arXiv subjects

Michael L. W. Thewalt

Publications and source records attributed to Michael L. W. Thewalt.

13 recordsLinked to original sources

arXiv2026

Multi-Qubit Entanglement of Unit Cell Pairs in SiMOS

Spin qubits in silicon-MOS (SiMOS) quantum dots have recently demonstrated compatibility with existing industry standard CMOS fabrication techniques. These devices have routinely achieved single- and two-qubit gate fidelities above 99% and demonstrated highly entangled two-qubit Bell states in isolated double quantum dot (DQD) unit cells, however coupling between unit cells has remained challenging. In this work, we present a two unit cell, four-qubit SiMOS processor with universal controllability and fully parallelised state initialisation and readout. We use this processor to generate maximally entangled three-qubit states, including the Greenberger-Horne-Zeilinger (GHZ) state, and certify multipartite entanglement through violation of the classical Mermin-witness bound. By using a fully symmetric dynamically decoupled gate sequence to create our entangled states, we are able to preserve the lifetime of the entanglement beyond $T_2^*$, to a time limited instead by $T_2^\textrm{Hahn}$. These demonstrations pave a road to the scalable operation of larger SiMOS processors, and achieving high purity, long-lived multi-qubit entangled states in them.

quant-ph↗

arXiv2026

Spectral tuning of single T centres by the Stark effect

Among the many solid-state emitters being explored for scalable quantum technologies, the silicon T centre is a leading candidate offering long-lived spin qubits, a telecommunications-band spin-photon interface, and integration with on-chip photonic circuits. However, nanophotonic integration broadens both the inhomogeneous spectral distribution and individual emitter linewidths. Here, we integrate single T centres into silicon nanophotonic cavities with p-i-n diodes for local electronic control. These devices enable Stark tuning up to 30 GHz, sufficient to bring 55(2)% of on-chip T centres into mutual resonance, and demonstrate tunable lifetime reduction across the cavity resonance. A model of the joint excitation probability shows an orders-of-magnitude increase in entanglement rate by tuning distinct emitters into mutual resonance. Luminescence modulation at high reverse biases reveals a transition to a dark charge state. Finally, bias-induced modulation of the optical transition splitting uncovers a potential mechanism for electrically driven excited-state spin mixing via spin-orbit coupling. Localized and individual spectral tuning increases the yield of performant silicon spin-photon interfaces and the number of devices per chip available for large-scale entanglement and quantum information technologies.

quant-ph↗

arXiv2026

Mid-circuit logic executed in the qubit layer of a quantum processor

Practical quantum computers need to continuously exchange data between classical and quantum subsystems during a computation. Mid-circuit measurements of a qubits state are transferred to the classical electronics layer, and their outcome can inform feedforward operations that close the loop back to the quantum layer. These operations are crucial for fault-tolerant quantum computers, but the quantum-classical loop must be completed before the qubits decohere, presenting a substantial engineering challenge for full-scale systems comprising millions of qubits. Here we perform the first mid-circuit measurements in a system of silicon spin qubits, and show that feedforward operations can be performed without needing to route information to the classical layer. This in-layer approach leverages a backaction-driven control technique that has previously been considered a source of error. We benchmark our in-layer strategy, together with the standard FPGA-enabled approach, and analyse the performance of both methods using gate set tomography. Our results provide the first step towards moving resource-intensive classical processing into the quantum layer, an advance that could solve key engineering challenges, and drastically reduce the power budget of future quantum computers.

