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On-chip thermometry at deep-cryogenic temperatures is vital in quantum computing applications to accurately quantify the effect of increased temperature on qubit performance. In this work, we present a sub-1 K temperature sensor in CMOS technology based on the temperature dependence of the critical current of a superconducting (SC) thin-film. The sensor is implemented in 22-nm fully depleted silicon on insulator (FDSOI) technology and comprises a 6 nA resolution current-output digital-to-analog converter (DAC), a transimpedance amplifier (TIA) with a SC thin-film as a gain element, and a voltage comparator. The circuit dissipates 1.5 uW and is demonstrated operating at ambient temperatures as low as 15 mK, providing a variable temperature resolution reaching sub-10 mK., Comment: This work has been submitted to the IEEE for possible publication

We explore the effects of stress on silicon donor bound exciton ($\mathrm{D^0X}$) transitions in bulk silicon and in microfabricated silicon devices. We first study $\mathrm{D^0X}$ transitions in an isotopically purified silicon-28 bulk doped sample under controlled uniaxial stress, confirming the validity of existing models in the low strain ($\lesssim 10^{-5}$) regime. We then demonstrate the localised photoconductive detection of a few thousand donors illuminated by a 1078 nm resonant laser with $4~\mathrm{\mu m}$ spot focused on a microfabricated device consisting of an implanted phosphorus layer between a pair of metallic contacts. We observe local variations in the strained exciton peak splitting from $10~\mathrm{\mu eV}$ to $200~\mathrm{\mu eV}$, and obtain scanning microscopy stress maps in good agreement with finite-element-model thermal stress simulations. Our results suggest a potential use of donor bound excitons for in-situ stress sensing, and demonstrate pathways for the miniaturisation of $\mathrm{D^0X}$ photoconductive detection., Comment: 8 pages, 4 figures

Leveraging the advanced manufacturing capabilities of the semiconductor industry promises to help scale up silicon-based quantum processors by increasing yield, uniformity and integration. Recent studies of quantum dots fabricated on 300 mm wafer metal-oxide-semiconductor (MOS) processes have shown control and readout of individual spin qubits, yet quantum processors require two-qubit interactions to operate. Here, we use a 300 mm wafer MOS process customized for spin qubits and demonstrate coherent control of two electron spins using the spin-spin exchange interaction, forming the basis of an entangling gate such as $\sqrt{\text{SWAP}}$. We observe gate dephasing times of up to $T_2^{*}\approx500$ ns and a gate quality factor of 10. We further extend the coherence by up to an order of magnitude using an echo sequence. For readout, we introduce a dispersive readout technique, the radiofrequency electron cascade, that amplifies the signal while retaining the spin-projective nature of dispersive measurements. Our results demonstrate an industrial grade platform for two-qubit operations, alongside integration with dispersive sensing techniques.

Quantum processing units will be modules of larger information processing systems containing also digital and analog electronics modules. Silicon-based quantum computing offers the enticing opportunity to manufacture all the modules using the same technology platform. Here, we present a cryogenic multi-module assembly for multiplexed readout of silicon quantum devices where all modules have been fabricated using the same fully-depleted silicon-on-insulator (FDSOI) CMOS process. The assembly is constituted by three chiplets: (i) a low-noise amplifier (LNA), (ii) a single-pole eight-throw switch (SP8T), and (iii) a silicon quantum dot (QD) array. We integrate the chiplets into modules and show respectively, (i) a peak gain over 35dB with a 3dB bandwidth from 709MHz to 827MHz and an average noise temperature of 6.2K (minimum 4.2K), (ii) an insertion loss smaller than 1.1dB and a noise temperature less than 1.1K over the 0-2GHz range, and (iii) single-electron box (SEB) charge sensors. Finally, we combine all modules into a single demonstration showing time-domain radio-frequency multiplexing of two SEBs paving the way to an all-silicon quantum computing system., Comment: This work has been submitted to the IEEE for possible publication

We study the photo-physical stability of ensemble near-surface nitrogen vacancy (NV) centers in diamond under vacuum and air. The optically detected magnetic resonance contrast of the NV centers was measured following exposure to laser illumination, showing opposing trends in air compared to vacuum (increasing by up to 9% and dropping by up to 25%, respectively). Characterization using Raman and X-ray photoelectron spectroscopies suggests a surface reconstruction: In air, atmospheric oxygen adsorption on surface leads to an increase in NV- fraction, whereas in vacuum, net oxygen desorption increases the NV0 fraction. NV charge state switching is confirmed by photoluminescence spectroscopy. Deposition of ~ 2 nm alumina (Al2O3) over the diamond surface was shown to stabilize the NV charge state under illumination in either environment, attributed to a more stable surface electronegativity. The use of alumina coating on diamond is therefore a promising approach to improve the resilience of NV sensors.

