Abstract: Quantum photonics via quantum key distribution (QKD) offers a route to unconditional security but photon absorption, scattering, and losses have been an obstacle to implementing an untrusted-source QKD in optical fiber links in terms of distances and data rates. Whilst entangled photonic qudits offer enhanced information efficiency, noise robustness, and security level of QKD frequency and time entangled qudits promise resources to implement robust QKD in optical fibers. However, the spectral range occupied by frequency and time entangled qudits, their bandwidth, represents a significant issue towards their use for QKD applications in optical fiber networks. To implement high secret key rates for untrusted-source QKD in fiber links it is necessary to increase qudit dimensionality whilst keeping the time-bandwidth product as low as possible. However, this is a challenging task to achieve with current photonic platforms.

Abstract: A method and a system for generating a hyper-entangled high-dimensional time-bin frequency-bin state, the method comprising generating a hyper-entangled state composed of a time-bin and frequency-bin encoded state, and individually modifying at least one of: i) the amplitude and ii) the phase of the state components at different frequency-bins and different time-bins of the hyper-entangled state. The system comprises a non-linear medium exited with multiple pulses in broad phase-matching conditions, a frequency mode separator and an amplitude/phase modulator, the frequency mode separator temporally and spatially separating frequency modes of the hyper-entangled state, the amplitude/phase modulator individually modifying at least one of: i) the amplitude (and ii) the phase of the state components at different frequency-bins and different time-bins of the hyper-entangled state.

Abstract: A system and a method for phase extraction of a multi-path interferometer, the method comprising generating a reference signal of a coherence length longer than an arm length difference of the multi-path interferometer; splitting the reference signal into a frequency shifted reference signal and an unshifted reference signal; recombining the frequency shifted reference signal and the unshifted reference signal into a polarization- and frequency-multiplexed reference signal, and feeding the polarization- and frequency-multiplexed reference signal to the multi-path interferometer; detecting frequency shifted and unshifted output signals of the multi-path interferometer; and determining the interferometer phase from the detected signal.

Abstract: A method and a system for generating a hyper-entangled high-dimensional time-bin frequency-bin state, the method comprising generating a hyper-entangled state composed of a time-bin and frequency-bin encoded state, and individually modifying at least one of: i) the amplitude and ii) the phase of the state components at different frequency-bins and different time-bins of the hyper-entangled state. The system comprises a non-linear medium exited with multiple pulses in broad phase-matching conditions, a frequency mode separator and an amplitude/phase modulator, the frequency mode separator temporally and spatially separating frequency modes of the hyper-entangled state, the amplitude/phase modulator individually modifying at least one of: i) the amplitude (and ii) the phase of the state components at different frequency-bins and different time-bins of the hyper-entangled state.

Abstract: A system and a method for phase extraction of a multi-path interferometer, the method comprising generating a reference signal of a coherence length longer than an arm length difference of the multi-path interferometer; splitting the reference signal into a frequency shifted reference signal and an unshifted reference signal; recombining the frequency shifted reference signal and the unshifted reference signal into a polarization- and frequency-multiplexed reference signal, and feeding the polarization- and frequency-multiplexed reference signal to the multi-path interferometer; detecting frequency shifted and unshifted output signals of the multi-path interferometer; and determining the interferometer phase from the detected signal.