The Laser Frequency Stabilizing And Phase Locking On Quantum Memory Based On Cold Atoms

Quantum information is a new interdiscipline of quantum physics and information science.It mainly includes quantum computing,quntum communication and quntum cryptography.In theory,quantum computer has tremendous calculating capacity,and the quantum cryptography provides an absolute safety of information during transmission.The theoretical works of quantum computing and quantum cryptography are very consummate.A number of principles and algorithms have been made.But during the progress of pragmatizing,both quantum computing and quantum cryptography encounter great difficulties.Quantum computing has to solve the problem of quantum memory to get more quantum bits in a system.Distant or global quantum cryptography communication needs a quantum repeater,which include a quantum memory.In order to achieve quantum memory,first of all a suitable storage medium for quantum should be found.The high propagation velocity and low loss make photon a very good medium for transmission of quantum information.And the stationarity and low couple with environment make cold atoms very suitable medium for the storage of quantum information.The advance of cold atomic technology will bring a breakthrough to quantum memory,and promote the process of practical applications in quantum information.Cold atoms mean rarefied atoms vapor at ultracold state,which is usually at tens ofμK scale.The thermal velocity of those atoms is very low,about 0.05m/s for ⁸⁷Rb,on such a state.As a reference,it is about 250m/s at room temperature, approximal to the speed of an airliner.At such a low velocity,the kinetic energy of atoms are very low and it is very easy to trapped those atoms in statinary location. The Doppler effect can be omitted.And,the rarefication property of cold atoms make the interaction between atoms is very week.Each atom can be treated as a stationary isolated system.Such a system,which is the dream of scientists,makes a very good medium for fundamental scientific research.The generation of cold atoms is mainly due to the advance in laser cooling technology of atoms at room temperature and laser trapping technology of cold atoms.Because of the tremendous contribution to laser cooling and laser trapping, Steven Chu et al won the 1997 Nobel Prize in physics.This made an upsurge on the research on cold atoms.Soon after that,the 2001 and 2005 Nobel Prize in physics were all direct related to cold atoms.Over the past decade,the laser cooling and trapping of atoms technology has been well developed,and in a variety of forms.But no matter what kind of cooling and trapping means,almost all of them make a harsh demand on laser,which plays an important role in cooling and trapping.One of the most important requirements is a very narrow linewidth and high frequency stability. In order to provide the most basic conditions for the platform of cold atoms experiment,the chapterâ…¡and chapterâ…¢of this paper are provided to solve the two problems respectively.In the chapterâ…¡,we narrowed the linewidth of diode laser by adding a grating feedback to the laser.And in the chapterâ…¢,we locked the laser to the hyperfine transition of atoms with super-stability by the means of RF-modulated saturated absorption.The locking system in this paper can substitute the expensive locking system imported from German.And we also analyse the mechanism of error signal generating in the frequency stabilizing method by RF-modulated saturated absorption.The theoretical curves of error signal under difference modulation frequencies are given.After an in-depth study of the mechanism of error signal generation,a new algorithm that adopts both dispersion and absorption to generate the error signal,which may boarden the locking range and narrow the linewidth simultaneously,are proposed.The content of error signal generating in frequency stabilizing by RF-modulated saturated absorption is rare refered by native literatures. The work of these two chapters lays the foundation for the obtaining of cold atoms.The particular characters of cold atoms make it widely used by many research field,especially by physical sciences at microscale,including quantum information science,the new interdiscipline.In order to start the experiment of quantum memory based on cold atomic ensembles,beside the devices to obtain the cold atoms,the means to operate the quantum states of cold atomic ensembles are also needed.For example,in the quantum memory experiment based on EIT,we need two lasers running at difference frequency and with fixed phase relationship.The work in chapterâ…£is to deal with this requirement.We design an analog OPLL(Optical Phase Lock Loop) and a digital OPLL to lock the phase between two lasers,providing necessary means for quantum memory experiments based on cold atomic ensembles.Our digital OPLL can lock the phase between two lasers that run at frequency deviated to 7GHz.Such a research of phase locking between two lasers has not been reported in domestic literatures.Apart from the work above,which is for quantum memory based on cold atom ensembles,I also assume the task of design the switching nodes in metro quantum telephone network.An 8-port programmable optical quantum switch is implemented, first one in China,ensuring that 8 subscribers can establish a quantum phone call or other secrety applications to each other within a radius of 20 kilometers.The implementation of such a switch makes a step forward in the current progress of pragmatizing and networking of quantum cryptography communication.Sum up the above,the main points of work and innovations of this paper are as follows:1.Using the RF-modulated saturated absorption method to stabilize the frequency of our homemade ECDLs,laying the foundation for the quantum memory experiments base on cold atomic ensembles.This frequency stabilizing system achieves the same performance of similar products imported from German.2.Lock the phase between two lasers that run at frequency deviated to 7GHz, by our digital OPLL,which has wide capture range of frequency,well stability and reproducibility.Such a research of phase locking between two lasers has not been reported in domestic literatures.3.Design and implement an 8 subscribers programmable optical quantum switch,which lays the foundation for pragmatizing and networking of quantum cryptography communication.