Dr. Tien Tien Yeh, NORDITA and UConn

Structured Light and Induced Vorticity in Superconductors

Questions of controlling the quantum states of matter via light have been at the forefront of research on driven phases. We demonstrate the effects of imprinted vorticity on superconducting coherent states using structured light. Within the framework of the generalized time-dependent Ginzburg-Landau equation, we show the induction of coherent vortex pairs moving in phase with electromagnetic wave oscillation. The structured light, generated by a Laguerre-Gaussian beam, provides light sources with various quantum properties, such as spin angular momentum and orbital angular momentum. This state of light is also well known as an optical vortex, characterized by a twisted phase front. In the current work, we investigate the optically induced dynamics of superconducting coherent states using both normal light sources and optical vortices. These results uncover rich hydrodynamics of superconducting states and suggest new optical applications for imprinting quantum states on superconducting materials.

Prof. Nikolay Prokofiev, UMass Amherst

Bi-polaron superconductivity in the low density limit

It has been assumed for decades that high values of Tc from the electron-phonon coupling are impossible. At weak-to-intermediate coupling strength this result follows from the Migdal-Eliashberg theory, while at strong coupling, when bipolarons form, the transition temperatures are low because of the exponential effective mass enhancement. However, the latter conclusion was based on numerical solutions of the Holstein model. I will discuss a different model with coupling based on the displacement modulated hopping of electrons and argue that much larger values of the bipolaron Tc can be achieved in this setup. Non-locality of the problem gives rise to small-size, yet relatively light bipolarons, which can be studied by an exact sign-problem-free quantum Monte Carlo approach even in the presence of strong Hubbard and Coulomb potentials. We find that Tc in this model generically and significantly exceeds typical upper bounds based on Migdal-Eliashberg theory or superfluidity of Holstein bipolarons, and, thus, offers a route towards the design of high-Tc superconductors via functional material engineering. Finally, there are indications for even better prospects in systems with non-linear electron-phonon coupling.

Prof. Claudio Chamon, Boston University

Circuit complexity and functionality: a thermodynamic perspective

We explore a link between complexity and physics for circuits of given functionality. Taking advantage of the connection between circuit counting problems and the derivation of ensembles in statistical mechanics, we tie the entropy of circuits of a given functionality and fixed number of gates to circuit complexity. We use thermodynamic relations to connect the quantity analogous to the equilibrium temperature to the exponent describing the exponential growth of the number of distinct functionalities as a function of complexity. This connection is intimately related to the finite compressibility of typical circuits. Finally, we use the thermodynamic approach to formulate a framework for the obfuscation of programs of arbitrary length – an important problem in cryptography – as thermalization through recursive mixing of neighboring sections of a circuit, which can viewed as the mixing of two containers with “gases of gates”. This recursive process equilibrates the average complexity and leads to the saturation of the circuit entropy, while preserving functionality of the overall circuit. The thermodynamic arguments hinge on ergodicity in the space of circuits which we conjecture is limited to disconnected ergodic sectors due to fragmentation. The notion of fragmentation has important implications for the problem of circuit obfuscation as it implies that there are circuits with same size and functionality that cannot be connected via local moves. Furthermore, we argue that fragmentation is unavoidable unless the complexity classes NP and coNP coincide.

Dr. Paul M. Eugenio, University of Connecticut

Tunable moire sublattices in twisted square homobilayers: exploiting fundamental principles for new technologies

Stacking and twisting atomically thin bilayers at small angles produces an approximate periodic pattern, due to the overlap of the crystal layers. These devices, dubbed “moire” bilayers, exhibit a high degree of tunability and variability: through choice of twist angle, constituent layers, and gating. To date, a number of such devices have been built which have demonstrated a plethora of novel phases, including non-trivial topology and Mott physics. Despite this explosion in moire research, moire bilayers have been almost exclusively formed from layers with triangular/hexagonal crystal geometry, and where the valence bands are centered on the Gamma or K/K’ high symmetry points. Here we theoretically demonstrate that moire devices formed from square bilayers can be used to simulate the ground state of the Hubbard model, but where the ratio of the nearest-neighbor (t) and next-to-nearest neighbor (t’) tunneling can be tuned between zero and infinity, in situ via an electric field. If experimentally realized, such a device would be the first of its kind, and would open a pathway toward the testing of a number of proposed exotic phases, such as a spin-liquid and d+id superconductivity. Most importantly, the square Hubbard model is a quintessential model for high-Tc in cuprates, where numerics has demonstrated the absence of superconductivity when t’=0.

