# Employment

• 2012-2018: Research Assistant, Center for Quantum Information and Control, University of New Mexico, Albuquerque, NM, USA 87131.

# Publications

## Articles

1. X. Qi, Y.-Y. Jau, and I. H. Deutsch, Enhanced cooperativity for quantum-nondemolition-measurement–induced spin squeezing of atoms coupled to a nanophotonic waveguide, Phys. Rev. A 97, 033829 (2018).

We study the enhancement of cooperativity in the atom-light interface near a nanophotonic waveguide for application to quantum nondemolition (QND) measurement of atomic spins. Here the cooperativity per atom is determined by the ratio between the measurement strength and the decoherence rate. Counterintuitively, we find that by placing the atoms at an azimuthal position where the guided probe mode has the lowest intensity, we increase the cooperativity. This arises because the QND measurement strength depends on the interference between the probe and scattered light guided into an orthogonal polarization mode, while the decoherence rate depends on the local intensity of the probe. Thus, by proper choice of geometry, the ratio of good-to-bad scattering can be strongly enhanced for highly anisotropic modes. We apply this to study spin squeezing resulting from QND measurement of spin projection noise via the Faraday effect in two nanophotonic geometries, a cylindrical nanofiber and a square waveguide. We find that, with about 2500 atoms and using realistic experimental parameters, ∼ 6.3 and ∼ 13 dB of squeezing can be achieved on the nanofiber and square waveguide, respectively.

@article{Qi2017Enhanced,
author = {Qi, Xiaodong and Jau, Yuan-Yu and Deutsch, Ivan H.},
title = {Enhanced cooperativity for quantum-nondemolition-measurement--induced spin squeezing of atoms coupled to a nanophotonic waveguide},
journal = {Phys. Rev. A},
year = {2018},
volume = {97},
pages = {033829},
month = mar,
arxiv = {1712.02916},
doi = {10.1103/PhysRevA.97.033829},
grantnumber = {NSF-PHY-1606989, DE-NA0003525},
groups = {SpinSqueezing, Nanofiber/Fiber, WaveGuide, NeutralAtoms},
gsid = {14406064568034581929},
issue = {3},
numpages = {11},
owner = {qxd},
publicationdate = {March 16, 2018},
publisher = {American Physical Society},
submissiondate = {December 21, 2017},
timestamp = {2018-03-16},
}

2. X. Qi, B. Q. Baragiola, P. S. Jessen, and I. H. Deutsch, Dispersive response of atoms trapped near the surface of an optical nanofiber with applications to quantum nondemolition measurement and spin squeezing, Phys. Rev. A 93, 023817 (2016).

We study the strong coupling between photons and atoms that can be achieved in an optical nanofiber geometry when the interaction is dispersive. While the Purcell enhancement factor for spontaneous emission into the guided mode does not reach the strong-coupling regime for individual atoms, one can obtain high cooperativity for ensembles of a few thousand atoms due to the tight confinement of the guided modes and constructive interference over the entire chain of trapped atoms. We calculate the dyadic Green’s function, which determines the scattering of light by atoms in the presence of the fiber, and thus the phase shift and polarization rotation induced on the guided light by the trapped atoms. The Green’s function is related to a full Heisenberg-Langevin treatment of the dispersive response of the quantized field to tensor polarizable atoms. We apply our formalism to quantum nondemolition (QND) measurement of the atoms via polarimetry. We study shot-noise-limited detection of atom number for atoms in a completely mixed spin state and the squeezing of projection noise for atoms in clock states. Compared with squeezing of atomic ensembles in free space, we capitalize on unique features that arise in the nanofiber geometry including anisotropy of both the intensity and polarization of the guided modes. We use a first-principles stochastic master equation to model the squeezing as a function of time in the presence of decoherence due to optical pumping. We find a peak metrological squeezing of ∼5 dB is achievable with current technology for ∼2500 atoms trapped 180 nm from the surface of a nanofiber with radius a=225 nm.

