Quantum Computing in the NISQ era and beyond (1801.00862v3)

Abstract: Noisy Intermediate-Scale Quantum (NISQ) technology will be available in the near future. Quantum computers with 50-100 qubits may be able to perform tasks which surpass the capabilities of today's classical digital computers, but noise in quantum gates will limit the size of quantum circuits that can be executed reliably. NISQ devices will be useful tools for exploring many-body quantum physics, and may have other useful applications, but the 100-qubit quantum computer will not change the world right away --- we should regard it as a significant step toward the more powerful quantum technologies of the future. Quantum technologists should continue to strive for more accurate quantum gates and, eventually, fully fault-tolerant quantum computing.

• The paper establishes that NISQ devices, despite high noise and error rates, can explore quantum phenomena beyond classical simulation.
• The paper emphasizes that reducing quantum gate errors is crucial to extend circuit sizes and advance toward scalable, fault-tolerant quantum computing.
• The paper highlights quantum algorithms like Shor’s and variational methods as key examples demonstrating both the potential benefits and challenges of quantum simulations.

Overview of "Quantum Computing in the NISQ Era and Beyond"

The paper "Quantum Computing in the NISQ Era and Beyond" by John Preskill delineates the status, potential, and challenges of quantum computing technology as we transition into the Noisy Intermediate-Scale Quantum (NISQ) era. This time period is characterized by quantum devices with 50-100 qubits that, despite being susceptible to noise, hold the capability to surpass classical digital computers for certain tasks. However, the prevailing noise will constrain the size of quantum circuits and the reliability of their execution.

Opportunities and Challenges in the NISQ Era

Preskill identifies the onset of the NISQ era as a pivotal turning point for quantum computing, providing unprecedented tools for exploring quantum phenomena and potentially developing useful applications beyond current capabilities. NISQ devices will facilitate the paper of many-body quantum physics, enabling deeper scientific understanding and potentially uncovering phenomena that are infeasible to simulate classically. Nonetheless, the practical implications for business and industry remain speculative, with near-term commercial breakthroughs being uncertain.

The paper underscores the challenges inherent in quantum computing, primarily due to noise and error rates. High error rates in quantum gates will limit the use of large quantum circuits, and quantum error correction remains a distant goal, given its requisite high overhead costs. Consequently, the immediate task is improving quantum gate accuracy, which will extend the utility of NISQ devices and lay the foundation for scalable, fault-tolerant quantum computing.

Theoretical Underpinnings and Quantum Complexity

Preskill argues that the theoretical promise of quantum computing is deeply tied to quantum complexity and quantum error correction, both rooted in the phenomenon of quantum entanglement. By leveraging entanglement, quantum computers are postulated to efficiently simulate natural processes that are intractable for classical computers. Key justifications for the superior potential of quantum computing include:

1. Quantum Algorithms for Intractable Problems: Quantum algorithms, such as Shor's factoring algorithm, address problems believed to be hard for classical computing.
2. Complexity Theory Evidence: Complexity theory offers arguments suggesting quantum states that are efficient to prepare have properties that cannot be classically replicated.
3. Lack of Classical Simulation Methods: The persistent absence of classical algorithms simulating quantum computations supports the unique power of quantum systems.

Quantum Applications and the Path Forward

Preskill outlines several potential quantum computing applications, such as quantum optimization algorithms like the Quantum Approximate Optimization Algorithm (QAOA) and Variational Quantum Eigensolvers (VQE), which could tackle both classical and quantum problems. Yet, these possibilities face substantial hurdles due to noise and the lack of fault-tolerant computing.

In addition, Preskill discusses the promise and limitations of quantum annealing, deep learning, matrix inversion, semidefinite programming, and analog quantum simulation, emphasizing their potential but also noting their experimental and theoretical challenges.

Conclusion

As we advance through the NISQ era, the focus remains on improving quantum hardware precision and exploring near-term applications cautiously, while simultaneously working towards the long-term goal of fault-tolerant quantum computing. The paper provides a balanced perspective, highlighting the immense challenges and cautiously optimistic potential of quantum computing advancements. The NISQ era represents a crucial developmental phase, but the transformative impact of quantum computing awaits further technological and scientific breakthroughs. Researchers are advised to direct efforts toward both reducing quantum gate error rates and developing noise-resilient algorithms, positioning NISQ technology as a stepping stone toward the eventual realization of fully scalable, fault-tolerant quantum systems.