Prof. Serbyn received an ERC starting grant of approximately 1.5mi Euro for theoretical studies of non-ergodic quantum systems. New open positions related to the grant will be soon announced, please stay tuned. More details are available here and in the official IST press release.

# Author: Maksym SERBYN

## Mixed phase space in quantum many-body systems

In the earlier work we identified the quantum scars as a mechanism of weak ergodicity breaking in interacting many-body systems. The name “quantum many-body scars” was inspired by the presence of unstable TDVP trajectory reported in Ref. [Phys. Rev. Lett. 122, 040603 (2019)], thus allowing to make parallels with quantum scars in single-particle billiards.

However, in a few-body quantum systems quantum scarring is not the only possible mechanism of dependence of relaxation on initial state. In fact, the mixed phase space in a few body systems presents another common mechanism. In the most recent work arXiv:1905.08564 we generalized the concept of mixed phase space to strongly interacting quantum systems.

## Colloquium: Many-body localization, thermalization, and entanglement

Quantum systems of many particles, which satisfy the ergodicity hypothesis, are conventionally described by statistical mechanics. However, not all quantum systems are ergodic, with many-body localization providing a generic mechanism of ergodicity breaking by disorder. Many-body localized (MBL) systems remain perfect insulators at non-zero temperature, which do not thermalize and therefore cannot be described using statistical mechanics. In the *Colloquium*: Many-body localization, thermalization, and entanglement written together with D. Abanin, E. Altman, and I. Bloch and recently published at Reviews of Modern Physics, we summarize recent theoretical and experimental advances in studies of MBL systems, focusing on the new perspective provided by entanglement and non-equilibrium experimental probes such as quantum quenches.

## Paper on analytically solvable RG appears at PRL

Recent work on the analytically solvable RG for MBL transition by Anna Goremykina in collaboration with Romain Vasseur & Maksym Serbyn appears at PRL, see more details here: Analytically solvable renormalization group for the many-body localization transition. In short, in this work we suggest that MBL transition may be described by the Kosterlitz-Thouless universality class. More support for this scenario is available in the follow-up work, arXiv:1811.03103.

## ISTPlus Postdoctoral Fellowship: last two calls in 2019

The last two calls for ISTPlus Postdoctoral fellowship are on 15th of March 2019 and15th of September 2019. ISTPlus Fellows are free to work with any research groups on campus and outside of IST. Potential applicants are strongly encouraged to contact Prof. Serbyn **at least one month before the deadline**.

## Weak ergodicity breaking from quantum many-body scars

Quantum scars present an example of weak ergodicity breaking in a context of quantum chaos. In our recent work in Nature Physics, we also generalized this concept to the many-body case.

#### Quantum scars in a billiard

Imagine a ball bouncing around in an oval stadium. It will bounce around chaotically, back and forth through the available space. As its motion is random, it will sooner or later visit every place in the stadium as is illustrated in the example below:

Amidst all the chaos, however, there might be a potential for order: if the ball happens to hit the wall at a special spot and at the “correct” angle of incidence, it might end up in a periodic orbit, visiting the same places in the stadium over and over and not visiting the others. Such a periodic orbit is extremely unstable as the slightest perturbation will divert the ball off its track and back into chaotic pondering around the stadium:

The same idea is applicable to quantum systems, except that instead of a ball bouncing around, we are looking at a wave, and instead of a trajectory, we are observing a probability function. Classical periodic orbits can cause a quantum wave to be concentrated in its vicinity, causing a “scar”-like feature in a probability that would otherwise be uniform. Such imprints of classical orbits on the probability function have been named “quantum scars”. Below we compare the “scarred“ eigenstate in the stadium with the more typical state:

#### Quantum many-body scars

We observed quantum-scarred eigenstates in the theoretical model that describes a chain of Rydberg atoms. All atoms in the chain can be in two possible states: excited and ground state. Moreover there exist a constraint that prohibits two excited atoms to be adjacent to each other. We found a coherent oscillations in such a system which underlie the quantum-scarred eigenstates. Below we show the animated cartoon of this trajectory for L=8 atoms.

Here the graph shows the space of all possible configurations, and the bottom shows the average density of excitation on each site. Animation shows oscillations between two patterns.

## Probing dynamics by matrix elements

In order to determine generic non-equilibrium dynamics of the generic local observable *O* one needs two ingredients. The *spectrum* of the many-body Hamiltonian and the *matrix elements* of the operator *O* in the basis of eigenstates completely specify evolution of the local observable *O*. Moreover, the matrix elements at small energy differences correspond to the long-time dynamics that may be very hard to access in the thermalizing system.

Using these insights, we studied the properties of the matrix elements across the many-body localization transition in the recent preprint arXiv:1610.02389. In particular, we used matrix elements to define the (many-body) Thouless energy *E*Th which corresponds to the inverse time scale of relaxation. The behavior of this Thouless energy reveals the critical fan preceding localization transition. In addition, we associate the critical region with the onset of broad distributions and typicality breakdown.

## First group members arrive in July 2017

The first group members Ziruo Zhang and Margarita Davydova arrived to IST in July. See People page for more details.