Quantum and Relativity Combine for Secure Position
Researchers have experimentally demonstrated a secure position verification protocol that combines quantum optics and relativity.
A new experiment has successfully combined quantum optics and relativity to securely verify an entity's physical location. The work, reported in Nature Physics, uses phase-randomized weak coherent states to achieve a positioning accuracy better than 75 meters over a 2-kilometer distance.
Classical methods for verifying a prover's position are fundamentally insecure against a dishonest prover, according to the source. An untrusted prover can deploy collaborating adversaries to intercept and forward challenge messages, generating the correct response without the delay that would reveal their true, distant location. This vulnerability exists because classical information can be copied and relayed undetectably.
The Quantum Solution
Integrating quantum information into the verification process offers a way to overcome this classical insecurity. Encoding information into quantum bits, or qubits, makes interception and resending detectable. Recent theoretical protocols have shown that attacking such quantum position verification (QPV) schemes requires quantum resources, like entanglement or quantum gates, that must scale with the size of the classical input. This creates a cryptographic asymmetry: verifying a position is easy for an honest prover but becomes prohibitively difficult for an adversary.
However, implementing these qubit-based protocols has been extremely challenging. The source lists stringent requirements on system latency, computational capability, and optical loss that have hindered practical realization.
A Practical Experimental Realization
The research team realized a complete QPV protocol by building upon and extending earlier theoretical schemes. They implemented high-repetition-rate, low-loss quantum communication alongside large-scale, low-latency classical communication. The core of their quantum approach was a security framework based on phase-randomized weak coherent states (PR-WCS).
"This resolves the multiphoton security issue and eliminates the need for single-photon sources," the source states. Using readily available coherent-state sources enabled a high-speed, low-loss polarization-encoding scheme. The team constructed a Sagnac architecture using a micro-assembled component for high-fidelity polarization-state preparation and used a high-voltage-driven optical switch with superconducting detectors for analysis. The overall quantum-optical efficiency reached 70%, with an error rate of 0.27%.
Tackling Latency and Computation
A major practical hurdle was the latency associated with processing large classical messages. The team developed dedicated classical communication links using dense-wavelength-division-multiplexed on-off keying (DWDM-OOK) signals transmitted through anti-resonant hollow-core fibre (AR-HCF) and detected with high-speed PIN photodiodes. This design provided a high-bandwidth, near-light-speed channel with negligible excess latency.
To handle the complex credential computations required by the protocol, the researchers implemented a hardware lookup table on a field-programmable gate array (FPGA) and double data rate (DDR) memory array. This system achieved a computation latency below 118 nanoseconds for a function space exceeding 10^13, corresponding to a classical input size of n = 40 bits.
The protocol's mechanism relies on the prover performing operations based on information from the verifiers and returning an outcome. The honest prover's round-trip time, bounded by the speed of light, allows the verifiers to determine a spatial bound for its position. The experimental setup involved two verifiers separated by 2 kilometers who coordinated to verify the prover's claimed location within a 75-meter radius.
