Quantum Physics

Z2 lattice gauge theories (LGTs) coupled to dynamical matter show rich physics, including topological phases with anyons (toric code) and fractionalized Fermi liquids, with potential realizations in strongly correlated quantum matter. In this talk I report on recent progress — theoretical and experimental — in performing analog quantum simulations of such models. Starting from several distinct zero-dimensional building blocks I will move on to discuss extensions to extended 1D and 2D systems, including the realization of the plaquette operators in 2D. Next I will discuss how experimental imperfects, such as gauge-symmetry breaking errors, impact quantum simulations, and how they can be overcome. Then I will show how the insights gained lead us to an inherently stable protocol for quantum simulations of Z2 LGTs with dynamical matter with existing Rydberg tweezer arrays. I will close with an outlook and by discussing possible near-term experimental goals ranging from disorder-free localization to finite-temperature deconfinement transitions.

Zoom link:  https://pitp.zoom.us/j/91839919649?pwd=Tm1uOVljWUV3R05aUkxFVkFzN3lIZz09

The NLTS (No Low-Energy Trivial State) conjecture of Freedman and Hastings [2014] posits that there exist families of Hamiltonians with all low energy states of non-trivial complexity (with complexity measured by the quantum circuit depth preparing the state). Our recent work https://arxiv.org/abs/2206.13228 (with Nikolas Breuckmann and Chinmay Nirkhe) proves this conjecture by showing that the recently discovered families of constant-rate and linear-distance QLDPC codes correspond to NLTS local Hamiltonians. This talk will provide background on the conjecture, its relevance to quantum many-body physics and quantum complexity theory, and touch upon the proof techniques.

Zoom link: https://pitp.zoom.us/j/94224635225?pwd=SUovNXA3MWlkRUJlcTIxV0pLQzQxdz09

The advent of ultracold molecules opens the possibility to explore chemical reactions with perfect control of the quantum states of the reactants. We report on several surprising results of our work with ultracold NaLi molecules. First, we demonstrate a factor of 100 control of reaction rates between NaLi molecules and Na atoms by changing the atom's spin state. This ability to slow reactions allowed us to demonstrate sympathetic cooling of molecules for the first time. Next, we explore two very different collisional resonances. A resonance in NaLi+Na reactions exemplifies the standard description of chemical resonances. The other, for NaLi+NaLi, is the first ultracold molecule-molecule resonance observed and runs completely counter to the standard description. Simple models relate the complex chemical dynamics to the simple physics of a Fabry-Perot resonantor and point a number of open questions in chemical dynamics that can be explored with ultracold molecules.

Turbulent radiation flow is ubiquitous in many physical systems where light–matter interaction becomes relevant. Photon bubble instabilities, in particular, have been identified as a possible source of turbulent radiation transport in astrophysical objects such as massive stars and black hole accretion disks. Here, we report on the experimental observation of a photon bubble instability in cold atomic gases, in the presence of multiple scattering of light. A two-fluid theory is developed to model the coupled atom–photon gas and to describe both the saturation of the instability in the regime of quasi-static bubbles and the low-frequency turbulent phase associated with the growth and collapse of photon bubbles inside the atomic sample. We also employ statistical dimensionality reduction techniques to describe the low-dimensional nature of the turbulent regime. The experimental results reported here, along with the theoretical model we have developed, may shed light on analogue photon bubble instabilities in astrophysical scenarios. Our findings are consistent with recent analyses based on spatially resolved pump–probe measurements.

"S. Tiwari, S. Rammohan, A. Mishra, A. Pendse, A. K. Chauhan, R. Nath, F. Engel, M. Wagner, R.Schmidt, F. Meinert, A. Eisfeld and S. Wüster Indian Institute of Science Education and Research, Bhopal India Palacký University, Olomouc, Czech Republic Indian Institute of Science Education and Research, Pune, India Max Planck Institute for the Physics of Complex Systems, Dresden, Germany Universität Stuttgart, Germany Max-Planck-Institute of Quantum Optics and MCQST, Garching, Germany Rydberg Atoms in highly excited electronic states with n=30-200 can be excited within BoseEinstein condensates (BECs), and while lifetimes are shorter than in vacuum [1,2], they live long enough to cause a response of the BEC mean field [3]. During this, thousands of ground-state atoms are present within the Rydberg orbit, allowing the study of atoms moving within atoms [4]. We present beyond-mean field models of the joint Rydberg-BEC dynamics, showing how either can be used to probe the other. For multiple Rydberg atoms in a single electronic state, we show that the phase coherence of thecondensate allows the tracking of mobile Rydberg impurities akin to the function of bubblechambers in particle physics [5]. For a single Rydberg atom with multiple electronic states, weprovide spectral densities of the BEC as a decohering environment [6], and show that the BECcan image a signature of the entangling evolution that causes Rydberg q-bit decoherence [7] or serve as non-Markovian environment for quantum simulations. [1] Schlagmüller et al. PRX 6 (2016) 031020. [2] Kanungo et al. PRA 102 (2020) 063317. [3] Balewski et al. Nature 502 (2013) 664. [4] Tiwari et al. arXiv:2111.05031 (2021) [5] Tiwari et al. PRA 99 (2019) 043616. [6] Rammohan et al. PRA 103 (2021) 063307. [7] Rammohan et al. PRA(Letters) 104 (2021) L060202."

