Quantum-Secure Ballots Demonstrated in the Lab
Election security has become a hot political issue, and quantum information technologies may offer potential solutions. Two teams of researchers have now experimentally implemented a quantum voting protocol that was proposed four years ago in which votes can be cast securely and anonymously without requiring trust in the election administrators [1, 2]. Although these experiments admit only a small number of voters, advances in quantum technologies may expand the scope of the method in the future.
Quantum voting “bases its security on the fundamental laws of physics,” says quantum communications expert Nicolas Laurent-Puig of Sorbonne University in France. Quantum cryptography can prevent tampering with votes once they are cast, but there’s another problem, Laurent-Puig says: If the central authority collecting the votes is corrupt, hacked, or otherwise compromised, “the integrity of the entire election falls apart.” For example, previous quantum protocols did not guarantee that individual voters will remain anonymous to the authority.
One classical solution to that problem is a protocol where votes can’t retain any imprint of the voter’s identity. Imagine that each vote is a binary bit value of 0 or 1 and that there are as many rounds of voting as there are voters. An autonomous system randomly and secretly assigns each voter a specific round in which they cast a vote; in other rounds they simply submit an assigned value. In each round everyone is assigned a random value, with the constraint that the total number of 1s is even. Only the round’s designated voter has the option to submit either their assigned value (say, for candidate A) or the opposite (for candidate B). Then only the designated voter’s vote determines if the total number of 1s is odd or even, and their single vote is recorded without any connection to their identity.
In such a scheme, any cheating by the other participants, including collusion by groups of voters, can be detected because each round’s total is publicly available for voters to check. But this method still depends on the honesty of the agent distributing the bit values. A wholly trustable system needs some way of ensuring that the assigned values have the right collective statistics without anyone knowing in advance which bit value each person will receive—with knowledge of the assignments, someone could determine the designated voter.
Quantum bits (qubits) enable these conditions. The quantum voting protocol involves creating a quantum state of many quantum-entangled qubits known as a Greenberger-Horne-Zeilinger (GHZ) state, with one qubit for each voter. The state can be prepared so that each qubit measurement randomly produces 0 or 1, but the entanglement guarantees that the total number of 1s is either even or odd. These states can be prepared using, for example, photons as qubits, with 0 and 1 corresponding to distinct polarization states.
Such a protocol was proposed in 2022 by quantum information theorist Federico Centrone of the Barcelona Institute of Science and Technology in Spain and his co-workers [3]. Implementing it requires that the voters be able to verify that they have been given a true GHZ state and not some other state that subverts the protocol. Such verifications can be carried out, but each qubit can only be used once—either for voting or for verification. So extra sets of GHZ states must be produced for multiple rounds of verification.
Two research teams have now demonstrated the fundamental features of the protocol using photons as qubits, although several practical challenges remain before it can be used in a real election. Joey Marcellino, a PhD student at the University of Geneva, and his co-workers randomly assign each round as either a verification or a voting round [1]. Meanwhile, Laurent-Puig and his colleagues (including Centrone) simply chose to postpone the verification aspect of the protocol for future work [2].
The key challenge for implementing the protocol is to produce and distribute fragile many-photon GHZ states—in effect, to entangle many photons simultaneously and robustly. Both teams produce them using spontaneous parametric down-conversion (SPDC), a process that occurs when photons pass through a special class of crystalline optical material. The current record number of photons in a GHZ state is 14, and even this number can’t always be produced reliably [4]. Laurent-Puig and colleagues have a more reliable method of making GHZ states, so their success rate for passing the verification procedure is around 96%, whereas for Marcellino and colleagues it is just 87%. The latter means that there is a 13% chance that a voter casts their vote in an invalid round, in which case the entire protocol has to be restarted.
Implementing this protocol in a population-scale voting process remains a distant prospect if it demands GHZ states with millions of entangled qubits. Such an election could be broken down into smaller municipal districts, Marcellino says, but those numbers still present an immense challenge. Still, he says a method currently being developed could supplement or replace SPDC using tiny semiconductor crystals (quantum dots) and should be easier to scale up. On the other hand, the first implementations of these election strategies, Laurent-Puig says, “will almost certainly be in high-stakes but small-scale scenarios like secure boardroom voting rather than large-scale national elections.”
Peng Huang, an expert in quantum cryptography and communication at Shanghai Jiao Tong Technical University in China says that the two demonstrations “solve a foundational security problem that has plagued all previous electronic voting systems: the need to trust a central authority.” Despite their current limitations, he says that these experimental implementations “mark a pivotal proof of principle, demonstrating that a voting system can be built on information-theoretic security rather than assumptions of institutional trust.” He adds that these protocols do not solve all potential problems that could undermine voting systems, such as securing voter identity or protecting voters from coercion—issues for which solutions are inevitably more political than technological.
–Philip Ball
Philip Ball is a freelance science writer in London. His next book, The Man Who Broke Reality, a biography of Niels Bohr, will be published in December.
References
- F. J. Marcellino, “Experimental quantum voting using photonic Greenberger-Horne-Zeilinger states,” Phys. Rev. Lett. 137, 060802 (2026).
- N. Laurent-Puig, “Experimental quantum electronic voting,” Phys. Rev. Lett. 137, 060803 (2026).
- F. Centrone et al., “Quantum protocol for electronic voting without election authorities,” Phys. Rev. Appl. 18, 014005 (2022).
- P. Thomas et al., “Efficient generation of entangled multiphoton graph states from a single atom,” Nature 608, 677 (2022).