An optical apparatus for transmitting light pulses to a further optical apparatus for generating a quantum cryptographic key according to at least one quantum cryptography technique; the quantum cryptography technique at least comparing the phase difference between at least two light pulses; the optical apparatus comprising: I) an integrated optical splitter configured to: A) receive at least one input light pulse, B) split the input light pulse into: i. a first light pulse propagating along a first integrated optical path; and, ii. a second light pulse propagating along a second integrated optical path, wherein the second integrated optical path comprises an optical path length that: iii. is longer than the first integrated optical path; and, and, II) an integrated optical intensity controller configured to: C) receive the temporally separated first and second light pulses from the respective first and second integrated optical paths; and, D) control the relative intensities of the first and second light pulses output from the integrated optical intensity controller for transmission to the further optical apparatus by increasing or decreasing the output intensity of one of the first and second light pulses relative to the other light pulse, and, III) a phase modulator configured to change the phase of at least one of the first or second light pulses output from the optical apparatus; the phase modulator being along any of: E) the first or second integrated optical paths; F) an output of the integrated optical intensity controller; G) an output of an integrated optical intensity modulator configured to receive any one or more of the first and second light pulses output from the integrated optical intensity controller. I) an integrated optical splitter configured to: A) receive at least one input light pulse, B) split the input light pulse into: i. a first light pulse propagating along a first integrated optical path; and, ii. a second light pulse propagating along a second integrated optical path, wherein the second integrated optical path comprises an optical path length that: iii. is longer than the first integrated optical path; and, and, A) receive at least one input light pulse, B) split the input light pulse into: i. a first light pulse propagating along a first integrated optical path; and, ii. a second light pulse propagating along a second integrated optical path, wherein the second integrated optical path comprises an optical path length that: iii. is longer than the first integrated optical path; and, i. a first light pulse propagating along a first integrated optical path; and, ii. a second light pulse propagating along a second integrated optical path, II) an integrated optical intensity controller configured to: C) receive the temporally separated first and second light pulses from the respective first and second integrated optical paths; and, D) control the relative intensities of the first and second light pulses output from the integrated optical intensity controller for transmission to the further optical apparatus by increasing or decreasing the output intensity of one of the first and second light pulses relative to the other light pulse, and, C) receive the temporally separated first and second light pulses from the respective first and second integrated optical paths; and, D) control the relative intensities of the first and second light pulses output from the integrated optical intensity controller for transmission to the further optical apparatus by increasing or decreasing the output intensity of one of the first and second light pulses relative to the other light pulse, and, III) a phase modulator configured to change the phase of at least one of the first or second light pulses output from the optical apparatus; the phase modulator being along any of: E) the first or second integrated optical paths; F) an output of the integrated optical intensity controller; G) an output of an integrated optical intensity modulator configured to receive any one or more of the first and second light pulses output from the integrated optical intensity controller. E) the first or second integrated optical paths; F) an output of the integrated optical intensity controller; G) an output of an integrated optical intensity modulator configured to receive any one or more of the first and second light pulses output from the integrated optical intensity controller.