The current state of quantum computing is referred to as the noisy intermediate-scale quantum (NISQ) era, characterized by quantum processors containing up to 1000 qubits which are not advanced enough yet for fault-tolerance or large enough to achieve quantum supremacy. These processors, which are sensitive to their environment (noisy) and prone to quantum decoherence, are not yet capable of continuous quantum error correction. This intermediate-scale is defined by the quantum volume, which is based on the moderate number of qubits and gate fidelity. The term NISQ was coined by John Preskill in 2018.
NISQ algorithms are designed for quantum processors in the NISQ era, such as the variational quantum eigensolver (VQE) and quantum approximate optimization algorithm (QAOA), which use NISQ devices but offload some calculations to classical processors. These algorithms have been successful in quantum chemistry and have potential applications in various fields including physics, materials science, data science, cryptography, biology, and finance. However, they often require error mitigation techniques to produce accurate results.
The creation of a computer with tens of thousands of qubits and enough error correction would eventually end the NISQ era. These beyond-NISQ devices would be able to, for example, implement Shor's algorithm for very large numbers and break RSA encryption.
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