@article{Qi2016Dispersive,
author = {Qi, Xiaodong and Baragiola, Ben Q. and Jessen, Poul S. and Deutsch, Ivan H.},
title = {Dispersive response of atoms trapped near the surface of an optical nanofiber with applications to quantum nondemolition measurement and spin squeezing},
journal = {Phys. Rev. A},
year = {2016},
volume = {93},
number = {2},
pages = {023817},
month = feb,
arxiv = {1509.02625},
doi = {10.1103/PhysRevA.93.023817},
file = {https://github.com/CQuIC/NanofiberPaper2014/releases},
grantnumber = {AFOSR-Y600242,NSF-PHY-1212445, NSF-PHY-1307520},
gsid = {14591647056169599678},
issue = {2},
numpages = {17},
owner = {qxd},
publicationdate = {February 11, 2016},
publisher = {American Physical Society},
repo = {https://github.com/CQuIC/NanofiberPaper2014},
submissiondate = {September 22, 2015},
timestamp = {2016.02.11},
}

3. G. Liu, Y. Ning, and X. Qi, Study of whispering-gallery-mode in a photonic crystal microcavity, Optoelectronics Letters 7, 105 (2011).

The whispering-gallery-mode (WGM) photonic crystal microcavity can be potentially used for miniaturized photonic devices, such as thresholdless lasers. In this paper, we use plane wave expansion (PWE) method and study the WGM of H2 photonic crystal microcavities which are formed by removing seven center air holes in a photonic crystal. The WGM in these large-size cavities has some advantages compared with single defect WGM in the view of real device applications. We analyze the nearby air hole effect on WGM and conclude that WGM is more sensitive to moving towards the outside rather than moving towards the inside of a nearby air hole. In our case, if a nearby air hole is moved 0.1a away from the center, the WGM will disappear. If a nearby air hole is moved 0.6a towards the center, however, the WGM will still exit. We also analyze the structure with an air hole (rm= 0.2a) in the center of the microcavity, and we find that the WGM is not affected by the central hole sensitively. As we increase rm, the WGM remains unchanged until rm is 0.64 times greater than period a. It is found that the tolerance of WGM to the displacement of nearby air holes and the occurance of central holes is large enough to fabricate electrical injection structure.

@article{Liu2011Study,
author = {Liu, Guangyu and Ning, Yongqiang and Qi, Xiaodong},
title = {Study of whispering-gallery-mode in a photonic crystal microcavity},
journal = {Optoelectronics Letters},
year = {2011},
volume = {7},
number = {2},
pages = {105--108},
doi = {10.1007/s11801-011-0137-5},
grantnumber = {National Natural Science Foundation of China 60636020, 60676034, 60706007, 10974012, 60876036 and 90923037, Jilin Province Science and Technology Development Project 20080335 and 20080516, CAS Innovative Fund},
gsid = {16334681384623386562}
}

4. X. Qi, Modeling and deciphering on two spin-polariton entanglement experiments in NV center of diamond, ArXiv e-Prints (2011).

This work is a theoretical investigation on the spin-polariton (polarized single photon) entanglement in nitrogen vacancy (NV) centers in diamond in order to interpret the results of two landmark experiments reported by the teams of Buckley and Togan in Science and Nature. A Jaynes-Cummings model is applied to analyze the off- and on-resonant dynamics of the electronic spin and polarized photon system. Combined with the analysis on the NV center’s electron structure and transition rules, this model consistently explained the Faraday effect, Optical Stark effect, pulse echo technology and energy level engineering technology in the way to realize the spin-polariton entanglement in diamond. All theoretical results are consistent well with the reported phenomena and data. This essay essentially aims at applying the fundamental skills the author has learned in Quantum Optics and Nonlinear Optics, especially to the interesting materials not covered in class, in assignments and examinations, such as calculation on matrix form of Hamiltonian, quantum optical dynamics with dressed state analysis, entanglement and so on.

