The maximum number of qubits a quantum computer can have without becoming unstable is not a fixed value that can be definitively stated. The stability of a quantum computer depends on various factors, including the specific hardware implementation, error correction techniques employed, and the overall quality of the qubits and control systems.
Quantum systems are susceptible to errors and decoherence, which refers to the loss of quantum information due to interactions with the environment. To address these issues, error correction techniques are used to protect the quantum information and extend the coherence time.
Currently, the largest stable quantum computers have reached around 100 qubits. However, these systems heavily rely on error correction methods to mitigate errors and maintain stability. The number of physical qubits needed to achieve a certain number of stable logical qubits (those protected by error correction) is typically much higher.
Researchers and engineers are actively working on improving the stability and scalability of quantum computers. The field of quantum computing is still in its early stages, and it is difficult to predict when large-scale, fault-tolerant quantum computers with thousands or millions of qubits will be realized. It will likely require significant advancements in hardware, error correction techniques, and control systems to achieve such a milestone.