How do quantum computers implement secure multi-party computation?

Quantum computers enable secure multi-party computation (SMPC) by leveraging quantum mechanics principles like superposition, entanglement, and no-cloning to create protocols that are inherently secure against certain classical attacks. In SMPC, multiple parties jointly compute a function without revealing their private inputs. Quantum SMPC protocols often use entangled qubits or quantum key distribution (QKD) to establish trust and ensure data privacy. For example, entangled particles allow parties to share correlated information that cannot be intercepted without disturbing the system, enabling detection of eavesdropping. These properties make quantum protocols theoretically resistant to attacks that exploit computational limits of classical systems, such as factoring large primes.

A practical example is a quantum-based version of oblivious transfer, a foundational SMPC primitive. In classical oblivious transfer, a sender shares data with a receiver in a way that the receiver learns only one piece of information, and the sender remains unaware of which was chosen. Quantum oblivious transfer can achieve this with unconditional security by encoding data into qubits. For instance, using BB84-like protocols, the sender transmits qubits in random bases, and the receiver measures them in randomly chosen bases. Discrepancies in basis choices reveal eavesdropping attempts, ensuring the protocol’s integrity. Similarly, quantum secret sharing splits a secret into qubits distributed among parties, requiring collaboration to reconstruct it, as individual qubits provide no usable information.

However, current quantum SMPC implementations face challenges. Real-world quantum systems are prone to noise and decoherence, which can corrupt qubit states and compromise security. Error correction and fault-tolerant designs are critical but still under development. Additionally, quantum SMPC often requires classical pre-processing or post-verification steps, such as using QKD to establish secure channels before transmitting computation results. While promising, these methods are not yet scalable for complex computations. Developers exploring this space should focus on hybrid approaches—combining quantum-secure primitives with classical SMPC frameworks—to balance security and practicality until quantum hardware matures.