# Atomic Physics Seminar

Wednesday, April 10th, 2019

Strong, long-range, resonant and dispersive dipole-dipole interactions between the atoms in high-lying Rydberg states makes them uniquely suited to simulate and study various spin-lattice models. In this seminar, I will discuss two proposals [1,2] to implement spin chains with interesting properties using Rydberg atoms in a lattice.

I will show that an XXZ spin model with strong nearest-neighbor interactions and tunable long-range hopping of excitations can be realized by a regular array of laser driven atoms, with an excited Rydberg state representing the spin-up state and a Rydberg-dressed ground state corresponding to the spin-down state [1]. This scheme permits the observation of coherent quantum dynamics of spin excitations - magnons, their scattering and exotic interaction-bound states.

I will next describe a lattice of Rydberg superatoms - collections of atoms in microtraps that can each accommodate at most one Rydberg excitation. The coupling of superatoms to the laser radiation is collectively enhanced and they can represent mesoscopic, strongly polarizable spins. We studied a regular array of such effective spins driven by a laser field tuned to compensate the interaction-induced level shifts between the neighboring superatoms [2]. After the initial transient with resonantly facilitated excitation of large clusters of superatoms, the system relaxes to the steady state having nearly universal excitation density of 2/3. This state is characterized by highly-nontrivial equilibrium dynamics of quasi-particles – excitation holes in the lattice of Rydberg excited superatoms.

[1] F. Letscher, D. Petrosyan, Phys. Rev. A 97, 043415 (2018)

[2] F. Letscher, D. Petrosyan, M. Fleischhauer, New J. Phys. 19, 113014 (2017)

I will show that an XXZ spin model with strong nearest-neighbor interactions and tunable long-range hopping of excitations can be realized by a regular array of laser driven atoms, with an excited Rydberg state representing the spin-up state and a Rydberg-dressed ground state corresponding to the spin-down state [1]. This scheme permits the observation of coherent quantum dynamics of spin excitations - magnons, their scattering and exotic interaction-bound states.

I will next describe a lattice of Rydberg superatoms - collections of atoms in microtraps that can each accommodate at most one Rydberg excitation. The coupling of superatoms to the laser radiation is collectively enhanced and they can represent mesoscopic, strongly polarizable spins. We studied a regular array of such effective spins driven by a laser field tuned to compensate the interaction-induced level shifts between the neighboring superatoms [2]. After the initial transient with resonantly facilitated excitation of large clusters of superatoms, the system relaxes to the steady state having nearly universal excitation density of 2/3. This state is characterized by highly-nontrivial equilibrium dynamics of quasi-particles – excitation holes in the lattice of Rydberg excited superatoms.

[1] F. Letscher, D. Petrosyan, Phys. Rev. A 97, 043415 (2018)

[2] F. Letscher, D. Petrosyan, M. Fleischhauer, New J. Phys. 19, 113014 (2017)

Host:

SAFFMAN

*Institute of Electronic Structure and Lasers, FORTH, Heraklion, Crete, Greece*

**Available Downloads:**

Room and Building:

5280 Chamberlin Hall

Tuesday, December 7th, 2010

Dipole matrix elements are required for calculations of numerous spectroscopic properties of excited atoms including oscillator strengths, polarisabilities and radiative lifetimes. There is no exact analytical solution for the calculation of radial matrix elements for alkali atoms, however many theoretical models have been developed that include numeric integration of Schroedinger's equation in the Coulomb approximation with quantum defects taken as input parameters or using various model potentials. In quasiclassical methods the radial matrix elements are calculated avoiding the direct numerical integration using transcendental functions. Despite the large number of existing theoretical models, lack of experimental data on dipole matrix elements for rubidium complicates their verification.

In my talk I will present the experimental setup built at The Open University, UK to investigate the ultracold Rydberg atoms. I will show our first results of the measurements of the dipole moments for the transitions to the Rydberg states by investigation of the electromagnetically induced transparency (EIT) in ultracold

The dipole matrix elements for transitions measured in our experiment are compared with theoretical methods including quasiclassical calculations based on Dyachkov-Pankratov model, and Coulomb approximations. A very good agreement with the theoretical calculations is observed.

I will conclude the talk with the outline of further projects with ultracold Rydberg atoms that are carried out at the Open University.

