two-state vector formalism

{{Short description|Description of quantum mechanics in which the present depends on both the past and future}}

The two-state vector formalism (TSVF) is a description of quantum mechanics in terms of a causal relation in which the present is caused by quantum states of the past and of the future taken in combination.

Theory

The two-state vector formalism is one example of a time-symmetric interpretation of quantum mechanics (see Interpretations of quantum mechanics). Time-symmetric interpretations of quantum mechanics were first suggested by Walter Schottky in 1921,{{cite journal

| author = Schottky, Walter

| title = Das Kausalproblem der Quantentheorie als eine Grundfrage der modernen Naturforschung überhaupt

| journal = Naturwissenschaften

| volume = 9

| issue = 25

| pages = 492–496

| year = 1921

| doi = 10.1007/bf01494985

| pmid =

| bibcode = 1921NW......9..492S

| s2cid = 22228793

| url = https://zenodo.org/record/1428318

}} and later by several other scientists. The two-state vector formalism was first developed by Satosi Watanabe{{cite journal

| author = Watanabe, Satosi

| title = Symmetry of physical laws. Part III. Prediction and retrodiction

| journal = Reviews of Modern Physics

| volume = 27

| issue = 2

| pages = 179–186

| year = 1955

| doi = 10.1103/RevModPhys.27.179

| pmid =

| bibcode = 1955RvMP...27..179W

| hdl = 10945/47584

| s2cid = 122168419

| hdl-access = free

}} in 1955, who named it the Double Inferential state-Vector Formalism (DIVF). Watanabe proposed that information given by forwards evolving quantum states is not complete; rather, both forwards and backwards evolving quantum states are required to describe a quantum state: a first state vector that evolves from the initial conditions towards the future, and a second state vector that evolves backwards in time from future boundary conditions. Past and future measurements, taken together, provide complete information about a quantum system. Watanabe's work was later rediscovered by Yakir Aharonov, Peter Bergmann and Joel Lebowitz in 1964, who later renamed it the Two-State Vector Formalism (TSVF).Yakir Aharonov, Lev Vaidman: Protective measurements of two-state vectors, in: Robert Sonné Cohen, Michael Horne, John J. Stachel (eds.): Potentiality, Entanglement and Passion-At-A-Distance, Quantum Mechanical Studies for A. M. Shimony, Volume Two, 1997, {{ISBN|978-0792344537}}, pp. 1–8, [https://books.google.com/books?id=DsNoIcQemTsC&pg=PA2 p. 2] Conventional prediction, as well as retrodiction, can be obtained formally by separating out the initial conditions (or, conversely, the final conditions) by performing sequences of coherence-destroying operations, thereby cancelling out the influence of the two state vectors.{{cite journal | last1=Aharonov | first1=Yakir | last2=Bergmann | first2=Peter G. | last3=Lebowitz | first3=Joel L. | title=Time Symmetry in the Quantum Process of Measurement | journal=Physical Review | publisher=American Physical Society (APS) | volume=134 | issue=6B | date=1964-06-22 | issn=0031-899X | doi=10.1103/physrev.134.b1410 | pages=B1410–B1416| bibcode=1964PhRv..134.1410A }}

The two-state vector is represented by:

{{Equation box 1

|indent =:

|equation = $\langle\Phi| \ \ \ |\Psi\rangle$

|cellpadding

|border

|border colour = #50C878

|background colour = #ECFCF4}}

where the state $\langle\Phi|$ evolves backwards from the future and the state $|\Psi\rangle$ evolves forwards from the past.

In the example of the double-slit experiment, the first state vector evolves from the electron leaving its source, the second state vector evolves backwards from the final location of the electron on the detection screen, and the combination of forwards and backwards evolving state vectors determines what occurs when the electron passes the slits.

The two-state vector formalism provides a time-symmetric description of quantum mechanics, and is constructed such as to be time-reversal invariant.Michael Dickson: Non-relativistic quantum mechanics, Jeremy Butterfield, John Earman (eds.): Philosophy of Physics, Handbook of the Philosophy of Science, North-Holland, Elsevier, pp. 275–416, [https://books.google.com/books?id=2oSn5I70WW0C&pg=PA327 Footnote on p. 327] It can be employed in particular for analyzing pre- and post-selected quantum systems. Building on the notion of two-state, Reznik and Aharonov constructed a time-symmetric formulation of quantum mechanics that encompasses probabilistic observables as well as nonprobabilistic weak observables.{{cite journal | last1=Reznik | first1=B. | last2=Aharonov | first2=Y. | title=Time-symmetric formulation of quantum mechanics | journal=Physical Review A | publisher=American Physical Society (APS) | volume=52 | issue=4 | date=1995-10-01 | issn=1050-2947 | doi=10.1103/physreva.52.2538 | pages=2538–2550| pmid=9912531 |arxiv=quant-ph/9501011| bibcode=1995PhRvA..52.2538R | s2cid=11845457 }}

Relation to other work

In view of the TSVF approach, and in order to allow information to be obtained about quantum systems that are both pre- and post-selected, Yakir Aharonov, David Albert and Lev Vaidman developed the theory of weak values.

