A trapped calcium ion and a photon stayed strongly correlated after the photon traveled through 25 kilometers of coiled telecom fiber, crossing a threshold relevant to quantum key sharing. The Saarland University team reports a promising link for a security approach that relies less on trusting measurement devices, but it did not extract a final secure key.
One trapped ion, one photon at the far end
Quantum key distribution lets two distant parties establish a shared secret by measuring quantum states. Device-independent versions aim to judge security from the observed correlations rather than assuming the measurement devices work exactly as designed. That is demanding over fiber: photons can be lost, and their polarization can drift before a remote station measures them.
In the new setup, a trapped ion served as a local quantum memory and emitted a photon linked to its state. The team shifted the photon's wavelength into a low-loss telecom band, sent it through a 25-kilometer fiber spool, then shifted it back for measurement. Automatic polarization correction helped preserve the relationship between the ion and the arriving photon. A detection signal from the remote station triggered the later ion measurement. The fiber was coiled in the laboratory, not installed as a 25-kilometer field network.
Strong correlations, but no secure key
The researchers measured a Bell-test correlation value of 2.75, above the 2.362 threshold that their device-independent key model requires for a positive key fraction in the asymptotic limit. In that model, the data yield a conservative estimate of 69 secret-key bits from 10,908 successful detected events. This is a model-based estimate, not 69 bits of key that the team actually delivered for use.
The analysis window was chosen using the same measurements used to assess the key-related result. The authors therefore do not claim device-independent security or a final extracted key.
