Compact and scalable quantum computer required physical qubits with low footprint and cryogenic electronics to be used in situ as standard control/readout system. The small spatial gap between the quantum chip and the cryogenic electronics limits the complexity of the wiring connections and the associated stray capacitance of the wires themselves, yielding a gain in terms of bandwidth and resolution. These improvements allow to design a fully-integrated CMOS readout based on current measurements with similar performances compared to the most common RF reflectometry-based readout technique. Here, we propose an acquisition system, operating at 4.2 K, which converts the spin dependent current coming from a single-electron transistor charge sensor into a digital output with a small power consumption (1 mW/qubit) and small occupation area (2). The system on chip consists of a low noise active integrator for the spin current, feeding a programmable threshold comparator to provide a digital output of the qubit state. It exhibits a resolution of tens of pA over an average input current of 1 nA with a time resolution of 1 μs.Compared to spin-qubit readout based on RF-reflectometry techniques, our solution does not require bulky off-chip compo-nents and to handle microwave signal and fast analog-to digital converters.
Compact and scalable quantum computer required physical qubits with low footprint and cryogenic electronics to be used in situ as standard control/readout system. The small spatial gap between the quantum chip and the cryogenic electronics limits the complexity of the wiring connections and the associated stray capacitance of the wires themselves, yielding a gain in terms of bandwidth and resolution. These improvements allow to design a fully-integrated CMOS readout based on current measurements with similar performances compared to the most common RF reflectometry-based readout technique. Here, we propose an acquisition system, operating at 4.2 K, which converts the spin dependent current coming from a single-electron transistor charge sensor into a digital output with a small power consumption (1 mW/qubit) and small occupation area (2). The system on chip consists of a low noise active integrator for the spin current, feeding a programmable threshold comparator to provide a digital output of the qubit state. It exhibits a resolution of tens of pA over an average input current of 1 nA with a time resolution of 1 μs.Compared to spin-qubit readout based on RF-reflectometry techniques, our solution does not require bulky off-chip compo-nents and to handle microwave signal and fast analog-to digital converters.
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.
Quantum computing holds the promise to solve problems that are currently intractable by classical computing platforms. Such computing framework relies on the properties of quantum bits (qubits) that operates at cryogenic temperatures well below 1K. Besides a quantum processor, an operational quantum computer needs a classical electronic controller reading out the electrical signals generated by the qubits and the providing electrical control to perform the required operations on the qubits. Since today’s quantum processors comprise a low number of qubits (<10), most controlling electronics can operate at room temperature and be connected by bulky interconnects to the cryogenic qubit chamber. However, when scaling up to the thousands of qubits required in a practical quantum algorithm, the classical electronics must be placed at cryogenic temperature to simplify the interconnects and allow a compact, reliable and scalable system. In this work, we propose the fully-integrated implementation in standard CMOS technology of the electronic read-out for spin qubits. Simulations show that the proposed current read-out circuit can achieve a 10MHz bandwidth that is 10X wider than current state-of-the-art read-outs while keeping comparable noise performance and operating at 4.2K. Such results are a promising indication that cryogenic CMOS circuits can provide the building blocks for the integration of the classical electronic controller for future scalable quantum computers.
In this paper, we provide a system level perspective on the design of control electronics for large scale quantum systems. Quantum computing systems with high-fidelity control and readout, coherent coupling, calibrated gates, and reconfigurable circuits with low error rates are expected to have superior quantum volumes. Cryogenic CMOS plays a crucial role in the realization of scalable quantum computers, by minimizing the feature size, lowering the cost, power consumption, and implementing low latency error correction. Our approach toward achieving scalable feed-back based control systems includes the design of memory based arbitrary waveform generators (AWG’s), wide band radio frequency analog to digital converters, integrated amplifier chain, and state discriminators that can be synchronized with gate sequences. Digitally assisted designs, when implemented in an advanced CMOS node such as 7 nm can reap the benefits of low power due to scaling. A qubit readout chain demands several amplification stages before the digitizer. We propose the co-integration of our in-house developed InP HEMT LNAs with CMOS LNA stages to achieve the required gain at the digitizer input with minimal area. Our approach using high impedance matching between the HEMT LNA and the cryogenic CMOS receiver can relax the design constraints of an inverter-based CMOS LNA, paving the way toward a fully integrated qubit readout chain. The qubit state discriminator consists of a digital signal processor that computes the qubit state from the digitizer output and a pre-determined threshold. The proposed system realizes feedback-based optimal control for error mitigation and reduction of the required data rate through the serial interface to room temperature electronics.
