arXiv subjects

J. J. L. Morton

Publications and source records attributed to J. J. L. Morton.

At least 19 recordsLinked to original sources

arXiv2025

A superinductor in a deep sub-micron integrated circuit

Superinductors are circuit elements characterised by an intrinsic impedance in excess of the superconducting resistance quantum ($R_\text{Q}\approx6.45~$k$Ω$), with applications from metrology and sensing to quantum computing. However, they are typically obtained using exotic materials with high density inductance such as Josephson junctions, superconducting nanowires or twisted two-dimensional materials. Here, we present a superinductor realised within a silicon integrated circuit (IC), exploiting the high kinetic inductance ($\sim 1$~nH/$\square$) of TiN thin films native to the manufacturing process (22-nm FDSOI). By interfacing the superinductor to a silicon quantum dot formed within the same IC, we demonstrate a radio-frequency single-electron transistor (rfSET), the most widely used sensor in semiconductor-based quantum computers. The integrated nature of the rfSET reduces its parasitics which, together with the high impedance, yields a sensitivity improvement of more than two orders of magnitude over the state-of-the-art, combined with a 10,000-fold area reduction. Beyond providing the basis for dense arrays of integrated and high-performance qubit sensors, the realization of high-kinetic-inductance superconducting devices integrated within modern silicon ICs opens many opportunities, including kinetic-inductance detector arrays for astronomy and the study of metamaterials and quantum simulators based on 1D and 2D resonator arrays.

quant-ph↗

arXiv2023

Pipeline quantum processor architecture for silicon spin qubits

Noisy intermediate-scale quantum (NISQ) devices seek to achieve quantum advantage over classical systems without the use of full quantum error correction. We propose a NISQ processor architecture using a qubit `pipeline' in which all run-time control is applied globally, reducing the required number and complexity of control and interconnect resources. This is achieved by progressing qubit states through a layered physical array of structures which realise single and two-qubit gates. Such an approach lends itself to NISQ applications such as variational quantum eigensolvers which require numerous repetitions of the same calculation, or small variations thereof. In exchange for simplifying run-time control, a larger number of physical structures is required for shuttling the qubits as the circuit depth now corresponds to an array of physical structures. However, qubit states can be `pipelined' densely through the arrays for repeated runs to make more efficient use of physical resources. We describe how the qubit pipeline can be implemented in a silicon spin-qubit platform, to which it is well suited to due to the high qubit density and scalability. In this implementation, we describe the physical realisation of single and two qubit gates which represent a universal gate set that can achieve fidelities of $\mathcal{F} \geq 0.9999$, even under typical qubit frequency variations.

quant-ph↗

arXiv2023

An elongated quantum dot as a distributed charge sensor

Increasing the separation between semiconductor quantum dots offers scaling advantages by fa- cilitating gate routing and the integration of sensors and charge reservoirs. Elongated quantum dots have been utilized for this purpose in GaAs heterostructures to extend the range of spin-spin interactions. Here, we study a metal-oxide-semiconductor (MOS) device where two quantum dot arrays are separated by an elongated quantum dot (340 nm long, 50 nm wide). We monitor charge transitions of the elongated quantum dot by measuring radiofrequency single-electron currents to a reservoir to which we connect a lumped-element resonator. We operate the dot as a single electron box to achieve charge sensing of remote quantum dots in each array, separated by a distance of 510 nm. Simultaneous charge detection on both ends of the elongated dot demonstrates that the charge is well distributed across its nominal length, supported by the simulated quantum-mechanical electron density. Our results illustrate how single-electron boxes can be realised with versatile foot- prints that may enable novel and compact quantum processor layouts, offering distributed charge sensing in addition to the possibility of mediated coupling.

cond-mat.mes-hall↗

arXiv2022

Fast high-fidelity single-shot readout of spins in silicon using a single-electron box

Three key metrics for readout systems in quantum processors are measurement speed, fidelity and footprint. Fast high-fidelity readout enables mid-circuit measurements, a necessary feature for many dynamic algorithms and quantum error correction, while a small footprint facilitates the design of scalable, highly-connected architectures with the associated increase in computing performance. Here, we present two complementary demonstrations of fast high-fidelity single-shot readout of spins in silicon quantum dots using a compact, dispersive charge sensor: a radio-frequency single-electron box. The sensor, despite requiring fewer electrodes than conventional detectors, performs at the state-of-the-art achieving spin read-out fidelity of 99.2% in less than 6 $μ$s. We demonstrate that low-loss high-impedance resonators, highly coupled to the sensing dot, in conjunction with Josephson parametric amplification are instrumental in achieving optimal performance. We quantify the benefit of Pauli spin blockade over spin-dependent tunneling to a reservoir, as the spin-to-charge conversion mechanism in these readout schemes. Our results place dispersive charge sensing at the forefront of readout methodologies for scalable semiconductor spin-based quantum processors.

