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In standard quantum mechanics, the Hamiltonian describing the physical system is generally Hermitian, so as to ensure that the system has real energy spectra and that the system’s evolution is unitary. In recent years, it has been found that non-Hermitian Hamiltonians with parity-time (
${\cal {PT}}$ ) symmetry also have real energy spectra, and there is a novel non-Hermitian exceptional point between${\cal {PT}}$ -symmetric phase and${\cal {PT}} $ -symmetry-broken phase, which is unique to non-Hermitian systems. Recently, people have realized${\cal {PT}} $ symmetric and anti-${\cal {PT}}$ symmetric non-Hermitian Hamiltonians in various physical systems and demonstrated novel quantum phenomena, which not only deepened our understanding of the basic laws of quantum physics, but also promoted the breakthrough of application technology. This review will introduce the basic physical principles of${\cal {PT}} $ symmetry and anti-${\cal {PT}}$ symmetry, summarize the schemes to realize${\cal {PT}} $ symmetry and anti-${\cal {PT}} $ symmetry in optical and atomic systems systematically, including the observation of${\cal {PT}} $ -symmetry transitions by engineering time-periodic dissipation and coupling in ultracold atoms and single trapped ion, the realization of anti-${\cal {PT}} $ symmetry in dissipative optical system by indirect coupling, and realizing anti-${\cal {PT}} $ -symmetry through fast atomic coherent transmission in flying atoms. Finally, we review the research on precision sensing using non-Hermitian exceptional points of${\cal {PT}} $ -symmetric systems. Near the exceptional points, the eigenfrequency splitting follows an${\varepsilon }^{\tfrac{1}{N}}$ -dependence, where the$\varepsilon$ is the perturbation and$ N $ is the order of the exceptional point. We review the${\cal {PT}}$ -symmetric system composed of three equidistant micro-ring cavities and enhanced sensitivity at third-order exceptional points. In addition, we also review the debate on whether exceptional-point sensors can improve the signal-to-noise ratio when considering noise, and the current development of exceptional-point sensors, which is still an open and challenging question.-
Keywords:
- parity-time symmetry /
- anti-parity-time symmetry /
- non-Hermitian /
- exceptional point
[1] Bender C M 2007 Rep. Prog. Phys. 70 947Google Scholar
[2] Bender C M, Boettcher S 1998 Phys. Rev. Lett. 80 5243Google Scholar
[3] Heiss W D 2004 J. Phys. A. Math. Gen. 37 2455Google Scholar
[4] Bender C M, Brody D C, Jones H F 2003 Am. J. Phys. 71 1095Google Scholar
[5] Bagchi B, Quesne C 2000 Phys. Lett. A 273 285Google Scholar
[6] Makris K G, El-Ganainy R, Christodoulides D N, Musslimani Z H 2008 Phys. Rev. Lett. 100 103904Google Scholar
[7] Longhi S 2009 Phys. Rev. Lett. 103 123601Google Scholar
[8] Klaiman S, Günther U, Moiseyev N 2008 Phys. Rev. Lett. 101 080402Google Scholar
[9] Rüter C E, Makris K G, El-Ganainy R, Christodoulides D N, Segev M, Kip D 2010 Nat. Phys. 6 192Google Scholar
