Realization Of Two-Atom Entanglement Based On Rydberg Interaction

The neutral single-atom system with controllable interaction and long coherence time has the advantage of providing large-scale integration of thousands of qubits,and is a powerful candidate for quantum simulation and quantum computing.In recent years,the neutral single-atom system has achieved rapid development in experiments,including the deterministic loading of dozens or hundreds of single atoms,the addressing and manipulation of single atoms in two-dimensional and three-dimensional arrays,and the coherence time of qubits.The extension of,the realization of a two-qubit quantum gate based on the Rydberg state,and the efficient readout of atomic states,etc.,have greatly promoted the application of this system in quantum simulation and quantum computing.These results have laid a solid foundation for the use of single-atom systems in quantum computing and quantum simulation.However,the realization of universal quantum computers still needs to overcome many challenges and problems,including the realization of high-fidelity entangling gate operations and the realization of non-destructive measurement and low crosstalk preparation and detection of atoms in the array.This paper mainly studies the physical limitation of the two-qubit entanglement gate based on the Rydberg interaction,and we propose to use the Off-Resonant Modulated Driving(ORMD)scheme to implement a two-qubit controlled phase gate to reduce the impact of the physical limitation,and on this basis,realize the two-bit entanglement gate.Atomic entanglement lays the foundation for the next step of quantum simulation and quantum calculation based on heteronuclear single-atom systems.The main research results obtained in this paper are as follows:1.The laser sources with narrow linewidth,ultra-low noise and high stability are realizedWe customize a ultra-stable Fabry-Perot cavity with high fineness and tunable cavity length.Using Pound-Drever-Hall(PDH)frequency stabilization method,we lock the length of the Fabry-Perot cavity on a commercial iodine stabilized laser,and the long-term drift of the iodine-stabilized laser frequency is less than 1kHz/0.5? in a few hours,making the long drift of the laser frequency less than 46kHz in a month.Then,the frequency of 780nm and 960nm laser is stabilized to the cavity by the PDH method.At the same time,the Fabry-Perot cavity is used as a low-pass filter to filter the high frequency phase noise of the laser induced by finite feedback bandwidth of locking loop,and the phase noise at 0.75MHz is suppressed by 30 dB.Then,the cavity filter light is used to injection locking to amplifying the power of laser.By this way,we end up with a narrow linewidth(<1 kHz),ultralow phase noise and good long-term stability of the laser source.2.The Rydberg coherent excitation with high efficiency and long decay time of a single atom is realizedThe Rydberg coherent excitation of a single atom is the basis for realizing quantum simulation and quantum entanglement.After the frequency stabilization of 780nm laser and 960nm laser,we realize coherent excitation from the ground state to the Rydberg state through a two-photon transition.The 780nm laser couples the ground state |g>and intermediate level |p>,and the 480nm laser(generated from frequency doubling of 960nm laser)couples the intermediate level 1p>and the Rydberg level 1r>.The detuning of 780nm laser from |p>is about-5.7GHz.By using a counter-propagating beams scheme,we realize the coherent excitation with a excitation efficiency of 98.5%.Then,we study how the coherent coupling between a ground-state and a Rydberg level of a single atom could be affffected in many ways by physical and technical limitations.We develop models for each detection error and dephasing sources obtaining good quantitative agreements with experimental observations.Finally,by optimizing these parameters,we achieve a Rabi oscillation with a decay time of 67?s.3.The influence of controll-qubit decoherence on two-qubit entangling gateBy analyzing the characteristics of the present '?-gap-?' two-qubit logic gate scheme,we find that another important factor affecting the fidelity of the two-qubit gate is the coherence of a single qubit except for experimental technical factors.Firstly,the coherent time ?gr between the ground state and the Rydberg state of a single atom is measured experimentally and the source of the dephasing factor is analyzed.Then,we measure the coherent time tcont of the control bit in the entanglement process,which is experimentally consistent with the coherent time ?gr of a single atom.Therefore,we conclude that the entanglement is actually limited by exp(-tgap/?gr),without considering the other errors prepared by the entanglement process,where tgap is the interval time between ?-gap-? pulse sequences?4.Realizing Cz gate by single-pulse ORMD pulse and two-atom entanglement by the Cz gateAfter analyzing that the fidelity of the general two-qubit logic gate scheme is limited by the coherence of the control qubits,we use an ORMD scheme to implement a controlled phase gate(Cz gate).When preparing the two-qubit gate using this method,the atoms are not completely populated into the Rydberg state,which greatly reduces the requirement for the coherence time of the ground-Rydberg state of a single atom.Based on this scheme,we have achieved a two-atom entangled state with a fidelity of 0.875.We believe that the fidelity can reach more than 0.95 when the imperfection factors in the experiment are improved.