cond-mat.mes-hall↗

arXiv2021

Hyperfine spectroscopy in a quantum-limited spectrometer

We report measurements of electron spin echo envelope modulation (ESEEM) performed at millikelvin temperatures in a custom-built high-sensitivity spectrometer based on superconducting micro-resonators. The high quality factor and small mode volume (down to 0.2pL) of the resonator allow to probe a small number of spins, down to $5\cdot 10^2$. We measure 2-pulse ESEEM on two systems: erbium ions coupled to $^{183}$W nuclei in a natural-abundance $\text{CaWO}_4$ crystal, and bismuth donors coupled to residual $^{29}$Si nuclei in a silicon substrate that was isotopically enriched in the $^{28}$Si isotope. We also measure 3- and 5-pulse ESEEM for the bismuth donors in silicon. Quantitative agreement is obtained for both the hyperfine coupling strength of proximal nuclei, and the nuclear spin concentration.

quant-ph↗

arXiv2021

Spatially-resolved decoherence of donor spins in silicon strained by a metallic electrode

Electron spins are amongst the most coherent solid-state systems known, however, to be used in devices for quantum sensing and information processing applications, they must be typically placed near interfaces. Understanding and mitigating the impacts of such interfaces on the coherence and spectral properties of electron spins is critical to realize such applications, but is also challenging: inferring such data from single-spin studies requires many measurements to obtain meaningful results, while ensemble measurements typically give averaged results that hide critical information. Here, we report a comprehensive study of the coherence of near-surface bismuth donor spins in 28-silicon at millikelvin temperatures. In particular, we use strain-induced frequency shifts caused by a metallic electrode to make spatial maps of spin coherence as a function of depth and position relative to the electrode. By measuring magnetic-field-insensitive clock transitions we separate magnetic noise caused by surface spins from charge noise. Our results include quantitative models of the strain-split spin resonance spectra and extraction of paramagnetic impurity concentrations at the silicon surface. The interplay of these decoherence mechanisms for such near-surface electron spins is critical for their application in quantum technologies, while the combination of the strain splitting and clock transition extends the coherence lifetimes by up to two orders of magnitude, reaching up to 300 ms at a mean depth of only 100nm. The technique we introduce here to spatially map coherence in near-surface ensembles is directly applicable to other spin systems of active interest, such as defects in diamond, silicon carbide, and rare earth ions in optical crystals.

quant-ph↗

arXiv2020

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.

cond-mat.mes-hall↗

arXiv2020

Multimode storage of quantum microwave fields in electron spins over 100 ms

A long-lived multi-mode qubit register is an enabling technology for modular quantum computing architectures. For interfacing with superconducting qubits, such a quantum memory should be able to store incoming quantum microwave fields at the single-photon level for long periods of time, and retrieve them on-demand. Here, we demonstrate the partial absorption of a train of weak microwave fields in an ensemble of bismuth donor spins in silicon, their storage for 100 ms, and their retrieval, using a Hahn-echo-like protocol. The long storage time is obtained by biasing the bismuth donors at a clock transition. Phase coherence and quantum statistics are preserved in the storage.

quant-ph↗

arXiv2019

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 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 (JPA), 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 (SNR), 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 JPA. Additionally, the JPA allows operation at a lower RF power while maintaining identical SNR. We determine a noise temperature of 200 mK with a contribution from the JPA (25%), cryogenic amplifier (25%) and the resonator (50%), showing routes to further increase the read-out speed.