[10] Ramezani H, Kottos T, El-Ganainy R, Christodoulides D N 2010 Phys. Rev. A 82 043803Google Scholar
[11] Chang L, Jiang X, Hua S, Yang C, Wen J, Jiang L, Li G, Wang G, Xiao M 2014 Nat. Photonics 8 524Google Scholar
[12] Peng B, Özdemir S K, Lei F, Monifi F, Gianfreda M, Long G L, Fan S, Nori F, Bender C M, Yang L 2014 Nat. Phys. 10 394Google Scholar
[13] Lin Z, Ramezani H, Eichelkraut T, Kottos T, Cao H, Christodoulides D N 2011 Phys. Rev. Lett. 106 213901Google Scholar
[14] Regensburger A, Bersch C, Miri M A, Onishchukov G, Christodoulides D N, Peschel U 2012 Nature 488 167Google Scholar
[15] Feng L, Xu Y L, Fegadolli W S, Lu M H, Oliveira J E B, Almeida V R, Chen Y F, Scherer A 2013 Nat. Mater. 12 108Google Scholar
[16] Feng L, Wong Z J, Ma R M, Wang Y, Zhang X 2014 Science 346 972Google Scholar
[17] Hodaei H, Miri M A, Heinrich M, Christodoulides D N, Khajavikhan M 2014 Science 346 975Google Scholar
[18] Miao P, Zhang Z, Sun J, Walasik W, Longhi S, Litchinitser N M, Feng L 2016 Science 353 464Google Scholar
[19] Zhang Z, Qiao X, Midya B, Liu K, Sun J, Wu T, Liu W, Agarwal R, Jornet J M, Longhi S, Litchinitser N M, Feng L 2020 Science 368 760Google Scholar
[20] Hang C, Huang G, Konotop V V 2013 Phys. Rev. Lett. 110 083604Google Scholar
[21] Zhang Z, Zhang Y, Sheng J, Yang L, Miri M A, Christodoulides D N, He B, Zhang Y, Xiao M 2016 Phys. Rev. Lett. 117 123601Google Scholar
[22] Sun Y, Tan W, Li H, Li J, Chen H 2014 Phys. Rev. Lett. 112 143903Google Scholar
[23] Yang X, Li J, Ding Y, Xu M, Zhu X F, Zhu J 2022 Phys. Rev. Lett. 128 065701Google Scholar
[24] Schindler J, Li A, Zheng M C, Ellis F M, Kottos T 2011 Phys. Rev. A 84 040101Google Scholar
[25] Wu Y, Liu W, Geng J, Song X, Ye X, Duan C K, Rong X, Du J 2019 Science 364 878Google Scholar
[26] Jing H, Özdemir S K, Geng Z, Zhang J, Lü X Y, Peng B, Yang L, Nori F 2015 Sci. Rep. 5 9663Google Scholar
[27] Schönleber D W, Eisfeld A, El-Ganainy R 2016 New J. Phys. 18 045014Google Scholar
[28] Zhu X, Ramezani H, Shi C, Zhu J, Zhang X 2014 Phys. Rev. X 4 031042
[29] Fleury R, Sounas D, Alù A 2015 Nat. Commun. 6 5905Google Scholar
[30] Bittner S, Dietz B, Günther U, Harney H L, Miski-Oglu M, Richter A, Schäfer F 2012 Phys. Rev. Lett. 108 024101Google Scholar
[31] Yang F, Liu Y C, You L 2017 Phys. Rev. A 96 053845Google Scholar
[32] Antonosyan D A, Solntsev A S, Sukhorukov A A 2015 Opt. Lett. 40 4575Google Scholar
[33] Wu J H, Artoni M, La Rocca G C 2014 Phys. Rev. Lett. 113 123004Google Scholar
[34] Ge L, Türeci H E 2013 Phys. Rev. A 88 53810Google Scholar
[35] Zhao J, Liu Y, Wu L, Duan C K, Liu Y, Du J 2020 Phys. Rev. Appl. 13 014053Google Scholar
[36] Bergman A, Duggan R, Sharma K, Tur M, Zadok A, Alù A 2021 Nat. Commun. 12 486Google Scholar
[37] Zhang X L, Jiang T, Chan C T 2019 Light Sci. Appl. 8 88Google Scholar
[38] Peng P, Cao W, Shen C, Qu W, Wen J, Jiang L, Xiao Y 2016 Nat. Phys. 12 1139Google Scholar
[39] Wu H C, Jin L, Song Z 2021 Phys. Rev. B 103 235110Google Scholar
[40] Xu H S, Jin L 2021 Phys. Rev. A 104 012218Google Scholar
[41] Jin L 2018 Phys. Rev. A 98 022117Google Scholar
