Abstract
We study probe corrections to the Eigenstate Thermalization Hypothesis (ETH) in the context of 2D CFTs with large central charge and a sparse spectrum of low dimension operators. In particular, we focus on observables in the form of non-local composite operators {mathcal{O}}_{mathrm{obs}}(x)={mathcal{O}}_L(x){mathcal{O}}_L(0) with hL ≪ c. As a light probe, {mathcal{O}}_{mathrm{obs}}(x) is constrained by ETH and satisfies {leftlangle {mathcal{O}}_{mathrm{obs}}(x)rightrangle}_{h_H}approx {leftlangle {mathcal{O}}_{mathrm{obs}}(x)rightrangle}_{mathrm{micro}} for a high energy energy eigenstate |hH〉. In the CFTs of interests, {{leftlangle {mathcal{O}}_{mathrm{obs}}(x)rightrangle}_h}_{{}_H} is related to a Heavy-Heavy-Light-Light (HL) correlator, and can be approximated by the vacuum Virasoro block, which we focus on computing. A sharp consequence of ETH for {mathcal{O}}_{mathrm{obs}}(x) is the so called “forbidden singularities”, arising from the emergent thermal periodicity in imaginary time. Using the monodromy method, we show that finite probe corrections of the form mathcal{O}left({h}_L/cright) drastically alter both sides of the ETH equality, replacing each thermal singularity with a pair of branch-cuts. Via the branch-cuts, the vacuum blocks are connected to infinitely many additional “saddles”. We discuss and verify how such violent modification in analytic structure leads to a natural guess for the blocks at finite c: a series of zeros that condense into branch cuts as c → ∞. We also discuss some interesting evidences connecting these to the Stoke’s phenomena, which are non-perturbative e−c effects. As a related aspect of these probe modifications, we also compute the Renyi-entropy Sn in high energy eigenstates on a circle. For subsystems much larger than the thermal length, we obtain a WKB solution to the monodromy problem, and deduce from this the entanglement spectrum.
Highlights
An isolated quantum system, on the other hand, always evolves unitarily
We study probe corrections to the Eigenstate Thermalization Hypothesis (ETH) in the context of 2D conformal field theories (CFTs) with large central charge and a sparse spectrum of low dimension operators
We studied probe corrections to ETH in 2D CFTs, focusing on observables in the form of non-local composite operators Oobs ∼ OL(x)OL(0)
Summary
Let us make explicit the thermodynamic limit taken in our context, and the notion of ETH related to it. At this point there are two choices for thermodynamic limit, the one we focus on in this paper corresponds to sending c → ∞ while holding the ratio hH /c finite. The observables we are interested in consist of non-local composite operators Oobs ∼ OL(x)OL(0) They come with a length scale x, which we take to be fixed in the thermodynamic limit we are taking. In this case, both x/L and βH /x are finite. There is a different thermodynamic limit one can take, namely by sending hH → ∞ but keeping c finite In this limit, at least one of the ratios (βH /x, x/L) needs to be vanishing. It has been proposed that the generalized Gibbs ensembles augmented by infinitely many KdV charges are required to capture ETH in this case [52,53,54,55,56]
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