@article{Qi2011Modeling,
author = {Qi, Xiaodong},
title = {Modeling and deciphering on two spin-polariton entanglement experiments in NV center of diamond},
journal = {ArXiv e-prints},
year = {2011},
month = {\#nov\#},
adsnote = {Provided by the SAO/NASA Astrophysics Data System},
archiveprefix = {arXiv},
eprint = {1111.5532},
keywords = {Quantum Physics, Physics - Atomic Physics, Physics - Optics},
owner = {QXD},
primaryclass = {quant-ph},
timestamp = {2011.11.24}
}

5. X. Qi, S. Ye, N. Zhang, L. Qin, and L. Wang, Surface-emitting distributed-feedback semiconductor lasers and grating-coupled laser diodes, Chinese Journal of Optics and Applied Optics 3, 415 (2010).

The principles and structures of Surface-emitting Distributed-feedback Bragg(SE-DFB)semiconductor lasers,especially curved-grating coupled SE-DFB lasers,are described,then,their characteristics are discussed and compared with that of other semiconductors.It points out that the SE-DFB lasers based on special diffractive characteristics of curved grating can achieve the mode control and two-dimensional leaky-mode coupling of laser arrays,and can obtain the laser with narrow line width(typically 0.08 nm),small divergence angle(typically 0.5 mrad),high brightness(3 W(CW)near-diffraction limit emitting from a single device)and high power(73 W maximum in a single device and kW level in arrays).After reviewing the development,present status and new opportunities in future of the SE-DFB devices,it emphasizes that as the curved-grating coupled SE-DFB has both strengths from side emitting and surface emitting devices,it will have great research significance and wide application prospect by introducing into semiconductor lasers and arrays with different material systems and structures.

@article{Qi2010Surfacea,
author = {Qi, Xiaodong and Ye, Shujuan and Zhang, Nan and Qin, Li and Wang, Lijun},
title = {Surface-emitting distributed-feedback semiconductor lasers and grating-coupled laser diodes},
journal = {Chinese Journal of Optics and Applied Optics},
year = {2010},
volume = {3},
pages = {415-431},
grantnumber = {国家自然科学基金重点资助项目(No.60636020), 国家自然科学基金资助项目(No.60676034 10974012), 吉林省科技发展项目(No.20080335), 中国科学院知识创新工程领域前沿项目, 国家自然科学基金重点支持项目(No.90923037)},
gsid = {11825572885569869085},
issue = {5},
url = {http://en.cnki.com.cn/Article_en/CJFDTOTAL-ZGGA201005003.htm}
}

6. S. Ye, L. Qin, X. Qi, Y. Hu, N. Zhang, Y. Ning, and L. Wang, Emission characteristics of second-order distributed feedback semiconductor lasers, Zhongguo Jiguang (Chinese Journal of Lasers) 37, 2371 (2010).

@article{Ye2010Emissiona,
author = {Ye, Shujuan and Qin, Li and Qi, Xiaodong and Hu, Yongsheng and Zhang, Nan and Ning, Yongqiang and Wang, Lijun},
title = {Emission characteristics of second-order distributed feedback semiconductor lasers},
journal = {Zhongguo Jiguang (Chinese Journal of Lasers)},
year = {2010},
volume = {37},
pages = {2371-2375},
gsid = {13677417736077898026},
issue = {9},
publisher = {Chinese Optical Society,| a P. O. Box 800-211| c Shanghai| z 201800| e zhgjgail. shcnc. ac. cn| u www. opticsjournal. net}
}

7. 叶淑娟, 秦莉, 戚晓东, 胡永生, 张楠, 宁永强, and 王立军, 二阶光栅分布反馈半导体激光器的出光特性, 中国激光 37, 2371 (2010).

