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arXiv:2103.06921 · Science

Pauli blocking of light scattering in degenerate fermions

Yair Margalit, Yu‐Kun Lu, Furkan Çağrı Top, Wolfgang Ketterle

Published Nov 18, 2021• 1 citation
arXivHTMLPDF

Abstract

Pauli blocking of spontaneous emission is responsible for the stability of atoms. Electrons cannot decay to lower-lying internal states that are already occupied. Pauli blocking also occurs when free atoms scatter light elastically (Rayleigh scattering) and the final external momentum states are already populated. This was predicted more than 30 years ago but is challenging to realize experimentally. Here, we report on Pauli blocking of light scattering in a dense quantum-degenerate Fermi gas of ultracold lithium atoms. When the Fermi momentum is larger than the photon recoil, most final momentum states are within the Fermi surface. At low temperature, we find that light scattered even at large angles is suppressed by 37% compared with higher temperatures, where atoms scatter at the single-atom Rayleigh scattering rate.

DOI
10.1126/science.abi6153

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Pauli blocking of spontaneous emission is responsible for the stability of atoms. Electrons cannot decay to lower-lying internal states that are already occupied. Pauli blocking also occurs when free atoms scatter light elastically (Rayleigh scattering) and the final external momentum states are already populated. This was predicted more than 30 years ago but is challenging to realize experimentally. Here, we report on Pauli blocking of light scattering in a dense quantum-degenerate Fermi gas of ultracold lithium atoms. When the Fermi momentum is larger than the photon recoil, most final momentum states are within the Fermi surface. At low temperature, we find that light scattered even at large angles is suppressed by 37% compared with higher temperatures, where atoms scatter at the single-atom Rayleigh scattering rate.
Pauli blocking of spontaneous emission by a single excited-state atom has been predicted to be dramatic at low temperature when the Fermi energy E F exceeds the recoil energy E R . The photon scattering rate of a ground-state Fermi gas can also be suppressed by occupation of the final states accessible to a recoiling atom; however, suppression is diminished by scattering events near the Fermi edge. We analyse two new approaches to improve the visibility of Pauli blocking in a trapped Fermi gas. Focusing the incident light to excite preferentially the high-density region of the cloud can increase the blocking signature by 14%, and is most effective at intermediate temperature. Spontaneous Raman scattering between imbalanced internal states can be strongly suppressed at low temperature, and is completely blocked for a final state E F > 4 E R in the high imbalance limit.
The Pauli exclusion principle forbids indistinguishable fermions from occupying the same quantum mechanical state. The implications of this are profound; for example, it accounts for the electronic shell structure of atoms. Here, we performed measurements on the scattering of light from ultracold ensembles of atoms. For a Fermi gas in the quantum degenerate regime, we observed a marked suppression in scattering compared with a similarly prepared Bose gas. The observed decrease in fluorescence and the corresponding increase in light transmission results from Pauli blocking, where Fermi-Dirac statistics limits the number of accessible states for a scattering atom in a large Fermi sea. Our work confirms a fundamental result on the optical response of quantum gases and may contribute to cooling and thermometry schemes.
Transition rates between coupled states in a quantum system depend on the density of available final states. The radiative decay of an excited atomic state has been suppressed by reducing the density of electromagnetic vacuum modes near the atomic transition. Likewise, reducing the density of available momentum modes of the atomic motion when it is embedded inside a Fermi sea will suppress spontaneous emission and photon scattering rates. Here we report the experimental demonstration of suppressed light scattering in a quantum degenerate Fermi gas. We systematically measured the dependence of the suppression factor on the temperature and Fermi energy of a strontium quantum gas and achieved suppression of scattering rates by up to a factor of 2 compared with a thermal gas.
Light scattering from a neutral Fermi gas can be suppressed by occupation of the final states accessible to a recoiling atom. A related effect, Pauli blocking of spontaneous emission by a single excited-state atom, has been predicted to be dramatic at low temperature when the Fermi energy EF exceeds the recoil energy ER. However suppression of light scattering from a large ground-state ensemble is diminished by scattering events near unoccupied states. We analyze two possible methods of enhancement: using a focused light beam to excite only the high-density region of the cloud, and Raman scattering between imbalanced internal states. We find that focusing can enhance suppression by 14 % of the unblocked scattering rate, and that Raman scattering can yield a full suppression at low temperature for EF> 4ER, even in the inhomogeneous gas. ‡ Present address: Lyman Laboratories, Harvard University, Cambridge MA 02138, USAEnhanced Pauli blocking of light scattering in a trapped Fermi gas 2 1.
The observation of Pauli blocking of atomic spontaneous decay via direct measurements of the atomic population requires the use of long-lived atomic gases where quantum statistics, atom recoil, and cooperative radiative processes are all relevant. We develop a theoretical framework capable of simultaneously accounting for all these effects in the many-body quantum degenerate regime. We apply it to atoms in a single 2D pancake or arrays of pancakes featuring an effective Λ level structure (one excited and two degenerate ground states). We identify a parameter window in which a factor of 2 extension in the atomic lifetime clearly attributable to Pauli blocking should be experimentally observable in deeply degenerate gases with ∼10^{3} atoms. We experimentally observe a suppressed excited-state decay rate, fully consistent with the theory prediction of an enhanced excited-state lifetime, on the ^{1}S_{0}-^{3}P_{1} transition in ^{87}Sr atoms.
Spontaneous emission by an excited fermionic atom can be suppressed due to the Pauli exclusion principle if the relevant final states after the decay are already occupied by identical atoms in the ground state. Here we discuss a setup where a single atom is prepared in the first excited state on a single site of an optical lattice under conditions of very tight trapping. We investigate these phenomena in the context of two experimental realizations: (1) with alkali-metal atoms, where the decay rate of the excited state is large, and (2) with alkaline-earth-metal-like atoms, where the decay rate from metastable states can be tuned in experiments. This phenomenon has potential applications towards reservoir engineering and dissipative many-body state preparation in an optical lattice.