cond-mat.mes-hall↗

arXiv2019

Magnon-photon coupling in a non-collinear magnetic insulator Cu$_2$OSeO$_3$

Anticrossing behavior between magnons in a non-collinear chiral magnet Cu$_2$OSeO$_3$ and a two-mode X-band microwave resonator was studied in the temperature range 5-100K. In the field-induced ferrimagnetic phase, we observed a strong coupling regime between magnons and two microwave cavity modes with a cooperativity reaching 3600. In the conical phase, cavity modes are dispersively coupled to a fundamental helimagnon mode, and we demonstrate that the magnetic phase diagram of Cu$_2$OSeO$_3$ can be reconstructed from the measurements of the cavity resonance frequency. In the helical phase, a hybridized state of a higher-order helimagnon mode and a cavity mode - a helimagnon polariton - was found. Our results reveal a new class of magnetic systems where strong coupling of microwave photons to non-trivial spin textures can be observed.

cond-mat.mes-hall↗

arXiv2018

A CMOS dynamic random access architecture for radio-frequency readout of quantum devices

Quantum computing technology is maturing at a relentless pace, yet individual quantum bits are wired one by one. As quantum processors become more complex, they require efficient interfaces to deliver signals for control and readout while keeping the number of inputs manageable. Digital electronics offers solutions to the scaling challenge by leveraging established industrial infrastructure and applying it to integrate silicon-based quantum devices with conventional CMOS circuits. Here, we combine both technologies at milikelvin temperatures and demonstrate the building blocks of a dynamic random access architecture for efficient readout of complex quantum circuits. Our circuit is divided into cells, each containing a CMOS quantum dot (QD) and a field-effect transistor that enables selective readout of the QD, as well as charge storage on the QD gate similar to 1T-1C DRAM technology. We show dynamic readout of two cells by interfacing them with a single radio-frequency resonator. Our results demonstrate a path to reducing the number of input lines per qubit and enable addressing of large-scale device arrays.

cond-mat.mes-hall↗

arXiv2018

Strain-induced spin resonance shifts in silicon devices

In spin-based quantum information processing devices, the presence of control and detection circuitry can change the local environment of a spin by introducing strain and electric fields, altering its resonant frequencies. These resonance shifts can be large compared to intrinsic spin line-widths and it is therefore important to study, understand and model such effects in order to better predict device performance. Here we investigate a sample of bismuth donor spins implanted in a silicon chip, on top of which a superconducting aluminium micro-resonator has been fabricated. The on-chip resonator provides two functions: first, it produces local strain in the silicon due to the larger thermal contraction of the aluminium, and second, it enables sensitive electron spin resonance spectroscopy of donors close to the surface that experience this strain. Through finite-element strain simulations we are able to reconstruct key features of our experiments, including the electron spin resonance spectra. Our results are consistent with a recently discovered mechanism for producing shifts of the hyperfine interaction for donors in silicon, which is linear with the hydrostatic component of an applied strain.

cond-mat.mes-hall↗

arXiv2017

Coherent spin dynamics of ytterbium ions in yttrium orthosilicate

We investigate the electron and nuclear spin coherence properties of ytterbium ($\mathrm{Yb}^{3+}$) ions with non-zero nuclear spin, within an yttrium orthosilicate (Y$_2$SiO$_5$) crystal, with a view to their potential application in quantum memories or repeaters. We find electron spin-lattice relaxation times are maximised at low magnetic field ($<100$ mT) where $g~\sim6$, reaching 5 s at 2.5 K, while coherence times are maximised when addressing ESR transitions at higher fields where $g\sim0.7$ where a Hahn echo measurement yields $T_2$ up to 73 $μ$s. Dynamical decoupling (XY16) can be used to suppress spectral diffusion and extend the coherence lifetime to over 0.5 ms, close to the limit of instantaneous diffusion. Using Davies electron-nuclear-double-resonance (ENDOR), we performed coherent control of the $^{173}\mathrm{Yb}^{3+}$ nuclear spin and studied its relaxation dynamics. At around 4.5 K we measure a nuclear spin $T_1$ and $T_2$ of 4 and 0.35 ms, respectively, about 4 and 14 times longer than the corresponding times for the electron spin.

quant-ph↗

arXiv2017

Magnetic resonance with squeezed microwaves

Vacuum fluctuations of the electromagnetic field set a fundamental limit to the sensitivity of a variety of measurements, including magnetic resonance spectroscopy. We report the use of squeezed microwave fields, which are engineered quantum states of light for which fluctuations in one field quadrature are reduced below the vacuum level, to enhance the detection sensitivity of an ensemble of electronic spins at millikelvin temperatures.} By shining a squeezed vacuum state on the input port of a microwave resonator containing the spins, we obtain a $1.2$\,dB noise reduction at the spectrometer output compared to the case of a vacuum input. This result constitutes a proof of principle of the application of quantum metrology to magnetic resonance spectroscopy.

