Simons Collaboration on It from Qubit Annual Meeting 2021

The It from Qubit (IFQ) Simons Collaboration held its sixth annual meeting on December 9–10, 2021. Due to the ongoing pandemic and associated travel restrictions, only about half of the collaboration’s principal investigators (PIs) and postdocs were able to attend the event in person. Nonetheless, with around 80 in-person attendees and 30 or so people following the talks remotely, the meeting was lively and productive. It was immediately preceded by a three-day IFQ workshop at the Institute for Advanced Study, providing a full week of exciting scientific interaction.

The meeting at the Simons Foundation featured seven speakers: five IFQ PIs, one former IFQ postdoc and one experimentalist. The talks kicked off with PI VIjay Balasubramanian (University of Pennsylvania) describing the analysis of circuit complexity in the sense of Nielsen for SYK models, highlighting in particular how chaotic versus integrable dynamics are reflected in the time-dependence of complexity. Former IFQ postdoc Nima Lashkari (now faculty at Purdue University) then explained how RG flows can be thought of as quantum error-correcting codes, in which the low-energy operators are the protected, or logical, observables, with a specific focus on the continuous MERA RG scheme. Crystal Noel (Duke University) described her group’s experiments realizing measurement-induced phase transitions in trapped ions. PI Daniel Harlow (Massachusetts Institute of Technology) explained how to understand the recent calculations of the Page curve for evaporating black holes in terms of the picture of holography as a quantum error-correcting code.

On the second day of the meeting, PI Brian Swingle (Brandeis University) developed a hydrodynamic theory to describe the universal “ramp” behavior in the spectral form factor of a chaotic quantum system. PI Alexei Kitaev (California Institute of Technology) then explained a new method for computing out-of-time-order correlators (OTOCs), which are used to diagnose chaos in many-body quantum systems. Finally, PI Juan Maldacena (Institute for Advanced Study) reviewed recent work on the transition in the spectrum of string theory between highly excited string states and small black holes.

In addition, a poster session on the first afternoon gave the IFQ postdocs the opportunity to present their latest research. As usual, the time between the talks was filled with intense scientific discussion. It from Qubit continues to be a vibrant community whose members are eager to make use of every opportunity to learn from each other, and the annual meeting is always a highlight of the calendar.

Vijay Balasubramanian
University of Pennsylvania

Complexity, Integrability, & Chaos

Recent discoveries suggest that the interior of an old evaporating black hole is dual to the Hawking radiation emitted from it. This seems surprising because conventional reasoning would say that instead there should be separate physical degrees of freedom in the black hole interior and in the distant radiation. Balasubramanian will show how the new picture can be consistent because of the complex structure of the encoding of the interior in the Hawking radiation. Specifically, the interior density matrix is correctable against the action of quantum operations on the radiation which (i) do not have prior access to details of the microstate, and (ii) do not have large, negative coherent information with respect to the maximally mixed state on the radiation, with a lower bound controlled by the black hole entropy. Thus, an experimentalist carrying out generic, low-rank quantum operations on the radiation cannot measure or modify the interior state of the black hole.

Daniel Harlow
Massachusetts Institute of Technology

Quantum Error Correction and the Page Curve

Harlow will discuss some new ideas on how to think about the emergence of the black hole interior for an old black hole.
 

Alexei Kitaev
California Institute of Technology

Out-of-Time-Order Correlators: A Two-Way Approach

Chaos in quantum many-body systems is described by out-of-time-order correlators (OTOCs) of four operators. One may regard two of them as a perturbation source and two others as a probe. A standard approach is to replace the actual source with some classical perturbation and to solve for the probe correlation function on the double Keldysh contour. In this talk, Kitaev shows how to obtain the OTOC by combining two such solutions, for perturbations propagating forward and backward in time. (This is a joint work with Yingfei Gu and Pengfei Zhang, currently in preparation.)
 

