Decentralized Cash

Blockchain has effectively solved the mechanism behind decentralized currency issuance. A central bank could effectively be run by a populace participating in governance rather than a select few managers. However, the issuance of this currency is inherently digital. The cryptographic signature can only efficiently be generated on a device like a computer, and creating paper cash with the same capabilities is prohibitively expensive.

Even if cash with cryptographic signatures was possible, even the decentralized currencies we have today are tied to the state of the blockchain. A token on Ethereum isn't really "cash" because once it's spent, inherently everyone else on the blockchain can at least observe and form conclusions about the transaction. Cash, when spent, should be completely informal. No ledgers, no records, just spendable material. Could be physical, but could be digital as long as the requirements suffice.

The quantum answer

Stephen Wiesner theorized about quantum money around 1969, in a paper called "Conjugate Coding." When I found this, I saw that the realization of a truly decentralized cash is possible.

The core idea leans on the no-cloning theorem: an unknown quantum state cannot be copied. That property is what paper cash approximates with holograms and security codes, and what digital money fakes with a ledger everyone has to agree on. A quantum banknote doesn't need either. The quantum state is preserved as the currency is passed from wallet to wallet, and any attempt to duplicate it destroys the information you'd need to duplicate it. Counterfeiting is literally forbidden by physics.

Wiesner's original scheme had one big flaw for our purposes: only the mint could verify a bill. That's a central bank with extra steps. The interesting progress came later. Scott Aaronson proposed public-key quantum money in 2009, where anyone can verify a note without talking to the issuer, and Aaronson and Christiano formalized a candidate construction in 2012. Then Mark Zhandry's "quantum lightning" work in 2019 pushed it further: quantum states so unclonable that not even the party who generated them could produce a second copy. That's the theoretical object closest to true decentralized cash. Notes that verify locally, transfer hand to hand, and leave no record anywhere.

The how

I am still thinking about the how though. Two paths:

Do it in a decentralized manner like Bitcoin, where issuance is governed by protocol and participation rather than an institution. Quantum lightning actually gestures at this, since notes could bolt onto a consensus layer only for issuance while circulation stays completely off-ledger.

Or gain some political position and issue it as a central bank would. Faster to legitimacy, but you've reintroduced the concentration risk.

Decisions.