  Quantum key distribution (QKD) allows the establishment of common
cryptographic keys among distant parties. Many of the QKD protocols that were
introduced in the past involve the challenge of monitoring the signal
disturbance over the communication line, in order to evaluate the information
leakage to a potential eavesdropper. Recently, a QKD protocol that circumvents
the need for monitoring signal disturbance, has been proposed and demonstrated
in initial experiments. Here, we propose a new version of this so-called
round-robin differential phase-shifting (RRDPS) protocol, in which both time
and phase degrees-of-freedom are utilized to enlarge the Hilbert space
dimensionality, without increasing experimental complexity or relaxing security
assumptions. We derive the security proofs of the round-robin differential
phase-time-shifting (RRDPTS) protocol in the collective attack scenario and
benchmark the new protocol against RRDPS for different experimental parameters.
Furthermore, a proof-of-concept experiment of the RRDPTS protocol, using weak
coherent pulses and decoy-state method, is demonstrated over 80 km of fiber
link. Our results show that the RRDPTS protocol can achieve higher secret key
rate in comparison with the RRDPS, in the condition of high quantum bit error
rate.
