Replacing the Breit-Wigner Amplitude by the Complex Delta Function to Describe Resonances

Madrid, Rafael de la

doi:10.1143/ptps.184.516

Cited by 1 publication

(1 citation statement)

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“…One of the earliest and most-studied appraoches involves the construction of an arrival time operator [6] by quantizing the classical result −mx/p (for an incoming classical particle with momentum p and position x). This typically leads to arrival time operators which are not self-adjoint [7], although self-adjoint variants have been proposed [8][9][10]. Even when self-adjoint, such operators are not obviously connected to a particular measurement scheme, since there is no obvious way of creating a physically realizable coupling between a measuring device and any of the proposed arrival time operators.…”

Section: Introductionmentioning

confidence: 99%

Quasiprobability for the arrival-time problem with links to backflow and the Leggett-Garg inequalities

Halliwell¹,

Beck²,

Lee³

et al. 2019

Phys. Rev. A

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The arrival time problem for the free particle in one dimension may be formulated as the problem of determining a joint probability for the particle being found on opposite sides of the x-axis at two different times. We explore this problem using a two-time quasi-probability linear in the projection operators, a natural counterpart of the corresponding classical problem. We show that it can be measured either indirectly, by measuring its moments in different experiments, or directly, in a single experiment using a pair of sequential measurements in which the first measurement is weak (or more generally, ambiguous). We argue that when positive, it corresponds to a measurementindependent arrival time probability. For small time intervals it coincides approximately with the time-averaged current, in agreement with semiclassical expectations. The quasi-probability can be negative and we exhibit a number of situations in which this is the case. We interpret these situations as the presence of "quantumness", in which the arrival time probability is not properly defined in a measurement-independent manner. Backflow states, in which the current flows in the direction opposite to the momentum, are shown to provide an interesting class of examples such situations. We also show that the quasi-probability is closely linked to a set of two-time Leggett-Garg inequalities, which test for macroscopic realism.represents the amplitude for the history in which the particle lies in x < 0 at times t 1 , t 2 , · · · t n−1 and is in x > 0 at t n , where P + (t) denotes the projection operator in the Heisenberg picture. For sufficiently close spacing of the times, this object is then a plausible candidate for the amplitude for the particle to make a left-right crossing of the origin, for the first time, during the time interval [t n−1 , t n ]. The probability for the crossing is then the norm of this state.Note that in an expression of the form Eq.(1.1), one would expect the Zeno effect [11] to come into play for sufficiently frequent measurements. This is indeed the case -it becomes significant when the time interval between projectors is smaller than /E, where E is the energy scale of the incoming packet [12,13].

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