Foundations of Physics

, Volume 22, Issue 3, pp 333–342 | Cite as

Can one detect the state of an individual system?

  • L. E. Ballentine
Part IV. Invited Papers Dedicated To Sir Karl Popper


Some interpretations of quantum mechanics regard a mixed quantum state as a ensemble, each individual member of which has a definite but unknown state vector. Other interpretations ascribe a state vector only to anensemble of similarly prepared systems, but not to anindividual. Previous attempts to detect the hypothetical individual state vectors have failed, essentially because the state operator (density matrix) enters the relevant equations linearly. An example from nonlinear dynamics, in which a density matrix enters nonlinearly, is examined because it might appear to circumvent this difficulty. However, it is shown that the hypothetical individual state vectors can not be detected this way, so the adequacy of theensemble interpretation survives a critical test.


Quantum Mechanic Nonlinear Dynamic Quantum State Density Matrix State Vector 
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Copyright information

© Plenum Publishing Corporation 1992

Authors and Affiliations

  • L. E. Ballentine
    • 1
  1. 1.Physics DepartmentSimon Fraser UniversityBurnabyCanada

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