quant-ph↗

arXiv2025

Electrically-triggered spin-photon devices in silicon

Quantum networking and computing technologies demand scalable hardware with high-speed control for large systems of quantum devices. Solid-state platforms have emerged as promising candidates, offering scalable fabrication for a wide range of qubits. Architectures based on spin-photon interfaces allow for highly-connected quantum networks over photonic links, enabling entanglement distribution for quantum networking and distributed quantum computing protocols. With the potential to address these demands, optically-active spin defects in silicon are one proposed platform for building quantum technologies. Here, we electrically excite the silicon T centre in integrated optoelectronic devices that combine nanophotonic waveguides and cavities with p-i-n diodes. We observe single-photon electroluminescence from a cavity-coupled T centre with $g^{(2)}(0)=0.05(2)$. Further, we use the electrically-triggered emission to herald the electron spin state, initializing it with $92(8)\%$ fidelity. This shows, for the first time, electrically-injected single-photon emission from a silicon colour centre and a new method of electrically-triggered spin initialization. These findings present a new telecommunications band light source for silicon and a highly parallel control method for T centre quantum processors, advancing the T centre as a versatile defect for scalable quantum technologies.

quant-ph↗

arXiv2024

Assessment of error variation in high-fidelity two-qubit gates in silicon

Achieving high-fidelity entangling operations between qubits consistently is essential for the performance of multi-qubit systems and is a crucial factor in achieving fault-tolerant quantum processors. Solid-state platforms are particularly exposed to errors due to materials-induced variability between qubits, which leads to performance inconsistencies. Here we study the errors in a spin qubit processor, tying them to their physical origins. We leverage this knowledge to demonstrate consistent and repeatable operation with above 99% fidelity of two-qubit gates in the technologically important silicon metal-oxide-semiconductor (SiMOS) quantum dot platform. We undertake a detailed study of these operations by analysing the physical errors and fidelities in multiple devices through numerous trials and extended periods to ensure that we capture the variation and the most common error types. Physical error sources include the slow nuclear and electrical noise on single qubits and contextual noise. The identification of the noise sources can be used to maintain performance within tolerance as well as inform future device fabrication. Furthermore, we investigate the impact of qubit design, feedback systems, and robust gates on implementing scalable, high-fidelity control strategies. These results are achieved by using three different characterization methods, we measure entangling gate fidelities ranging from 96.8% to 99.8%. Our analysis tools identify the causes of qubit degradation and offer ways understand their physical mechanisms. These results highlight both the capabilities and challenges for the scaling up of silicon spin-based qubits into full-scale quantum processors.

quant-ph↗

arXiv2023

High-fidelity operation and algorithmic initialisation of spin qubits above one kelvin

The encoding of qubits in semiconductor spin carriers has been recognised as a promising approach to a commercial quantum computer that can be lithographically produced and integrated at scale. However, the operation of the large number of qubits required for advantageous quantum applications will produce a thermal load exceeding the available cooling power of cryostats at millikelvin temperatures. As the scale-up accelerates, it becomes imperative to establish fault-tolerant operation above 1 kelvin, where the cooling power is orders of magnitude higher. Here, we tune up and operate spin qubits in silicon above 1 kelvin, with fidelities in the range required for fault-tolerant operation at such temperatures. We design an algorithmic initialisation protocol to prepare a pure two-qubit state even when the thermal energy is substantially above the qubit energies, and incorporate radio-frequency readout to achieve fidelities up to 99.34 per cent for both readout and initialisation. Importantly, we demonstrate a single-qubit Clifford gate fidelity of 99.85 per cent, and a two-qubit gate fidelity of 98.92 per cent. These advances overcome the fundamental limitation that the thermal energy must be well below the qubit energies for high-fidelity operation to be possible, surmounting a major obstacle in the pathway to scalable and fault-tolerant quantum computation.

