Primary thermometry of a single reservoir using cyclic electron tunneling to a quantum dot

At the nanoscale, local and accurate measurements of temperature are of particular relevance when testing quantum thermodynamical concepts or investigating novel thermal nanoelectronic devices. Here, we present a primary electron thermometer that allows probing the local temperature of a single-electron reservoir in single-electron devices. The thermometer is based on cyclic electron tunneling between a system with discrete energy levels and the reservoir. When driven at a finite rate, close to a charge degeneracy point, the system behaves like a variable capacitor whose full width at half maximum depends linearly with temperature. We demonstrate this type of thermometer using a quantum dot in a silicon nanowire transistor. We drive cyclic electron tunneling by embedding the device in a radio-frequency resonator which in turn allows reading the thermometer dispersively. Overall, the thermometer shows potential for local probing of fast heat dynamics in nanoelectronic devices and for seamless integration with silicon-based quantum circuits

Spin readout of a CMOS quantum dot by gate reflectometry and spin-dependent tunnelling

Silicon spin qubits are promising candidates for realising large scale quantum processors, benefitting from a magnetically quiet host material and the prospects of leveraging the mature silicon device fabrication industry. We report the measurement of an electron spin in a singly-occupied gate-defined quantum dot, fabricated using CMOS compatible processes at the 300 mm wafer scale. For readout, we employ spin-dependent tunneling combined with a low-footprint single-lead quantum dot charge sensor, measured using radiofrequency gate reflectometry. We demonstrate spin readout in two devices using this technique, obtaining valley splittings in the range 0.5-0.7 meV using excited state spectroscopy, and measure a maximum electron spin relaxation time ($T_1$) of $9 \pm 3$ s at 1 Tesla. These long lifetimes indicate the silicon nanowire geometry and fabrication processes employed here show a great deal of promise for qubit devices, while the spin-readout method demonstrated here is well-suited to a variety of scalable architectures

Fast Gate-Based Readout of Silicon Quantum Dots Using Josephson Parametric Amplification

Spins in silicon quantum devices are promising candidates for large-scale quantum computing. Gate-based sensing of spin qubits offers a compact and scalable readout with high fidelity, however, further improvements in sensitivity are required to meet the fidelity thresholds and measurement timescales needed for the implementation of fast feedback in error correction protocols. Here, we combine radio-frequency gate-based sensing at 622 MHz with a Josephson parametric amplifier, that operates in the 500–800 MHz band, to reduce the integration time required to read the state of a silicon double quantum dot formed in a nanowire transistor. Based on our achieved signal-to-noise ratio, we estimate that singlet-triplet single-shot readout with an average fidelity of 99.7% could be performed in 1     μ s , well below the requirements for fault-tolerant readout and 30 times faster than without the Josephson parametric amplifier. Additionally, the Josephson parametric amplifier allows operation at a lower radio-frequency power while maintaining identical signal-to-noise ratio. We determine a noise temperature of 200 mK with a contribution from the Josephson parametric amplifier (25%), cryogenic amplifier (25%) and the resonator (50%), showing routes to further increase the readout speed

Radio frequency measurements of tunnel couplings and singlet–triplet spin states in Si:P quantum dots

Spin states of the electrons and nuclei of phosphorus donors in silicon are strong candidates for quantum information processing applications given their excellent coherence times. Designing a scalable donor-based quantum computer will require both knowledge of the relationship between device geometry and electron tunnel couplings, and a spin readout strategy that uses minimal physical space in the device. Here we use radio frequency reflectometry to measure singlet–triplet states of a few-donor Si:P double quantum dot and demonstrate that the exchange energy can be tuned by at least two orders of magnitude, from 20 μeV to 8 meV. We measure dot–lead tunnel rates by analysis of the reflected signal and show that they change from 100 MHz to 22 GHz as the number of electrons on a quantum dot is increased from 1 to 4. These techniques present an approach for characterizing, operating and engineering scalable qubit devices based on donors in silicon

Compilation and scaling strategies for a silicon quantum processor with sparse two-dimensional connectivity

Funder: UKRI Future Leaders Fellowship (grant number MR/V023284/1)AbstractInspired by the challenge of scaling-up existing silicon quantum hardware, we propose a 2d spin-qubit architecture with low compilation overhead. The architecture is based on silicon nanowire split-gate transistors which form 1d chains of spin-qubits and allow the execution of two-qubit operations among neighbors. We introduce a silicon junction which can couple four nanowires into 2d arrangements via spin shuttling andSwapoperations. We then propose a modular sparse 2d spin-qubit architecture with unit cells of diagonally-oriented squares with nanowires along the edges and junctions on the corners. Targeting noisy intermediate-scale quantum (NISQ) demonstrators, we show that the proposed architecture allows for compilation strategies which outperform methods for 1d chains, and exhibits favorable scaling properties which enable trading-off compilation overhead and colocation of control electronics within each square by adjusting the nanowire length. An appealing feature of the proposed architecture is its manufacturability using complementary-metal-oxide-semiconductor (CMOS) fabrication processes.</jats:p

Beyond-adiabatic Quantum Admittance of a Semiconductor Quantum Dot at High Frequencies: Rethinking Reflectometry as Polaron Dynamics

