A new type of carbon resistance thermometer with excellent thermal contact at millikelvin temperatures

Using a new brand of commercially available carbon resistor we built a cryogenic thermometer with an extremely good thermal contact to its thermal environment. Because of its superior thermal contact the thermometer is insensitive to low levels of spurious radio frequency heating. We calibrated our thermometer down to 5mK using a quartz tuning fork He-3 viscometer and measured its thermal resistance and thermal response time.Comment: 5 pages, 4 figure

Low disorder and high valley splitting in silicon

The electrical characterisation of classical and quantum devices is a critical step in the development cycle of heterogeneous material stacks for semiconductor spin qubits. In the case of silicon, properties such as disorder and energy separation of conduction band valleys are commonly investigated individually upon modifications in selected parameters of the material stack. However, this reductionist approach fails to consider the interdependence between different structural and electronic properties at the danger of optimising one metric at the expense of the others. Here, we achieve a significant improvement in both disorder and valley splitting by taking a co-design approach to the material stack. We demonstrate isotopically-purified, strained quantum wells with high mobility of 3.14(8)$\times$10$^5$ cm$^2$/Vs and low percolation density of 6.9(1)$\times$10$^{10}$ cm$^{-2}$. These low disorder quantum wells support quantum dots with low charge noise of 0.9(3) $\mu$eV/Hz$^{1/2}$ and large mean valley splitting energy of 0.24(7) meV, measured in qubit devices. By striking the delicate balance between disorder, charge noise, and valley splitting, these findings provide a benchmark for silicon as a host semiconductor for quantum dot qubits. We foresee the application of these heterostructures in larger, high-performance quantum processors

CMOS-based cryogenic control of silicon quantum circuits

The most promising quantum algorithms require quantum processors hosting millions of quantum bits when targeting practical applications. A major challenge towards large-scale quantum computation is the interconnect complexity. In current solid-state qubit implementations, a major bottleneck appears between the quantum chip in a dilution refrigerator and the room temperature electronics. Advanced lithography supports the fabrication of both CMOS control electronics and qubits in silicon. When the electronics are designed to operate at cryogenic temperatures, it can ultimately be integrated with the qubits on the same die or package, overcoming the wiring bottleneck. Here we report a cryogenic CMOS control chip operating at 3K, which outputs tailored microwave bursts to drive silicon quantum bits cooled to 20mK. We first benchmark the control chip and find electrical performance consistent with 99.99% fidelity qubit operations, assuming ideal qubits. Next, we use it to coherently control actual silicon spin qubits and find that the cryogenic control chip achieves the same fidelity as commercial instruments. Furthermore, we highlight the extensive capabilities of the control chip by programming a number of benchmarking protocols as well as the Deutsch-Josza algorithm on a two-qubit quantum processor. These results open up the path towards a fully integrated, scalable silicon-based quantum computer

Ultra-low temperature studies of the even denominator fractional quantum Hall states

We have constructed a specialized experimental setup with integrated magnetic field independent thermometry, which has enabled us to cool the charge carriers in two dimensional electron gases down to 5 mK, and reliably measure the temperature. Using this setup, we have conducted studies of ν=5/2 fractional quantum Hall state(FQHS) in so far unexplored regions of the parameter space. Using a sample with a tunable density, we observe, for the first time, an evidence of a transition at ν=5/2 filling factor. This transition takes place at the lowest density at which ν=5/2 state had been measured to date, around 6x1010cm-2. Using a different set of samples, we also demonstrate a consistent way to account for the disorder contribution to the energy gap of ν=5/2 FQHS for several samples of vastly different densities. This lets us quantify, for the first time, the dependence of the experimentally measured intrinsic gap at ν=5/2 on Landau level mixing alone. Finally, we have conducted an ultra-low temperature study of the fractional quantum Hall states in the 1/3\u3cν\u3c2/5 region. Due to the residual interaction of composite fermions, this region is thought to support next generation FQHS. Similarly to an earlier report, at the relatively high temperature of 51mK, the magnetoresistance exhibits developing FQHS at ν=4/11; 5/13, 6/17 and 3/8. However, we find that at lower temperatures only the ν=4/11 and 5/13 develop incompressibility, while the ν=6/17 and 3/8 remain compressible

A Si/SiGe based quantum dot with floating gates for scalability

Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.QCD/Vandersypen LabQN/Veldhorst LabQID/Ishihara LabQuantum Circuit Architectures and TechnologyQuTechQN/Vandersypen La

Nonlinear Response and Crosstalk of Electrically Driven Silicon Spin Qubits

Micromagnet-based electric dipole spin resonance offers an attractive path for the near-term scaling of dense arrays of silicon spin qubits in gate-defined quantum dots while maintaining long coherence times and high control fidelities. However, accurately controlling dense arrays of qubits using a multiplexed drive will require an understanding of the cross-talk mechanisms that may reduce operational fidelity. We identify an unexpected cross-talk mechanism whereby the Rabi frequency of a driven qubit is drastically changed when the drive of an adjacent qubit is turned on. These observations raise important considerations for scaling single-qubit control. </p

Nonlinear response and crosstalk of strongly driven silicon spin qubits

Micromagnet-based electric dipole spin resonance (EDSR) offers an attractive path for the near-term scaling of dense arrays of silicon spin qubits in gate-defined quantum dots while maintaining long coherence times and high control fidelities. However, accurately controlling dense arrays of qubits using a multiplexed drive will require an understanding of the crosstalk mechanisms that may reduce operational fidelity. We identify a novel crosstalk mechanism whereby the Rabi frequency of a driven qubit is drastically changed when the drive of an adjacent qubit is turned on. These observations raise important considerations for scaling single-qubit control.Comment: 12 pages, 9 figure

Quantum logic with spin qubits crossing the surface code threshold

High-fidelity control of quantum bits is paramount for the reliable execution of quantum algorithms and for achieving fault tolerance—the ability to correct errors faster than they occur1. The central requirement for fault tolerance is expressed in terms of an error threshold. Whereas the actual threshold depends on many details, a common target is the approximately 1% error threshold of the well-known surface code2,3. Reaching two-qubit gate fidelities above 99% has been a long-standing major goal for semiconductor spin qubits. These qubits are promising for scaling, as they can leverage advanced semiconductor technology4. Here we report a spin-based quantum processor in silicon with single-qubit and two-qubit gate fidelities, all of which are above 99.5%, extracted from gate-set tomography. The average single-qubit gate fidelities remain above 99% when including crosstalk and idling errors on the neighbouring qubit. Using this high-fidelity gate set, we execute the demanding task of calculating molecular ground-state energies using a variational quantum eigensolver algorithm5. Having surpassed the 99% barrier for the two-qubit gate fidelity, semiconductor qubits are well positioned on the path to fault tolerance and to possible applications in the era of noisy intermediate-scale quantum devices.QCD/Vandersypen LabQuTechBUS/TNO STAFFQCD/Scappucci LabQN/Vandersypen La