Harald Homulle

A reconfigurable cryogenic platform for the classical control of quantum processors

The implementation of a classical control infrastructure for large-scale quantum computers is challenging due to the need for integration and processing time, which is constrained by coherence time. We propose a cryogenic reconfigurable platform as the heart of the control infrastructure implementing the digital error-correction control loop. The platform is implemented on a field-programmable gate array (FPGA) that supports the functionality required by several qubit technologies and that can operate close to the physical qubits over a temperature range from 4 K to 300 K. This work focuses on the extensive characterization of the electronic platform over this temperature range. All major FPGA building blocks (such as look-up tables (LUTs), carry chains (CARRY4), mixed-mode clock manager (MMCM), phase-locked loop (PLL), block random access memory, and IDELAY2 (programmable delay element)) operate correctly and the logic speed is very stable. The logic speed of LUTs and CARRY4 changes less then 5%, whereas the jitter of MMCM and PLL clock managers is reduced by 20%. The stability is finally demonstrated by operating an integrated 1.2 GSa/s analog-to-digital converter (ADC) with a relatively stable performance over temperature. The ADCs effective number of bits drops from 6 to 4.5 bits when operating at 15 K.

Cryo-CMOS for quantum computing

Cryogenic CMOS, or cryo-CMOS circuits and systems, are emerging in VLSI design for many applications, in primis quantum computing. Fault-tolerant quantum bits (qubits) in surface code configurations, one of the most accepted implementations in quantum computing, operate in deep sub-Kelvin regime and require scalable classical control circuits. In this paper we advocate the need for a new generation of deep-submicron CMOS circuits operating at deep-cryogenic temperatures to achieve the performance required in a fault-tolerant qubit system. We outline the challenges and limitations of operating CMOS in near-zero Kelvin regimes and we propose solutions. The paper concludes with several examples showing the suitability of integrating fault-tolerant.qubits with CMOS.

LinoSPAD: a time-resolved 256×1 CMOS SPAD line sensor system featuring 64 FPGA-based TDC channels running at up to 8.5 giga-events per second

LinoSPAD is a reconfigurable camera sensor with a 256×1 CMOS SPAD (single-photon avalanche diode) pixel array connected to a low cost Xilinx Spartan 6 FPGA. The LinoSPAD sensor’s line of pixels has a pitch of 24 μm and 40% fill factor. The FPGA implements an array of 64 TDCs and histogram engines capable of processing up to 8.5 giga-photons per second. The LinoSPAD sensor measures 1.68 mm×6.8 mm and each pixel has a direct digital output to connect to the FPGA. The chip is bonded on a carrier PCB to connect to the FPGA motherboard. 64 carry chain based TDCs sampled at 400 MHz can generate a timestamp every 7.5 ns with a mean time resolution below 25 ps per code. The 64 histogram engines provide time-of-arrival histograms covering up to 50 ns. An alternative mode allows the readout of 28 bit timestamps which have a range of up to 4.5 ms. Since the FPGA TDCs have considerable non-linearity we implemented a correction module capable of increasing histogram linearity at real-time. The TDC array is interfaced to a computer using a super-speed USB3 link to transfer over 150k histograms per second for the 12.5 ns reference period used in our characterization. After characterization and subsequent programming of the post-processing we measure an instrument response histogram shorter than 100 ps FWHM using a strong laser pulse with 50 ps FWHM. A timing resolution that when combined with the high fill factor makes the sensor well suited for a wide variety of applications from fluorescence lifetime microscopy over Raman spectroscopy to 3D time-of-flight.

15.5 Cryo-CMOS circuits and systems for scalable quantum computing

Quantum computing holds the promise to achieve unprecedented computation power and to solve problems today intractable. State-of-the-art quantum processors consist of arrays of quantum bits (qubits) operating at a very low base temperature, typically a few tens of mK, as shown in Fig. 15.5.1 The qubit states degrade naturally after a certain time, upon loss of quantum coherence. For proper operation, an error-correcting loop must be implemented by a classical controller, which, in addition of handling execution of a quantum algorithm, reads the qubit state and performs the required corrections. However, while few qubits (∼10) in todays quantum processors can be easily connected to a room-temperature controller, it appears extremely challenging, if not impossible, to manage the thousands of qubits required in practical quantum algorithms [1].

