Quantum Coherence And Its Applications In Quantum Information Processing

As one of the fundamental features that marks the distinction between quantum and classical,quantum coherence,as also the superposition in quantum states,not only constitutes a powerful resource for quantum metrology and entanglement preparation,but also is the root of a lot of intriguing characteristics in quantum optics,quantum information,solid state physics,and thermodynamics.For the past few years,a number of researches have been done in both theoretic and experimental studying on uncertainty principle,macrorealism,local-realism,etc.However,there still exists some loopholes and technical difficulties in experimental testing such quantum principle.Thus,designing the effective experiment to demonstrate the basic principle of quantum,not only is very important for us to understand the nature of the quantum world,also provide important theoretical and technical supports to make use of the quantum resource in quantum information processing.In this thesis,by using quantum photon sources generated by parametric down-conversion process and the linear optical elements,the basic principles including uncertainty principle and macrorealism are studied.The applications in quantum information processes of quantum metrology and quantum simulations are also investigated.Details as follows:1).Two new stronger state-dependent uncertainty relations which are constructed in the sum of variances are tested experimentally.The Heisenberg-Robertson relations expressed in the term of the product of the variance does not fully capture the notion of incompatible obeservables,where the product is zero when either of the two variances is zero even if the other variance(here variance is used as a measure of uncertainty)is nonzero.Our experiments in qubit and qutrit systems fully capture the notion of incompatible observables in contrast to the Heisenberg-Robertson inequality by testing the uncertainty relations in the form of sum,thus making the uncertainty relations much stronger.2).The higher violations of Leggett-Garg inequality on a three-level system are observed experimentally.The Leggett-Garg inequality which quantifies the temporal correlations in quantum mechanics has become the standard laboratory tools for ruling out macrorealism in the same way that Bell’s inequality which quantifies the spatial correlations rules out local realism.By implementing the ideal negative measurement that admit three distinct measurement outcomes in qutrit system,rather than the usual two,we obtain violations of threeand four-time Leggett-Garg inequalities that are significantly in excess of those obtainable in standard Leggett-Garg tests.These results highlight the differences between the spatial and temporal correlations in quantum mechanics.3).Using ambiguous measurements and unambiguous measurements,the violations of Leggett-Garg inequality without signaling is observed experimentally.Ambiguous measurements of the qutrit states are realized by blocking one out from the three modes at any one time.Using these measurements we construct a test of a Leggett-Garg inequality as well as tests of no-signaling-in-time for the measurements.Moreover,we tailor the qutrit dynamics such that both ambiguous and unambiguous measurements are simultaneously nonsignaling.The general principle used for constructing ambiguous Leggett-Garg inequality tests without signaling could potentially be scaled up to larger,massive objects.4).Two kinds of error-tolerant quantum witness inequalities are proposed and tested in this work.The first witness heralds the invasiveness of a blind measurement;its maximum violation has been shown to grow with the dimensionality of the system under study.The second witness heralds the invasiveness of a generic quantum channel and can achieve its maximum violation in any dimension;Compared to the conventional quantum witness equality which can be easily violated by the error in experiment,such error-tolerant inequalities are more useful to witness quantum.5).We experimental demonstrate quantum metrology in different kinds of noisy channels with the results show that entanglement is useful for phase estimation to defending decoherence.Different kinds of noisy channels are realized by implementing space-multiplexed dual interferometers with quantum photonic inputs.We demonstrate the advantage of entanglementassisted protocols in a phase estimation experiment run with either a single-probe or multiprobe approach.These results establish that entanglement with ancillae is a valuable approach for delivering quantum-enhanced metrology.Our approach to entanglement assisted quantum metrology via a simple linear-optical interferometric network with easy-to-prepare photonic inputs provides a path towards practical quantum metrology.6).Dynamic quantum phase transitions characterized by many-body quench dynamics between distinct Floquet topological phases are studied by using photonic quantum walks experimentally.The quench dynamics between many-body Floquet topological phases which associated with different effective Hamiltonian is mapped into a single-photon walks where the quench process dynamics happen in different decoupled quasimomentum sectors coherent evolute with time.Thus we can simulate the quantum dynamic topological phase transition under different situations including the parity-time-symmetric preserving unitary and nonunitary evolutions,parity-time-symmetric broken nonunitary evolution and with the initial states in pure and mixed by using interference-based measurements experimentally.All these results give a strong proof that the quantum walks is a powerful tool to simulate the complex physics phenomenon.7).We experimentally simulate the quench dynamics of parity – time-symmetric topological systems,and reveal skyrmion structures in quantum walks for the first time.By using single-photon discrete-time quantum walks to simulate the quench dynamics between many-body Floquet topological phases and applying projective combined with interferencebased measurements in position space to reconstruct the time-dependent density matrix,the emergence of skyrmions in parity-time-symmetric preserving unitary and nonunitary evolutions are observed in quantum walks.We also detected the disappearance of the skyrmions structures during the quench dynamic happens in the same Floquet topological phases and parity–timesymmetric broken area.This work experimentally reveals the interplay of parity-time-symmetry and quench dynamics in inducing emergent topological structures,and highlights the application of discrete-time quantum walks for the study of dynamic topological phenomena.In summary,the research of this paper is helpful to further understand the uncertainty relations and macrorealism,which provides the certain reference values to boundary the quantum and classical.In the aspect of applications,we not only experimentally emphasize the usefulness of entanglement in defending quantum decoherence in quantum metrology,also use the platform of quantum walks to simulate many-body quench dynamics between distinct Floquet topological phases where we have observed the dynamic quantum phase transitions and2-dimensional dynamic skyrmion structures.