Silicon technology offers the enticing opportunity for monolithic integration of quantum and classical electronic circuits. However, the power consumption levels of classical electronics may compromise the local chip temperature and hence the fidelity of qubit operations. Here, we utilize a quantum-dot-based thermometer embedded in an industry-standard silicon field-effect transistor (FET), to assess the local temperature increase produced by an active FET placed in close proximity. We study the impact of both static and dynamic operation regimes. When the FET is operated statically, we find a power budget of 45 nW at 100 nm separation whereas at 216 $\mu$m the power budget raises to 150 $\mu$W. When operated dynamically, we observe negligible temperature increase for the switch frequencies tested up to 10 MHz. Our work describes a method to accurately map out the available power budget at a distance from a solid-state quantum processor and indicate under which conditions cryoelectronics circuits may allow the operation of hybrid quantum-classical systems.

Accurate on-chip temperature sensing is critical for the optimal performance of modern CMOS integrated circuits (ICs), to understand and monitor localized heating around the chip during operation. The development of quantum computers has stimulated much interest in ICs operating a deep cryogenic temperatures (typically 0.01 - 4 K), in which the reduced thermal conductivity of silicon and silicon oxide, and the limited cooling power budgets make local on-chip temperature sensing even more important. Here, we report four different methods for on-chip temperature measurements native to complementary metal-oxide-semiconductor (CMOS) industrial fabrication processes. These include secondary and primary thermometry methods and cover conventional thermometry structures used at room temperature as well as methods exploiting phenomena which emerge at cryogenic temperatures, such as superconductivity and Coulomb blockade. We benchmark the sensitivity of the methods as a function of temperature and use them to measure local excess temperature produced by on-chip heating elements. Our results demonstrate thermometry methods that may be readily integrated in CMOS chips with operation from the milliKelivin range to room temperature.

Quantum computing requires a universal set of gate operations; regarding gates as rotations, any rotation angle must be possible. However a real device may only be capable of $B$ bits of resolution, i.e. it might support only $2^B$ possible variants of a given physical gate. Naive discretization of an algorithm's gates to the nearest available options causes coherent errors, while decomposing an impermissible gate into several allowed operations increases circuit depth. Conversely, demanding higher $B$ can greatly complexify hardware. Here we explore an alternative: Probabilistic Angle Interpolation (PAI). This effectively implements any desired, continuously parametrised rotation by randomly choosing one of three discretised gate settings and postprocessing individual circuit outputs. The approach is particularly relevant for near-term applications where one would in any case average over many runs of circuit executions to estimate expected values. While PAI increases that sampling cost, we prove that a) the approach is optimal in the sense that PAI achieves the least possible overhead and c) the overhead is remarkably modest even with thousands of parametrised gates and only $7$ bits of resolution available. This is a profound relaxation of engineering requirements for first generation quantum computers where even $5-6$ bits of resolution may suffice and, as we demonstrate, the approach is many orders of magnitude more efficient than prior techniques. Moreover we conclude that, even for more mature late-NISQ hardware, no more than $9$ bits will be necessary., Comment: 15 pages, 5 figures -- includes proof of optimality of protocol, generalisation to non-uniform settings etc

Quantum memories capable of storing and retrieving coherent information for extended times at room temperature would enable a host of new technologies. Electron and nuclear spin qubits using shallow neutral donors in semiconductors have been studied extensively but are limited to low temperatures ($\le$10 K); however, the nuclear spins of ionized donors have potential for high temperature operation. We use optical methods and dynamical decoupling to realize this potential for an ensemble of 31P donors in isotopically purified 28Si and observe a room temperature coherence time of over 39 minutes. We further show that a coherent spin superposition can be cycled from 4.2 K to room temperature and back, and report a cryogenic coherence time of 3 hours in the same system., Comment: 5 pages, 4 figures

