Kerr Photon-blockade Sources And Applications In Quantum Key Distributions

Quantum key distribution(QKD)applies quantum mechanics principles to guarantee secure private communication and enables two to produce a shared random secret key known only to them.It shows great significance in communication,economics and national defense.The decoy state method can help generate secure key in a practical QKD system with only imperfect single-photon sources.As a result,the method has been widely used at present.Recently,the weak coherent sources(WCS)seem to be widely used in most QKD systems.As WCS has very limited single-photon state,the key rate of WCS is significantly lower than the perfect single photon source.Therefore,we propose new practically feasible sources that can raise the key rate in several aspects as shown below.We have also provided some practical optimizations.First,we have studied the single-photon blockade by coherently driving laser pulses for Kerr nonlinear resonators.In the blockade,multi-photon states are greatly suppressed due to the strong nonlinear interaction of photons.And an effectvie collection of single photons is achieved due to the existence of cavities.Then we use quantum trajectory method to simulate blockade systems and give photon number distributions out of the cavity.And the method is based on the evolution of a Monte Carlo wave function(MCWF)of small systems and essentially equivalent to the master-equation treatment.The non-classical light field we obtain has a high probability of single-photon state,also showing sub-poissonian statistics and anti-bunching.Further we show how such quasi photon sources can be applied to decoy state QKD systems,which could improve the key rate drastically.Second,photon sources also require high repetition rate in quantum communication.To raise both the repetition rate and the single-photon probability,we propose a coupled model for cascaded photon-blockade systems by coherently driving laser pulses for Kerr nonlinear resonators.By cascading cavities,we can effectively raise the repetition rate and the nonlinear interaction strength.Based on the quantum trajectory method in cascaded systems,we also give photon number distributions out of cavities.And further we propose a general N-cavity model which could raise both the repetition rate and single-photon probability much more.Third,in realistic systems,photon fluctuation cannot be ignored.To estimate our light sources,we calculate the QKD key rate under fluctuation,and we find it has much higher key rate and longer transmission distance,which shows its practical uses.