Talks

Quantum state tomography (QST) is the art of reconstructing an unknown quantum state through measurements. It is a key primitive for developing quantum technologies. Neural network quantum state tomography (NNQST), which aims to reconstruct the quantum state via a neural network ansatz, is often implemented via a basis-dependent cross-entropy loss function. State-of-the-art implementations of NNQST are often restricted to characterizing a particular subclass of states, to avoid an exponential growth in the number of required measurement settings. In this talk, I will discuss an alternative neural-network-based QST protocol that uses shadow-estimated infidelity as the loss function, named “neural-shadow quantum state tomography” (NSQST). After introducing NNQST and the classical shadow formalism, I will present numerical results on the advantage of NSQST over NNQST at learning the relative phases, NSQST’s noise robustness, and NSQST’s advantage over direct shadow estimation. I will also briefly discuss the future prospects of the protocol with different variational ansatz and randomized measurements, as well as its experimental feasibility.

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Zoom link https://pitp.zoom.us/j/94167105773?pwd=TXR3TUtwNjV4VFB4SEpvTkhqd29SUT09

To develop a quantum gravity theory, it is fundamental to move away from the gauge-fixing approach and instead employ a gauge-free analysis. There is an increasing body of evidence suggesting that the symmetries employed for gauge-fixing might carry charge. Consequently, setting the associated fields to zero is a physical constraint on the system, which should be avoided. In this talk, we will examine a partial fixing of the Fefferman-Graham (FG) gauge, referred to as the Weyl-Fefferman-Graham (WFG) gauge, which restores boundary Weyl covariance. We will show that the diffeomorphism mapping WFG to FG can be charged and discuss how this relates to holography.

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Zoom link https://pitp.zoom.us/j/93359035909?pwd=aHo0TUFXekZaeCtrT2FzNDZKUW15Zz09

We investigate perturbations of the Schwarzschild geometry using a linearization of the Einstein vacuum equations within a Bondi-Sachs, or null cone, formalism. We develop a numerical method to calculate the quasinormal modes, and present results for the case ℓ= 2. The values obtained are different than those of a Schwarzschild black hole, and we interpret them as quasinormal modes of a Schwarzschild white hole.

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Zoom link https://pitp.zoom.us/j/93648313308?pwd=akZMZXFKejIwdFphOVU0ZjFpeW13dz09

We will present a teleportation protocol which uses the properties of superoscillatory wavefunctions. Then we will briefly introduce a different protocol, the port based teleportation. We will use it to study teleportation over infinitesimally small distances, where the vacuum of a quantum field serves as the source of entanglement. We find that the resulting motion is equivalent to a quantum teleportation-induced Brownian motion. Purifying the interactions, from measurements to unitary operations, leads to motion described by Schrodinger’s equation. Therefore, we find that the notions of teleportation and continuous quantum and classical motion are unified in this sense.

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Zoom link https://pitp.zoom.us/j/93504138534?pwd=VUFPSXpRd2g0TVNxVTNlY3Rxa1ZjUT09

Black hole thermodynamics plays a central role in the theoretical landscape, acting as both synthesizer and sieve for concepts in field theory, quantum gravity, information theory, and more. While its formulation and applications are well understood in asymptotically flat and anti-de Sitter spaces, significant difficulties arise when generalizing to cosmological settings. In this talk, I will discuss how the thermodynamic properties of black holes can be understood in such scenarios through the Euclidean path integral. I will also discuss black holes in a more generalized, semi-classical setting, and suggest a consistent framework for describing a wide variety of dynamical black hole models and ultracompact objects, with consequences for horizon formation, back-reaction, and the growth of astrophysical black holes.

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Zoom link https://pitp.zoom.us/j/97647270696?pwd=bXhFYWYyYUlrTEUyVXhtOUNDakhhQT09

Long-range entangled quantum matter encompasses a wealth of fascinating phenomena including fractionalization and criticality.  I will show how quantum dynamics involving measurements can both enable new kinds of long-range entangled states and facilitate their realization on quantum simulators.  In the first part, I will illustrate how competing measurements along with unitary time evolution can give rise to distinct universality classes of non-equilibrium criticality.  In the second part, I will show how measurements and unitary evolution conditioned on the measurement outcomes (“adaptive quantum circuits”) enable efficient preparation of long-range entangled matter.  Finally, I will demonstrate how environmental measurement (decoherence) can remarkably enrich quantum critical pure states, giving rise to renormalization group flows between quantum channels with important implications on the entanglement structure of the resulting critical mixed states.  

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Zoom link https://pitp.zoom.us/j/97087115629?pwd=NHdlWG9oM0xHUDIzU05sUWRKdWlFdz09

Holography with branes and/or cutoff surfaces presents a promising approach to studying quantum gravity beyond asymptotically anti-de Sitter spacetimes. However, this generalized holography is known to face several inconsistencies, including potential violations of causality and fundamental entropic inequalities. In this talk, we address these challenges by investigating the bulk scattering process and its holographic realization. Specifically, we propose that causality of a radially propagating excitation should be an induced one originating from a fictitious boundary behind the brane/cutoff surface. We present its consistency by checking the connected wedge theorem supported from quantum cryptography and (strong) subadditivity of holographic entanglement entropies. While the induced light cone seemingly permits superluminal signaling, we argue that this causality violation can be an artifact of state preparation in our picture. This talk is based on 2308.00739 [10.1007/JHEP10(2023)104] with Beni Yoshida.

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Zoom link https://pitp.zoom.us/j/98252951858?pwd=RVNZUjNVM2JkZXRlSXJVbEZ6cUpsUT09

The recent data releases by multiple pulsar timing array (PTA) experiments show evidence for Hellings-Downs angular correlations indicating that the observed stochastic common spectrum can be interpreted as a stochastic gravitational wave background. We study whether the signal may originate from gravitational waves induced by high-amplitude primordial curvature perturbations. Such large perturbations may be accompanied by the generation of a sizable primordial black hole (PBH) abundance. We discuss in which scenarios the inclusion of non-Gaussianities in the computation of the abundance can lead to a signal compatible with the PTA experiments without overproducing PBHs.

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Zoom link https://pitp.zoom.us/j/95261778825?pwd=QndRd0xQVFpNVzk0VXpRUkNqR1JXZz09