Some answers on quantum computing

Fifth installment of the series, answering four fundamental questions raised by the LinkedIn community: information storage, computing basis, memory architecture, and RSA-2048 decryption. Originally published in French.

Quantum information storage

Information is encoded in the qubits themselves as quantum states, inherently temporary (coherence times from µs to minutes depending on architecture). The no-cloning theorem forbids duplication: any read attempt collapses the superposition. Computation results, once measured, become classical and can be stored in conventional architectures.

Computing basis and memory architecture

The computing basis remains |0⟩ and |1⟩, but superposition enables simultaneous processing of all states. Quantum computers have no memory in the classical sense (registers, cache, RAM). Future architectures will combine classical systems (possibly HPC for qubit stabilization) with quantum components.

Shor’s algorithm in detail

The article pedagogically walks through Shor’s four steps — choosing a random integer a coprime with N, finding the period r via quantum Fourier transform, deducing factors through GCD — with a complete example of factoring N=21.

RSA-2048 decryption and post-quantum cryptography

Qubit estimates range from 372 physical qubits (questionable Chinese paper) to 4,100 logical qubits (~4 million physical transmon qubits). With cat-qubits’ 90% error correction reduction, the threshold would be approximately 400,000 stable physical qubits — a 15-20 year horizon. Lattice-based cryptography is identified as the best RSA/ECC replacement candidate.

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Key takeaways

Information storage: information is encoded in the qubits themselves as quantum states, inherently temporary (µs to minutes depending on coherence). No memory in the classical sense — the no-cloning theorem forbids duplication (any read attempt collapses the superposition).
Computing basis: computational basis |0⟩ and |1⟩, like classical binary. The fundamental difference: 2 bits encode 00, 01, 10, 11 sequentially; 2 qubits encode |00⟩, |01⟩, |10⟩, |11⟩ simultaneously. Upon measurement, collapse to a single classical state.
Memory architecture: no registers, cache, or RAM in the classical sense. Information is volatile by nature. Future architectures will combine classical systems (possibly HPC for qubit stabilization) with quantum components.
Shor's algorithm detailed: factorization problem reduced to order-finding → quantum Fourier transform to find period r → factor deduction via GCD. Complete pedagogical example: factoring N=21 with a=2, r=6.
RSA-2048 decryption: estimates range from 372 physical qubits (questionable Chinese arxiv paper) to 4,100 logical qubits / ~4M physical transmon qubits (IBM). With cat-qubits (−90% correction): ~400K stable physical qubits. Realistic horizon: 15-20 years.
Post-quantum cryptography: lattice-based cryptography is resistant to Shor's algorithm and is the best candidate to replace RSA/ECC in the post-quantum era.