Nima Lashkari
Purdue University

Renormalization Group and Error Correction

In renormalization group (RG) flow, the low energy states form a code subspace that is protected against the local short-distance errors. Lashkari will discuss the similarities and differences between this approximate error correction code and the classical and quantum codes that appear in the degenerate vacua of local many-body quantum systems. Lashkari will consider continuous MERA as a concrete example and discuss how well the low-energy operators are protected. The trade-off bounds set a bound on the amount of quantum information at a scale. Finally, Lashkari will comment on the connections to the quantum error correction codes in holography and the high-energy states of chaotic quantum systems.
 

Juan Maldacena
Institute for Advanced Study

String Scale Black Holes

Maldacena will explore the transition between small black holes and string states. He will show that they are not continuously connected as classical solutions in the type II string theory but they are in the heterotic case. Maldacena will also discuss applications to some charged black holes.
 

Crystal Noel
Duke University

Observing a Measurement-Induced Quantum Phase Transition

In this talk, Noel will introduce our trapped ion quantum computer based on 171Yb+ clock qubits (previously at University of Maryland and now located at the new Duke Quantum Center). The system is a fully connected random-access universal quantum computer with up to 13 qubits in regular operation and individual addressing of each qubit. Noel will describe experiments designed to explore open quantum systems via random quantum circuits. Here, the system evolution is represented by unitary gates with interspersed projective measurements. As the measurement rate is varied, a purification phase transition is predicted to emerge at a critical point, akin to a fault-tolerant threshold. Noel finds evidence of the phases associated with the transition and show numerically that, with modest system scaling, critical properties of the transition emerge. Finally, Noel will highlight other recent applications using the system and give an outlook for the future of ion trap quantum computing.
 

Brian Swingle
Brandeis University

Hydrodynamics and Random Matrix Theory

Ensembles of quantum chaotic systems are expected to exhibit random matrix universality in their energy spectrum. The presence of this universality can be diagnosed by looking for a linear in time ‘ramp’ in the spectral form factor, but for realistic systems, this feature is typically only visible after a sufficiently long time. It is important to understand the emergence of this universality and how it connects to the larger body of phenomena associated with quantum chaos. This talk will present a hydrodynamic theory of the spectral form factor in systems with slow modes. The formalism predicts the linear ramp at sufficiently late time and gives a quantitative framework for computing the approach to ramp.

5:00-6:15 PM

A public lecture will be presented the evening before the official start of the conference. Participants are encouraged to register and attend should their travel and other schedules align.

Simons Foundation Lectures are free public colloquia related to basic science and mathematics. These high-level talks are intended for professors, students, postdocs and business professionals, but interested people from the metropolitan area are welcome as well.

More information is available at the lecture’s page.

Participation is optional; separate registration is required.
 

Vijay Balasubramanian
University of Pennsylvania

Complexity, Quantum Mechanics and the Structure of Space-Time

Quantum mechanics and general relativity are the two pillars of 20th-century physics. However, paradoxes arise when we attempt to combine these theories to arrive at a quantum theory of space and time, often because the appearance of black holes and spacetime singularities seem to destroy quantum information.

In this lecture, Vijay Balasubramanian will discuss how we can resolve these paradoxes by drawing on concepts of complexity and information from theoretical computer science, communications theory and cryptography. He will use techniques from these fields to explain diverse, interlinked aspects of fundamental physics, including: the dynamics of quantum chaos and thermalization; the recovery of information from beyond the horizon of black holes; and, most fundamentally, causality in physical processes.
 

To attend this in-person event, you will need to register in advance and provide:

• Acceptable proof of vaccination (vaccine card/certificate, a copy or photo of vaccine card/certificate or electronic NYS Excelsior Pass or NJ Docket Pass)
• Photo ID
• Eventbrite ticket confirmation email with QR code
• Simons Foundation Health Screening Questionnaire approval email

Guests are expected to complete these requirements each time they visit the Simons Foundation and entrance will not be granted without this documentation.

On-site registration will not be permitted. Walk-in entry will be denied.