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Quantum Supremacy Is Both Closer and Farther than It Appears (1807.10749v3)

Published 27 Jul 2018 in quant-ph, cs.DC, and cs.ET

Abstract: As quantum computers improve in the number of qubits and fidelity, the question of when they surpass state-of-the-art classical computation for a well-defined computational task is attracting much attention. The leading candidate task for this milestone entails sampling from the output distribution defined by a random quantum circuit. We develop a massively-parallel simulation tool Rollright that does not require inter-process communication (IPC) or proprietary hardware. We also develop two ways to trade circuit fidelity for computational speedups, so as to match the fidelity of a given quantum computer --- a task previously thought impossible. We report massive speedups for the sampling task over prior software from Microsoft, IBM, Alibaba and Google, as well as supercomputer and GPU-based simulations. By using publicly available Google Cloud Computing, we price such simulations and enable comparisons by total cost across hardware platforms. We simulate approximate sampling from the output of a circuit with 7x8 qubits and depth 1+40+1 by producing one million bitstring probabilities with fidelity 0.5%, at an estimated cost of $35184. The simulation costs scale linearly with fidelity, and using this scaling we estimate that extending circuit depth to 1+48+1 increases costs to one million dollars. Scaling the simulation to 10M bitstring probabilities needed for sampling 1M bitstrings helps comparing simulation to quantum computers. We describe refinements in benchmarks that slow down leading simulators, halving the circuit depth that can be simulated within the same time.

Citations (107)

Summary

  • The paper presents a novel simulation tool, Rollright, that uses a hybrid Schrödinger-Feynman algorithm to efficiently mimic complex quantum circuits.
  • It demonstrates significant speed and cost improvements through massively-parallel processing on cloud servers without requiring specialized hardware.
  • The research analyzes fidelity trade-offs and cost scalability, prompting a re-evaluation of current quantum supremacy benchmarks.

An Analysis of Quantum Supremacy Simulation and Implications for Classical Computing

The paper "Quantum Supremacy Is Both Closer and Farther than It Appears" provides an important contribution to the field by presenting advances in simulation algorithms that address the challenges associated with quantum computational supremacy. The research focuses on simulating the behavior of complex quantum circuits on classical computers, positing that progress in this area directly influences the perceived timeline of achieving quantum supremacy.

Traditional simulations of quantum circuits face significant computational challenges due to the exponential growth of complexity with the number of qubits. The research introduces a new simulation tool called Rollright, which is designed to efficiently simulate quantum circuits using a massively-parallel approach without relying on interprocess communication (IPC) or specialized hardware. These design choices enable the simulation to operate on more widely accessible cloud-server infrastructures, as opposed to exclusive supercomputing environments.

Highlights of the Rollright Simulation

The Rollright simulator utilizes a hybrid Schrödinger-Feynman algorithm, enabling the simulation of quantum wave functions with increased efficiency. Notably, the simulator demonstrates substantial speed and memory usage improvements over existing platforms like Microsoft QDK and IBM QISKit. It achieves massive computational gains by leveraging:

  1. Parallel Processing Efficiency: The simulator employs parallel processing across numerous servers, significantly reducing total simulation time.
  2. Fidelity Trade-Off: Rollright introduces an innovative method to balance fidelity and computational cost, where simulation fidelity is explicitly matched with circuit fidelity, providing a practical approach for approximate simulations.
  3. Scalability and Cost Assessment: The paper provides a detailed account of the computational costs associated with various simulation scenarios, highlighting significant reductions in operational costs, thus offering a new dimension by comparing simulation costs across different hardware platforms.

Numerical Results and Benchmarks

The research presents simulations of circuits with configurations such as 7×87 \times 8 qubits with depth $1+40+1$. These simulations yield strong numerical results, demonstrating the production of one million bitstring probabilities with a fidelity of 0.5% for an estimated cost of $35,184. This evaluation underscores the practical and economical feasibility of using classical simulations to validate quantum supremacy claims.

An emergent concern highlighted is the impact of circuit depth on cost, with the simulation costs scaling linearly with fidelity, revealing an exponential growth in costs for deeper circuits. This finding is pertinent for assessing the long-term viability and competitive edge of quantum computers over classical systems.

Implications and Future Prospects

The implications of this research are manifold. In a practical context, the success of Rollright in simulating large-scale quantum circuits using commercially available computing resources illustrates the potential for widespread accessibility in quantum research, democratizing participation in quantum exploration beyond high-resource institutions.

Theoretically, the introduction of approximation methods challenges existing notions of quantum supremacy, suggesting that the current benchmarks might need re-evaluation. The development of second-generation benchmarks, such as replacing CZ gates with iSWAP gates to make simulation more challenging, will need ongoing scrutiny to ensure fair assessments of quantum superiority.

Looking ahead, as quantum and classical computational capabilities continue to improve, the boundaries defining quantum supremacy will shift. To maintain accuracy in demonstrating quantum advantage, there needs to be a focus on improving both simulation efficiency and the reliability of quantum computations.

This research stands as a significant contribution to the field, encouraging further investigation and innovation in quantum circuit simulation, which will undoubtedly shape the landscape of computational supremacy in the years to come.

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