Session Z05: Rydberg Atoms; Spectroscopy and Quantum Simulations

Universal quantum gates using nuclear spin qubits in an optical tweezer array of 171Yb atoms

Rydberg-mediated entanglement between neutral atoms in optical tweezer arrays is a rapidly developing platform for quantum science. An emerging frontier within this field is the use of alkaline earth-like atoms (AEAs) such as ytterbium (Yb). The rich internal structure of these atoms affords numerous unique capabilities, including narrow-line cooling and imaging [1], an optically active ion core for Rydberg state trapping [2] and gate addressing [3], and, in fermionic isotopes, highly coherent qubit storage in the nuclear spin. In this talk, we present a universal set of quantum gate operations on a qubit in the I=½ nucleus of ¹⁷¹Yb [4]. We observe long qubit coherence times, T₂* = 1.24(5) s due to the negligible differential light shift on the qubit state from the optical tweezer. We also demonstrate single-qubit gate operations with a randomized benchmarking fidelity F_1Q = 0.99959(6), as well as entangling gates through the Rydberg state. 

[1] S Saskin et al. PRL 122, 143002 (2019)

[2] J Wilson et al. PRL 128, 033201 (2022) 

[3] A P Burgers et al. arXiv: 2110.06902 (2021)

[4] S Ma et al. arXiv: 2112.06799 (2021)   

Quasi-adiabatic preparation of antiferromagnetic-like state of Rydberg excitations of atoms in a lattice

We examine the adiabatic preparation of spatially-ordered Rydberg excitations of atoms in finite one-dimensional lattices by frequency-chirped laser pulses, as realized in a number of recent experiments simulating quantum Ising model. Our aims are to unravel the microscopic mechanism of the phase transition from the unexcited state of atoms to the antiferromagnetic-like state of Rydberg excitations by traversing an extended gapless phase, and to estimate the preparation fidelity of the target state in a moderately sized system. We find that the system climbs the ladder of Rydberg excitations predominantly along the strongest-amplitude paths towards the final ordered state. We show that, despite its complexity, the interacting many-body system can be described as an effective two-level system involving a pair of lowest-energy instantaneous collective eigenstates of the time-dependent Hamiltonian. The final preparation fidelity of the target state can then be well approximated by the Landau-Zener formula involving the minimal energy gap, extrapolation of which allows us to estimate the preparation fidelity for much larger systems.   

The effect of atom motion in Rydberg atom quantum simulators of spin models

Rydberg atom arrays are a widely used platform to simulate lattice spin models, but some experiments reveal decoherences that we do not fully understand. Inspired by strong decoherence observed by recent experiments [Guardado-Sanchez et al. Phys. Rev. X 8, 021069 (2018)] that conjectured this to be caused by atom motion, we consider the coupling of atom motion to the spin dynamics. Our results show that, in ongoing experiments with either optical lattices or microtraps, atom motion causes decoherence that can be significant. This decoherence comes from the initial spread of atoms' positions and momenta, as well as the interaction-induced motion during the dynamics. We suggest possible ways to reduce this type of decoherence, such as using heavier atoms or deeper traps.   

Interacting Quantum Matter with Rydberg or Molecule Synthetic Dimensions

Synthetic dimension platforms offer unique ways of exploring quantum matter.  Building on recent success building a synthetic lattice using six Rydberg states on a single atom in ultracold ⁸⁴Sr atoms (arXiv:2101.02871), we theoretically study the many-body physics of atoms with internal synthetic lattices loaded in microtrap arrays and uniform synthetic tunneling. In this setup, different atoms interact via strong dipole-dipole angular-momentum-exchange interactions. We predict that string-like ground states of finite synthetic width occur in the absence of synthetic tunneling and show that these string-like states survive at finite temperatures and tunneling before phase transitioning into a disordered state. We show that finite-temperature and quantum phase transitions occur, including first-order and second-order transitions, with a rich dependence on tunneling, temperature and size of the synthetic lattice. Furthermore, we will discuss how Rydberg atom and ultracold molecule synthetic dimensions platform opens a wide variety of physics to explore by using different tunneling and level schemes.   

Quantum simulation of the central spin model with a Rydberg atom and polar molecules in optical tweezers

Central spin models, where a single spinful particle interacts with a spin environment, find wide application in quantum information technology and can be used to describe e.g. the decoherence of a qubit over time. We propose a method of realizing an ultracold quantum simulator for the central spin model. The proposed system consists of a single Rydberg atom ("central spin") and surrounding diatomic molecules ("environment spins"), polarized by an external electric field and coupled to each other via dipole-dipole interactions. By mapping internal particle states to spin states, spin-exchanging interactions can be simulated. We demonstrate that this setup allows realizing a range of central spin models of high scientific interest. More precise control over the model can be exerted by directly manipulating the placement of environment spins. As an example, we consider a ring-shaped arrangement of environment spins, and show how the time evolution of the central spin is affected by the tilt angle of the ring.   

