Wideband Solid-state Quantum Memory And Its Applications In Quantum Communication And Tests Of Fundamental Physics

As the key components for building quantum networks,quantum memory has de-veloped rapidly in the past decade.In quantum computing,it can be used as a syn-chronization tool that matches various process within a quantum computer.In quantum communication,it can be used to realize quantum repeater combining with entangle-ment swapping technique,enabling quantum communication over long-distance.The solid-state quantum memory has the advantages of long lifetime,large bandwidth,high fidelity and high multimode capability,which make it become one of the most promi-nent candidates for realizing quantum repeater.In addition,as a light-matter interfaced system,quantum memory and its matching single-photon source can also be utilized to perform research on some fundamental quantum physics problems,such as the obser-vatioin of anomalous trajectories of single photons and tests of macroscopic realism.During my PhD,I mainly focus on the study of wideband solid-state quantum memory based on Nd-doped crystals,including its application in quantum communica-tion and tests of fundamental quantum physics.The followings are the main research works of my doctoral dissertation:1.Spectroscopic investigations of 143Nd:YVO4 for quantum memory applica-tions.Because of the stable nuclear spin energy levels,Nd-143 ions doped crystal can be considered as a promising candidate system for realizing long-lived quantum memory in the future.For the first time,we performed basic spectroscopic investigations in isotope pure 143Nd:YV04 crystals.Furthermore,we achieved the preparation of the atomic frequency comb(AFC)and implemented the two-level AFC quantum storage.These studies provide feasibility for the realization of spin-wave quantum memory if we can transfer optical excitations into ground-state spin levels in this system.2.Semihierarchical quantum repeaters based on moderate lifetime quantum memories.The construction of large-scale quantum networks relies on the development of practical quantum repeaters.Many approaches have been proposed with the goal of outperforming the direct transmission of photons,but most of them are inefficient or difficult to implement with current technology.Here,we present a protocol that uses a semihierarchical structure to improve the entanglement distribution rate while reducing the requirement of memory time to a range of tens of milliseconds.This protocol can be implemented with a fixed distance of elementary links and fixed requirements on quantum memories,which are independent of the total distance.This configuration is especially suitable for scalable applications in large-scale quantum networks.3.Experimental observation of anomalous trajectories of single photons.The contradiction between quantum mechanics and classical world become espe-cially sharp in case one consider trajectories of truly quantum objects such as single photons.From a classical point of view,trajectories are well defined for particles,but not for waves.The wave-particle duality forces a breakdown of this dichotomy and quantum mechanics resolves this in a remarkable way:Trajectories can be well de-fined,but they are utterly different from classical trajectories.We give an operational definition to the trajectory of a single photon by introducing a technique to mark its path using its spectral composition.The method demonstrates that the frequency degree of freedom can be used as a bona fide quantum measurement device(meter).The analysis of a number of setups,using our operational definition,leads to anomalous trajecto-ries which are noncontinuous and in some cases do not even connect the source of the photon to where it is detected.We carried out an experimental demonstration of these anomalous trajectories using a nested interferometer.We show that the two-state vector formalism provides a simple explanation for the results.4.Strict experimental test of macroscopic realism in a light-matter interfaced system.Macroscopic realism is a classical worldview that a macroscopic system is always determinately in one or other of the macroscopically distinguishable states available to it,and so is never in a superposition of these states.The question that whether there is a fundamental limitation on the possibility to observe quantum phenomena at the macroscopic scale remains unclear.We implement a strict and simple protocol to test macroscopic realism in a light-matter interfaced system.We create a micro-macro en-tanglement with two macroscopically distinguishable solid-state components and rule out those theories which would deny coherent superpositions of up to 76 atomic ex-citations shared by 1010 ions in two separated solids.These results provide a general method to enhance the size of superposition states of atoms by utilizing quantum mem-ory techniques and to push the envelope of macroscopicity at higher levels.