[42] Makris K G, El-Ganainy R, Christodoulides D N, Musslimani Z H 2011 Int. J. Theor. Phys. 50 1019Google Scholar
[43] Zyablovsky A A, Vinogradov A P, Pukhov A A, Dorofeenko A V, Lisyansky A A 2014 Physics-Uspekhi 57 1063Google Scholar
[44] Feng L, El-Ganainy R, Ge L 2017 Nat. Photonics 11 752Google Scholar
[45] El-Ganainy R, Makris K G, Khajavikhan M, Musslimani Z H, Rotter S, Christodoulides D N 2018 Nat. Phys. 14 11Google Scholar
[46] Özdemir S K, Rotter S, Nori F, Yang L 2019 Nat. Mater. 18 783Google Scholar
[47] Krasnok A, Nefedkin N, Alu A 2021 IEEE Antennas Propag. Mag. 63 110Google Scholar
[48] Hang C, Huang G 2017 Adv. Phys. X 2 737
[49] Qi B, Chen H, Ge L, Berini P, Ma R 2019 Adv. Opt. Mater. 7 1900694Google Scholar
[50] Konotop V V, Yang J, Zezyulin D A 2016 Rev. Mod. Phys. 88 035002Google Scholar
[51] Suchkov S V, Sukhorukov A A, Huang J, Dmitriev S V, Lee C, Kivshar Y S 2016 Laser Photon. Rev. 10 177Google Scholar
[52] Shankar R 1994 Principles of Quantum Mechanics (New York: Springer US) pp145–147
[53] Guo A, Salamo G J, Duchesne D, Morandotti R, Volatier-Ravat M, Aimez V, Siviloglou G A, Christodoulides D N 2009 Phys. Rev. Lett. 103 093902Google Scholar
[54] Li J, Harter A K, Liu J, de Melo L, Joglekar Y N, Luo L 2019 Nat. Commun. 10 855Google Scholar
[55] Ding L Y, Shi K Y, Zhang Q X, Shen D N, Zhang X, Zhang W 2021 Phys. Rev. Lett. 126 083604Google Scholar
[56] Wiersig J 2014 Phys. Rev. Lett. 112 203901Google Scholar
[57] Chen W, Özdemir S K, Zhao G, Wiersig J, Yang L 2017 Nature 548 192Google Scholar
[58] Hodaei H, Hassan A U, Wittek S, Garcia-Gracia H, El- Ganainy R, Christodoulides D N, Khajavikhan M 2017 Nature 548 187
[59] Lau H K, Clerk A A 2018 Nat. Commun. 9 4320Google Scholar
[60] Zhang M, Sweeney W, Hsu C W, Yang L, Stone A D, Jiang L 2019 Phys. Rev. Lett. 123 180501Google Scholar
[61] Lai Y H, Lu Y K, Suh M G, Yuan Z, Vahala K 2019 Nature 576 65Google Scholar
[62] Xiao Z, Li H, Kottos T, Alù A 2019 Phys. Rev. Lett. 123 213901Google Scholar
[63] Kononchuk R, Cai J, Ellis F, Thevamaran R, Kottos T 2022 Nature 607 697Google Scholar
[64] Kepesidis K V, Milburn T J, Huber J, Makris K G, Rotter S, Rabl P 2016 New J. Phys. 18 095003Google Scholar
[65] Schomerus H 2010 Phys. Rev. Lett. 104 233601Google Scholar
[66] Ge L, Stone A D 2014 Phys. Rev. X 4 031011
[67] Ge L, Makris K G, Christodoulides D N, Feng L 2015 Phys. Rev. A 92 062135Google Scholar
[68] Malzard S, Poli C, Schomerus H 2015 Phys. Rev. Lett. 115 200402Google Scholar
[69] Miri M A, Heinrich M, El-Ganainy R, Christodoulides D N 2013 Phys. Rev. Lett. 110 233902Google Scholar
[70] Heinrich M, Miri M A, Stützer S, El-Ganainy R, Nolte S, Szameit A, Christodoulides D N 2014 Nat. Commun. 5 3698Google Scholar
[71] Christodoulides D, Yang J 2018 Parity-Time Symmetry and Its Applications (Singapore: Springer Singapore) pp513–534
[72] Yang J 2017 Opt. Lett. 42 4067Google Scholar
[73] Nixon S, Yang J 2016 Phys. Rev. A 93 031802Google Scholar
[74] He Z, Li L, Cui W, Wang Y, Xue W, Xu H, Yi Z, Li C, Li Z 2021 New J. Phys. 23 053015Google Scholar