Semiconductor quantum dots operated dynamically are the basis of many quantum technologies such as quantum sensors and computers. Hence, modelling their electrical properties at microwave frequencies becomes essential to simulate their performance in larger electronic circuits. Here, we develop a self-consistent quantum master equation formalism to obtain the admittance of a quantum dot tunnel-coupled to a charge reservoir under the effect of a coherent photon bath. We find a general expression for the admittance that captures the well-known semiclassical (thermal) limit, along with the transition to lifetime and power broadening regimes due to the increased coupling to the reservoir and amplitude of the photonic drive, respectively. Furthermore, we describe two new photon-mediated regimes: Floquet broadening, determined by the dressing of the QD states, and broadening determined by photon loss in the system. Our results provide a method to simulate the high-frequency behaviour of QDs in a wide range of limits, describe past experiments, and propose novel explorations of QD-photon interactions.Winton Programme for the Physics of Sustainability at Cambridge Universit

Engineering the Photoresponse of InAs Nanowires

We report on individual-InAs nanowire optoelectronic devices which can be tailored to exhibit either negative or positive photoconductivity (NPC or PPC). The NPC photoresponse time and magnitude is found to be highly tunable by varying the nanowire diameter under controlled growth conditions. Using hysteresis characterization, we decouple the observed photoexcitation-induced hot electron trapping from conventional electric field-induced trapping to gain a fundamental insight into the interface trap states responsible for NPC. Furthermore, we demonstrate surface passivation without chemical etching which both enhances the field-effect mobility of the nanowires by approximately an order of magnitude and effectively eliminates the hot carrier trapping found to be responsible for NPC, thus restoring an "intrinsic" positive photoresponse. This opens pathways toward engineering semiconductor nanowires for novel optical-memory and photodetector applications

Research data supporting "Engineering the Photoresponse of InAs Nanowires"

Raw data from the figures in "Engineering the Photoresponse of InAs Nanowires". InAs nanowires were grown by metal–organic chemical vapor deposition (MOCVD). The growth conditions were chosen to grow wurtzite crystal structures with minimal stacking faults, minimal tapering, and hexagonal cross sections with {11̅00} side facets, as confirmed by transmission electron microscopy (TEM). The nanowire diameter was tightly controlled by selecting the diameter of the Au catalyst. Nanowire diameters, inclusive of surface oxide, of 30 ± 5 nm, 40 ± 5 nm, 65 ± 5 nm, and 110 ± 5 nm, respectively, were obtained, as confirmed by SEM. For TEM measurements, nanowires were mechanically transferred to a holey carbon grid. TEM was performed using a JEOL 2100F instrument operated at 200 keV. The nanowires were transferred to a doped Si wafer with 300 nm of thermally grown SiO2 which served as a global back gate. Contacts with a separation of 1 μm were patterned using e-beam lithography and sputter deposition of 70 nm Ni, followed by lift-off. To obtain low contact resistivity, prior to Ni deposition, the contact region of the nanowire was etched in 2% aqueous (NH4)2S solution at 40 °C for 10 min. Electrical measurements were carried out using a probe station connected to a Keithley 4200-SCS semiconductor characterization system. Illumination of the samples to measure the photoresponse was applied using a 3200 K halogen lamp with a power density of 30 mW cm–2. All measurements were carried out at room temperature under ambient conditions unless otherwise specified. For ALD passivation, after device fabrication, a 90 nm capping layer of Al2O3 was deposited using a Cambridge NanoTech ALD system at 120 °C using trimethylaluminum and H2O precursors. The probe pads were then exposed by etching through the Al2O3 with phosphoric acid

Non-symmetric Pauli spin blockade in a silicon double quantum dot

Acknowledgements: We thank M. Benito and C. Lainé for valuable discussions. This research has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under grant agreements No 688539 and 951852. T.L. acknowledges support from the EPSRC Cambridge NanoDTC, EP/L015978/1. D.J.I. is supported by the Bristol Quantum Engineering Centre for Doctoral Training, EPSRC Grant No. EP/L015730/1. M.F.G.-Z. acknowledges support from UKRI Future Leaders Fellowship [grant number MR/V023284/1]. C.L. and J.W.A.R. acknowledge the EPSRC through the Core-to-Core International Network “Oxide Superspin” (EP/P026311/1) and the “Superconducting Spintronics” Programme Grant (EP/N017242/1).Funder: UKRI Future Leader Fellowship MR/V023284/1AbstractSpin qubits in gate-defined silicon quantum dots are receiving increased attention thanks to their potential for large-scale quantum computing. Readout of such spin qubits is done most accurately and scalably via Pauli spin blockade (PSB), however, various mechanisms may lift PSB and complicate readout. In this work, we present an experimental study of PSB in a multi-electron low-symmetry double quantum dot (DQD) in silicon nanowires. We report on the observation of non-symmetric PSB, manifesting as blockaded tunneling when the spin is projected to one QD of the pair but as allowed tunneling when the projection is done into the other. By analyzing the interaction of the DQD with a readout resonator, we find that PSB lifting is caused by a large coupling between the different electron spin manifolds of 7.90 μeV and that tunneling is incoherent. Further, magnetospectroscopy of the DQD in 16 charge configurations, enables reconstructing the energy spectrum of the DQD and reveals the lifting mechanism is energy-level selective. Our results indicate enhanced spin-orbit coupling which may enable all-electrical qubit control of electron spins in silicon nanowires.</jats:p