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Digest of technical papers/Digest of technical papers - IEEE International Solid-State Circuits Conference

19.1 A Scalable Cryo-CMOS 2-to-20GHz Digitally Intensive Controller for 4×32 Frequency Multiplexed Spin Qubits/Transmons in 22nm FinFET Technology for Quantum Computers

Bishnu Patra, Jeroen P. G. van Dijk, Sushil Subramanian, Andrea Corna, Xiao Xue, Charles Jeon, Farhana Sheikh, Esdras Juarez-Hernandez, Brando Perez Esparza, Huzaifa Rampurawala, Brent R. Carlton, Nodar Samkharadze, Surej Ravikumar, Carlos F. Nieva, Sungwon Kim, Hyung‐Jin Lee, Amir Sammak, Giordano Scappucci, Menno Veldhorst, Lieven M. K. Vandersypen, Masoud Babaie, Fabio Sebastiano, Edoardo Charbon A, Stefano Pellerano

Published Feb 1, 2020• 110 citations

Abstract

Quantum computers (QC), comprising qubits and a classical controller, can provide exponential speed-up in solving certain problems. Among solid-state qubits, transmons and spin-qubits are the most promising, operating ≪ 1K. A qubit can be implemented in a physical system with two distinct energy levels representing the |0) and |1) states, e.g. the up and down spin states of an electron. The qubit states can be manipulated with microwave pulses, whose frequency f matches the energy level spacing E = hf (Fig. 19.1.1). For transmons, f ~ 6GHz, for spin qubits f ~ 20GHz, with the desire to lower it in the future. Qubit operations can be represented as rotations in the Bloch sphere. The rotation axis is set by the phase of the microwave signal relative to the qubit phase, which must be tracked for coherent operations. The pulse amplitude and duration determine the rotation angle. A π-rotation is typically obtained using a 50ns Gaussian pulse for transmons and a 500ns rectangular pulse for spin qubits with powers of -60dBm and -45dBm, respectively.

DOI: 10.1109/isscc19947.2020.9063109

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Quantum computers (QC), comprising qubits and a classical controller, can provide exponential speed-up in solving certain problems. Among solid-state qubits, transmons and spin-qubits are the most promising, operating ≪ 1K. A qubit can be implemented in a physical system with two distinct energy levels representing the |0) and |1) states, e.g. the up and down spin states of an electron. The qubit states can be manipulated with microwave pulses, whose frequency f matches the energy level spacing E = hf (Fig. 19.1.1). For transmons, f ~ 6GHz, for spin qubits f ~ 20GHz, with the desire to lower it in the future. Qubit operations can be represented as rotations in the Bloch sphere. The rotation axis is set by the phase of the microwave signal relative to the qubit phase, which must be tracked for coherent operations. The pulse amplitude and duration determine the rotation angle. A π-rotation is typically obtained using a 50ns Gaussian pulse for transmons and a 500ns rectangular pulse for spin qubits with powers of -60dBm and -45dBm, respectively.

Building a large-scale quantum computer requires the co-optimization of both the quantum bits (qubits) and their control electronics. By operating the CMOS control circuits at cryogenic temperatures (cryo-CMOS), and hence in close proximity to the cryogenic solid-state qubits, a compact quantum-computing system can be achieved, thus promising scalability to the large number of qubits required in a practical application. This work presents a cryo-CMOS microwave signal generator for frequency-multiplexed control of 4 × 32 qubits (32 qubits per RF output). A digitally intensive architecture offering full programmability of phase, amplitude, and frequency of the output microwave pulses and a wideband RF front end operating from 2 to 20 GHz allow targeting both spin qubits and transmons. The controller comprises a qubit-phase-tracking direct digital synthesis (DDS) back end for coherent qubit control and a single-sideband (SSB) RF front end optimized for minimum leakage between the qubit channels. Fabricated in Intel 22-nm FinFET technology, it achieves a 48-dB SNR and 45-dB spurious-free dynamic range (SFDR) in a 1-GHz data bandwidth when operating at 3 K, thus enabling high-fidelity qubit control. By exploiting the on-chip 4096-instruction memory, the capability to translate quantum algorithms to microwave signals has been demonstrated by coherently controlling a spin qubit at both 14 and 18 GHz.

