# Wiesner Quantum Money

The classical money scheme involves the Bank distributing notes to untrusted users. Each note has a unique serial number attached to it and this number provides a basis for the verification of the note when the user wants to use it for a transaction. However, in the classical world, nothing prevents a user with sufficient resources to be able to forge the note and create more notes than what he originally had in possession. In the 1980s, Wiesner proposed the idea of quantum money to create unforgeable bank notes. The unforgeability of the note relied on the no-cloning property of quantum mechanics. In this example protocol, the banknotes are several BB84 states prepared by the Bank, who then distributes them to the untrusted users. When the user needs to carry out a transaction with his note, he sends it to the Bank for verification, who then authenticates the validity of the note. Based on the no-cloning property of quantum mechanics, Wiesner showed information-theoretic security against a forger of bank notes.

Tags: Multi Party Protocols, non-local games, Quantum Enhanced Classical Functionality, Specific Task

## Assumptions

• The quantum money state which is a two-state system must be isolated from the rest of universe, roughly.
• When Wiesner wrote his thesis, there was no device operating in which the phase coherence of a two-state system was preserved for longer than about a second.

## Outline

Let the money have ${\displaystyle n}$ isolated systems ${\displaystyle S_{i}\in \{a,b,\alpha ,\beta \},i=1,...,n}$.

• The Mint creates two random binary sequences of ${\displaystyle n}$ digits ${\displaystyle M_{i},N_{i}\in \{0,1\}}$ where ${\displaystyle i=1,...,n}$. Then, two-state systems are placed in one of four states ${\displaystyle a,b,\alpha ,\beta }$.
1. Bank prepares a pair of orthonormal base states for each state system. Then the two-state system is located in one of four states ${\displaystyle a,b,\alpha ,\beta }$
2. The bank records all polarizations and their serial numbers. On the banknote/quantum money the serial number is plain, while polarizations are kept hidden.
3. If the money is returned to the Mint, it checks whether each isolated system is still in its initial state or not.

Note that since no one except the Mint knows ${\displaystyle M_{i}}$ and ${\displaystyle N_{i}}$, even if someone copies the money, he cannot recover the polarization.

## Notation

• ${\displaystyle S_{i}}$= Isolated system
• ${\displaystyle M_{i},N_{i}}$= Random binary sequences
• ${\displaystyle a,b,\alpha ,\beta }$= States

## Requirements

• Network stage: quantum memory network

## Properties

• The scheme requires a central bank for verifying the money
• Pairs of conjugate variables has the same relation with Heisenberg uncertainty principle
• The success probability of the adversary in guessing the state of the target quantum money is ${\displaystyle (3/4)^{n}}$

## Protocol Description

Input: ​Product state of ${\displaystyle N}$ qubit and a serial number
Output: ​approved/rejected
Stage 1: Preparation

1. The Mint generate a quantum money composed of two component ${\displaystyle (|S\rangle ,k)}$ where ${\displaystyle k}$ is the serial number of the banknote and ${\displaystyle S}$ is a product state of ${\displaystyle N}$ qubits. Each qubit is randomly chosen from the set ${\displaystyle \{|0\rangle ,|1\rangle ,|+\rangle ,|-\rangle \}}$
2. Serial numbers and their states are recorded and kept at the Mint

Stage 2: Verification

1. The Mint looks for the serial number and the corresponding measurement basis in its database. Thus, each qubit is measured in the right basis,${\displaystyle \{|0\rangle ,|1\rangle \}}$ or ${\displaystyle \{|+\rangle ,|-\rangle \}}$.
2. The Mint outputs 1 if the result of the measurement corresponds with the data stored in its database, otherwise it returns 0.

## References

contributed by Gözde Üstün