[75] Zhu X, Peng X, Qiu Y, Wang H, He Y 2020 New J. Phys. 22 033035Google Scholar
[76] Jing H, Özdemir S K, Lü X Y, Zhang J, Yang L, Nori F 2014 Phys. Rev. Lett. 113 53604Google Scholar
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图 2 传统和
$\cal {PT} $ 对称耦合光学系统 (a) 复折射率的实部($ {\mathit{n}}_{\mathrm{R}} $ , 红线)和虚部($ {\mathit{n}}_{\mathrm{I}} $ , 绿线)分布; (b) 传统和PT对称系统的叠加态; (c) 对于传统和$\cal {PT} $ 对称系统, 当系统在通道1或通道2处被激发时的光波传播情况 [9]Figure 2. Conventional and
$\cal {PT} $ -symmetric optical systems: (a) The distribution of real part ($ {n}_{\mathrm{R}} $ , red line) and imaginary part ($ {n}_{\mathrm{I}} $ green line) of the complex refractive index; (b) superposition state of conventional and PT-symmetric systems; (c) light wave propagation when the system is excited at channel 1 or channel 2 [9].图 3 (a) 在冷原子系统中实现
$\cal {PT}$ 对称的示意图[54]; (b) 在单个囚禁离子系统中实现$\cal {PT} $ 对称的镱离子$ {}_{}{}^{171}{\mathrm{Y}\mathrm{b}}^{+} $ 的能级示意图[55]; (c) 系统密度矩阵测量图[55]; (d) 系统的相图[55], 红色和黄色区域对应$\cal {PT} $ 对称相, 蓝色区域对应$\cal {PT} $ 对称破缺相Figure 3. (a) Schematic diagram of realizing
$\cal {PT} $ symmetry in cold atom system[54]; (b) schematic diagram of energy levels of ytterbium ion$ {}_{}{}^{171}{\mathrm{Y}\mathrm{b}}^{+} $ for realizing$\cal {PT} $ symmetry in a single trapped ion system[55]; (c) system density matrix measurement diagram[55]; (d) the phase diagram of the system[55]. The red and yellow areas correspond to the$\cal {PT} $ -symmetric phase, and the blue area corresponds to the$\cal {PT} $ -symmetry-broken phase.图 4 (a) 耦合波导示意图; (b) 耦合波导的截面示意图, 波导
$ c $ 红色部分表示存在较大耗散; (c), (d) 波导本征模式的特性; (e) 波导场强的特性; 数据点是有限元模拟结果, 实线是理论计算结果[31]Figure 4. (a) Schematic diagram of coupled waveguide; (b) cross section diagram of coupled waveguide, the red part of waveguide c indicates large dissipation; (c), (d) characteristics of waveguide eigenmodes; (e) property of waveguide field strength. Data points are finite element simulation results, and solid lines are theoretical calculation results[31].
图 5 波导内的光场演化图 [31] (a), (b)
$\cal {PT} $ 对称相和$\cal {PT} $ 对称破缺相的传播特性, 数据点是有限元模拟结果, 实线是理论计算结果; (c) 传统厄米系统和$\cal {PT} $ 反对称系统的光场分布对比图; (d) 分束比例对波长的依赖特性, 红色线是$ \cal {PT} $ 反对称系统, 蓝色线是传统厄米系统Figure 5. Evolution diagram of light field in the waveguides[31]: (a) (b) The propagation characteristics of
$\cal {PT} $ -symmetric phase and$\cal {PT} $ -symmetry-broken phase, respectively, the data points are the result of finite element simulation, and the solid lines are the result of theoretical calculation; (c) comparison diagram of light field distribution between traditional Hermitian system and anti-PT-symmetric system; (d) the dependence of beam splitting ratio on wavelength, the red line is the anti-$\cal {PT} $ -symmetric system, and the blue line is the traditional Hermitian system.图 6 光学微腔构型I (a)和构型II (b)及相应本征频率在复平面上的演化(c)(d), 数据点是有限元模拟结果, 实线是理论计算结果 [31]
Figure 6. Optical microcavity configuration I (a) and configuration II (b) and the corresponding eigenfrequencies on the complex plane. Data points are finite element simulation results, and solid lines are theoretical calculation results[31].
