Quantum voting system aims to keep ballots secret

Two independent research teams have successfully demonstrated a quantum-based voting system that guarantees ballot secrecy, according to new research published in Physical Review Letters. Led by Federico Centrone at the Barcelona Institute of Technology and Rob Thew at the University of Geneva, the systems employ a quantum protocol that ensures every voter's choice remains completely anonymous.

The proposed quantum protocol, first introduced in 2022 by Centrone and colleagues, leverages entangled qubits to maintain voter anonymity. By using a system of entangled qubits, each voter's choice is concealed from external observation, preventing any manipulation or tampering during the counting process.

In conventional voting systems, ballots could be hacked or corrupted during the count, leading to rigged outcomes or compromised anonymity. In 2007, Anne Broadbent and Alain Tapp proposed an e-voting protocol to address this issue by dividing an election into multiple rounds, secretly assigning each participant to be the "real" voter in only one designated round. In the other rounds, pre-assigned values are submitted on behalf of each voter, maintaining the secrecy of their true choice.

While the conventional scheme is theoretically secure, it does not guarantee the trustworthiness of the e-voting system itself. In 2022, Centrone's team proposed using entanglement between quantum bits, or qubits, to close this loophole. By harnessing entanglement, each voter is assigned a single qubit prepared as part of a joint entangled state spanning all voters.

When measured, each qubit randomly appears as either a 0 or 1, but due to the entanglement, the total number of 1s across all voters is guaranteed to have the expected parity.

Joey Marcellino of Thew's team explained that the entangled qubits were created using spontaneous parametric down-conversion (SPDC), a process where a laser is fired into a nonlinear crystal, splitting single photons into pairs of lower-energy photons that are entangled in polarization states. These entangled photons were then used to create a single large entangled state, with each photon being measured to determine the voting outcome.

Both teams achieved success rates of around 96% and 87%, respectively, in their experimental tests. While the current success rates are promising, further improvements are needed for real-world applications. Nonetheless, the research demonstrates the feasibility of quantum-guaranteed e-voting systems using existing technologies such as single-photon sources, detectors, and entanglement generation schemes.