Alice & Bob's research estimates that approximately 29,600 physical qubits could crack the Bitcoin key.

A study by quantum computing company Alice & Bob estimates that a fault-tolerant quantum computer with approximately 29,600 physical qubits could solve the 256-bit elliptic curve cryptography problem used by Bitcoin in about one hour. This research uses the cat-qubits technique, and the results are theoretical resource estimates; existing quantum computers do not yet possess this capability. If a sufficiently powerful quantum computer becomes available in the future, approximately 6.9 million Bitcoins would be at risk due to their public keys being publicly available. (Source: Bitcoin.com News)