Quantum voting system aims to keep ballots secret

At risk Any voting system can be compromised, but quantum physics could help make elections more secure. (Courtesy: iStock/Stocknshares) A voting system with a high level of security intrinsically guaranteed by quantum mechanics has been demonstrated by two independent teams – one led by Federico Centrone at the Barcelona Institute of Technology and the other led by Rob Thew at the University of Geneva. Their systems employ a quantum protocol that was first proposed by Centrone and colleagues in 2022 and uses a system of entangled qubits to ensure every voter has complete anonymity. A conventional voting system – paper or electronic – could be hacked or corrupted at the counting stage, resulting in a rigged outcome or lack of anonymity. In 2007, Anne Broadbent and Alain Tapp at the University of Montreal proposed a possible solution: an e-voting protocol in which an election is broken into as many rounds as there are voters. Before anyone casts a vote, Broadbent and Tapp’s system secretly assigns each participant to be the “real” voter in exactly one designated round. In that round, the chosen voter picks a value (either 0 or 1) representing their actual choice. Secret ballot In every other round, the system automatically submits a pre-assigned value on the voter’s behalf, engineered so the round’s total has a fixed parity – either always odd or always even. The real voter’s free choice then either flips this total to the opposite parity or leaves it unchanged, and the final odd or even result is recorded. Once every round is complete, each voter will have had their real say – but from the outside, the identity of the real voter in any given round is completely hidden. Although this scheme is watertight in principle, there is still no guarantee that the e-voting system itself can be fully trusted. In 2022, a team led by Centrone proposed how this loophole could be closed by harnessing entanglement between quantum bits, or qubits. In the quantum protocol, each voter is assigned a single qubit, prepared as part of one joint entangled state spanning all voters together. When measured, each qubit is randomly either a 0 or 1, But, because of the type of entanglement between the qubits, the total number of 1s across all voters is guaranteed to have the expected parity. Since not even the e-voting system itself can know the outcome of any individual measurement in advance, the protocol removes the need to trust whoever is distributing the values. Easily adapted Thew’s team realised that it could create this protocol in the lab. “We had just finished another experiment concerning distributed polarization entanglement and realized we could ‘easily’ adapt our setup to produce the entangled states required for the voting protocol,” says Geneva’s Joey Marcellino. Both teams used spontaneous parametric down-conversion (SPDC) to create the entangled qubits, which were photons. Here, a laser is fired into a nonlinear crystal, splitting single photons into pairs of lower-energy photons that are entangled in terms of their polarization states. SPDC is then repeated using the newly-entangled photons, chaining pairs together until reaching the same number of entangled photons as there are voters – albeit a small number in these demonstrations. The result is one single, large entangled state. Each individual photon then passes through a 50/50 beam splitter, is coupled into an optical fibre, and sent to a detector, where its state is measured. Fidelity and success In their first experimental tests of the technique, Centrone and colleagues were able to create the entanglement required for the voting protocol at success rate of about 96%. Meanwhile, Thew’s team achieved an 87% success rate. New findings shorten the road to cryptographically relevant quantum computers Read more Commenting on his colleagues’ result, Marcellino says, “This suffices as a proof-of-principle for the protocol and our modified approach, although obviously we would want to improve these numbers significantly for real-world applications”. For now, the result demonstrates that e-voting systems with quantum-guaranteed security are achievable, using existing technologies including single-photon sources, detectors, and entanglement generation schemes. “Moreover, in the end the protocol is just a way to anonymously distribute a bit with perfect security,” Marcellino adds. “While this is obviously useful for voting, it could also work as an anonymous message board, or to enable anonymous distributed computation.” The research is described in Physical Review Letters: Thew and colleagues; Centrone and colleagues. Want to read more? Registration is free, quick and easy Note: The verification e-mail to complete your account registration should arrive immediately. However, in some cases it takes longer. Don't forget to check your spam folder. If you haven't received the e-mail in 24 hours, please contact customerservices@ioppublishing.org. E-mail Address Register