Quantum computing is predicted to provide unprecedented enhancements in computational power. A quantum computer requires implementation of a well-defined and controlled quantum system of many interconnected qubits, each defined using fragile quantum states. The interest in a spin-based quantum computer in silicon stems from demonstrations of very long spin-coherence times, high-fidelity single spin control and compatibility with industrial mass-fabrication. Industrial scale fabrication of the silicon platform offers a clear route towards a large-scale quantum computer, however, some of the processes and techniques employed in qubit demonstrators are incompatible with a dense and foundry-fabricated architecture. In particular, spin-readout utilises external sensors that require nearly the same footprint as qubit devices. In this thesis, improved readout techniques for silicon quantum devices are presented and routes towards implementation of a scalable and high-sensitivity readout architecture are investigated. Firstly, readout sensitivity of compact gate-based sensors is improved using a high-quality factor resonator and Josephson parametric amplifier that are fabricated separately from quantum dots. Secondly, an integrated transistor-based control circuit is presented using which sequential readout of two quantum dot devices using the same gate-based sensor is achieved. Finally, a large-scale readout architecture based on random-access and frequency multiplexing is introduced. The impact of readout circuit footprint on readout sensitivity is determined, showing routes towards integration of conventional circuits with quantum devices in a dense architecture, and a fault-tolerant architecture based on mediated exchange is introduced, capable of relaxing the limitations on available control circuit footprint per qubit. Demonstrations are based on foundry-fabricated transistors and few-electron quantum dots, showing that industry fabrication is a viable route towards quantum computation at a scale large enough to begin addressing the most challenging computational problems.
Quantum computing is predicted to provide unprecedented enhancements in computational power. A quantum computer requires implementation of a well-defined and controlled quantum system of many interconnected qubits, each defined using fragile quantum states. The interest in a spin-based quantum computer in silicon stems from demonstrations of very long spin-coherence times, high-fidelity single spin control and compatibility with industrial mass-fabrication. Industrial scale fabrication of the silicon platform offers a clear route towards a large-scale quantum computer, however, some of the processes and techniques employed in qubit demonstrators are incompatible with a dense and foundry-fabricated architecture. In particular, spin-readout utilises external sensors that require nearly the same footprint as qubit devices. In this thesis, improved readout techniques for silicon quantum devices are presented and routes towards implementation of a scalable and high-sensitivity readout architecture are investigated. Firstly, readout sensitivity of compact gate-based sensors is improved using a high-quality factor resonator and Josephson parametric amplifier that are fabricated separately from quantum dots. Secondly, an integrated transistor-based control circuit is presented using which sequential readout of two quantum dot devices using the same gate-based sensor is achieved. Finally, a large-scale readout architecture based on random-access and frequency multiplexing is introduced. The impact of readout circuit footprint on readout sensitivity is determined, showing routes towards integration of conventional circuits with quantum devices in a dense architecture, and a fault-tolerant architecture based on mediated exchange is introduced, capable of relaxing the limitations on available control circuit footprint per qubit. Demonstrations are based on foundry-fabricated transistors and few-electron quantum dots, showing that industry fabrication is a viable route towards quantum computation at a scale large enough to begin addressing the most challenging computational problems.
Recent advances in quantum error correction codes for fault-tolerant quantum computing and physical realizations of high-fidelity qubits in multiple platforms give promise for the construction of a quantum computer based on millions of interacting qubits. However, the classical-quantum interface remains a nascent field of exploration. Here, we propose an architecture for a silicon-based quantum computer processor based on complementary metal-oxide-semiconductor (CMOS) technology. We show how a transistor-based control circuit together with charge-storage electrodes can be used to operate a dense and scalable two-dimensional qubit system. The qubits are defined by the spin state of a single electron confined in quantum dots, coupled via exchange interactions, controlled using a microwave cavity, and measured via gate-based dispersive readout. We implement a spin qubit surface code, showing the prospects for universal quantum computation. We discuss the challenges and focus areas that need to be addressed, providing a path for large-scale quantum computing.