基于耦合模理论,分析了二阶光栅分布反馈（DFB）激光器的综合出光特性,包括阈值增益、光子密度分布、外微分量子效率等。数值计算结果表明,对于波长为1.55μm的给定结构器件,二阶光栅占空比对其出光特性影响较大。最后得到优化的光栅占空比为0.43,优化后,腔内光子密度分布均匀,边模抑制比达35 dB,外微分量子效率达47%。

@article{Ye2010Emission,
author = {叶淑娟 and 秦莉 and 戚晓东 and 胡永生 and 张楠 and 宁永强 and 王立军},
title = {二阶光栅分布反馈半导体激光器的出光特性},
journal = {中国激光},
year = {2010},
volume = {37},
pages = {2371-2375},
gsid = {13677417736077898026},
issue = {9},
url = {http://www.cqvip.com/QK/95389X/201009/35393683.html}
}

8. 戚晓东, 叶淑娟, 张楠, 秦莉, and 王立军, 面发射分布反馈半导体激光器及光栅耦合半导体激光器, 中国光学与应用光学 514 (2010).

@article{Qi2010Surface,
author = {戚晓东 and 叶淑娟 and 张楠 and 秦莉 and 王立军},
title = {面发射分布反馈半导体激光器及光栅耦合半导体激光器},
journal = {中国光学与应用光学},
year = {2010},
pages = {514-431},
issue = {5},
url = {http://www.cqvip.com/qk/60129x/201005/35847714.html}
}

9. 李德华, 戚晓东, and 刘盛纲, 光整流法产生THz辐射转化率的理论分析, 中国科学：E辑 39, 745 (2009).

从理论上分析了超短激光脉冲在二阶非线性晶体中光整流效应产生THz辐射的过程，讨论了影响激光能量转换为THz辐射能量的效率的各种因素；入射激光脉冲宽度对转换效率影响非常大．对于一般二阶非线性晶体，当晶体厚度等于相干长度Lc时，光对THz的转换效率最高．对于周期性极化的非线性晶体，在忽略介质吸收的情况下，转换效率与介质厚度成正比．考虑介质对THz的吸收，晶体的有效长度Leff=0．63／α,再增大晶体长度，转化效率将没有明显增加，并最终趋于常数．

@article{Li2009Optical,
author = {李德华 and 戚晓东 and 刘盛纲},
title = {光整流法产生THz辐射转化率的理论分析},
journal = {中国科学：E辑},
year = {2009},
volume = {39},
pages = {745-750},
issue = {4},
url = {http://www.cqvip.com/qk/98492x/200904/30256010.html}
}

10. D. Li, X. Qi, and S. Liu, A theoretical analysis of optical-to-THz conversion efficiency via optical rectification, Science in China Series-E 51, 2080 (2008).

A theoretical analysis of an ultra-short pulse converted to Terahertz radiation via optical rectification in nonlinear optical crystal is presented here; several factors that affect optical-to-THz conversion efficiencies are discussed; pulse durations affect the conversion efficiency effectively: when crystal length is equal to the optimal crystal length lc, optical-to-THz conversion efficiency is the highest, but for the periodically-inverted electro-optic crystals, conversion efficiency is almost proportional to the crystal length when absorption can be neglected. Taking account of the absorption of crystals, effective length of crystal is Leff=0.63/α, there is no apparent increase of conversion efficiency and the conversion efficiency approaches to a constant eventually when the crystal length is increased.

@article{Li2008theoretical,
author = {Li, Dehua and Qi, Xiaodong and Liu, Shenggang},
title = {A theoretical analysis of optical-to-THz conversion efficiency via optical rectification},
journal = {Science in China Series-E},
year = {2008},
volume = {51},
number = {12},
pages = {2080-2088},
doi = {10.1007/s11431-008-0309-0},
gsid = {16256416162266910954}
}


## Theses

1. X. Qi, Dispersive Quantum Interface with Atoms And Nanophotonic Waveguides, PhD thesis, University of New Mexico, 2018. (352 pages with 7 main Chapters and 19 Appendices.)