quant-ph↗

arXiv2023

Improved Single-Shot Qubit Readout Using Twin RF-SET Charge Correlations

High fidelity qubit readout is critical in order to obtain the thresholds needed to implement quantum error correction protocols and achieve fault-tolerant quantum computing. Large-scale silicon qubit devices will have densely-packed arrays of quantum dots with multiple charge sensors that are, on average, farther away from the quantum dots, entailing a reduction in readout fidelities. Here, we present a readout technique that enhances the readout fidelity in a linear SiMOS 4-dot array by amplifying correlations between a pair of single-electron transistors, known as a twin SET. By recording and subsequently correlating the twin SET traces as we modulate the dot detuning across a charge transition, we demonstrate a reduction in the charge readout infidelity by over one order of magnitude compared to traditional readout methods. We also study the spin-to-charge conversion errors introduced by the modulation technique, and conclude that faster modulation frequencies avoid relaxation-induced errors without introducing significant spin flip errors, favouring the use of the technique at short integration times. This method not only allows for faster and higher fidelity qubit measurements, but it also enhances the signal corresponding to charge transitions that take place farther away from the sensors, enabling a way to circumvent the reduction in readout fidelities in large arrays of qubits.

cond-mat.mes-hall↗

arXiv2022

Memory and transduction prospects for silicon T centre devices

The T centre, a silicon-native spin-photon interface with telecommunications-band optical transitions and long-lived microwave qubits, offers an appealing new platform for both optical quantum memory and microwave to optical telecommunications band transduction. A wide range of quantum memory and transduction schemes could be implemented on such a platform, with advantages and disadvantages that depend sensitively on the ensemble properties. In this work we characterize T centre spin ensembles to inform device design. We perform the first T ensemble optical depth measurement and calculate the improvement in centre density or resonant optical enhancement required for efficient optical quantum memory. We further demonstrate a coherent microwave interface by coherent population trapping (CPT) and Autler-Townes splitting (ATS). We then determine the most promising microwave and optical quantum memory protocol for such ensembles. By estimating the memory efficiency both in free-space and in the presence of a cavity, we show that efficient optical memory is possible with forecast densities. Finally, we formulate a transduction proposal and discuss the achievable efficiency and fidelity.

quant-ph↗

arXiv2022

On-demand electrical control of spin qubits

Once called a "classically non-describable two-valuedness" by Pauli , the electron spin is a natural resource for long-lived quantum information since it is mostly impervious to electric fluctuations and can be replicated in large arrays using silicon quantum dots, which offer high-fidelity control. Paradoxically, one of the most convenient control strategies is the integration of nanoscale magnets to artificially enhance the coupling between spins and electric field, which in turn hampers the spin's noise immunity and adds architectural complexity. Here we demonstrate a technique that enables a \emph{switchable} interaction between spins and orbital motion of electrons in silicon quantum dots, without the presence of a micromagnet. The naturally weak effects of the relativistic spin-orbit interaction in silicon are enhanced by more than three orders of magnitude by controlling the energy quantisation of electrons in the nanostructure, enhancing the orbital motion. Fast electrical control is demonstrated in multiple devices and electronic configurations, highlighting the utility of the technique. Using the electrical drive we achieve coherence time $T_{2,{\rm Hahn}}\approx50 μ$s, fast single-qubit gates with ${T_{π/2}=3}$ ns and gate fidelities of 99.93 % probed by randomised benchmarking. The higher gate speeds and better compatibility with CMOS manufacturing enabled by on-demand electric control improve the prospects for realising scalable silicon quantum processors.

cond-mat.mes-hall↗

arXiv2018

Characterization of the Si:Se+ spin-photon interface

Silicon is the most developed electronic and photonic technological platform and hosts some of the highest-performance spin and photonic qubits developed to date. A hybrid quantum technology harnessing an efficient spin-photon interface in silicon would unlock considerable potential by enabling ultra-long-lived photonic memories, distributed quantum networks, microwave to optical photon converters, and spin-based quantum processors, all linked using integrated silicon photonics. However, the indirect bandgap of silicon makes identification of efficient spin-photon interfaces nontrivial. Here we build upon the recent identification of chalcogen donors as a promising spin-photon interface in silicon. We determined that the spin-dependent optical degree of freedom has a transition dipole moment stronger than previously thought (here 1.96(8) Debye), and the T1 spin lifetime in low magnetic fields is longer than previously thought (> 4.6(1.5) hours). We furthermore determined the optical excited state lifetime (7.7(4) ns), and therefore the natural radiative efficiency (0.80(9) %), and by measuring the phonon sideband, determined the zero-phonon emission fraction (16(1) %). Taken together, these parameters indicate that an integrated quantum optoelectronic platform based upon chalcogen donor qubits in silicon is well within reach of current capabilities.

