NISQ Computers: A Path to Quantum Supremacy

TL;DR

The paper surveys the NISQ era as researchers pursue quantum supremacy and practical near-term applications. It synthesizes major supremacy milestones across superconducting and photonic platforms, detailing Google's Sycamore, Zuchongzhi, Jiuzhang, and Borealis achievements, as well as debates about classical simulability. It also maps the patent landscape and highlights applied quantum computing efforts in ML, chemistry, NLP, finance, optimization, and numerical methods, illustrating a diverse path toward quantum advantage. The findings underscore substantial progress toward fault-tolerant quantum computing, while acknowledging the persistent challenges of noise, limited qubit counts, and error correction requirements, and they point to a future where early practical quantum applications begin to materialize across science and industry.

Abstract

The quest for quantum advantage, wherein quantum computers surpass the computational capabilities of classical computers executing state-of-the-art algorithms on well-defined tasks, represents a pivotal race in the domain of quantum computing. NISQ (Noisy Intermediate-Scale Quantum) computing has witnessed remarkable advancements, culminating in significant milestones on the journey towards the realization of universal fault-tolerant quantum computers. This transformative turning point, known as quantum supremacy, has been achieved amid a series of breakthroughs, signifying the dawn of the quantum era. Quantum hardware has undergone substantial integration and architectural evolution, contrasting with its nascent stages. In this review, we critically examine the quantum supremacy experiments conducted thus far, shedding light on their implications and contributions to the evolving landscape of quantum computing. Additionally, we endeavor to illuminate a range of cutting-edge proof-of-principle investigations in the realm of applied quantum computing, providing an insightful overview of the current state of applied quantum research and its prospective influence across diverse scientific, industrial, and technological frontiers.

Figures (9)

• Figure 1: The developmental trajectory of quantum information processing. This development can be conceptualized as a progression through seven distinct stages, each of which relies on and complements the preceding stages while requiring continuous refinement in parallel with the others. Over time, the hope is to advance from the earlier stages to the more advanced ones, building upon the foundations laid by the preceding phases. Presently, quantum computing platforms are primarily situated between the third stage, involving (QND) Quantum Non-Demolition measurements, and the fourth stage dedicated to logical memory. In this transitional phase, computers are becoming capable of implementing error-correcting codes to construct logical qubits from physical qubits. It's important to note that, with the exception of the last two stages that mark the beginning of the fault-tolerant era, the earlier stages mostly belong to what is known as the NISQ era. Additionally, it's crucial to emphasize that progressing from a lower to a higher stage does not negate the ongoing need for improving the techniques required for the lower stages. For instance, enhancing the control of individual physical qubits remains an essential goal throughout the entire developmental timeline, even after logical qubits replace physical ones in computational applications.
• Figure 2: Proportion of patent applications in the field of 2G quantum simulation for which a patent was granted, highlighting the United States' significant role with over 37% of International Patent Families securing IP rights. It also notes other regions' substantial but relatively lower proportions, including Japan (14%), Australia (12%), China (9%), and Europe (8%, based on granted European patents). Data sourced from EPO23sim.
• Figure 3: Google's Sycamore quantum processor layout. This processor composed of a rectangular-array of $54$ programmable Superconducting transmon qubits. Each qubit linked to the four nearest neighbors via couplers. As one qubit did not operate perfectly during the supremacy experiment, the quantum processor has $53$-qubits and $86$-couplers.
• Figure 4: A $m$-cycle circuit schematic diagram of the Sycamore 53-qubit RQCs. Each cycle joining a layer of random single-qubit gates (chosen at random from the gate set $\{\sqrt{X},\, \sqrt{Y},\, \sqrt{W} \}$), followed by a layer of 2-qubit gates, labeled A, B, C, or D. Layers in longer circuits repeat in the order A; B; C; D -- C; D; A; B. It is worth noting that single-qubit gates do not repeat sequentially, also there is an additional layer of one-qubit gates prior measurements.
• Figure 5: The photonics-based quantum computing device developed by USTC, known as Jiuzhang, operates by intricately controlling light through an array of optical devices. A photographic representation of the Jiuzhang photonic network reveals the experimental configuration, which is situated on an optical table spanning approximately three square meters. Within this setup, 25 Two-Mode Squeezed States (TMSSs) are introduced into the photonic network, resulting in the collection of 25 phase-locked light signals. To further elaborate, the output modes of the Jiuzhang photonic network are meticulously separated into 100 distinct spatial modes through the utilization of mini-mirrors and Polarizing Beam Splitters (PBSs). This achievement marks the second quantum computer claiming to attain quantum computational advantages, following Google's Sycamore quantum processor. Reproduced under Creative Commons license CC BY from jiuzhang.