non-Hermitian quantum mechanics
{{Short description|Concept in physics}}{{Improve lead|date=June 2023}}
In physics, non-Hermitian quantum mechanics describes quantum mechanical systems where Hamiltonians are not Hermitian.
History
The first paper that has "non-Hermitian quantum mechanics" in the title was published in 1996 {{Cite journal|last1=Hatano|first1=Naomichi |last2=Nelson|first2=David R.|date=1996-07-15|title=Localization Transitions in Non-Hermitian Quantum Mechanics|journal=Physical Review Letters|volume=77|issue=3|pages=570–573|arxiv=cond-mat/9603165|bibcode=1996PhRvL..77..570H|doi=10.1103/PhysRevLett.77.570|s2cid=43569614}} by Naomichi Hatano and David R. Nelson. The authors mapped a classical statistical model of flux-line pinning by columnar defects in high-Tc superconductors to a quantum model by means of an inverse path-integral mapping and ended up with a non-Hermitian Hamiltonian with an imaginary vector potential in a random scalar potential. They further mapped this into a lattice model and came up with a tight-binding model with asymmetric hopping, which is now widely called the Hatano-Nelson model. The authors showed that there is a region where all eigenvalues are real despite the non-Hermiticity.
Parity–time (PT) symmetry was initially studied as a specific system in non-Hermitian quantum mechanics.N. Moiseyev, "Non-Hermitian Quantum Mechanics", Cambridge University Press, Cambridge, 2011{{Cite web|date=2015-07-20|title=Non-Selfadjoint Operators in Quantum Physics: Mathematical Aspects|url=https://www.wiley.com/en-us/Non+Selfadjoint+Operators+in+Quantum+Physics:+Mathematical+Aspects-p-9781118855287|access-date=2018-06-12|website=Wiley.com|language=en-us}} In 1998, physicist Carl Bender and former graduate student Stefan Boettcher published a paper{{Cite journal|last1=Bender|first1=Carl M.|last2=Boettcher|first2=Stefan|date=1998-06-15|title=Real Spectra in Non-Hermitian Hamiltonians Having $\mathsc{P}\mathsc{T}$ Symmetry|journal=Physical Review Letters|volume=80|issue=24|pages=5243–5246|arxiv=physics/9712001|bibcode=1998PhRvL..80.5243B|doi=10.1103/PhysRevLett.80.5243|s2cid=16705013 }} where they found non-Hermitian Hamiltonians endowed with an unbroken PT symmetry (invariance with respect to the simultaneous action of the parity-inversion and time reversal symmetry operators) also may possess a real spectrum. Under a correctly-defined inner product, a PT-symmetric Hamiltonian's eigenfunctions have positive norms and exhibit unitary time evolution, requirements for quantum theories.{{Cite journal|last=Bender|first=Carl M.|date=2007|title=Making sense of non-Hermitian Hamiltonians|journal=Reports on Progress in Physics|volume=70|issue=6|pages=947–1018|arxiv=hep-th/0703096|bibcode=2007RPPh...70..947B|doi=10.1088/0034-4885/70/6/R03|s2cid=119009206 |issn=0034-4885}} Bender won the 2017 Dannie Heineman Prize for Mathematical Physics for his work.{{Cite web | url=https://www.aps.org/programs/honors/prizes/heineman.cfm | title=Dannie Heineman Prize for Mathematical Physics}}
A closely related concept is that of pseudo-Hermitian operators, which were considered by physicists Paul Dirac,{{cite journal |title=Bakerian Lecture - The physical interpretation of quantum mechanics |journal=Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences |date=18 March 1942 |volume=180 |issue=980 |pages=1–40 |doi=10.1098/rspa.1942.0023|bibcode=1942RSPSA.180....1D |doi-access=free |last1=Dirac |first1=P. A. M. }} Wolfgang Pauli,{{cite journal |last1=Pauli |first1=W. |title=On Dirac's New Method of Field Quantization |journal=Reviews of Modern Physics |date=1 July 1943 |volume=15 |issue=3 |pages=175–207 |doi=10.1103/revmodphys.15.175|bibcode=1943RvMP...15..175P }} and Tsung-Dao Lee and Gian Carlo Wick.