In my talk I will present the experimental setup built at The Open University, UK to investigate the ultracold Rydberg atoms. I will show our first results of the measurements of the dipole moments for the transitions to the Rydberg states by investigation of the electromagnetically induced transparency (EIT) in ultracold

^{87}Rb gas. The absorption profile of a weak probe laser beam on 5S_{1/2}→ 5P_{3/2}transition is observed in the presence of a strong coupling laser beam at 480 nm driving the 5P_{3/2}→ nD_{5/2}transition for the Rydberg states with principal quantum numbers in the range 20≤n≤48. The strong dependence of the shape of the EIT spectrum on the Rabi frequency of the transition between the first excited state and Rydberg states allowed us to directly measure the dipole moments of the transitions involved for several values of principal number n.The dipole matrix elements for transitions measured in our experiment are compared with theoretical methods including quasiclassical calculations based on Dyachkov-Pankratov model, and Coulomb approximations. A very good agreement with the theoretical calculations is observed.

I will conclude the talk with the outline of further projects with ultracold Rydberg atoms that are carried out at the Open University.

Host:

Mark Saffman

*The Open University, Milton Keynes UK*

**Available Downloads:**

Room and Building:

5280 Chamberlin

Thursday, January 31st, 2019

We have developed an approach to continuously load ultracold 85Rb2 vibrational ground state molecules into a crossed optical dipole trap from a magneto-optical trap. The technique relies on a single high-power light beam with a broad spectrum superimposed onto a narrow peak at an energy of about 9400 cm-1. This single laser source performs all the required steps, namely the photoassociation, vibrational cooling, and optical trapping of the molecules. We have also performed depletion spectroscopy, which allows us to determine that 75% of the vX= 0 molecules are in J= 0, 1 and 2 rotational states. The lifetime of the ultracold molecules in the optical dipole trap is limited by off-resonant light scattering to about 70 ms. The proposed technique may open perspectives for the formation of new molecular species in the ultracold domain, which are not yet accessible by well-established approaches.

Host:

Saffman

*Universidade de Sao Paulo, Brazil*

**Available Downloads:**

Room and Building:

5310 Chamberlin Hall

The Global Network of Optical Magnetometers for Exotic physics (GNOME) comprises an array of atomic

magnetometers used for searching for beyond the standard model spin interactions. It presently

consists of 12 active magnetometer stations spread around the globe with gps-time stamped data being

continuously streamed to a central server for analysis. Of special interest is the search for dark matter in

the form of interactions with axion - like particles (ALPs) which may form ALP domain walls and ALP

stars. Such interactions will have the signature of temporally coincident transients occurring in multiple

sensors in the network. I will in this talk describe the experimental details of the network and review some

data analysis ideas.

magnetometers used for searching for beyond the standard model spin interactions. It presently

consists of 12 active magnetometer stations spread around the globe with gps-time stamped data being

continuously streamed to a central server for analysis. Of special interest is the search for dark matter in

the form of interactions with axion - like particles (ALPs) which may form ALP domain walls and ALP

stars. Such interactions will have the signature of temporally coincident transients occurring in multiple

sensors in the network. I will in this talk describe the experimental details of the network and review some

data analysis ideas.

Host:

Thad Walker

*Bucknell University*

**Available Downloads:**

Room and Building:

5310 Chamberlin

Wednesday, December 12th, 2018

Optical trapping and imaging of atoms plays an essential role in cold-atom physics, ranging from precision measurement to the study of correlated many body systems. Due to the diffraction limit, trapping and imaging are typically limited to length scales on the order of the wavelength of the light. The nonlinear response of three-level atoms, however, supports a dark state with spatial structures much smaller than the wavelength. In this talk, I will present the experimental use of such dark state spatial structure to both create optical potentials and probe the atomic wave function with a resolution of lambda/50, far below the diffraction limit. The optical potential physically realizes a Kronig-Penney lattice of near delta-function barriers with widths below 10nm. The coherent nature of our approach also provides a fast temporal resolution (500 ns), with which we could observe the quantum motion of atoms inside the unit cell of an optical lattice.

Host:

Saffman

*Joint Quantum Institute, National Institute of Standards and Technology and the University of Maryland*

**Available Downloads:**

Room and Building:

5310 Chamberlin Hall

Thursday, October 11th, 2018

The rich internal structures of diatomic molecules enable a wide range of experiments in regimes not accessible with atoms. Uses of molecules range from measurement of symmetry-violating effects that probe interesting phenomena in nuclear and particle physics, to the study of highly correlated quantum systems, to the control of novel phenomena in chemical reactions. Despite this broad interest, methods for cooling and trapping molecules have been far less advanced than those for atoms. In particular, direct laser cooling of molecules was long considered infeasible: the same complex internal structure that makes molecules useful also makes laser cooling more difficult. Over the past several years, our group and others have found methods to overcome this obstacle. Now, most of the standard tools of atomic laser cooling and trapping have been demonstrated to work, with appropriate modifications and for certain molecules. In this talk I will review progress in laser cooling and trapping of molecules, and give an outlook for future directions enabled by these rapidly-developing methods.

