Quantum Money

Quantum states serving as unforgeable currency, exploiting the no-cloning theorem to prevent counterfeiting.

Quantum money is a theoretical concept where currency exists as quantum states that cannot be copied, making counterfeiting physically impossible.

The Idea

Classical money can be counterfeited by copying. But the no-cloning theorem says you can’t copy an unknown quantum state. So: make money from quantum states!

Wiesner’s Scheme (1983)

The original quantum money proposal:

Creation (by bank)

For each bill:

1. Generate random serial number $s$
2. Create $n$ qubits, each randomly in $|0⟩$, $|1⟩$, $|+⟩$, or $|-⟩$
3. Record the serial number and basis choices in secret database
4. Bill = serial number + quantum state

Verification

1. Customer presents bill to bank
2. Bank looks up bases for that serial number
3. Measures each qubit in the recorded basis
4. If all correct → valid; if errors → counterfeit

Why It’s Unforgeable

Counterfeiter doesn’t know the bases. Any measurement to learn the state disturbs it. Can’t clone without knowing.

Limitations of Wiesner’s Scheme

Public-Key Quantum Money

Can anyone verify without bank involvement?

Requirements

• Public verification: Anyone can verify
• Unforgeability: Still can’t copy

This is much harder. Some proposals exist but are less practical.

Types of Quantum Money

Private-Key (Wiesner-type)

• Only issuer can verify
• Simpler to achieve

Public-Key

• Anyone can verify
• Much more challenging
• Active research area

Quantum Tokens

• Single-use credentials
• Simpler security requirements

Current Status

Quantum money remains largely theoretical:

• Requires long-lived quantum memory
• No practical systems deployed
• Active research in quantum-computational money (based on computational assumptions)

Related Concepts

Why It Matters

Quantum money demonstrates:

• Practical implications of no-cloning
• Quantum advantages for security
• The frontier of quantum cryptography

See also: No-Cloning Theorem, Quantum Key Distribution