with in-situ deposited Al contacts, we obtained excellent ohmic contacts and very low bulk resistivities. We developed a unique regrowth technique to achieve spatially localized doping, which is required for many unipolar devices. The method is based on selective MBE growth of chlorine doped ZnSe on an in-situ deposited and pre-structured gate dielectric, introducing a versatile platform for device fabrication. We fabricated first test devices and characterized the transport properties for different sample configurations at room- and cryogenic-temperature.
Anisotropic electron spin lifetimes in strained undoped $(\mathrm{In},\mathrm{Ga})\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ (110) quantum wells of different widths and heights are investigated by time-resolved Faraday rotation and time-resolved transmission and are compared to the (001) orientation. From the suppression of spin precession as a function of transverse magnetic fields, the ratio of in-plane to out-of-plane spin lifetimes is calculated. While the ratio increases with In concentration in agreement with theory, an unprecedented high anisotropy of 480 is observed for the broadest quantum well at the low In concentration, when expressed in terms of spin relaxation times.
The development of a fault tolerant quantum computer necessitates an expansion to millions of physical qubits while maintaining high coherence and minimal error rates. Gated Si/SiGe spin qubits are one of the main contenders to achieve this goal, thanks to the compatibility with existing industrial CMOS processes, which heralds vast capacities and elevated quality benchmarks. Elegant architectural approaches towards scalability of qubits include the electron shuttling over long distances, which promise a reduction of peripherals and opens up space for control electronics. The concept of Si/SiGe spin qubits and shuttling structures is based on the z-direction confinement of 2D electron gas in a Si quantum well, which is additionally tailored in the x-y-plane by potentials that arise from multiple gate layers. To trace the viability of these complex devices, it is advisable to first check basic functionality of the gate layers with teststructures, like field effect transistors (FETs). It is particularly relevant to characterize at temperatures comparable to the target operating temperature of Gated Si/SiGe spin qubits. Recently, Fraunhofer IAF has installed a 300 mm wafer prober that can perform device characterization at temperatures as low as 2K. In the related research project, the wafer prober is implemented in a fully industry compatible feedbackloop within heterostructure growth at IHP and device fabrication at Infineon. We present in this paper the viability of this approach by comparing and discussing the current-voltage characteristics and the resulting threshold voltage measured at room temperature and at 2K of 213 FETs on a 200mm wafer. We show the significance of characterizing devices at cryogenic temperatures rather than at room temperature. At 2K, observations were made regarding the presence or absence of off-current, resulting in the clustering of I-V curves and variations in threshold voltage. The origin of these phenomena is scrutinized, examining whether they stem from the cryogenic wafer prober system or are inherent to the FET design. Possible causes are discussed and explored. Funded by the German Federal Ministry of Education and Research (BMBF) within the QUASAR ”Semiconductor quantum processor with shuttling-based scalable architecture” project.
Artificial molecules containing just one or two electrons provide a powerful platform for studies of orbital and spin quantum dynamics in nanoscale devices. A well-known example of these dynamics is tunnelling of electrons between two coupled quantum dots triggered by microwave irradiation. So far, these tunnelling processes have been treated as electric-dipole-allowed spin-conserving events. Here we report that microwaves can also excite tunnelling transitions between states with different spin. We show that the dominant mechanism responsible for violation of spin conservation is the spin-orbit interaction. These transitions make it possible to perform detailed microwave spectroscopy of the molecular spin states of an artificial hydrogen molecule and open up the possibility of realizing full quantum control of a two-spin system through microwave excitation.
