Friday Quantum Seminar

Abstract:  Many applications of quantum simulation require qubit representations of a fixed number of fermions (F) in a larger number of possible modes (M). Representing such states is possible with I := ⌈log(M choose F)⌉ qubits, but existing constructions achieving this level of compactness result in fermion operators with gate complexity exponential in I. We show that a small amount of redundancy enables efficiency, presenting a second quantized fermion encoding using I + O( F ) qubits such that fermion operators can be implemented in depth O( log M ) and gate complexity O(I).

Abstract: Calculating the equilibrium properties of condensed matter systems is one of the promising applications of near-term quantum computing. Recently, hybrid quantum-classical time-series algorithms have been proposed to efficiently extract these properties (time evolution up to short times t). In this work, we study the operation of this algorithm on a present-day quantum computer. Specifically, we measure the Loschmidt amplitude for the Fermi-Hubbard model on a 16-site ladder geometry (32 orbitals) on the Quantinuum H2-1 trapped-ion device.

Abstract: In this talk, I will introduce a family of dynamically generated quantum error correcting codes that we call “hyperbolic Floquet codes.” These codes are defined by a specific sequence of non-commuting two-body measurements arranged periodically in time that stabilize a topological code on a hyperbolic manifold with negative curvature. We focus on a family of lattices for n qubits that, according to our prescription that defines the code, provably achieve a finite encoding rate (1/8+2/n) and have a depth-3 syndrome extraction circuit.

Abstract: We analyze a model of qubits which we argue has an emergent quantum gravitational description similar to the fermionic Sachdev-Ye-Kitaev (SYK) model. The model we consider is known as the quantum q-spin model because it features q-local interactions between qubits.

Abstract: Optical spectroscopy is used to study a material by measuring the intensity of light modes that scatter off it. In this work, we develop a theory for G2 spectroscopy of correlated materials, where instead of measuring the intensity of scattered photons, one measures the second order coherence between pairs of photons scattered off a material. We map this correlation function of the photons to the correlation functions of the material being probed.

Abstract:  We use time-dependent density functional theory to investigate the possibility of hosting organic color centers in (6, 6) armchair single-walled carbon nanotubes, which are known to be metallic. Our calculations show that in short segments of (6, 6) nanotubes ∼5 nm in length there is a dipole-allowed singlet transition related to the quantum confinement of charge carriers in the smaller segments. The introduction of sp3 defects to the surface of (6, 6) nanotubes results in new dipole-allowed excited states.

Abstract: Taking the ground state of two weakly coupled Sachdev-Ye-Kitaev models and perturbing one side, the excitation emerges on the other side, oscillating back and forth, consistent with a particle traversing an eternal wormhole. We show that spin systems with no simple gravitational dual exhibit similar behavior. In particular, 1+1-dimensional conformal field theories realize an eternal wormhole phase. We provide a microscopic description in terms of operator spreading resulting in a speed-up of information transfer.

Abstract: Qubit-based noise spectroscopy (QNS) techniques, where the dephasing of a probe qubit is exploited to study a system of interest, underlie some of the most common quantum sensing and noise characterization protocols. They have a variety of applications, ranging from designing effective quantum control protocols to investigating properties (phase transitions, thermodynamics, etc.) of quantum many-body systems.

Abstract: The two most prevalent classes of ordered states in quantum materials are those arising from spontaneous symmetry breaking (SSB) and from topological order. However, a systematic study for their coexistence in interacting systems is still lacking. In this talk, I will discuss how the topological configuration in order parameter spaces from SSB (classical topology) interplays with the symmetry protected/enriched topological orders (quantum topology) in two spatial dimensions (2d). Three examples of such systems will be given.

Abstract: At the heart of modern quantum technologies is the interference in the radiation of quantum emitters mediated by common vacuum modes. When there are many atoms interfering in the emission process, one observes enhancement or suppression of decay rate coefficient, which is called superradiance and subradiance, respectively [1]. When there are transitions from different excited levels interfering in the emission process, the intensity of the emitted light is modulated at the frequency of the excited level splittings, which is called quantum beats.