Investigations of self-pumped phase conjugation using Brillouin-enhanced four-wave mixing in TiCl4 at 1 μm are presented. We have (1) observed both forward and backward Brillouin scattering into the optical cavity used for self-pumped phase conjugation during a single pump laser pulse and enhanced the forward-scattered component relative to the backward-scattered component by changing the angle of incidence of the pump beam, (2) demonstrated up to 80% coupling of pump radiation into cavity radiation, (3) examined quenching of the radiation in the optical cavity and eliminated both multiple Stokes orders from Brillouin scattering and Raman scattering as possible mechanisms, (4) observed mode beating in the cavity intensity profile coincident with the quenching, and (5) studied ring-resonator geometries, demonstrating both forward and backward scattering, unidirectional oscillation, and bidirectional oscillation leading to a triple-shifted phase conjugate.
A bstract The backwards-forwards map, introduced as a generalization of the non-isometric holographic maps of the black hole interior of Akers, Engelhardt, Harlow, Penington, and Vardhan to include non-trivial dynamics in the effective description, has two possible formulations differing in when the post-selection is performed. While these two forms are equivalent on the set of dynamically generated states — states formed from unitary time evolution acting on well-defined initial configurations of infalling matter — they differ on the generic set of states necessary to describe the apparent world of the infalling observer. We show that while both versions successfully reproduce the Page curve, the version involving post-selection as the final step, dubbed the backwards-forwards-post-selection (BFP) map, has the desirable properties of being non-isometric but isometric on average and providing state-dependent reconstruction of bulk operators, while the other version does not. Thus the BFP map is a suitable non-isometric code describing the black hole interior including interior interactions.
We show that the emergence of time evolution in an otherwise timeless nonrelativistic closed quantum system -- viewed as a poor man's model of generally covariant quantum theory -- can be understood from the perspective of the path integral representation. As often happens in the functional integral approach, this viewpoint offers a more intuitive account of features that become cumbersome in the operator/Hilbert-space formulation. We show how Schrödinger evolution emerges once a clock degree of freedom is identified and placed in a suitable semiclassical `good-clock state'.
Our analysis has a consequence that extends to path integral formulations of generally covariant systems with action
S (including gravity). In such theories certain transition amplitudes take the form
exp(iS/ℏ)+exp(−iS/ℏ) rather than the expected `forward propagating'
exp(iS/ℏ). This feature, known as the {\em cosine problem}, appears in concrete regularizations of the path integral, for example in the spin foam representation defining the physical inner product between spin network states in loop quantum gravity. Both formally and in explicit regularizations, this apparent difficulty has led some authors to seek modifications of the basic amplitudes to eliminate backward propagation.
Our model shows that the cosine problem is instead a natural consequence of time-reversal invariance of the fundamental dynamics together with the time-neutral boundary states commonly used in transition amplitudes. When a suitable clock system is identified and placed in a semiclassical `good-clock state', it introduces a time arrow selecting the `forward propagating'
exp(iS/ℏ), without modifying the fundamental dynamics.
The analysis clarifies how time emerges under suitable conditions and emphasizes that, in the canonical formulation, quantum gravity is fundamentally timeless.