Quantum entanglement and superposition, in addition to revealing interesting physics in their own right, can be harnessed as computational resources in a machine, enabling a range of algorithms for classically intractable problems. In recent years, experiments with small numbers of qubits have been demonstrated in a range of solid-state systems, but this is far from the numbers required to realise a useful quantum computer. In addition to the qubits themselves, quantum operation requires a host of classical electronics for control and readout, and current techniques used in few-qubit systems are not scalable. This thesis presents a series of techniques for control and readout of solid-state qubits, working towards scalability by integrating classical control with the quantum technology. Two techniques for reducing the footprint associated with readout of gallium arsenide spin qubits are demonstrated. Gate electrodes, used to define the quantum dot, are also shown to be sensitive state detectors. These gate-sensors, and the more conventional Quantum Point Contacts, are then multiplexed in the frequency domain, where three-channel qubit readout and ten-channel QPC readout are demonstrated. Two types of superconducting devices are also explored. The loss in superconducting coplanar waveguide resonators is measured, and a suppression of coupling to the parasitic electromagnetic environment is demonstrated. The thesis also details software for the simulation of Josephson-junction based circuits including features beyond what is available in commercial products. Finally, an architecture for managing control of a scalable machine is proposed where classical components are distributed throughout a cryostat and cryogenic switches route control pulses to the appropriate qubits. A simple implementation of the architecture is demonstrated that incorporates a double quantum dot, a gallium arsenide switch matrix, frequency multiplexed readout, and cryogenic classical computation.
Abstract Recent advances in quantum error correction codes for fault-tolerant quantum computing and physical realizations of high-fidelity qubits in multiple platforms give promise for the construction of a quantum computer based on millions of interacting qubits. However, the classical-quantum interface remains a nascent field of exploration. Here, we propose an architecture for a silicon-based quantum computer processor based on complementary metal-oxide-semiconductor (CMOS) technology. We show how a transistor-based control circuit together with charge-storage electrodes can be used to operate a dense and scalable two-dimensional qubit system. The qubits are defined by the spin state of a single electron confined in quantum dots, coupled via exchange interactions, controlled using a microwave cavity, and measured via gate-based dispersive readout. We implement a spin qubit surface code, showing the prospects for universal quantum computation. We discuss the challenges and focus areas that need to be addressed, providing a path for large-scale quantum computing.
Quantum computing promises to revolutionize our lives, achieving unprecedented computational powers and unlocking new possibilities in drug discovery, chemical simulations and cryptography. The fundamental unit of computation of a quantum computer is the quantum bit (qubit). It can be implemented in a number of technologies, such as superconducting qubits, spin qubits, photonic systems, trapped ions and NV-centers. Solid-state superconducting qubits and spin qubits seem to be the most promising ones, as shown by the recent demonstration of quantum supremacy with a superconducting qubit processor. Such quantum processors, to operate in the quantum regime, require very low temperatures around 10 mK, and they are hence kept inside dilution fridges. The electronics required to read out and control such processors is typically realized by discrete components or bulky scientific instruments at room temperature wired into the dilution fridge. Such approach is only feasible until few (< 100) qubits are handled, but it is not scalable for million-qubit processors required for fault-tolerant operation. In this thesis, readout and control electronics is proposed to be integrated and operated directly at cryogenic temperatures, in close proximity with the qubits and potentially even co-integrated. CMOS technology is advocated for its scalability, compactness and low prototyping cost, to create a comprehensive cryo-CMOS class of fully-integrated transceivers to read out and control solid-state qubits. In particular, the thesis focuses on cryo-CMOS radio-frequency integrated circuits for the readout of silicon quantum computers, looking to integrate the whole readout system, from the quantum layer to the room temperature user interface. Cryogenic CMOS devices are explored first, to lay the foundations of integrated circuit design. The realization of quantum dots in standard CMOS technology is explored as a platform to realize solid-state qubit arrays, demonstrating functionality at 50 mK and good reproducibility. Moreover, the operation of integrated transistors at 50 mK and 4.2 K is explored to understand their basic cryogenic functionality, through characterization and modeling. Then also integrated passives, such as capacitors, inductors and transformers, are studied and modeled at cryogenic temperatures. Then, a fully-integrated readout platform spanning across all the blocks needed for dispersive readout is proposed. A fully-integrated matrix of quantum dots with radio-frequency gate-based readout capabilities is demonstrated along with time- and frequency-multiplexing. This quantum-classical circuit shows all the features for a scalable approach and is the core of the platform following it. A cryogenic CMOS circulator based on a new all-pass filter architecture is demonstrated, to be used as integrated replacement for current bulky counterparts; it allows power reduction, insertion loss and bandwidth enhancement with respect to state-of-the-art integrated circulators. The rest of the front-end is also explored with dedicated test multiplexers, a low noise-amplifier and an oscillator, that are finally merged in a single fully-integrated low-noise receiver with frequency synthesizer operating at 3.5 K for scalable multiplexed readout of silicon qubits. The platform presented in this thesis, across temperature boundaries, carries all the features required for future scalable silicon quantum computing platforms and could therefore form their core.