quant-ph↗

arXiv2016

Electron nuclear double resonance with donor-bound excitons in silicon

We present Auger-electron-detected magnetic resonance (AEDMR) experiments on phosphorus donors in silicon, where the selective optical generation of donor-bound excitons is used for the electrical detection of the electron spin state. Because of the long dephasing times of the electron spins in isotopically purified $^{28}$Si, weak microwave fields are sufficient, which allow to realize broadband AEDMR in a commercial ESR resonator. Implementing Auger-electron-detected ENDOR, we further demonstrate the optically-assisted control of the nuclear spin under conditions where the hyperfine splitting is not resolved in the optical spectrum. Compared to previous studies, this significantly relaxes the requirements on the sample and the experimental setup, e.g. with respect to strain, isotopic purity and temperature. We show AEDMR of phosphorus donors in silicon with natural isotope composition, and discuss the feasibility of ENDOR measurements also in this system.

physics.ins-det↗

arXiv2016

A photonic platform for donor spin qubits in silicon

Donor impurity spins in silicon-28 are highly competitive qubits for upcoming solid-state quantum technologies, yet a proven scalable strategy for multi-qubit devices remains conspicuously absent. These CMOS-compatible, atomically identical qubits offer significant advantages including 3-hour coherence ($T_2$) lifetimes, as well as simultaneous qubit initialization, manipulation and readout fidelities near $\sim\!99.9\%$. These properties meet the requirements for many modern quantum error correction protocols, which are essential for constructing large-scale universal quantum technologies. However, a method of reliably coupling spatially-separated qubits, which crucially does not sacrifice qubit quality and is robust to manufacturing imperfections, has yet to be identified. Here we present such a platform for donor qubits in silicon, by exploiting optically-accessible `deep' chalcogen donors. We show that these donors emit highly uniform light, can be optically initialized, and offer long-lived spin qubit ground states without requiring milliKelvin temperatures. These combined properties make chalcogen donors uniquely suitable for incorporation into silicon photonic architectures for single-shot single-qubit readout as well as for multi-qubit coupling. This unlocks clear pathways for silicon-based quantum computing, spin to photon conversion, photonic memories, silicon-integrated triggered single photon sources and all-optical silicon switches.

quant-ph↗

arXiv2011

Electron spin coherence exceeding seconds in high purity silicon

Silicon is undoubtedly one of the most promising semiconductor materials for spin-based information processing devices. Its highly advanced fabrication technology facilitates the transition from individual devices to large-scale processors, and the availability of an isotopically-purified $^{28}$Si form with no magnetic nuclei overcomes what is a main source of spin decoherence in many other materials. Nevertheless, the coherence lifetimes of electron spins in the solid state have typically remained several orders of magnitude lower than what can be achieved in isolated high-vacuum systems such as trapped ions. Here we examine electron spin coherence of donors in very pure $^{28}$Si material, with a residual $^{29}$Si concentration of less than 50 ppm and donor densities of $10^{14-15}$ per cm$^3$. We elucidate three separate mechanisms for spin decoherence, active at different temperatures, and extract a coherence lifetime $T_2$ up to 2 seconds. In this regime, we find the electron spin is sensitive to interactions with other donor electron spins separated by ~200 nm. We apply a magnetic field gradient in order to suppress such interactions and obtain an extrapolated electron spin $T_2$ of 10 seconds at 1.8 K. These coherence lifetimes are without peer in the solid state by several orders of magnitude and comparable with high-vacuum qubits, making electron spins of donors in silicon ideal components of a quantum computer, or quantum memories for systems such as superconducting qubits.

cond-mat.mtrl-sci↗