Abstract: An apparatus including: addition circuitry configured to receive simultaneously a probe signal that has passed through a channel having a complex transmission and a reference signal and configured to produce at least: a plurality of weighted additions including an addition of the probe signal and the reference signal with different relative phase rotations between the probe signal and the reference signal for each weighted addition; and processing circuitry configured to use at least two of the plurality of weighted additions to determine a first quantitative parameter for the channel dependent upon the complex transmission for the channel.

Abstract: The disclosure relates to secured wireless communications. A sender device emits randomly photons in a first polarisation, a second polarisation and a third polarisation without aligning a polarisation system in a plane perpendicular to the first and second polarisations with a recipient device. The recipient device is adapted for detection of events in association with six polarisations. The recipient device selects randomly polarisation basis for measurement of the received photons. Information of detected events in association with three basis is communicated to the sender device. The information from the recipient device and information stored in the sender device is processed to determine events where same polarisation basis was used by the sender device and the recipient device.

Abstract: An apparatus including: addition circuitry configured to receive simultaneously a probe signal that has passed through a channel having a complex transmission and a reference signal and configured to produce at least: a plurality of weighted additions including an addition of the probe signal and the reference signal with different relative phase rotations between the probe signal and the reference signal for each weighted addition; and processing circuitry configured to use at least two of the plurality of weighted additions to determine a first quantitative parameter for the channel dependent upon the complex transmission for the channel.