{{cite journal |last1=Lee |first1=T.D. |last2=Wick |first2=G.C. |title=Negative metric and the unitarity of the S-matrix |journal=Nuclear Physics B |date=February 1969 |volume=9 |issue=2 |pages=209–243 |doi=10.1016/0550-3213(69)90098-4|bibcode=1969NuPhB...9..209L }} Pseudo-Hermitian operators were discovered (or rediscovered) almost simultaneously by mathematicians Mark Krein and collaboratorsM. G. Krein, “A generalization of some investigations of A. M. Lyapunov on linear differential equations with periodic coefficients,” Dokl. Akad. Nauk SSSR N.S. 73, 445 (1950) (Russian).M. G. Krein, Topics in Differential and Integral Equations and Operator Theory (Birkhauser, 1983).I. M. Gel’fand and V. B. Lidskii, “On the structure of the regions of stability of linear canonical systems of differential equations with periodic coefficients,” Usp. Mat. Nauk 10:1(63), 3−40 (1955) (Russian).V. Yakubovich and V. Starzhinskii, Linear Differential Equations with Periodic Coefficients (Wiley, 1975), Vol. I. as G-Hamiltonian{{What|date=June 2023}} in the study of linear dynamical systems. The equivalence between pseudo-Hermiticity and G-Hamiltonian is easy to establish.{{Cite journal|last1=Zhang|first1=Ruili|last2=Qin|first2=Hong|last3=Xiao|first3=Jianyuan|date=2020-01-01|title=PT-symmetry entails pseudo-Hermiticity regardless of diagonalizability|url=http://aip.scitation.org/doi/10.1063/1.5117211|journal=Journal of Mathematical Physics|language=en|volume=61|issue=1|pages=012101|doi=10.1063/1.5117211|issn=0022-2488|arxiv=1904.01967|bibcode=2020JMP....61a2101Z |s2cid=102483351 }}
In the early 1960s, Olga Taussky, Michael Drazin, and Emilie Haynsworth demonstrated that the necessary and sufficient criteria for a finite-dimensional matrix to have real eigenvalues is that said matrix is pseudo-Hermitian with a positive-definite metric.
In 2002, Ali Mostafazadeh showed that diagonalizable PT-symmetric Hamiltonians belong to the class of pseudo-Hermitian Hamiltonians.{{Cite journal|last=Mostafazadeh|first=Ali|date=2002|title=Pseudo-Hermiticity versus symmetry: The necessary condition for the reality of the spectrum of a non-Hermitian Hamiltonian|journal=Journal of Mathematical Physics|volume=43|issue=1|pages=205–214|arxiv=math-ph/0107001|doi=10.1063/1.1418246|bibcode=2002JMP....43..205M|s2cid=15239201 |issn=0022-2488}}{{Cite journal|last=Mostafazadeh|first=Ali|date=2002|title=Pseudo-Hermiticity versus PT-symmetry. II. A complete characterization of non-Hermitian Hamiltonians with a real spectrum|journal=Journal of Mathematical Physics|volume=43|issue=5|pages=2814–2816|arxiv=math-ph/0110016|doi=10.1063/1.1461427|bibcode=2002JMP....43.2814M |s2cid=17077142 |issn=0022-2488}}{{Cite journal|last=Mostafazadeh|first=Ali|date=2002|title=Pseudo-Hermiticity versus PT-symmetry III: Equivalence of pseudo-Hermiticity and the presence of antilinear symmetries|journal=Journal of Mathematical Physics|volume=43|issue=8|pages=3944–3951|arxiv=math-ph/0107001|doi=10.1063/1.1489072|bibcode=2002JMP....43.3944M |s2cid=7096321 |issn=0022-2488}} In 2003, it was proven that in finite dimensions, PT-symmetry is equivalent to pseudo-Hermiticity regardless of diagonalizability,{{Cite journal|last=Scolarici|first=G.|date=2003-10-01|title=On the pseudo-Hermitian nondiagonalizable Hamiltonians|journal=Journal of Mathematical Physics|volume=44|issue=10|pages=4450-4459|arxiv=quant-ph/0211161|doi=10.1063/1.1609031}}
thereby applying to the physically interesting case of non-diagonalizable Hamiltonians at exceptional points. This indicates that the mechanism of PT-symmetry breaking at exception points, where the Hamiltionian is usually not diagonalizable, is the Krein collision between two eigenmodes with opposite signs of actions.