We report experimental measurements of the rates of blackbody-radiation-induced transitions between highlying (n > 60) S, P, and D Rydberg levels of rubidium atoms in a magneto-optical trap using a hybrid field ionization and state-selective depumping technique. Our results reveal significant deviations of the measured transition rates from theory for well-defined ranges of the principal quantum number. We assume that the most likely cause for those deviations is a modified blackbody spectrum inside the glass cell in which the magneto-optical trap is formed, and we test this assumption by installing electrodes to create an additional microwave cavity around the cell. From the results, we conclude that it should be possible to use such external cavities to control and suppress the blackbody-radiation-induced transitions."

I will talk about the applicability of a new kind of gas sensor based on Rydberg excitations. From a gas mixture the molecule in question is excited to a Rydberg state. By succeeding collisions with all other gas components this molecule ionizes and the emerging electrons can be measured as a current. I will show Doppler-free spectra for the A <- X transition, an estimate of the excitation efficiency dependent on the used laser powers, the applied charge-extraction voltage as well as the overall gas pressure and a first Stark map of NO Rydberg states recorded with cw excitation.

Electric field sensors based on warm vapors of Rydberg atoms have distinguishing features that offer new application possibilities. A single sensor can operate over a wide spectrum of frequencies, from DC to THz, with a consistent instantaneous baseband bandwidth of approximately 10MHz. The sensor head containing the vapor is highly transparent and can be made small relative to the electric field wavelengths, enabling accurate measurements with sub-wavelength spatial resolution. Presently Rydberg sensors rely on the spectroscopic method of electromagnetically induced transparency (EIT) for preparing and probing the atoms, and though simple and effective, this places limits on the sensitivity and instantaneous bandwidth of the sensor. I will discuss these limitations and the optimal EIT parameter regime considering presently available laser technology, and show performance of a promising new prototype vertical external cavity surface emitting laser (VECSEL). Finally, I will present results on recent demonstrations, such as a Rydberg-based spectrum analyzer with sensitivity of -145dBm/Hz and dynamic range >80 dB.

The strong interaction of optically excited Rydberg atoms with external fields has made them promising for the detection of radio frequency (RF) electric fields with high sensitivity. Such Rydberg-atom based sensors offer advantages over conventional metal antennas in RF transparency and self-calibration, enabled by all-dielectric construction and extremely well-known atomic properties. In this talk, we describe recent advances in the sensing of low amplitude RF electric fields and the timing of sub-microsecond RF pulses using room temperature Cesium vapour cells. We examine their transient response to RF pulses with durations ranging from 10 μs to 50 ns, identifying the dependence of atomic time scales on Rabi frequencies and dephasing mechanisms. We present a method for extracting the arrival time of RF pulses in a typical two-photon setup using a matched filter tailored to the atomic response, achieving a field sensitivity down to ~240 nV cm-1 Hz-1/2 and a timing precision of ~30 ns. On the other hand, practical operation at room temperature results in the self-calibration and sensitivity of this setup being limited by residual Doppler broadening. We therefore develop a novel sub-Doppler approach using a colinear three-photon scheme, which extends the self-calibrated Autler-Townes regime to significantly weaker RF electric fields. With this setup, we achieve a ~200 kHz spectral linewidth of the Rydberg atoms’ electromagnetically induced transparency within a room temperature vapour cell. The results demonstrate the potential of Rydberg atombased sensors for use in test and measurement, communications, and radar applications. "

"Neutral atoms have emerged as a competitive platform for digital quantum simulations and computing. In this talk, we discuss recent results on the design of time-optimal two- and three-qubit gates for neutral atoms, where entangling gates are implemented via the strong and long-range interactions provided by highly excited Rydberg states. We combine numerical and semi-analytical quantum optimal control techniques to obtain theoretically laser pulses that are “smooth”, time-optimal and “global” -- that is, they do not require individual addressability of the atoms. This technique improves upon current implementations of the controlled-Z (CZ) and the three-qubit C2Z gates with just a limited set of variational parameters, demonstrating the potential of quantum optimal control techniques for advancing quantum computing with Rydberg atoms."

"Neutral atom quantum computers with Rydberg mediated entangling gates are rapidly advancing as a leading platform for quantum information processing. I will present recent results running quantum algorithms for preparation of multi-qubit GHZ states, phase estimation, and hybrid quantum/classical optimization. Future fault tolerant quantum processors will require large numbers of qubits, high fidelity gates, and error correcting protocols. Work in progress towards fault tolerance including preparation of arrays of more than 1000 atoms, mid-circuit measurements, and multi-qubit gates will be presented"