图 7 (a) 通过热
$ {}_{}{}^{87}\mathrm{R}\mathrm{b} $ 蒸汽池中的快速原子相干传输, 实现$\cal {PT} $ 反对称性的示意图; (b) 两个通道中的三能级${\Lambda }$ 型EIT构型 [38]Figure 7. (a) Schematic diagram of realizing anti-
$ \cal {PT} $ -symmetry through fast atomic coherent transmission in hot 87Rb vapor cell; (b) three level Λ-type EIT configuration in two channels[38].图 8 (a) 3个等距微环腔构成的
$\cal {PT} $ 对称系统示意图, 两侧的谐振腔具有平衡的增益和损耗, 而中间的谐振腔是中性的; (b) 系统处于三阶非厄米奇异点的激光模式的强度分布; (c) 相邻激光谱线之间的分裂随微扰强度$\varepsilon $ 的变化, 数据点是实验测量结果, 实线是理论计算结果[58]Figure 8. (a) Schematic diagram of
$\cal {PT} $ -symmetric system composed of three equidistant micro-ring cavities, the resonators on both sides have balanced gain and loss, while the resonators in the middle are neutral; (b) the intensity distribution of the laser mode with the system at the third-order non-Hermitian exception point; (c) splitting between adjacent laser spectral lines with perturbation intensity$\varepsilon$ . Data points are experimental measurement results, and solid lines are theoretical calculation results[58]. -
[1] Bender C M 2007 Rep. Prog. Phys. 70 947Google Scholar
[2] Bender C M, Boettcher S 1998 Phys. Rev. Lett. 80 5243Google Scholar
[3] Heiss W D 2004 J. Phys. A. Math. Gen. 37 2455Google Scholar
[4] Bender C M, Brody D C, Jones H F 2003 Am. J. Phys. 71 1095Google Scholar
[5] Bagchi B, Quesne C 2000 Phys. Lett. A 273 285Google Scholar
[6] Makris K G, El-Ganainy R, Christodoulides D N, Musslimani Z H 2008 Phys. Rev. Lett. 100 103904Google Scholar
[7] Longhi S 2009 Phys. Rev. Lett. 103 123601Google Scholar
[8] Klaiman S, Günther U, Moiseyev N 2008 Phys. Rev. Lett. 101 080402Google Scholar
[9] Rüter C E, Makris K G, El-Ganainy R, Christodoulides D N, Segev M, Kip D 2010 Nat. Phys. 6 192Google Scholar
[10] Ramezani H, Kottos T, El-Ganainy R, Christodoulides D N 2010 Phys. Rev. A 82 043803Google Scholar
[11] Chang L, Jiang X, Hua S, Yang C, Wen J, Jiang L, Li G, Wang G, Xiao M 2014 Nat. Photonics 8 524Google Scholar
[12] Peng B, Özdemir S K, Lei F, Monifi F, Gianfreda M, Long G L, Fan S, Nori F, Bender C M, Yang L 2014 Nat. Phys. 10 394Google Scholar
[13] Lin Z, Ramezani H, Eichelkraut T, Kottos T, Cao H, Christodoulides D N 2011 Phys. Rev. Lett. 106 213901Google Scholar
[14] Regensburger A, Bersch C, Miri M A, Onishchukov G, Christodoulides D N, Peschel U 2012 Nature 488 167Google Scholar
[15] Feng L, Xu Y L, Fegadolli W S, Lu M H, Oliveira J E B, Almeida V R, Chen Y F, Scherer A 2013 Nat. Mater. 12 108Google Scholar
[16] Feng L, Wong Z J, Ma R M, Wang Y, Zhang X 2014 Science 346 972Google Scholar
[17] Hodaei H, Miri M A, Heinrich M, Christodoulides D N, Khajavikhan M 2014 Science 346 975Google Scholar
[18] Miao P, Zhang Z, Sun J, Walasik W, Longhi S, Litchinitser N M, Feng L 2016 Science 353 464Google Scholar