Ultracold heavy Rydberg system formed from long-range molecules

We propose a scheme to realize a heavy Rydberg system (HRS), a bound pair of oppositely charged ions, from a gas of ultracold atoms. The intermediate step to achieve large internuclear separations is the creation of a unique class of ultra-long-range Rydberg molecules bound in a stairwell potential energy curve. Here, a ground-state atom is bound to a Rydberg atom in an oscillatory potential emerging due to attractive singlet p-wave electron scattering. The utility of our approach originates in the large electronic dipole transition element between the Rydberg and the ionic molecule, while the nuclear configuration of the ultracold gas is preserved. The Rabi coupling between the Rydberg molecule and the heavy Rydberg system is typically in the MHz range and the permanent electric dipole moments of the HRS can be as large as one kilo-Debye. We identify specific transitions which place the creation of the heavy Rydberg system within immediate reach of experimental realization. HRS can serve as a source for ultracold negative ions as well as for mass balanced plasmas.   

Resolving the rotationally excited states of ultralong-range Rydberg molecules

We report experimental observations of rotational structure in photo-associative spectroscopy of ultralong-range Rydberg molecules (ULRRMs) in an ultracold gas of ⁸⁶Sr. ULRRM spectroscopy probes scattering wave functions at much larger internuclear separations than does photo-associative spectroscopy to low-lying electronic states [1]. At such separations (R_n=1400a₀ at n=31), the kinetic energy of the initial colliding pairs supports higher partial wave scattering channels, which, coupled with the recoil momentum resulting from photoexcitation, allows creation of rotationally excited ULRRMs. The visibility of the rotational structure is further enhanced by the suppression of the s-wave channel due to the near-resonant scattering properties of ⁸⁶Sr (a_s=811a₀). Results are discussed with the aid of theory that accounts for the recoil momentum associated with photoexcitation and the large s-wave scattering length in the entrance channel.

    

Coherent Control of Processes that Break the Dipole Blockade

The Rydberg excitation blockade has enabled impressive achievements in quantum information and simulation.  However, state-mixing processes may compromise the single-excitation behavior of the blockade.  When ultracold atoms are excited to Rydberg states near Förster resonance, a significant number of atoms can be found in dipole-coupled product states immediately after excitation.  Under certain experimental conditions, this is due to laser excitation of mixed three-atom states.   We use a rotary echo technique to demonstrate the coherence of the evolution of these three-atom states.  We find good agreement between our data and a theoretical model for laser excitation in a coupled three-atom basis¹.

¹ PRL 102, 013004 (2009)   

Isolated core excitation Stark shifts of Yb Rydberg states without autoionization

Isolated core excitation (ICE) provides an efficient means of applying an AC Stark shift to bound Rydberg state of a cold two electron Rydberg atom. ICE is the laser excitation of the second valence, or core, electron, with the Rydberg electron remaining a spectator, resulting in a doubly excited Rydberg state. When applied to low $\ell$ Rydberg states the result is most often rapid autoionization, which seems to preclude the use of ICE as a nondestructive tool. However, a quantitative description of ICE using multi channel quantum defect theory (MQDT) shows that there are zeros in the ICE cross section, due to the overlap integral between the bound and doubly excited rydberg states, which were shown to exist some time ago. The same MQDT model yields an AC Stark shift of the bound Rydberg state proportional to the principal part integral over the ICE  cross section. Since the zeros can be close to the peak ICE cross section, the shift can be large where the ICE cross section vanishes. Using bound $6sns$ states of cold Yb atoms in a magneto optical trap we have verified that the ICE cross section to the $6p_{1/2}ns$ states has the expected zeros and observed substantial shifts of the $6sns$ states at these zeros. More generally, we have shown that the shifts are well described by the MQDT model.   

The route to quantum light via semiconductor excitons

Manipulating light down to the level of single photons is an important goal of quantum optics. Semiconductor excitons afford a convenient integrable platform for this purpose. However, the optical nonlinearities of such systems are typically too weak to induce significant quantum correlations. Here, we present new strategies for this purpose that are effective in the presence of strong dissipation. These possibilities are opened up by high-lying excitonic Rydberg states with unusually strong interactions. We discuss recent progress with Rydberg excitons in bulk and two-dimensional systems along with their specific challenges, which we show can be overcome with new theoretical proposals. The results promise record photon correlations and, ultimately, provide a positive outlook on quantum optics with semiconductor excitons.