Rare-earth doped crystals have long coherence times and the potential to provide quantum interfaces between microwave and optical photons. Such applications benefit from a high cooperativity between the spin ensemble and a microwave cavity -- this motivates an increase in the rare earth ion concentration which in turn impacts the spin coherence lifetime. We measure spin dynamics of two rare-earth spin species, $^{145}$Nd and Yb doped into Y$_{2}$SiO$_{5}$, coupled to a planar microwave resonator in the high cooperativity regime, in the temperature range 1.2 K to 14 mK. We identify relevant decoherence mechanisms including instantaneous diffusion arising from resonant spins and temperature-dependent spectral diffusion from impurity electron and nuclear spins in the environment. We explore two methods to mitigate the effects of spectral diffusion in the Yb system in the low-temperature limit, first, using magnetic fields of up to 1 T to suppress impurity spin dynamics and, second, using transitions with low effective g-factors to reduce sensitivity to such dynamics. Finally, we demonstrate how the `clock transition' present in the $^{171}$Yb system at zero field can be used to increase coherence times up to $T_{2} = 6(1)$ ms., Comment: 8 pages, 5 figures

Layered van der Waals (vdW) magnets can maintain a magnetic order even down to the single-layer regime and hold promise for integrated spintronic devices. While the magnetic ground state of vdW magnets was extensively studied, key parameters of spin dynamics, like the Gilbert damping, crucial for designing ultra-fast spintronic devices, remains largely unexplored. Despite recent studies by optical excitation and detection, achieving spin wave control with microwaves is highly desirable, as modern integrated information technologies predominantly are operated with these. The intrinsically small numbers of spins, however, poses a major challenge to this. Here, we present a hybrid approach to detect spin dynamics mediated by photon-magnon coupling between high-Q superconducting resonators and ultra-thin flakes of Cr$_2$Ge$_2$Te$_6$ (CGT) as thin as 11\,nm. We test and benchmark our technique with 23 individual CGT flakes and extract an upper limit for the Gilbert damping parameter. These results are crucial in designing on-chip integrated circuits using vdW magnets and offer prospects for probing spin dynamics of monolayer vdW magnets.

Impurity spins in crystal matrices are promising components in quantum technologies, particularly if they can maintain their spin properties when close to surfaces and material interfaces. Here, we investigate an attractive candidate for microwave-domain applications, the spins of group-VI Impurity spins in crystal matrices are promising components in quantum technologies, particularly if they can maintain their spin properties when close to surfaces and material interfaces. Here, we investigate an attractive candidate for microwave-domain applications, the spins of group-VI $^{125}$Te$^+$ donors implanted into natural Si at depths as shallow as 20~nm. We show that surface band-bending can be used to ionise such near-surface Te to spin-active Te$^+$ state, and that optical illumination can be used further to control the Te donor charge state. We examine spin activation yield, spin linewidth, relaxation ($T_1$) and coherence times (\ttwo) and show how a zero-field 3.5~GHz `clock transition' extends spin coherence times to over 1~ms, which is about an order of magnitude longer than other near-surface spin systems.

We investigated the use of dielectric layers produced by atomic layer deposition (ALD) as an approach to strain mitigation in composite silicon/superconductor devices operating at cryogenic temperatures. We show that the addition of an ALD layer acts to reduce the strain of spins closest to silicon/superconductor interface where strain is highest. We show that appropriately biasing our devices at the hyperfine clock transition of bismuth donors in silicon, we can remove strain broadening and that the addition of ALD layers left $T_2$ (or temporal inhomogeneities) unchanged in these natural silicon devices.

As in conventional computing, key attributes of quantum memories are high storage density and, crucially, random access, or the ability to read from or write to an arbitrarily chosen register. However, achieving such random access with quantum memories in a dense, hardware-efficient manner remains a challenge, for example requiring dedicated cavities per qubit or pulsed field gradients. Here we introduce a protocol using chirped pulses to encode qubits within an ensemble of quantum two-level systems, offering both random access and naturally supporting dynamical decoupling to enhance the memory lifetime. We demonstrate the protocol in the microwave regime using donor spins in silicon coupled to a superconducting cavity, storing up to four multi-photon microwave pulses in distinct memory modes and retrieving them on-demand up to 2~ms later. A further advantage is the natural suppression of superradiant echo emission, which we show is critical when approaching unit cooperativity. This approach offers the potential for microwave random access quantum memories with lifetimes exceeding seconds, while the chirped pulse phase encoding could also be applied in the optical regime to enhance quantum repeaters and networks.