Quantum computing promises exponential speed-up in solving certain complex problems that would be intractable by classical computers. However, thousands or millions of qubits might be required to solve useful problems. High-precision and low-noise electrical signals are required to manipulate and read the state of a qubit and to control qubit-to-qubit interactions. Current systems use room temperature electronics with many coax cables routed to the qubit chip inside a dilution refrigerator. This approach does not scale to large number of qubits, due to form factor, cost, power consumption and thermal load to the fridge. To address this challenge, a cryogenic qubit controller has been proposed [1]. The first integrated implementation of a cryogenic pulse modulator has been presented in [2], demonstrating the capability of manipulating (drive) the state of superconducting qubits. The work in [3] extends the capability of the controller with 3 main features: frequency-multiplexing to reduce the number of RF cables per qubit, an arbitrary I/Q pulse generation for improved control fidelity and a digitally-intensive architecture with integrated instruction set to enable integration in existing quantum control stacks. This work further advances the prior art by integrating the capability of reading the qubit state and generating the voltage pulses required for drive, readout, 2-qubit operations and qubit characterization. The SoC can drive up to 16 spin qubits by frequency multiplexing over a single RF line, read the state of up to 6 qubits simultaneously and control up to 22 gate potentials. The SoC also integrates a

\mu

-controller for increased flexibility in implementing the control instruction set. The proposed cryogenic controller can replace all the high-speed control electronics used in conventional solutions today, paving the way towards scalable quantum computers.

Quantum computing holds the promise to solve many of today's intractable problems. A solid-state quantum computer (QC) is generally made of an array of qubits implemented in one of many solid-state technologies and operating at deep-cryogenic temperatures (10-to-20mK). Silicon spin qubits are a promising candidate for scalable QCs, due to their size, long coherence times and potential for co-integration with the required classical control and readout electronics. Recently, semiconductor spin qubits have been demonstrated to operate at ~1K, thus accelerating the achievement of a compact QC [1]. Classical qubit control electronics has also progressed, with the demonstration of fully-integrated control of spin qubits [2] and transmons [3] implemented in a cryo-CMOS technology. While a cryo-CMOS integrated circuit has been co-integrated with quantum dots [4], fully-integrated readout electronics has not yet been addressed in the literature.

Cryogenic CMOS (cryo-CMOS) support of quantum processors is becoming a necessity to ensure the continuous growth of qubit count, so as to achieve scalable, fault-tolerant quantum computers. The first paper of the session describes an integrated controller for spin qubits that performs state manipulation, readout, and gate pulsing fabricated in 22nm FinFET CMOS technology. The second paper presents a fully integrated SoC for spin qubit interface based on RF reflectometry of quantum dots, all implemented in 40nm CMOS technology, for scalable quantum systems operating at 3.5K. The third paper also focuses on scalable RF based readout of spin qubits for a record 0.17mW/qubit power requirement, operating at 4.2K. The fourth paper proposes a 1GS/s A/D converter for low-power digitization of the signal measured from a qubit readout that achieves a FOM of 15fJ/conv.-step at 4K.

A scalable, non-multiplexed cryogenic 14-nm FinFET quantum bit (qubit) state controller (QSC) for use in the semi-autonomous control of superconducting transmon qubits is reported. The QSC includes an augmented general-purpose digital processor that supports waveform generation and phase rotation operations combined with a low-power current-mode single sideband upconversion

{I}/{Q}

mixer-based RF arbitrary waveform generator (AWG). Implemented in the 14-nm CMOS FinFET technology, the QSC generates control signals in its target 4.5–5.5-GHz-frequency range, achieving an spurious free dynamic range (SFDR) > 50 dB for a signal bandwidth of 500 MHz. With the controller operating in the 4 K stage of a cryostat and connected to a transmon qubit in the cryostat’s millikelvin stage, measured transmon

T_{1}

and

T_{2}

coherence times were 75.7 and 73

\mu \text{s}

, respectively, in each case comparable to results achieved using conventional room temperature (RT) controls. In further tests with transmons, a qubit-limited error rate of 7.76 × 10−4 per Clifford gate is achieved, again comparable to the results achieved using RT controls. The QSC’s maximum RF output power is −18 dBm, and power dissipation per qubit under active control is 23 mW.