Strong coupling between atoms and light is critical for quantum information processing and precise sensing. A nanophotonic waveguide is a promising platform for realizing an atom-light interface that reaches the strong coupling regime. In this dissertation, we study the dispersive response theory of the nanowaveguide system as the means to create an entangling atom-light interface, with applications to quantum non-demolition (QND) measurement and spin squeezing. We calculate the dyadic Green’s function, which determines the scattering of light by atoms in the presence of a nanowaveguide, and thus the phase shift and polarization rotation induced on the guided light. The Green’s function is related to the full Heisenberg-Langevin treatment of the dispersive response of the quantized field to tensor polarizable atoms. Using the Green’s tensors, we calculate the modified spontaneous emission rates for classical dipoles and quantum alkali atoms. We model QND measurement and spin squeezing using first-principles stochastic master equations. Based on the birefringence effect, we propose a spin squeezing protocol for the spins encoded in the clock transition of cesium-133. We generalize the concept of cooperativity, which is determined by the ratio between the measurement strength and the decoherence rate in the context of the dispersive waveguide interface. By maximizing the cooperativity per atom, we find the optimal choice of quantization axis that defines the clock states. With this, we predict a peak squeezing of 4.7 dB with 2500 atoms trapped along a realistic nanofiber. To enhance the squeezing and for applications in magnetometry, we propose a protocol based on the Faraday effect for a nanofiber and a square waveguide. Counterintuitively, by placing the atoms at an azimuthal position where the guided probe mode has the lowest intensity, we increase the cooperativity. This arises because the measurement strength depends on the interference between the probe and scattered light into an orthogonal mode, while the decoherence rate depends on the local intensity of the probe. We find 6.3 dB and 13 dB of peak squeezing for the nanofiber and the square waveguide, respectively, with 2500 atoms.

@phdthesis{Qi2018Dispersive,
author = {Qi, Xiaodong},
title = {Dispersive Quantum Interface with Atoms and
Nanophotonic Waveguides},
school = {University of New Mexico},
year = {2018},
month = jul,
note = {352 pages with 7 main Chapters and 19 Appendices.},
doi = {10.5281/zenodo.1216258},
file = {https://github.com/i2000s/PhD_Thesis/releases},
repo = {https://github.com/i2000s/PhD_Thesis},
timestamp = {2018-06-08},
url = {https://doi.org/10.5281/zenodo.1216258}
}

2. X. Qi, The Effects of Multi-Exciton Interactions on Optical Cavity Emission, Master’s thesis, Queen’s University, 2012.

This thesis presents a theoretical study of the collective effects of a large number of photon emitters coupled to optical cavities. The ensemble effects are accounted for by considering both the light emitting and scattering by the photon emitters. It suggests that, to correctly estimate the emitters ensemble coupled cavity mode, it is necessary to consider the existence of the excited excitons ensemble and optical pumps. This thesis shows that optical pumps can excite more excitons and scattering channels as pumping power increases. The change in exciton population can lead to comprehensive spectral behaviors by changing the cavity spectral shapes, bandwidth and resonance positions, through the inhomogeneous broadening and frequencies repulsion effects of collective emissions. The existence of the exciton ensemble can also enhance optical coupling effects between target excitons and the cavity mode. The target exciton, which has a relatively large coupling strength and is close to the cavity peak, can affect the properties of the background dipoles and their coupling to the cavity. All these collective effects are sensitive to the number, the resonances distribution, and the optical properties of the background excitons in the frequency domain and the property of the target exciton, if any. This study provides a perspective on the control of the optical properties of cavities and individual excitons through collective excitation.

@mastersthesis{Qi2012effects,
author = {Qi, Xiaodong},
title = {The effects of multi-exciton interactions on optical cavity emission},
school = {Queen's University},
year = {2012},
month = jul,
file = {https://github.com/i2000s/Thesis_Queens/releases},
owner = {qxd},
repo = {https://github.com/i2000s/Thesis_Queens},
timestamp = {2012.07.23},
url = {http://hdl.handle.net/1974/7337}
}