Host:

Saffman

*Yale University*

**Available Downloads:**

Room and Building:

5310 Chamberlin Hall

Tuesday, October 16th, 2018

Controllable, coherent quantum many-body systems can provide insights into fundamental properties of quantum matter, enable the realization of exotic quantum phases, and ultimately offer a platform for quantum information processing that could surpass any classical approach. Recently, reconfigurable arrays of neutral atoms with programmable Rydberg interactions have become promising systems to study such quantum many-body phenomena, due to their isolation from the environment, and high degree of control. Using this approach, we demonstrate high fidelity manipulation of individual atoms and entangled atomic states. Furthermore, we realize a programmable Ising-type quantum spin model with tunable interactions and system sizes up to 51 qubits. Within this model, we observe transitions into ordered states that break various discrete symmetries. Varying the rate at which the quantum phase transition is crossed allows us to observe the power-law scaling of the correlation length, as predicted by the Kibble-Zurek mechanism. The scaling exponent observed is consistent with theoretical predictions for the Ising universality class when sweeping into a Z2-ordered phase, and with the 3-state Chiral Clock Model for transitions into the Z3-ordered phase.
An alternative, hybrid approach for engineering interactions is the coupling of atoms to nanophotonic structures in which guided photons mediate interactions between atoms. I will discuss our progress towards entangling two atoms that are coupled to a photonic crystal cavity and outline the exciting prospects of scaling these systems to many qubits and to quantum networks over large distances.

Host:

Saffman

*University of Chicago*

**Available Downloads:**

Room and Building:

5310 Chamberlin Hall

Friday, July 20th, 2018

Understanding the ionization properties of diatomic molecules is of fundamental importance to chemical physics and many spectroscopic applications. Here, I present rotational-state-selective field ionization spectra of highly excited triplet nd H2 Rydberg states. A fast 6 keV beam of metastable c3∏u- 2pπ molecules is excited to v = 0, R(1)nd1 (n = 17-27) Rydberg states by a frequency-doubled tunable dye laser and ionized by a static electric field as large as ~35 kV/cm. For each Rydberg state, we observe, as expected, an ion yield that corresponds to diabatic field ionization into the N+ = 1 continuum. At higher fields, we observed an additional ion yield. A model, which considers a diabatic traverse of the Stark map, expects the N+ = 1 ion yield and allows for characterization of the second ion yield as ionization into the N+ = 3 continuum and the result of a rotational-state population transfer. Candidates for the population-transfer mechanism are discussed.

Host:

Shimon Kolkowitz

*Thomas Morgan's group, Wesleyan University*

**Available Downloads:**

Room and Building:

Chamberlin 5310

Thursday, August 2nd, 2018

What sets the boundaries of what humans can perceive? From time and frequency standards, to molecular biology, the limits of what we can measure depend on two related factors:

1. How much information we get in a single measurement

2. How well we can combine and average these measurements.

I will tell two stories from these two different fields of metrology.

The first story is about a new atomic clock design aimed at measuring time to the 19th decimal place. By using a Fermi-degenerate gas in a three-dimensional optical lattice, we controlled all quantum degrees of freedom of our atomic frequency references and suppressed atomic interactions. This allowed us to increase our single-shot frequency sensitivity by both extending the atom-light coherence time and by using more atoms to reduce the quantum projection noise. This new technology enabled new records in clock stability and the corresponding improvements in our ability to evaluate and stabilize systematic shifts.

The second story is about pushing the limits of cryogenic transmission electron microscopy (cryo-EM). Cryo-EM is rapidly usurping x-ray crystallography for determining protein structure, as it allows the visualization of molecules in their native environments, without the need for crystallization. Information from the nearly-transparent specimen manifests as a small phase shift on the electron wavefunction, which goes undetected unless the microscope is intentionally defocused. Defocusing compromises resolution and still results in low contrast at low spatial frequencies. Reaching atomic resolution requires using low-frequency information to align ~100,000 2D projections of randomly-oriented particles before averaging. The need for sufficient low-frequency information has limited the scope of cryo-EM to large macromolecular complexes. Zernike phase contrast converts phase to amplitude by applying a 90 degree phase shift to the unscattered electron beam, but has yet to be widely implemented, as all previous phase plate designs degrade under the charged electron beam. Laser-based electron optics offer stable, tunable operation, without material objects in the electron beam path. We phase shift the unscattered electron wavefunction via the ponderomotive force of a tightly-focused laser in a near-concentric buildup cavity, which reaches 100 GW/cm^2 continuous intensity.

This aims to be a light, introductory talk with many pictures of my cat and Fourier transforms of my cat. Pickles has a lot of 1/f noise.