A fault tolerant scalable quantum computer requires more than 1000 Qubits to perform real time state correction and to improve the Qubit coherence time. The dense wiring connections required between the quantum processor placed at a 100 mK temperature and the room temperature control and readout electronics serve as a limiting factor in this system. Qubit control and readout electronics operating at 4 K temperature, placed closer to a quantum processor inside the dilution refrigerator avoids the dense wiring connections to room temperature electronics, and hence, serve as a potential solution to realize a scalable quantum computer. However, with no CMOS process technology models available at cryogenic temperatures, the design of a fully integrated, ultra-low power Qubit control and readout electronics at a sub-1 mW power consumption proves to be extremely challenging. In recent years, there has been a huge rise in interest among the research communities to understand and solve the device and circuit level challenges at deep cryogenic temperatures. This paper presents an overview of the recently published active and passive device nonidealities, circuit design challenges and solutions, and the future research directions in the Qubit control and readout electronics operating at deep cryogenic temperatures.
Quantum computers hold a promise to solve intractable problems that can not be solved with classical computers. In quantum computers, information is processed in qubits as opposed to bits. The proper operation of qubits requires them to be operated in a quantum state; hence they need to be cooled down to temperatures well below l1K. Using qubits and their quantum mechanical property, quantum computers can achieve exponential speedup in solving certain computing problems compared to classical computers. However, this can only be achieved by employing thousands of qubits. At the current state of play, quantum computers are still far from practical. The current state-of-the-art solution dictates that each qubit needs multiple interconnects towards room temperature and requires bulky room temperature instruments for qubit readout and control. Clearly, this current solution will not scale well for a quantum computer with many qubits. In an effort to solve this issue, it has been proposed to realize dedicated integrated circuits for control and readout of the qubits and operate them closer to the qubits' operating temperature to enable scalability. The thesis presents the implementation of an integrated RF receiver for spin and superconducting qubit readout applications. The work aims to replace the bulky room temperature electronics with an integrated solution that operates at 4K to enable scalability and simplification of the quantum computing architecture. The receiver has been designed in the TSMC 40nm CMOS technology and has been characterized in room temperature and 4K. The measured performance shows that the receiver features a gain of 58 dB and a minimum double-sideband noise figure of 0.6 dB at 4K with an operating frequency of 6-8 GHz; a comparable performance to the current state-of-the-art room temperature electronics used for qubit readout. Moreover, this work demonstrates a qubit readout experiment with the integrated receiver in both room temperature and 4K. The qubit readout experiment shows a comparable qubit readout performance with the integrated receiver at room temperature. However, due to oscillation problems, a degradation in qubit readout performance with the chip operating at 4K is observed.
Solid-state quantum computers require classical electronics to control and readout individual qubits and to enable fast classical data processing [1-3]. Integrating both subsystems at deep cryogenic temperatures [4], where solid-state quantum processors operate best, may solve some major scaling challenges, such as system size and input/output (I/O) data management [5]. Spin qubits in silicon quantum dots (QDs) could be monolithically integrated with complementary metal-oxide-semiconductor (CMOS) electronics using very-large-scale integration (VLSI) and thus leveraging over wide manufacturing experience in the semiconductor industry [6]. However, experimental demonstrations of integration using industrial CMOS at mK temperatures are still in their infancy. Here we present a cryogenic integrated circuit (IC) fabricated using industrial CMOS technology that hosts three key ingredients of a silicon-based quantum processor: QD arrays (arranged here in a non-interacting 3x3 configuration), digital electronics to minimize control lines using row-column addressing and analog LC resonators for multiplexed readout, all operating at 50 mK. With the microwave resonators (6-8 GHz range), we show dispersive readout of the charge state of the QDs and perform combined time- and frequency-domain multiplexing, enabling scalable readout while reducing the overall chip footprint. This modular architecture probes the limits towards the realization of a large-scale silicon quantum computer integrating quantum and classical electronics using industrial CMOS technology.