Session Q24: Quantum Computing Hardware

Enhancing the Capabilities of Superconducting Quantum Processors with Microwave Quantum Networks*

Superconducting electronic circuits are ideally suited for studying quantum physics and its applications. Since complex circuits containing hundreds or thousands of elements can be designed, fabricated, and operated with relative ease, they are one of the prime contenders for realizing quantum computers. Large-scale universal fault-tolerant quantum computers operated using quantum-error-corrected logical qubits will likely require millions of physical qubits. Operating this many physical qubits will benefit from a modular approach to realizing quantum computers. In this talk I will present work in which we explore approaches to network modular superconducting quantum processors using microwave-frequency quantum channels. We previously used such channels to transfer quantum states and create entanglement between two physically separated modules in a single cryostat [1] and between modules housed in two cryostats over meter-scale distances [2]. In a recent experiment, we have operated a coherent microwave quantum channel to deterministically entangle a pair of superconducting qubits across 30 meters [3]. Performing fast, and high-fidelity qubit readout along randomly chosen bases we have demonstrated a loophole-free violation of Bell’s inequality with superconducting circuits. We have evaluated a CHSH-type Bell inequality for more than one million experimental trials and determined an average S-value of 2.0747 ± 0.0033, violating Bell’s inequality by more than 22 standard deviations [3]. Our work demonstrates that non-locality is a viable new resource in networked quantum information processors based on superconducting circuits. This work also points at the potential of creating general-purpose microwave-quantum networks for applications in quantum computing, communication, and sensing, and for fundamental physics.

[1] P. Kurpiers et al., Nature 558, 264–267 (2018)

[2] P. Magnard et al., Phys. Rev. Lett. 125, 260502 (2020)

[3] S. Storz et al., Nature 617, 265-270 (2023)   

Semiconductor spin qubits: Playing the long game

Today's noisy intermediate-scale quantum (NISQ) processors support of order 1000 qubits, yet resource estimates suggest that more than one million physical qubits will be required to achieve fault- tolerant quantum computation with the surface code [1]. Of all of the qubit technologies being pursued, semiconductor spin qubits most closely resemble conventional transistors, which can be fabricated at scale with ~100 billion transistors on a chip. It is therefore prudent to pursue long-game approaches to fault-tolerant quantum computing with semiconductor spin qubits. In this lecture, I will describe how the international research community has addressed some of the most pressing challenges facing silicon spin qubits (decoherence, large effective mass, and multiple conduction band valleys). Recent progress includes multi-qubit control with fidelities that are on par with competing technologies [2], as well as a variety of approaches for coupling modules of densely packed spin qubits [3,4,5]. I will also describe new device platforms that open the door to the fabrication of large two-dimensional spin qubit arrays [6].   

Gate model quantum computing with atom arrays

The talk will present progress on gate model quantum computing with atom arrays. Large two-dimensional arrays provide a scalable architecture based on stationary atoms and rapid scanning of focused control beams. Long range interactions and entanglement are mediated by Rydberg interactions which provide a viable approach for implementing non-local qLDPC codes. Approaches to realizing mid-circuit measurements and quantum error correction based on either hyperfine shelving of single species Cs atom arrays or dual species Cs and Rb atom arrays will be presented. Integration of atom arrays with photon collection optics provides a path towards distributed quantum processing.   

Path to large-scale photonic quantum computing

Many efforts around the world are now pursuing the ambitious goal of utility-scale, fault-tolerant quantum computing. Consistent themes are emerging across the field, as teams attempt to scale from existing small systems to the millions of qubits needed for useful applications. Systems partitioning, manufacturability, cooling power, networking, and control electronics are recurring challenges across all qubit technologies. PsiQuantum is pursuing a photonic approach, based on qubits implemented using optical photons propagating in lithographically fabricated waveguides. In this talk we will give a broad overview of recent progress, framed against these major scaling challenges. We will describe the basic requirements for photonic quantum computing, the advantages and disadvantages of our approach, and the current state of maturity of the technology.