Dr. J. I. A. Li, Brown University

The Superconducting Diode Effect And Spontaneous Symmetry Breaking In Multi-Layer Graphene

The superconducting diode effect, defined as nonreciprocity in the critical supercurrent, provides a unique window into the nature of the superconducting phase. It has been argued that a zero-field diode effect in the superconducting transport requires inversion and time-reversal symmetries to be simultaneously broken. Along this vein, the zero-field superconducting diode effect in multi-layer graphene provides direct evidence of the microscopic coexistence between superconductivity and time-reversal symmetry breaking. In this talk, I will discuss our recent efforts that utilize the angle-resolved measurement of transport nonreciprocity to directly probe the nature of spontaneous symmetry breaking in the normal phase. By investigating the interplay between transport nonreciprocity, ferromagnetism, and superconductivity, our findings suggest that the exchange-driven instability in the momentum space plays a key role in the zero-field superconducting diode effect.

Prof. Jed Pixley, Rutgers University

Novel Strongly Correlated Phases in Stacked TMD Bilayers

Two-dimensional transition metal dichalcogenides (TMDs) have emerged as an exciting platform to stack and twist bilayers to engineer strongly correlated quantum phases. Here we present a theory to describe the recent realization of a heavy fermion state in stacked MoTe2/WSe2 bilayers. An extension of this theory that allows for the formation of unconventional superconductivity through repulsive nearest neighbor interactions will be used to show how to realize the p-wave BEC to BCS transition.

Dr. Jon Curtis, UCLA

Multimode cavity control of ferroelectric fluctuations

Electromagnetic cavities and metamaterials have been used to great effect in the field of AMO physics and electrical engineering. By shaping the spatial, spectral, or polarization characteristics of the electromagnetic environment, the coherent interaction between light and matter can be focused and amplified, leading to phenomena such as lasing, the Purcell effect, the Casimir effect, and superradiance. In this talk I will show how these ideas may be extended and applied to solid state quantum materials. In particular, I will consider polarization fluctuations in a quantum paraelectric insulator, and consider their coupling to a Fabry-Perot type optical cavity. By using the full multimode continuum description of the system, I will show how the ferroelectric fluctuations respond in a local, spatially resolved manner. The presence of the cavity indeed is shown to renormalize the soft-mode frequency, with effects primarily confined to the surface, and thus for thin films this effect can be pronounced. The temperature dependence shows this effect only onsets at low temperatures, indicating its origin from quantum electrodynamics effects – in close analogy with the Casimir effect.

Prof. Alicia Kollár, University of Maryland

Lattices in Circuit QED

The field of circuit QED has emerged as a rich platform for both quantum computation and quantum simulation. These systems exhibit a high degree of both spatial and temporal control which can be used to create synthetic lattice systems. Spatial lattices can be formed using periodic arrays of resonators. Combined with strong qubitphoton interactions, these systems can be used to study dynamical phase transitions, many-body phenomena, and spin models in driven-dissipative systems. I will show that lattices of coplanar waveguide (CPW) resonators permit the creation of unique devices which host photons in curved spaces, gapped flat bands, and novel forms of qubit-qubit interaction [1,2]. I will show that graph theory is the natural language for describing these microwave photonic systems and present preliminary data on the development of a new generation of CPW lattice devices with unconventional band structures. Periodic modulation in superconducting-qubit systems also provides a route to Floquet systems with topological band structures. I will present preliminary experimental steps toward the realization of a topological energy pump which can “boost” smaller non-clalssical states of light into larger ones [3].

[1] A. J. Koll´ar et al., Nature 45, 571 (2019).

[2] A. J. Koll´ar et al., Comm. Math. Phys. 376, 1909 (2020).

[3] D. Long et al., Phys. Rev. Lett. 128, 183602 (2022).

Prof. Steve Lamoreaux, Yale University

Title: TBA