Experimental Quantum Information With Novel Optical Quantum Channels

Among many quantum systems which are implemented as tools to realize quantum information processing,photons are good media of quantum information:they are a type of flying qubits and are difficult to interact with the environment,henceforth being decohered.In addition,photons have many degrees of freedom that can be used to encode quantum information,including the path degree of freedom(DoF),polarization DoF,orbital angular momentum DoF,frequency DoF,time DoF,etc.All these advantages make photons an adequate transmission carrier in developing quantum information.As an indispensable structural unit for the transmission of quantum information,quantum channels result in different dynamical evolution results after the quantum states are transmitted and pass through the channels of different structures.During my postgraduate period,I mainly conducted experimental research in quantum information on optical platforms,including the simulation of quantum channels,the dynamics of quantum coherent resources in the decoherent quantum channels,and an experimental realization of the causal structure distinction of all two-point quantum correlations.The main research results obtained in this paper are as follows:1.Experimentally observed the sudden change phenomenon of quantum coherence in optical decoherent channel,including coherence sudden death and sudden rebirth.In quantum mechanics,the coherence of an open quantum system will gradually decay over time due to interactions with the external environment,an effect called decoherence.We utilize the frequency degrees of freedom of photon to simulate the environment use the polarization degrees of freedom to code the system,a quantum decoherence channel is constructed by groups of specially-made inclined quartz plates in the space environment.For the first time,we experimentally observed the sudden chance phenomenon(SCP)of quantum coherence resources in the open quantum system.At the same time,we also define n-order coherent sudden death mathematically,and also observed the high-ordered SCP of quantum coherence experimentally.In addition,by adding non-Markovian noise to the environment,we also experimentally observed that the quantum coherence suddenly rebirth from 0 to non-zero,that is,the controllable recovery of the quantum coherence in the non-Markovian environment occurs.2.The first experimental verification of the super-additivity of coherent information in quantum channels,which lays the foundation for in-depth experimental research on quantum information theory.Channel capacity is a key parameter for the measurement of the communication capability of a quantum noisy channel,and its super-additivity is too weak to be experimentally observed in the common quantum channels.Here we construct a quantum channel which combines both "dephase" and "erasure" properties-the dephrasure channel,with which we could experimentally and directly observe the super-additivity of quantum coherence for the first time in the quantum channel capacity.In the experiment,we built a set of quantum channels(n-fold quantum channel)with n multiplexing capability.The super-additivity of quantum coherence is reflected in the experiment by the fact that the coherent information measured under two channels with single multiplexing capability is not equivalent to coherent information under one channel with double multiplexing capability.At the same time,we also observed that when using double-and triple-fold channels,the coherence information is significantly different from the results measured using single-fold channels:that is,When the amount of the coherence information measured under single-fold channels is zero,the coherent information in the double-and triple-fold channels still exists,which again proves the super-additivity of quantum coherent information.This result is instructive for both proving the super-additivity of quantum coherent information and measuring the quantum channel capacity.3.Implementing a parity-time-(PT)enhanced quantum sensor on optical system,whose sensitivity is 8.86 times higher than the traditional Hermitian sensor.The parity-time(PT)-symmetry theory was developed to extend quantum mechanics.The so-called PT-symmetry refers to the inverse of the parity transformation and time of the Hamiltonian.The essential difference between the Hermitian system and the non-Hermitian one is that the observables in the former quantum system are required to be Hermitian,while in the PT-symmetric quantum system it can be non-Hermitian.In this work,we construct a weak-measurement-assisted quantum PT-symmetric system with the help of both the weak measurement and an auxiliary system.That is,we embed a n-dimensional parity-time symmetric Hamiltonian system into a 2n-dimensional Hermitian system,in the case of reducing the auxiliary system,the channel evolution of the high-dimensional Hermitian system can be equivalent to the Hamiltonian channel evolution of the low-dimensional PT-symmetric system,so that the real part and the imaginary system can be directly obtained by weak measurement.This dilation of Hilbert space is suitable for both the PT-symmetry broken and unbroken region,so that we could obtain the full energy spectrum of the symmetric Hamiltonian.Based on this system,when applying a small perturbation to the system at the exceptional point(EP),there will be a large energy level splitting,then we experimentallly realize a PT-symmetrically enhanced quantum sensor for the first time,and investigate various properties related to the optimal conditions for enhanced sensitivity.The experimental results show that the sensitivity of this quantum sensor is 8.86 times higher than that of the traditional quantum sensor when perturbations are set at the EP of the PTsymmetric system.In addition,by detecting the real and imaginary parts of the energy splitting separately,information on the perturbation direction can also be obtained.This work is a major step forward towards non-Hermitian quantum sensor technology,and also provides a paradigm for introducing interesting classical PT-symmetry phenomena and their applications into the quantum realm.4.Discrimination of causal structures for all two-point quantum correlations based with optical system.This work is divided into two parts.First,a special quantum non-unital channel is constructed by linear optical components.The two-point quantum temporal correlation through this channel will surpass the two-point quantum spatial correlation.The special geometric structure,makes the distinguishable parameters between these two quantum correlations extend from single-valued points to the interval range.From the perspective of causal inference,the classical temporal correlation can be regarded as a direct cause causal structure,while the classical spatial correlation can be regarded as a common cause causal structure.Our experimental results broaden the scope of the direct identification of the two-point quantum correlations between these two quantum causal structures,but this distinguishable range of work is still limited,and all associated distinguishable tasks cannot be achieved.Therefore,the second part of our work further improves the two-point quantum correlation function that does not belong to the distinguishable parameter interval and uses a quantum random switch to realize a general experimental distinguishing platform on the optical platform.It is based on a mathematical logic algorithm in operation when the value of the quantum correlation function of the two points to be distinguished is known;only the unitary evolution operation and a few logical judgments are performed on the observables to realize the causal structure of all the quantum correlation functions of the two points identification.In classical causal inference,only knowing two facts of correlation cannot directly realize the distinction of causal structure.Usually,other operations are performed on the system,and the system’s state under test is forcibly changed by means of intervention.Our quantum causal discrimination experiment,which requires no system intervention,is the first in quantum inference.Together,these two works enrich the application of causal inference in the quantum world.