What is a quantum computer?

A quantum computer works with qubits, which can hold a combination of 0 and 1 and can be entangled with each other. A cryptographically relevant one would be large and reliable enough to break the public-key cryptography that wallets and websites rely on.

Also called cryptographically relevant quantum computer, CRQC or quantum computing.

How a quantum computer differs

An ordinary computer stores each bit as a 0 or a 1. The information in a quantum computer is carried by qubits, which can be entangled so that their states depend on one another.

That structure lets a quantum computer do certain jobs faster than a classical one. For factoring large numbers, a quantum algorithm does far less work than any known classical method, and Shor’s algorithm is the best-known example.

What a cryptographically relevant machine needs

  • It must run Shor’s algorithm, or a method like it, on keys of the size in use.
  • Noise limits how long a circuit can run, so its qubits must be error corrected, and error correction takes many physical qubits for each reliable one.
  • Gidney’s 2025 estimate says that a machine with fewer than a million noisy qubits could factor a 2048-bit RSA integer in less than a week.
  • Gidney and Ekerå estimated in 2019 that 20 million noisy qubits could do the same job in 8 hours.

Why today’s machines are not one

The largest working machines have far fewer noisy qubits than the estimates require, and their gates are too noisy for long calculations. Preskill coined the term NISQ, for noisy intermediate-scale quantum, to describe machines of this kind, and he noted that noise limits how large a circuit they can run reliably.

In Redacted

Bunker mode runs your private account entirely on hash-based cryptography, the kind expected to hold up against quantum computers. Its proofs are STARKs, its signatures use SLH-DSA, and nothing it writes on chain is encrypted, so there is nothing stored there for a future computer to decrypt.

Bunker mode →

Related terms

Shor’s algorithmShor’s algorithm is a quantum method, described by Peter Shor in 1994, that factors large numbers and computes discrete logarithms in polynomial time. A large enough quantum computer running it would break RSA and elliptic-curve signatures such as ECDSA.Grover’s algorithmGrover’s algorithm is a quantum search method that finds one item among N in about the square root of N steps. Against symmetric keys and hashes, it halves the strength in bits, so a 256-bit key keeps about 128 bits of strength against it.Elliptic curve cryptographyElliptic curve cryptography builds key pairs from points on an elliptic curve over a finite field. It gives about the strength of 3072-bit RSA with 256-bit keys, and Bitcoin signs with the secp256k1 curve.Post-quantum cryptographyPost-quantum cryptography is ordinary cryptography designed to stay secure when large quantum computers exist. It replaces the key exchanges and signatures that Shor’s algorithm breaks, such as RSA and elliptic curves, with schemes built on lattices, hash functions or error-correcting codes.Harvest now, decrypt laterHarvest now, decrypt later means recording encrypted data today to decrypt it once quantum computers are strong enough. Anything encrypted on a public blockchain is stored for good, so it is open to this by design.Bunker modeBunker mode is Justin Drake’s name for a calm, early move of crypto to addresses whose public keys stay hidden behind a hash, in case elliptic curves break. In Redacted, one click puts your account on 25 words, with no wallet key behind it.

Sources: Quantum Computing in the NISQ era and beyond, John Preskill, 2018 · How to factor 2048 bit RSA integers with less than a million noisy qubits, Craig Gidney, 2025 · How to factor 2048 bit RSA integers in 8 hours using 20 million noisy qubits, Craig Gidney and Martin Ekerå, 2019

Updated November 1, 2026. All terms