What is meant by "Noisy Intermediate-Scale Quantum" (NISQ) technology?

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"Noisy Intermediate-Scale Quantum" (NISQ) is a term coined by John Preskill in 2017 to describe a specific era in the development of quantum computing technology. The term highlights the current state of quantum computers, which are characterized by limited qubit counts, relatively short coherence times, and the presence of noise and errors in quantum operations.

The key features of NISQ technology include:

Intermediate-Scale: NISQ devices have a moderate number of qubits, typically ranging from a few to a few dozen qubits. While these qubit numbers are still far from the hundreds or thousands required for full-scale fault-tolerant quantum computing, they are large enough to demonstrate quantum computational power and to explore quantum algorithms and applications.

Noisy: Quantum devices in the NISQ era are subject to various sources of noise, which can arise from factors such as thermal fluctuations, device imperfections, and interactions with the environment. Noise can introduce errors in quantum operations and limit the coherence time of qubits.

Quantum Supremacy and Quantum Advantage: Despite the noise and limitations, NISQ devices have the potential to achieve "quantum supremacy" or "quantum advantage." Quantum supremacy refers to the point where a quantum computer can perform certain tasks faster than classical computers, even considering the noise. Quantum advantage implies that a quantum computer can solve specific problems more efficiently than classical computers, making it a valuable tool for certain applications.

Algorithm Exploration: NISQ devices are used for exploring and testing quantum algorithms, error mitigation techniques, and new quantum applications. Researchers are developing quantum algorithms that are specifically tailored to the capabilities and constraints of NISQ devices.

Hybrid Approaches: To overcome the limitations of NISQ devices, researchers often use hybrid approaches that combine classical and quantum computation. In hybrid algorithms, quantum processors perform specific subtasks while classical computers handle other aspects of the computation, such as error correction or optimization.

The NISQ era is seen as an important transitional phase in the development of quantum computing technology. While NISQ devices are not yet capable of solving complex problems that are intractable for classical computers, they offer the opportunity to explore and experiment with quantum algorithms, learn more about quantum error correction, and develop novel applications that can leverage the unique properties of quantum systems.

As quantum hardware improves, and as error correction and noise mitigation techniques advance, researchers aim to move beyond the NISQ era toward the development of large-scale fault-tolerant quantum computers capable of solving more complex and practical problems.