1. How much information we get in a single measurement

2. How well we can combine and average these measurements.

I will tell two stories from these two different fields of metrology.

The first story is about a new atomic clock design aimed at measuring time to the 19th decimal place. By using a Fermi-degenerate gas in a three-dimensional optical lattice, we controlled all quantum degrees of freedom of our atomic frequency references and suppressed atomic interactions. This allowed us to increase our single-shot frequency sensitivity by both extending the atom-light coherence time and by using more atoms to reduce the quantum projection noise. This new technology enabled new records in clock stability and the corresponding improvements in our ability to evaluate and stabilize systematic shifts.

The second story is about pushing the limits of cryogenic transmission electron microscopy (cryo-EM). Cryo-EM is rapidly usurping x-ray crystallography for determining protein structure, as it allows the visualization of molecules in their native environments, without the need for crystallization. Information from the nearly-transparent specimen manifests as a small phase shift on the electron wavefunction, which goes undetected unless the microscope is intentionally defocused. Defocusing compromises resolution and still results in low contrast at low spatial frequencies. Reaching atomic resolution requires using low-frequency information to align ~100,000 2D projections of randomly-oriented particles before averaging. The need for sufficient low-frequency information has limited the scope of cryo-EM to large macromolecular complexes. Zernike phase contrast converts phase to amplitude by applying a 90 degree phase shift to the unscattered electron beam, but has yet to be widely implemented, as all previous phase plate designs degrade under the charged electron beam. Laser-based electron optics offer stable, tunable operation, without material objects in the electron beam path. We phase shift the unscattered electron wavefunction via the ponderomotive force of a tightly-focused laser in a near-concentric buildup cavity, which reaches 100 GW/cm^2 continuous intensity.

This aims to be a light, introductory talk with many pictures of my cat and Fourier transforms of my cat. Pickles has a lot of 1/f noise.

Host:

Shimon Kolkowitz

*Postdoc in Holger Müller's group, UC Berkeley*

**Available Downloads:**

Room and Building:

5310, Chamberlin Hall

Thursday, April 19th, 2018

Triatomic molecules are deceptively simple. Even though there is only one additional atom compared to a diatomic molecule, this leads to non-trivial additional motional degrees of freedom and new associated quantum numbers. This, plus the larger density of states, realizes a quantum object whose complexity leads to new chemistry and physics research opportunities and concomitantly presents new challenges in molecular control. The science opportunities include the development of accurate and precise manipulation of chemical reactions and collisions in a qualitatively more complex species. But the reach of triatomics also includes dramatically improved, novel approaches to searches for physics beyond the Standard Model, and enhanced platforms for quantum computing using molecular tweezer arrays, both of which are aided by the low lying bending modes present in triatomic molecules. All of these research frontiers with triatomics, and their symmetric and asymmetric top brethren, either require or are greatly enhanced by chilling them to ultracold temperatures where they can be prepared in exquisitely well-defined internal and external motional states.

The recent experimental advances in direct laser cooling of diatomic molecules and triatomic molecules clearly indicates that full extension of laser tools - the creation of a magneto-optical trap (MOT) plus sub-Doppler cooling - to triatomic species should be possible. Recently in our laboratory we achieved a magneto-optical trap of diatomic molecules with CaF, sub-Doppler cooling to 40 µK, and loading of these molecules into an optical dipole trap. We also accomplished the first laser cooling and bichromatic force deflection of a polyatomic molecule, using SrOH. In addition, in 2016 we proposed the laser cooling of more complex polyatomic molecules using the methods we have now demonstrated. In particular, symmetric top molecules like CaOCH3 (and, possibly, related asymmetric top molecules) look extremely promising for direct laser cooling. The experimental prospects for a MOT of CaOH, YbOH, and CaOCH3 will be discussed.

The recent experimental advances in direct laser cooling of diatomic molecules and triatomic molecules clearly indicates that full extension of laser tools - the creation of a magneto-optical trap (MOT) plus sub-Doppler cooling - to triatomic species should be possible. Recently in our laboratory we achieved a magneto-optical trap of diatomic molecules with CaF, sub-Doppler cooling to 40 µK, and loading of these molecules into an optical dipole trap. We also accomplished the first laser cooling and bichromatic force deflection of a polyatomic molecule, using SrOH. In addition, in 2016 we proposed the laser cooling of more complex polyatomic molecules using the methods we have now demonstrated. In particular, symmetric top molecules like CaOCH3 (and, possibly, related asymmetric top molecules) look extremely promising for direct laser cooling. The experimental prospects for a MOT of CaOH, YbOH, and CaOCH3 will be discussed.

Host:

Saffman

*Harvard*

**Available Downloads:**

Room and Building:

5310 Chamberlin