The current state ofquantum computing[1] is referred to as thenoisy intermediate-scale quantum (NISQ)era,[2][3] characterized by quantum processors containing up to 1,000qubits which are not advanced enough yet forfault-tolerance or large enough to achievequantum advantage.[4][5] These processors, which are sensitive to their environment (noisy) and prone toquantum decoherence, are not yet capable of continuousquantum error correction. This intermediate-scale is defined by thequantum volume, which is based on the moderate number of qubits andgate fidelity. The term NISQ was coined byJohn Preskill in 2018.[6][2]
According toMicrosoft Azure Quantum's scheme, NISQ computation is considered level 1, the lowest of the quantum computing implementation levels.[7][8]
In October 2023, the 1,000 qubit mark was passed for the first time by Atom Computing's 1,180 qubit quantum processor.[9] However, as of 2024, only two quantum processors have over 1,000 qubits, with sub-1,000 quantum processors still remaining the norm.[10]
NISQ algorithms arequantum algorithms designed for quantum processors in the NISQ era. Common examples are thevariational quantum eigensolver (VQE) andquantum approximate optimization algorithm (QAOA), which use NISQ devices but offload some calculations to classical processors.[2] These algorithms have been successful inquantum chemistry and have potential applications in various fields including physics, materials science, data science, cryptography, biology, and finance.[2] However, due to noise during circuit execution, they often require error mitigation techniques.[11][5][12][13]These methods constitute a way of reducing the effect of noise by running a set of circuits and applying post-processing to the measured data. In contrast toquantum error correction, where errors are continuously detected and corrected during the run of the circuit, error mitigation can only use the outcome of the noisy circuits.
The creation of a computer with tens of thousands of qubits and enough error correction would eventually end the NISQ era.[4] These beyond-NISQ devices would be able to, for example, implementShor's algorithm for very large numbers and breakRSA encryption.[14]
In April 2024, researchers at Microsoft announced a significant reduction in error rates that required only 4 logical qubits, suggesting that quantum computing at scale could be years away instead of decades.[15]
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