There is currently a worldwide effort towards the realisation of large-scale quantum computers that exploit quantum phenomena for information processing. While these computing systems could potentially redefine the technological landscape, harnessing quantum effects is challenging due to their inherently fragile nature and the experimentally demanding environments in which they arise. In order for quantum computation to be viable it is first necessary to demonstrate the operation of two-level quantum systems (qubits) which have long coherence times, can be quickly read out, and can be controlled with high fidelity. Focusing on these key requirements, this thesis presents four experiments towards scalable solid state quantum computing using gate-defined quantum dot devices based on gallium arsenide (GaAs) heterostructures. The first experiment investigates a phonon emission process that limits the charge coherence in GaAs and potentially complicates the microwave control of multi-qubit devices. We show that this microwave analogy to Raman spectroscopy can provide a means of detecting the unique phonon spectral density created by a nanoscale device. Experimental results are compared to a theoretical model based on a non-Markovian master equation and approaches to suppressing electron-phonon coupling are discussed. The second experiment demonstrates a technique involving in-situ gate electrodes coupled to lumped-element resonators to provide high-bandwidth dispersive read-out of the state of a double quantum dot. We characterise the charge sensitivity of this method in the few-electron regime and benchmark its performance against quantum point contact charge sensors. The third experiment implements a low-loss, chip-level frequency multiplexing scheme for the readout of scaled-up spin qubit arrays. Dispersive gate-sensing is realised in combination with charge detection based on two radio frequency quantum point contacts to perform multiplexed readout of a double quantum dot in the few-electron regime. Demonstration of a 10-channel multiplexing device is achieved and limitations in scaling spin qubit readout to large numbers using multiplexed channels discussed. The final experiment ties previously presented results together by realising a micro-architecture for controlling and reading out qubits during the execution of a quantum algorithm. The basic principles of this architecture are demonstrated via the manipulation of a semiconductor qubit using control pulses that are cryogenically routed using a high-electron mobility transistor switching matrix controlled by a field programmable gate array. Finally, several technical results are also presented including the development of printed circuit board solutions to allow the high-frequency measurement of nanoscale devices at cryogenic temperatures and the design of on-chip interconnects used to suppress electromagnetic crosstalk in high-density spin qubit device architectures.
Practical quantum computers require a large network of highly coherent qubits, interconnected in a design robust against errors. Donor spins in silicon provide state-of-the-art coherence and quantum gate fidelities, in a platform adapted from industrial semiconductor processing. Here we present a scalable design for a silicon quantum processor that does not require precise donor placement and leaves ample space for the routing of interconnects and readout devices. We introduce the flip-flop qubit, a combination of the electron-nuclear spin states of a phosphorus donor that can be controlled by microwave electric fields. Two-qubit gates exploit a second-order electric dipole-dipole interaction, allowing selective coupling beyond the nearest-neighbor, at separations of hundreds of nanometers, while microwave resonators can extend the entanglement to macroscopic distances. We predict gate fidelities within fault-tolerance thresholds using realistic noise models. This design provides a realizable blueprint for scalable spin-based quantum computers in silicon.Quantum computers will require a large network of coherent qubits, connected in a noise-resilient way. Tosi et al. present a design for a quantum processor based on electron-nuclear spins in silicon, with electrical control and coupling schemes that simplify qubit fabrication and operation.