[19] Zhang Z, Qiao X, Midya B, Liu K, Sun J, Wu T, Liu W, Agarwal R, Jornet J M, Longhi S, Litchinitser N M, Feng L 2020 Science 368 760Google Scholar
[20] Hang C, Huang G, Konotop V V 2013 Phys. Rev. Lett. 110 083604Google Scholar
[21] Zhang Z, Zhang Y, Sheng J, Yang L, Miri M A, Christodoulides D N, He B, Zhang Y, Xiao M 2016 Phys. Rev. Lett. 117 123601Google Scholar
[22] Sun Y, Tan W, Li H, Li J, Chen H 2014 Phys. Rev. Lett. 112 143903Google Scholar
[23] Yang X, Li J, Ding Y, Xu M, Zhu X F, Zhu J 2022 Phys. Rev. Lett. 128 065701Google Scholar
[24] Schindler J, Li A, Zheng M C, Ellis F M, Kottos T 2011 Phys. Rev. A 84 040101Google Scholar
[25] Wu Y, Liu W, Geng J, Song X, Ye X, Duan C K, Rong X, Du J 2019 Science 364 878Google Scholar
[26] Jing H, Özdemir S K, Geng Z, Zhang J, Lü X Y, Peng B, Yang L, Nori F 2015 Sci. Rep. 5 9663Google Scholar
[27] Schönleber D W, Eisfeld A, El-Ganainy R 2016 New J. Phys. 18 045014Google Scholar
[28] Zhu X, Ramezani H, Shi C, Zhu J, Zhang X 2014 Phys. Rev. X 4 031042
[29] Fleury R, Sounas D, Alù A 2015 Nat. Commun. 6 5905Google Scholar
[30] Bittner S, Dietz B, Günther U, Harney H L, Miski-Oglu M, Richter A, Schäfer F 2012 Phys. Rev. Lett. 108 024101Google Scholar
[31] Yang F, Liu Y C, You L 2017 Phys. Rev. A 96 053845Google Scholar
[32] Antonosyan D A, Solntsev A S, Sukhorukov A A 2015 Opt. Lett. 40 4575Google Scholar
[33] Wu J H, Artoni M, La Rocca G C 2014 Phys. Rev. Lett. 113 123004Google Scholar
[34] Ge L, Türeci H E 2013 Phys. Rev. A 88 53810Google Scholar
[35] Zhao J, Liu Y, Wu L, Duan C K, Liu Y, Du J 2020 Phys. Rev. Appl. 13 014053Google Scholar
[36] Bergman A, Duggan R, Sharma K, Tur M, Zadok A, Alù A 2021 Nat. Commun. 12 486Google Scholar
[37] Zhang X L, Jiang T, Chan C T 2019 Light Sci. Appl. 8 88Google Scholar
[38] Peng P, Cao W, Shen C, Qu W, Wen J, Jiang L, Xiao Y 2016 Nat. Phys. 12 1139Google Scholar
[39] Wu H C, Jin L, Song Z 2021 Phys. Rev. B 103 235110Google Scholar
[40] Xu H S, Jin L 2021 Phys. Rev. A 104 012218Google Scholar
[41] Jin L 2018 Phys. Rev. A 98 022117Google Scholar
[42] Makris K G, El-Ganainy R, Christodoulides D N, Musslimani Z H 2011 Int. J. Theor. Phys. 50 1019Google Scholar
[43] Zyablovsky A A, Vinogradov A P, Pukhov A A, Dorofeenko A V, Lisyansky A A 2014 Physics-Uspekhi 57 1063Google Scholar
[44] Feng L, El-Ganainy R, Ge L 2017 Nat. Photonics 11 752Google Scholar
[45] El-Ganainy R, Makris K G, Khajavikhan M, Musslimani Z H, Rotter S, Christodoulides D N 2018 Nat. Phys. 14 11Google Scholar
[46] Özdemir S K, Rotter S, Nori F, Yang L 2019 Nat. Mater. 18 783Google Scholar
[47] Krasnok A, Nefedkin N, Alu A 2021 IEEE Antennas Propag. Mag. 63 110Google Scholar
