Solid-state Quantum Memory

Photons are the natural carriers of information in quantum networks. However, because photon loss scales exponentially with the channel length, quantum communication protocols that utilize the direct transmission of photons are limited to distances of approximately300km. To overcome such limitation, the concept of a quantum repeater has been proposed, which utilizes entanglement swapping and quantum memory to efficiently create long-distance entanglement so that to make the long distance quantum communication network over500km possible. On the other hand, distributed quantum computation network based on quantum memory can carry out quantum parallel computation among different places. So the quantum memory is key components of a quantum network. The practical realization of quantum networks depends on the physical realization of a high-profile quantum memory.Rare-earth ions doped solids show abundant optical transitions. Their4f electrons interact weakly with the environments because that they are shielded by the outer electrons. The related4f-4f transitions have long coherence time under low temperature and are suitable for storing single photons. Here we study the solid-state quantum memory based on rare-earth ion doped crystals. Quantum memory can serve as an important interface among various physical systems so the key experimental challenges are the quantum interface between various systems and the quantum memory. My PhD study focused on the quantum interface between quantum light sources and the solid-state quantum memory. My primary works are detailed as follow.1. Phase compensation enhancement of the on-demand entanglement generated from a semiconductor quantum dot.Because the three-dimensional spatial confinement of carriers, self-assembled quantum dot has discrete energy levels. When a single quantum dot is lifted to the biexciton levels, it can emit cascaded photon pairs and work as a deterministic source of entanglement. However, the exciton fine-structure splittings can introduce phase differences between the two biexciton decay paths that greatly reduce the entanglement. We analyzed this problem in the frequency domain and proposed a practicable method to compensate the phase difference by inserting a spatial light modulator, which substantially improved the entanglement of the photon pairs without any loss.2. Realization of ultra-reliable solid-state quantum memory for photonic polarization qubits.Logical qubits with photons can be encoded in several ways, for example, via intensity, phase, time-bin or polarization. The polarization degree of freedom is particularly useful because it is the most convenient and robust way to carry information. Because of the strongly polarization-dependent absorption of ions in crystal, however, all previous experiments have been conducted with a single predefined polarization. We propose a unique memory design with a "Sandwich"-like structure, where two pieces of crystals sandwiching a45°half-wave plate. The memory can work for arbitary polarization states. Due to the high stability of the memory design, we measured an unprecedentedly high fidelity of99.9%for the memory process.3. Theoretical and experimental studies on the optical precursors in solid-state medium.To resolve the apparent contradictions between fast light propagation and the theory of relativity, optical precursors were introduced by Sommerfeld and Brillouin in1914. The theory states that the front edges of an ideal step-modulated pulse propagate at speed c because of the finite response time of any physical material. We noticed that the absorption profile of rare-earth ion doped solids can be arbitrarily tailored with pump light so that to achieve fast light, slow-light and no-dispersion regimes. We proposed a method to identify the speed of optical wave fronts using polarization-based interference scheme. Then we experimentally observed optical precursors in optically pumped Nd:YVO4crystals and confirmed that the speed of optical precursor is the same for various dispersion regimes.4. Solid-state quantum memory based on Nd:YLF crystals.We measured the spectroscopic properties of Nd:YLF crystals with various thickness and doping levels under several configurations of sample temperature and magnetic fields. A storage efficiency of approximately35%was achieved with a sample temperature of1.5K and a magnetic field of6.6T. The isotope shifts of Nd were directly resolved in the optical absorption spectrum. This work stimulated us to further consider the isotope-purified Nd-143as a candidate system for the realization of spin-wave quantum memory. Then we performed spectroscopic measurements on the143Nd:YVO4crystals for quantum memory applications.5. Solid-state quantum memory for narrow-band photons generated with spontaneous-parametric down-conversion and the experimental tests on LGI.Entangled photon pairs are generated with spontaneous-parametric down-conversion process (SPDC) in a nonlinear crystal. To match the bandwidth of the quantum memory, the bandwidth of the photon source is filtered to approximately700MHz using optical gratings and calibrated etalon. Then we successfully demonstrated the storage of such photons in our solid-state quantum memory. We proposed the frequency-detuned and polarization dependent atomic-frequency comb to control the phase evolution of collective atomic excitation in quantum memory. We first demonstrated the experimental tests on LGI in a light-matter interface system and confirmed the persistence of quantum coherent evolution in a macroscopic system.6. Solid-state quantum memory for deterministic single photons generated from a quantum dot.In addition to serve as a deterministic photon source, the semiconductor quantum dot can also enable some quantum logical operations on qubits based on its electron spin. So the quantum dot can be an important quantum node in future quantum networks and the quantum interface between semiconductor quantum dot and solid-state quantum memory are of great importance. We first generated single photons with wavelength of880nm using specially fabricated InGaAs quantum dot. The exact emission wavelength was finely tuned using local heating effects and calibrated with an optical etalon. We measured storage fidelity of91%for polarization encoded single photons. We further demonstrated temporal multimode operation of the quantum memory.100temporal modes were successfully stored and retrieved in the memory. This is the best result reported for the temporal multimode storage of true single photons.7. Solid-state quantum memory for high-dimensional orbital-angular-momentum entanglement.The orbital-angular-momentum (OAM) of a photon describes the transverse structure of the wavefron. Because it has an unlimited quantum number, it attracts many research interests and has proven to be an outstanding degree of freedom for carrying high-dimensional entanglement and for spatial multimode operations. We demonstrated the first experimental realization of solid-state quantum memory for photonic OAM states. High-dimension OAM entanglement can be generated with nonlinear crystals. However, the typical bandwidth of such photon source is the order of THz, which is impractical for interface with atomic memory. We utilized plano-plano cavity for spectral filtering to obtain a narrow-band and high-dimensional OAM entangled photon pairs. We stored such high-dimensional entanglement with fidelity greater than99%. The spatial-multimode capacity of the quantum memory was assessed through the visibility of the stored superposition states in higher dimensional spaces. The solid-state quantum is shown to be capable of working with dimensions (spatial modes) of up to51. These works systematically investigated the quantum storage of OAM states in a solid. These results pave the way towards the construction of high-dimensional and multiplexed quantum repeaters for large-scale quantum networks.Based on these works, we established the experimental systems for the narrow-band photon source and the solid-state quantum memory. We established the quantum interface among three heterogeneous quantum systems, namely, the semiconductor quantum dot, the narrow-band SPDC and the rare-earth ion doped crystal. The future quantum work will be combination of communication network and computation network. Such complex network will necessarily constructed based various quantum systems. These results are important steps towards the practical realization of long-distance quantum communication and distributed quantum computations. The experimental techniques developed here also can find fascinating applications in experimental tests on fundamental physical theories.