In 2005, PT symmetry was introduced to the field of optics by the research group of Gonzalo Muga by noting that PT symmetry corresponds to the presence of balanced gain and loss.{{Cite journal|last1=Ruschhaupt|first1=A|last2=Delgado|first2=F|last3=Muga|first3=J G|date=2005-03-04|title=Physical realization of -symmetric potential scattering in a planar slab waveguide|url=https://iopscience.iop.org/article/10.1088/0305-4470/38/9/L03|journal=Journal of Physics A: Mathematical and General|volume=38|issue=9|pages=L171–L176|doi=10.1088/0305-4470/38/9/L03|issn=0305-4470|arxiv=1706.04056|s2cid=118099017}} In 2007, the physicist Demetrios Christodoulides and his collaborators further studied the implications of PT symmetry in optics.{{Cite journal|last=Bender|first=Carl|date=April 2016|title=PT symmetry in quantum physics: from mathematical curiosity to optical experiments|url=https://www.europhysicsnews.org/articles/epn/abs/2016/02/epn2016472p17/epn2016472p17.html|journal=Europhysics News|volume=47, 2|issue=2|pages=17–20|doi=10.1051/epn/2016201|bibcode=2016ENews..47b..17B|doi-access=free}}{{Cite journal|last1=Makris|first1=K. G.|last2=El-Ganainy|first2=R.|last3=Christodoulides|first3=D. N.|last4=Musslimani|first4=Z. H.|date=2008-03-13|title=Beam Dynamics in $\mathcal{P}\mathcal{T}$ Symmetric Optical Lattices|journal=Physical Review Letters|volume=100|issue=10|pages=103904|bibcode=2008PhRvL.100j3904M|doi=10.1103/PhysRevLett.100.103904|pmid=18352189}} The coming years saw the first experimental demonstrations of PT symmetry in passive and active systems.{{Cite journal|last1=Guo|first1=A.|last2=Salamo|first2=G. J.|last3=Duchesne|first3=D.|last4=Morandotti|first4=R.|author-link4=Roberto Morandotti |last5=Volatier-Ravat|first5=M.|last6=Aimez|first6=V.|last7=Siviloglou|first7=G. A.|last8=Christodoulides|first8=D. N.|date=2009-08-27|title=Observation of $\mathcal{P}\mathcal{T}$-Symmetry Breaking in Complex Optical Potentials|journal=Physical Review Letters|volume=103|issue=9|pages=093902|bibcode=2009PhRvL.103i3902G|doi=10.1103/PhysRevLett.103.093902|pmid=19792798}}{{Cite journal|last1=Rüter|first1=Christian E.|last2=Makris|first2=Konstantinos G.|last3=El-Ganainy|first3=Ramy|last4=Christodoulides|first4=Demetrios N.|last5=Segev|first5=Mordechai|last6=Kip|first6=Detlef|date=March 2010|title=Observation of parity–time symmetry in optics|journal=Nature Physics|volume=6|issue=3|pages=192–195|bibcode=2010NatPh...6..192R|doi=10.1038/nphys1515|issn=1745-2481|doi-access=free}} PT symmetry has also been applied to classical mechanics, metamaterials, electric circuits, and nuclear magnetic resonance.{{Cite journal|last=Miller|first=Johanna L.|date=October 2017|title=Exceptional points make for exceptional sensors|journal=Physics Today|volume=10, 23|issue=10|pages=23–26|doi=10.1063/PT.3.3717|bibcode=2017PhT....70j..23M|doi-access=free}}
In 2017, a non-Hermitian PT-symmetric Hamiltonian was proposed by Dorje Brody and Markus Müller that "formally satisfies the conditions of the Hilbert–Pólya conjecture."{{Cite journal|last1=Bender|first1=Carl M.|last2=Brody|first2=Dorje C.|last3=Müller|first3=Markus P.|date=2017-03-30|title=Hamiltonian for the Zeros of the Riemann Zeta Function|journal=Physical Review Letters|volume=118|issue=13|pages=130201|doi=10.1103/PhysRevLett.118.130201|pmid=28409977|arxiv=1608.03679|bibcode=2017PhRvL.118m0201B|s2cid=46816531 }}{{Cite news|url=https://www.quantamagazine.org/quantum-physicists-attack-the-riemann-hypothesis-20170404/|title=Quantum Physicists Attack the Riemann Hypothesis {{!}} Quanta Magazine|work=Quanta Magazine|access-date=2018-06-12}}
References
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