It from Qubit Collaboration
Virtual Seminar Series
The It from Qubit Simons Collaboration is continuing its virtual seminar series, with a new talk roughly every couple of weeks.
Past seminars
Thursday, November 5, 2020
4pm U.S. Eastern / 1p U.S. Pacific
Speaker: Daniel Jafferis (Harvard)
Title: Inside the Hologram
Abstract: I will discuss using probe black holes to explore the reconstruction of bulk operators. Under some assumptions, modular flow of the probes evolves nearby bulk operators in proper time along the probe trajectory. This includes cases where the evolution goes behind horizons in the spacetime being probed.
Wednesday, October 21, 2020
3pm U.S. Eastern / noon U.S. Pacific
Speaker: Pablo Bueno (Instituto Balseiro, Bariloche)
Title: Reflected entropy, free fields and symmetries
Abstract: A well-defined notion of von Neumann entropy associated to pairs of spatial subregions has been recently proposed both in the holographic context and for general QFTs. I will show that in the case of Gaussian systems ---and similarly to the entanglement entropy (EE)--- this "reflected entropy” can be obtained in terms of correlation functions of the fields. In particular, I will present general formulas valid for free scalars and fermions in arbitrary dimensions. I will apply the results to various free theories in 1+1 and 2+1 dimensions, verifying that the conjectural monotonicity property $R(A,BC)\geq R(A,B)$ and the general inequality $R(A,B)\geq I(A,B)$ hold in all cases. The results obtained suggest that for general regions characterized by length-scales $L_A\sim L_B \sim L$ and separated a distance $\ell$, the reflected entropy in the large-separation regime ($x\equiv L/\ell \ll 1$) is related to the mutual information by: $R(x) \sim −I(x) \log x$ for general CFTs in arbitrary dimensions. Finally, I will argue that the notion of reflected entropy can be canonically generalized in a way which is particularly suitable for theories obtained by restricting the full algebra of operators to those which are neutral under global symmetry groups. A key role in the discussion is played by type-I von Neumann algebras, which differ from the usual type-III algebras associated to spatial subregions in QFT. I will perform various explicit comparisons between both types of algebras.
Thursday, July 16, 2020
3pm U.S. Eastern / noon U.S. Pacific
Speaker: Hong Liu (MIT)
Title: Entanglement entropies of equilibrated pure states and replica wormholes
Abstract: Consider a quantum many-body system initially in a far-from-equilibrium pure state.
If the system is non-integrable, the state should approach equilibrium after some time.
We develop an approximation to calculate quantum informational properties of such an "equilibrated pure state.” Applied to gravity systems, the approximation leads to a derivation of replica wormholes discussed recently in the context of Page curves for black holes, elucidating their mathematical origin, physical interpretation, and why they lead to answers which are consistent with unitarity.
Based on work to appear with Shreya Vardhan.
Tuesday, June 16, 2020
4pm U.S. Eastern / 1pm U.S. Pacific
Speaker: Jordan Cotler (Stanford)
Title: AdS3 gravity and random CFT
Abstract: We compute the path integral of three-dimensional gravity with negative cosmological constant on spaces which are topologically a torus times an interval. These are Euclidean wormholes, which smoothly interpolate between two asymptotically Euclidean AdS3 regions with torus boundary. From our results we obtain the spectral correlations between BTZ black hole microstates near threshold, as well as extract the spectral form factor at fixed momentum, which has linear growth in time with small fluctuations around it. The low-energy limit of these correlations is precisely that of a double-scaled random matrix ensemble with Virasoro symmetry. Our findings suggest that if pure three-dimensional gravity has a holographic dual, then the dual is an ensemble which generalizes random matrix theory.
Thursday, May 21, 2020
3pm U.S. Eastern / noon U.S. Pacific
Speaker: Hrant Gharibyan (Caltech)
Title: The Python's Lunch: geometric obstructions to decoding Hawking radiation
Abstract: Harlow and Hayden [arXiv:1301.4504] argued that distilling information out of Hawking radiation is computationally hard despite the fact that the quantum state of the black hole and its radiation is relatively un-complex. I will trace this computational difficulty to a geometric obstruction in the Einstein-Rosen bridge connecting the black hole and its radiation. Inspired by tensor network models, I will present a conjecture that relates the computational hardness of distilling information to geometric features of the wormhole - specifically to the exponential of the difference in generalized entropies between the two non-minimal quantum extremal surfaces that constitute the obstruction. Due to its shape, this obstruction was dubbed "Python's Lunch", in analogy to the reptile's postprandial bulge.
Thursday, May 7, 2020
3pm U.S. Eastern / noon U.S. Pacific
Speaker: Vijay Balasubramanian (Penn)
Title: Geometric secret sharing in a model of Hawking radiation
Abstract: We consider a black hole in three dimensional AdS space entangled with an auxiliary radiation system. We then model the microstates of the black hole in terms of a field theory living on an end of the world brane behind the horizon, and allow this field theory to itself have a holographic dual geometry. This geometry is also a black hole since entanglement of the microstates with the radiation leaves them in a mixed state. This "inception black hole" can be purified by entanglement through a wormhole with an auxiliary system which is naturally identified with the external radiation, giving a realization of the ER=EPR scenario. In this context, we propose an extension of the Ryu-Takayanagi (RT) formula, in which extremal surfaces computing entanglement entropy are allowed to pass through the brane into its dual geometry. This new rule reproduces the Page curve for evaporating black holes, consistently with the recently proposed "island formula". We then separate the radiation system into pieces. Our extended RT rule shows that the entanglement wedge of the union of radiation subsystems covers the black hole interior at late times, but the union of entanglement wedges of the subsystems may not. This result points to a secret sharing scheme in Hawking radiation wherein reconstruction of certain regions in the interior is impossible with any subsystem of the radiation, but possible with all of it.
Tuesday, April 21, 2020
4pm U.S. Eastern / 1pm U.S. Pacific
Speaker: Ying Zhao (IAS)
Title: A quantum circuit interpretation of evaporating black hole geometry
Abstract: When Alice shares thermofield double with Bob, her time evolution can make the wormhole grow. We identify different kinds of operations Alice can do as being responsible for the growth of different parts of spacetime and see how it fits together with subregion duality. With this, we give a quantum circuit interpretation of evaporating black hole geometry. We make an analogy between the appearance of island for evaporating black hole and the transition from two-sided to one-sided black hole in the familiar example of perturbed thermofield double. If Alice perturbs thermofield double and waits for scrambling time, she will have a one-sided black hole with interior of her own. We argue that by similar mechanism the radiation gets access to the interior (island forms) after Page time. The growth of the island happens as a result of the constant transitions from two-sided to one-sided black holes.
Tuesday, April 9, 2020
4:30pm U.S. Eastern / 1:30pm U.S. Pacific
Speaker: Don Marolf (UCSB)
Title: Baby Universes and Black Hole Information
Abstract: In the 1980’s, work by Coleman and by Giddings and Strominger linked the physics of spacetime wormholes to ‘baby universes’ and an ensemble of theories. We revisit such ideas, using features associated with a negative cosmological constant and asymptotically AdS boundaries to strengthen the results, introduce a change in perspective, and connect with recent replica wormhole discussions of the Page curve. A key new feature is an emphasis on the role of null states. We explore this structure in detail in simple topological models of the bulk that allow us to compute the full spectrum of associated boundary theories. We also argue that similar properties must hold in any consistent gravitational path integral.