[48] Hang C, Huang G 2017 Adv. Phys. X 2 737
[49] Qi B, Chen H, Ge L, Berini P, Ma R 2019 Adv. Opt. Mater. 7 1900694Google Scholar
[50] Konotop V V, Yang J, Zezyulin D A 2016 Rev. Mod. Phys. 88 035002Google Scholar
[51] Suchkov S V, Sukhorukov A A, Huang J, Dmitriev S V, Lee C, Kivshar Y S 2016 Laser Photon. Rev. 10 177Google Scholar
[52] Shankar R 1994 Principles of Quantum Mechanics (New York: Springer US) pp145–147
[53] Guo A, Salamo G J, Duchesne D, Morandotti R, Volatier-Ravat M, Aimez V, Siviloglou G A, Christodoulides D N 2009 Phys. Rev. Lett. 103 093902Google Scholar
[54] Li J, Harter A K, Liu J, de Melo L, Joglekar Y N, Luo L 2019 Nat. Commun. 10 855Google Scholar
[55] Ding L Y, Shi K Y, Zhang Q X, Shen D N, Zhang X, Zhang W 2021 Phys. Rev. Lett. 126 083604Google Scholar
[56] Wiersig J 2014 Phys. Rev. Lett. 112 203901Google Scholar
[57] Chen W, Özdemir S K, Zhao G, Wiersig J, Yang L 2017 Nature 548 192Google Scholar
[58] Hodaei H, Hassan A U, Wittek S, Garcia-Gracia H, El- Ganainy R, Christodoulides D N, Khajavikhan M 2017 Nature 548 187
[59] Lau H K, Clerk A A 2018 Nat. Commun. 9 4320Google Scholar
[60] Zhang M, Sweeney W, Hsu C W, Yang L, Stone A D, Jiang L 2019 Phys. Rev. Lett. 123 180501Google Scholar
[61] Lai Y H, Lu Y K, Suh M G, Yuan Z, Vahala K 2019 Nature 576 65Google Scholar
[62] Xiao Z, Li H, Kottos T, Alù A 2019 Phys. Rev. Lett. 123 213901Google Scholar
[63] Kononchuk R, Cai J, Ellis F, Thevamaran R, Kottos T 2022 Nature 607 697Google Scholar
[64] Kepesidis K V, Milburn T J, Huber J, Makris K G, Rotter S, Rabl P 2016 New J. Phys. 18 095003Google Scholar
[65] Schomerus H 2010 Phys. Rev. Lett. 104 233601Google Scholar
[66] Ge L, Stone A D 2014 Phys. Rev. X 4 031011
[67] Ge L, Makris K G, Christodoulides D N, Feng L 2015 Phys. Rev. A 92 062135Google Scholar
[68] Malzard S, Poli C, Schomerus H 2015 Phys. Rev. Lett. 115 200402Google Scholar
[69] Miri M A, Heinrich M, El-Ganainy R, Christodoulides D N 2013 Phys. Rev. Lett. 110 233902Google Scholar
[70] Heinrich M, Miri M A, Stützer S, El-Ganainy R, Nolte S, Szameit A, Christodoulides D N 2014 Nat. Commun. 5 3698Google Scholar
[71] Christodoulides D, Yang J 2018 Parity-Time Symmetry and Its Applications (Singapore: Springer Singapore) pp513–534
[72] Yang J 2017 Opt. Lett. 42 4067Google Scholar
[73] Nixon S, Yang J 2016 Phys. Rev. A 93 031802Google Scholar
[74] He Z, Li L, Cui W, Wang Y, Xue W, Xu H, Yi Z, Li C, Li Z 2021 New J. Phys. 23 053015Google Scholar
[75] Zhu X, Peng X, Qiu Y, Wang H, He Y 2020 New J. Phys. 22 033035Google Scholar
[76] Jing H, Özdemir S K, Lü X Y, Zhang J, Yang L, Nori F 2014 Phys. Rev. Lett. 113 53604Google Scholar
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