cond-mat.mes-hall↗

arXiv2017

Inductive-detection electron-spin resonance spectroscopy with $\mathbf{65}\,$spins$/\sqrt{\text{Hz}}$ sensitivity

We report electron spin resonance spectroscopy measurements performed at millikelvin temperatures in a custom-built spectrometer comprising a superconducting micro-resonator at $7$ GHz and a Josephson parametric amplifier. Owing to the small ${\sim}10^{-12}λ^3$ magnetic resonator mode volume and to the low noise of the parametric amplifier, the spectrometer sensitivity reaches $260\pm40$ spins$/$echo and $65\pm10$ $\mathrm{spins}/\sqrt{\text{Hz}}$, respectively.

quant-ph↗

arXiv2017

Conditional dispersive readout of a CMOS quantum dot via an integrated transistor circuit

Quantum computers require interfaces with classical electronics for efficient qubit control, measurement and fast data processing. Fabricating the qubit and the classical control layer using the same technology is appealing because it will facilitate the integration process, improving feedback speeds and offer potential solutions to wiring and layout challenges. Integrating classical and quantum devices monolithically, using complementary metal-oxide-transistor (CMOS) processes, enables the processor to profit from the most mature industrial technology for the fabrication of large scale circuits. Here we demonstrate the integration of a single-electron charge storage CMOS quantum dot with a CMOS transistor for control of the readout via gate-based dispersive sensing using a lumped element $LC$ resonator. The control field-effect transistor (FET) and quantum dot are fabricated on the same chip using fully-depleted silicon-on-insulator technology. We obtain a charge sensitivity of $δq=165\, μe \mathrm{Hz}^{-1/2}$ when the quantum dot readout is enabled by the control FET. Additionally, we observe a single-electron retention time of the order of a second when storing a single-electron charge on the quantum dot at milli-Kelvin temperatures. These results demonstrate first steps towards time-based multiplexing of gate-based dispersive qubit readout in CMOS technology opening the path for the development of an all-silicon quantum-classical processor.

cond-mat.mes-hall↗

arXiv2015

Controlling spin relaxation with a cavity

Spontaneous emission of radiation is one of the fundamental mechanisms by which an excited quantum system returns to equilibrium. For spins, however, spontaneous emission is generally negligible compared to other non-radiative relaxation processes because of the weak coupling between the magnetic dipole and the electromagnetic field. In 1946, Purcell realized that the spontaneous emission rate can be strongly enhanced by placing the quantum system in a resonant cavity -an effect which has since been used extensively to control the lifetime of atoms and semiconducting heterostructures coupled to microwave or optical cavities, underpinning single-photon sources. Here we report the first application of these ideas to spins in solids. By coupling donor spins in silicon to a superconducting microwave cavity of high quality factor and small mode volume, we reach for the first time the regime where spontaneous emission constitutes the dominant spin relaxation mechanism. The relaxation rate is increased by three orders of magnitude when the spins are tuned to the cavity resonance, showing that energy relaxation can be engineered and controlled on-demand. Our results provide a novel and general way to initialise spin systems into their ground state, with applications in magnetic resonance and quantum information processing. They also demonstrate that, contrary to popular belief, the coupling between the magnetic dipole of a spin and the electromagnetic field can be enhanced up to the point where quantum fluctuations have a dramatic effect on the spin dynamics; as such our work represents an important step towards the coherent magnetic coupling of individual spins to microwave photons.

quant-ph↗

arXiv2015

Reaching the quantum limit of sensitivity in electron spin resonance

We report pulsed electron-spin resonance (ESR) measurements on an ensemble of Bismuth donors in Silicon cooled at 10mK in a dilution refrigerator. Using a Josephson parametric microwave amplifier combined with high-quality factor superconducting micro-resonators cooled at millikelvin temperatures, we improve the state-of-the-art sensitivity of inductive ESR detection by nearly 4 orders of magnitude. We demonstrate the detection of 1700 bismuth donor spins in silicon within a single Hahn echo with unit signal-to-noise (SNR) ratio, reduced to just 150 spins by averaging a single Carr-Purcell-Meiboom-Gill sequence. This unprecedented sensitivity reaches the limit set by quantum fluctuations of the electromagnetic field instead of thermal or technical noise, which constitutes a novel regime for magnetic resonance.

quant-ph↗