Virtual state

{{short description|In quantum physics, a very short-lived, unobservable quantum state}}

{{for|the metastable state of a certain class of Feshbach resonance|Feshbach resonance#Unstable state}}

{{Use dmy dates|date=February 2014}}

Image:Raman energy levels.svg, including virtual energy states.]]

In quantum physics, a virtual state is a very short-lived, unobservable quantum state.{{cite book | chapter-url = https://books.google.com/books?id=LDkrAAAAYAAJ&q=%22virtual+state%22+fictitious+quantum&pg=PA61 | chapter = A glossary of terms in nuclear science and technology | series = A series of nine sections | work = National Research Council (U.S.). | title = Conference on Glossary of Terms in Nuclear Science and Technology | publisher = American Society of Mechanical Engineers | date = 1953 | page = 61 }}

In many quantum processes a virtual state is an intermediate state, sometimes described as "imaginary"{{cite journal | vauthors = Robinson AL | title = Tunable Far IR Molecular Lasers Developed: Stimulated Raman scattering associated with a series of closely spaced rotational states is the key to wavelength tunability | journal = Science | location = New York, N.Y. | volume = 227 | issue = 4688 | pages = 736–7 | date = February 1985 | pmid = 17796721 | doi = 10.1126/science.227.4688.736 }} in a multi-step process that mediates otherwise forbidden transitions. Since virtual states are not eigenfunctions of any operator,{{cite book | vauthors = Masters BR | chapter = Historical Development of Non-linear Optical Microscopy and Spectroscopy | veditors = Masters BR, So P | title = Handbook of Biomedical Nonlinear Optical Microscopy | publisher = Oxford University Press| location = US | date = 2008 | isbn = 978-0-19-516260-8 | pages = 10 | chapter-url = https://books.google.com/books?id=4mki1ThMgGYC&pg=PA10 }} normal parameters such as occupation, energy and lifetime need to be qualified. No measurement of a system will show one to be occupied,{{cite thesis |last1=Wardle |first1=David |title=Raman scattering in optical fibres |year=1999 |hdl=2292/433 |hdl-access=free |page=22 }} but they still have lifetimes derived from uncertainty relations.{{cite book | url = https://books.google.com/books?id=bnuGcAZR14IC&pg=PA139 | title = Nonlinear Optics and Laser Spectroscopy | veditors = Abbi SC, Ahmad SA | publisher = Alpha Science International, Limited | date = 2001 | page = 139 | isbn = 978-81-7319-354-5 }}{{cite book | vauthors = Norman P, Ruud K | chapter = Microscopic theory of nonlinear optics. | veditors = Papadopoulos MG, Sadlej AJ, Leszczynski J | title = Non-Linear Optical Properties of Matter | date = 2006 | pages = 3 | publisher = Springer | location = Dordrecht | chapter-url =https://books.google.com/books?id=eeE8OrohX3sC&pg=PA3 | isbn = 978-1-4020-4849-4}} While each virtual state has an associated energy, no direct measurement of its energy is possible{{cite book | vauthors = Belkic D | chapter = The Dyson Perturbation Expansion of the Evolution Operator | title = Principles of quantum scattering theory | chapter-url = https://books.google.com/books?id=V9YY6Bmg6ngC&pg=PA70 | page = 70 | publisher = CRC Press | date = 2004 | isbn = 978-0-7503-0496-2 }} but various approaches have been used to make some measurements (for example see{{Cite journal| vauthors = Saleh BE, Jost BM, Fei HB, Teich MC |date = April 1998 |title=Entangled-Photon Virtual-State Spectroscopy|url=http://people.bu.edu/teich/pdfs/PRL-80-3483-1998.pdf|journal=Physical Review Letters |volume=80 |issue=16 |pages=3483–3486 |doi=10.1103/PhysRevLett.80.3483 |bibcode = 1998PhRvL..80.3483S }} and related work{{cite journal|title=Entangled biphoton virtual-state spectroscopy of the A2Σ+–X2Π system of OH| vauthors = Kojima J, Nguyen QV |date=1 October 2004|journal=Chemical Physics Letters |volume=396 |issue=4 |pages=323–328 |doi=10.1016/j.cplett.2004.08.051| bibcode = 2004CPL...396..323K }}{{Cite journal| vauthors = Lee DI, Goodson III T | veditors = Nunzi JM |date=2007|title=Quantum spectroscopy of an organic material utilizing entangled and correlated photon pairs|url=|journal=Linear and Nonlinear Optics of Organic Materials VII|publisher=International Society for Optics and Photonics|volume=6653|pages=66530V|doi=10.1117/12.745492 |bibcode=2007SPIE.6653E..0VL|s2cid=122068309}} on virtual state spectroscopy) or extract other parameters using measurement techniques that depend upon the virtual state's lifetime.{{Cite journal|title=Measuring photon bunching at ultrashort timescale by two-photon absorption in semiconductors| vauthors = Boitier F, Godard A, Rosencher E, Fabre C |date=13 April 2009 |journal=Nature Physics |volume=5 |issue=4 |pages=267–270 |doi=10.1038/nphys1218 |bibcode=2009NatPh...5..267B |doi-access=free }} The concept is quite general and can be used to predict and describe experimental results in many areas including Raman spectroscopy,{{cite book | url = https://books.google.com/books?id=ZecrNiUkHToC&q=%22&pg=PA16 | vauthors = Griffiths PR, De Haseth JA | title = Fourier Transform Infrared Spectrometry | volume = 83 | edition = second | publisher = Wiley-Interscience | date = 2007 | isbn = 978-0-470-10629-7 | page = 16 }} non-linear optics generally, various types of photochemistry,{{cite book | vauthors = Strehmel B, Strehmel V | chapter = Two-photon physical, organic, and polymer chemistry: theory, techniques, chromophore design, and applications. | title = Advances in Photochemistry |series=| date = January 2007 | volume = 29 | pages = 111–354 (116) | chapter-url = https://books.google.com/books?id=Ik0bDuWzEHYC&pg=PA118 | publisher = John Wiley and Sons | doi = 10.1002/047003758X.ch3 | isbn = 978-0-471-68240-0}} and nuclear processes.{{cite journal | vauthors = Breit G | title = Virtual Coulomb excitation in nucleon transfer | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 57 | issue = 4 | pages = 849–55 | date = April 1967 | pmid = 16591541 | pmc = 224623 | doi = 10.1073/pnas.57.4.849 | bibcode = 1967PNAS...57..849B | doi-access = free }}