---
title: "two-state vector formalism"
type: "entity"
entity_kind: "concept"
status: "hub"
canonical_name: "two-state vector formalism"
aliases: []
first_seen: "2026-10-23T00:00:00.000Z"
writer_model: "claude-sonnet-5"
connects_to: ["weak value","pre- and post-selection","Yakir Aharonov","Lev Vaidman","ABL formula"]
seek_code_commit: "d40ee39"
---


A time-symmetric reformulation of quantum mechanics, developed by [[entity-yakir-aharonov|Yakir Aharonov]] and [[entity-lev-vaidman|Lev Vaidman]], in which a system between a pre-selection and a later post-selection is described jointly by a forward-evolving and a backward-evolving state vector, built on the Aharonov-Bergmann-Lebowitz (ABL) formula (1964). In the vault it is the interpretive framework underlying [[entity-weak-value|weak values]] and the quantum pigeonhole principle's "no two particles share a box" result.

## References
- [[claim-aharonov-2016-quantum-pigeonhole-three-particles-two-boxes-weak-value]]