200 MS/s ADC implemented in a FPGA employing TDCs

Analog signals are used in many applications and systems, such as cyber physical systems, sensor networks and automotive applications. These are also applications where the use of FPGAs is continuously growing. To date, however there is no direct integration between FPGAs, which are digital, and the analog world (except for the newest generation of FPGAs). Currently, an external analog-to-digital converter (ADC) has to be added to the system, thus limiting its overall compactness and flexibility. To address this issue we propose a novel architecture implementing a high speed ADC in reconfigurable devices. The system exploits picosecond resolution time-to-digital converters (TDCs) to reach a conversion as fast as its clock speed. The resulting analog-through-time-to-digital converter (ATDC) can achieve a sampling rate of 200 MS/s with a 7 bit resolution for signals ranging from 0 to 2.5 V. Except for the external resistor needed for the analog reference ramp, the system is fully integrated inside the target FPGA. Moreover, our design can be easily scaled for multichannel ADCs, proving the suitability of reconfigurable devices for applications requiring a deep integration between analog and digital world.

Ten years of biophotonics single-photon SPAD imager applications - retrospective and outlook

SPAD (single-photon avalanche diode) arrays are single-photon imagers that can provide unparalleled timeresolved sensing performance. Since their inception in standard CMOS technologies (2003), a host of architectures and target applications have been explored, ranging from simple pixel arrays, with off-chip data processing electronics, to fully integrated “smart” imagers with in-pixel time-stamping electronics and/or on-chip data processing fabric. Applications include (endoscopic) FLIM, (multi-beam multiphoton) FLIM-FRET, SPIMFCS, time-resolved Raman, NIROT, super-resolution microscopy, and PET, to name a few. We will review some representative sensors and applications, the corresponding challenges, and provide an outlook on the future of this fascinating technology

CryoCMOS hardware technology a classical infrastructure for a scalable quantum computer

We propose a classical infrastructure for a quantum computer implemented in CMOS. The peculiarity of the approach is to operate the classical CMOS circuits and systems at deep-cryogenic temperatures (cryoCMOS), so as to ensure physical proximity to the quantum bits, thus reducing thermal gradients and increasing compactness. CryoCMOS technology leverages the CMOS fabrication infrastructure and exploits the continuous effort of miniaturization that has sustained Moores Law for over 50 years. Such approach is believed to enable the growth of the number of qubits operating in a fault-tolerant fashion, paving the way to scalable quantum computing machines.

Characterization of bipolar transistors for cryogenic temperature sensors in standard CMOS

This paper presents the cryogenic characterization of the bipolar substrate PNPs that are typically employed as sensing elements in CMOS integrated temperature sensors. PNPs realized in a standard 160-nm CMOS technology were characterized over the temperature range from 7 K to 294 K. Although PNP non-idealities, such as finite current gain and parasitic base resistance, deteriorate at lower temperature, device operation similar to room temperature is observed down to 70 K, while operation at lower temperatures is limited by carrier freeze-out in the base region and limited current gain. These results demonstrate the feasibility of temperature sensors in standard CMOS at cryogenic temperature.

Tunable single hole regime of a silicon field effect transistor in standard CMOS technology

The electrical properties of a Single Hole Field Effect Transistor (SH-FET) based on CMOS technology are analyzed in a cryogenic environment. Few electron–hole Coulomb diamonds are observed using quantum transport spectroscopy measurements, down to the limit of single hole transport. Controlling the hole filling of the SH-FET is made possible by biasing the top gate, while the bulk contact is employed as a back gate that tunes the hole state coupling with the contacts and their distance from the interface. We compare the cryogenic Coulomb blockade regime with the room temperature regime, where the device operation is similar to that of a standard p-MOSFET.

Nanometer CMOS characterization and compact modeling at deep-cryogenic temperatures

The characterization of nanometer CMOS transistors of different aspect ratios at deep-cryogenic temperatures (4 K and 100 mK) is presented for two standard CMOS technologies (40 nm and 160 nm). A detailed understanding of the device physics at those temperatures was developed and captured in an augmented MOS11/PSP model. The accuracy of the proposed model is demonstrated by matching simulations and measurements for DC and time-domain at 4 K and, for the first time, at 100 mK.