Research

Our new research group will investigate questions directly relevant to applications in quantum science, in particular, quantum metrology and quantum many-body physics.

The experimental platform of our choice is arrays of individually controlled alkaline-earth(-like) (AEL) atoms, which have been at the forefront of exploring new frontiers in quantum science over recent years1. AEL atoms possess two valence electrons, leading to a rich electronic structure with broad and narrow optical transitions. In particular, their ultra-narrow optical clock transition has made AEL species like strontium and ytterbium instrumental in developing ever-improving optical lattice clocks2.

Electronic states in alkaline-earth(-like) atoms
Structure of the lowest-lying electronic states in most alkaline-earth(-like) atoms and relevant transitions.

We are particularly interested in questions that emerge from equipping an optical atomic clock with programmability and viewing it as an ensemble of qubits with single-particle control. For example, interactions can then be harnessed for preparing entangled states—in direct analogy to gates in a quantum computer. Realizing these entangling operations in a clock can enable quantum-enhanced measurement sensitivity or realize robust decoherence-free subspaces3.

In addition, we are also interested in using similar experimental techniques for exploring open questions in condensed matter theory. For example, using quantum gas microscopy to detect individual AEL atoms in optical lattices could enable probing the rich quantum many-body physics emerging from SU(N)\mathrm{SU}(N) symmetric or multiorbital interactions4.

Past Research

Quantum metrology with optical clocks

Spectral response of the multiorbital Fermi polaron
Comparison of the measurement variance for a classical coherent spin state (CSS) and an entangled spin-queezed state (SSS) at variable analysis phase α. From W. Eckner et al., Nature 621 (2023)

Today’s state-of-the-art optical lattice clocks require astronomical scales to describe their accuracy and precision2. Remarkably, they would lose or gain less than a second over the estimated 13.8 billion years since the Big Bang. This unprecedented level of sensitivity has led to several interesting applications, for example, the measurement of gravitational redshifts on the millimeter scale5. To further improve optical atomic clocks and expand their range of applications, it will be necessary to simultaneously improve ultra-stable laser technology and control of the atomic ensemble and its environment.

In the Kaufman group and Ye group at JILA, we have explored different ways to improve strontium-based optical clocks through better control of atomic ensembles: Our approaches include engineering many-body states with reduced sensitivity to perturbations arising from atomic interactions6, as well as utilizing entangled states that feature a measurement precision below the standard quantum limit.7.

Quantum many-body physics with ultracold fermions in optical lattices

Spectral response of the multiorbital Fermi polaron
Signatures of a quasiparticle in a multiorbital Fermi gas. From N. Darkwah Oppong et al., PRL 122 (2019)

Trapping fermionic atoms in an optical lattice is an elegant way to simulate the physics of itinerant electrons in solids using a particularly clean and well-controlled environment8. This technique has become a powerful tool to improve our understanding of strongly correlated electron systems, which are often extremely challenging to simulate on a classical computer. However, most of the previous work in this field has been focused on the case of single-orbital physics, in particular, the single-band Hubbard model.

In the Bloch group at LMU Munich, we have used the ground and clock state of alkaline earth-like atoms to extend such simulations to the multiorbital regime9. Electrons in multiple orbitals with distinct mobility and interaction properties are relevant for important phenomena known from condensed matter physics like the Kondo effect or heavy fermions10.

  1. A. M. Kaufman and K.-K. Ni, Quantum science with optical tweezer arrays of ultracold atoms and molecules, Nat. Phys. 17, 1324 (2021).

  2. A. D. Ludlow et al., Optical atomic clocks, Rev. Mod. Phys. 87, 637 (2015).

  3. L. Pezzè et al., Quantum metrology with nonclassical states of atomic ensembles, Rev. Mod. Phys. 90, 035005 (2018).

  4. A. V. Gorshkov et al., Two-orbital SU(N)SU(N) magnetism with ultracold alkaline-earth atoms, Nat. Phys. 6, 289 (2010).

  5. T. Bothwell et al., Resolving the gravitational redshift across a millimetre-scale atomic sample, Nature 602, 420 (2022); also see. X. Zheng et al., A lab-based test of the gravitational redshift with a miniature clock network, Nat. Comm. 14, 4886 (2023).

  6. S. L. Campbell et al., A Fermi-degenerate three-dimensional optical lattice clock Science 358, 90 (2017).

  7. W. Eckner et al., Realizing spin squeezing with Rydberg interactions in an optical clock, Nature 621, 734 (2023).

  8. C. Gross and I. Bloch, Quantum simulations with ultracold atoms in optical lattices, Science 357, 995 (2017)

  9. L. Riegger et al., Localized Magnetic Moments with Tunable Spin Exchange in a Gas of Ultracold Fermions, Phys. Rev. Lett. 120, 143601 (2018); N. Darkwah Oppong et al., Observation of Coherent Multiorbital Polarons in a Two-Dimensional Fermi Gas, Phys. Rev. Lett. 122, 193604 (2019); N. Darkwah Oppong et al., Probing Transport and Slow Relaxation in the Mass-Imbalanced Fermi-Hubbard Model, Phys. Rev. X 12, 031026 (2022)

  10. A. C. Hewson, The Kondo Problem to Heavy Fermions (Cambridge University Press, 1993).