In TSVF, causality is time-symmetric; that is, the usual chain of causality is not simply reversed. Rather, TSVF combines causality both from the past (forward causation) and the future (backwards causation, or retrocausality).

Similarly as the de Broglie–Bohm theory, TSVF yields the same predictions as standard quantum mechanics.Yakir Aharonov, Lev Vaidmann: About position measurements which do not show the Bohmian particle position, in: James T. Cushing, Arthur Fine, Sheldon Goldstein (eds.): Bohmian mechanics and quantum theory: an appraisal, Kluwer Academic Publishers, 1996, pp. 141–154, [https://books.google.com/books?id=EFrDTg3ztfEC&pg=PA141 p. 141], [https://books.google.com/books?id=EFrDTg3ztfEC&pg=PA147 147] Lev Vaidman emphasizes that TSVF fits very well with Hugh Everett's many-worlds interpretation,Yakir Aharonov, Lev Vaidman: The Two-State Vector Formalism of Quantum Mechanics: an Updated Review. In: Juan Gonzalo Muga, Rafael Sala Mayato, Íñigo Egusquiza (eds.): Time in Quantum Mechanics, Volume 1, Lecture Notes in Physics 734, pp. 399–447, 2nd ed., Springer, 2008, {{ISBN|978-3540734727}}, DOI 10.1007/978-3-540-73473-4_13, [https://arxiv.org/abs/quant-ph/0105101v2 arXiv:quant-ph/0105101v2] (submitted 21 May 2001, version of 10 June 2007) [https://books.google.com/books?id=InKru6zHQWgC&pg=PA443 p. 443] with the difference that initial and final conditions single out one branch of wavefunctions (our world).{{Cite journal|last1=Aharonov|first1=Yakir|last2=Cohen|first2=Eliahu|last3=Landsberger|first3=Tomer|date=2017-03-12|title=The Two-Time Interpretation and Macroscopic Time-Reversibility|journal=Entropy|language=en|volume=19|issue=3|pages=111|doi=10.3390/e19030111|bibcode=2017Entrp..19..111A|issn=1099-4300|doi-access=free|hdl=1983/8998b8f3-71c3-430d-813b-a7b4479bcaa8|hdl-access=free}}

The two-state vector formalism has similarities with the transactional interpretation of quantum mechanics proposed by John G. Cramer in 1986, although Ruth Kastner has argued that the two interpretations (Transactional and Two-State Vector) have important differences as well.Ruth E. Kastner, talk presented at Cambridge 2014 Conference, Free Will and Retrocausality in the Quantum World, [https://www.youtube.com/watch?v=DMZuBbW6Rcg]Avshalom C. Elitzur, Eliahu Cohen: The Retrocausal Nature of Quantum Measurement Revealed by Partial and Weak Measurements, AIP Conf. Proc. 1408: Quantum Retrocausation: Theory and Experiment (13–14 June 2011, San Diego, California), pp. 120-131, {{doi|10.1063/1.3663720}} ([https://archive.today/20120713100902/http://proceedings.aip.org/resource/2/apcpcs/1408/1/120_1 abstract]) It shares the property of time symmetry with the Wheeler–Feynman absorber theory by Richard Feynman and John Archibald Wheeler and with the time-symmetric theories of Kenneth B. Wharton and Michael B. Heaney{{cite journal

| author = Heaney, Michael B.

| title = A symmetrical interpretation of the Klein-Gordon equation

| journal = Foundations of Physics

| volume = 43

| issue = 6

| pages = 733–746

| year = 2013

| doi = 10.1007/s10701-013-9713-9

| pmid =

| arxiv = 1211.4645

| bibcode = 2013FoPh...43..733H

| hdl =

| s2cid = 118770571

}}[https://arxiv.org/pdf/1211.4645.pdf]

two-state vector formalism

Theory

Relation to other work

See also

References

Further reading