  Quantum key distribution (QKD) is moving from research laboratories towards
applications. As computing becomes more mobile, cashless as well as cardless
payment solutions are introduced, and a need arises for incorporating QKD in a
mobile device. Handheld devices present a particular challenge as the
orientation and the phase of a qubit will depend on device motion. This problem
is addressed by the reference frame independent (RFI) QKD scheme. The scheme
tolerates an unknown phase between logical states that varies slowly compared
to the rate of particle repetition. Here we experimentally demonstrate the
feasibility of RFI QKD over a free-space link in a prepare and measure scheme
using polarisation encoding. We extend the security analysis of the RFI QKD
scheme to be able to deal with uncalibrated devices and a finite number of
measurements. Together these advances are an important step towards mass
production of handheld QKD devices.

Quantum Physics