Efficient Single-Photon Interface Of Rydberg Superatom And Its Applications

To realize quantum entanglement of long-distance,one feasible approach is to make use of efficient quantum repeater,the core function of which is the efficient singlephoton interface and the ability of quantum memory.As a commonly used quantum memory medium benefitting from Doppler cooling technology,the cold atom ensemble has the inherent photon interface coming from collective enhancement effect.And it can also achieve long-lived memory with methods such as clock states encoding.Recent years,there are a lot of improvements of the cold atom system in storage lifetime and photon efficiency.Also,some applications such as remote entanglement generation have been demonstrated by combining frequency conversion technology and fiber transmission.In order to provide the fundamental resource of quantum network,we need to establish high-fidelity atom-light entanglement,which can be achieved by spontaneous Raman scattering.However,this approach has a certain probability of releasing multiple photons.Thus we always need to keep low excitation rates,which will limit entanglement rate a lot.The corresponding solution is to use deterministic platform,such as confining the cold atom ensemble in an area of several micrometers and exciting to Rydberg state.We can intensely suppress high-order excitation with Rydberg blockade mechanism.By doing this,we can obtain collective excitation of single Rydberg atom with high quality,namely,the Rydberg superatom.With this platform,we can deterministically prepare the quantum state with efficiency of almost unity.In addition,the Rydberg interaction can also be used for quantum gate operations between atoms,enabling tasks such as entanglement exchange in quantum repeater experiment.Due to the restricted geometry size and atomic density,previous Rydberg experiments have low single-photon efficiencies.A certain level of improvement can be achieved by simply increasing the atomic density,but high density will also cause additional collisional decoherence.In this thesis,we use low-finesse optical cavities to enhance the coupling of light and atoms by several times combined with properly increasing of the optical depth.We measure and characterize the photon quality,high-order events and other factors.The results show that our experimental system can be used for subsequent high quality entanglement generation.In addition,we demonstrate the deterministic measurement of atomic states.To do that,we use high-efficiency photon interface to quickly detect several photons,and the photon emmission can be switched on or off with the interaction of controll atom.This scheme has the advantages of high speed and non-destructive,and thus can play an important role in applications such as quantum networks.Using a high efficiency single-photon interface,we further demonstrate multiphoton entangling states with single Rydberg superatom platform.By encoding multiple energy levels and creating interaction,we can directly create multiphoton entanglement without using external devices.Its efficiency shows better scaling factor than probabilistic systems such as spontaneous parametric down-conversion platform,and it’s also better than current experimental results of quantum dots.Meanwhile,we systematically study the decreasing trendency and limiting factors of entanglement efficiency and fidelity.The results show that Rydberg superatom have the potential of producing large-scale multiphoton states,and it can be further improved by reducing losses and improving the performance of optical cavity.In the future,it is expected to paving the way for applications such as one-way quantum repeater which is the quantum repeater of next generation.Except the generation of multiphoton entanglement states,we also demonstrate the entanglement between two Rydberg superatoms.To do that,we build another system with common standards.First,we manipulate two systems independently,which can both realize entanglement between time-bin encoded photon and atomic internal states.After that,we transmit photons from both sides to the intermediate node through optical fibers to realize interference.Then we can herald the entanglement between two Rydberg superatoms with the detection of two-photon coincidence.We measure the entanglement fidelity and Bell’s inequality,which both prove high entanglement quality.Moreover,the entanglement rate is improved by two orders of magnitude compared with previous cold atom ensemble experiments,and show the efficiency advantage of deterministic platform.We also demostrate another scheme by preparing fock state encoded single photon,and then herald the entanglement by detecting one photon in intermediate node.Due to the reduction of detected photon number,single-photon scheme can show higher success probability compared with two-photon scheme,in the condition of long-distance transmission.But it will reduce the fidelity,which can be traded off between heralding probability.Also in the experiment,we need to keep stable phase relation between two systems,thus fast phase measurement and feedback is implemented with the use of electronic feedback system.Current experimental results show that with the increasing distance the single-photon scheme will be more advantageous if both with cavity enhancement.The works in this thesis show that Rydberg superatom with cavity enhancement can greatly increase the rate of entanglement generation,and efficient detection of atomic state can significantly improve the success probability of operations such as entanglement connection in quantum repeater applications.It can lay the foundation for future applications in quantum repeater and quantum network.In the future,we can further improve photon efficiency,and higher fidelity can be achieved,and it will promote the practical use of Rydberg superatom.