# Selected Talks and Conferences:

• June 26, 2018: seminar talk at the Hefei National Laboratory of Physical Sciences at the Microscale, Division of Quantum Physics and Quantum Information, Hefei, China. Visit USTC from June 26 to 27.
• June 25, 2018: seminar talk at the Wuhan Institute of Physics and Mathematics, Wuhan, China. Visit WIPM from June 23 to 25.
• June 19, 2018: colloquium talk at the Institute of Laser Spectroscopy, Shanxi University, Taiyuan, China. Visit Shanxi University from June 18 to 22.
• June 15, 2018: seminar talk at the Department of Physics, Tsinghua University, Beijing, China.
• June 13, 2018: seminar talk at the Academy of Mathematics and Systems Science, CAS, Beijing, China. Visit AMSS from June 13 to 17. Visit the Institute of Physics on June 14.
• May 20th–25th, 2018: annual retreat talk. Work closely with experimental collaborators from the University of Arizona, at Flagstaff, AZ, USA.
• April 18th, 2018: Dissertation defense talk – Dispersive quantum interface with atoms and nanophotonic waveguides (video and PPT available).
• Feb 22nd–24th, 2018: 20th Annual SQuInT Workshop presentation
• June 9, 2017: APS 48th Annual Meeting of DAMOP 2017 presentation (Generation of atomic spin squeezed states in nanophotonic waveguides using QND measurement, slides and audio records can be downloaded from the link), Sacramento, California, USA.
• February 24, 2017: 19th Annual SQuInT Workshop 2017 talk (Spin Squeezing on Nanophotonic Waveguides, slides can be downloaded form the link), Baton Rouge, Louisiana, USA.
• Nov 16th, 2016: Annual research report, CQuIC @ Albuquerque, NM, USA.
• July 31-Aug 5, 2016: Gordon Research Conference – Quantum Entanglement, New States of Matter, and Correlated Dynamics, Easton, MA, USA. (poster)
• July 30-31, 2016: Gordon Research Seminar – Quantum Simulation, Entanglement and Dynamics of Condensed Matter Systems and Field Theories, Easton, MA, USA. (poster)
• Feb 18-20, 2016: 18th Annual SQuInT workshop, Albuquerque, NM, USA. (poster)
• Dec 29-30, 2015: Academic visit and a Seminar talk in IPhy, CAS @ Beijing, China.
• Dec 25-26, 2015: Academic visit and a Seminar talk in Hefei National Lab of Microscale Matter Physics, USTC @ Hefei, China. (slides repo)
• Dec 25, 2015: JuliaQuantum and software ecosystem in the perspective of quantum information science, public talk in USTC.
• Dec 23-24, 2015: Academic visit and a Seminar talk in the Key Laboratory of Quantum Information, USTC @ Hefei, China.
• Nov 27-Dec 2 and Dec 29-31, 2015: Academic visit and a Seminar talk on Strong dispersive response of atoms trapped near to an optical nanofiber, AMSS, CAS @ Beijing, China.
• Sep 29, 2015: Annual research report, CQuIC @ Albuquerque, NM, USA.
• June 10, 2015: 46th Annual Meeting of the APS DAMOP oral presentation, 60 (7), Columbus, OH, USA.
• May 12-15, 2015: Retreat research presentation with collaborators from UofA and Sandia National Labs @ Flagstaff, AZ, USA.
• Feb 21, 2015: JuliaMeetup opening up talk @ Berkeley, CA, USA.
• Feb 18-21, 2015: Poster presentation at the 17th SQuInT workshop, Berkeley, CA, USA.
• Oct, 1, 2014: PhD Candidate exam presentation, Department of physics and astronomy, UNM @ Albuquerque, NM, USA. (slides repo)
• July 27-July 1, 2014: Poster presentation, Gordon Research Workshop on Quantum Science, Easton, MI, USA.
• July 26, 2014: Poster presentation, Gordon Research Seminar on Quantum Science, Easton, MI, USA.
• May 26-30, 2014: Research retreat with collaborators from UofA, @ Flagstaff, AZ, USA.
• Feb, 2014: Poster presentation in the 16th SQuInT Workshop, Santa Fe, NM, USA.
• Oct, 2013: CQuIC seminar talk, @ Albuquerque, NM, USA.