High-Sensitivity rf Spectroscopy of a Strongly Interacting Fermi Gas

Abstract

rf spectroscopy is one of the most powerful probing techniques in the field of ultracold gases. We report on a novel rf spectroscopy scheme with which we can detect very weak signals of only a few atoms. Using this method, we extended the experimentally accessible photon-energies range by an order of magnitude compared to previous studies. We directly verify a universal property of fermions with short-range interactions which is a power-law scaling of the rf spectrum tail all the way up to the interaction scale. We also determine, with high precision, the trap average contact parameter for different interaction strength. Finally, we employ our technique to precisely measure the binding energy of Feshbach molecules in an extended range of magnetic fields. These data are used to extract a new calibration of the Feshbach resonance between the two lowest energy levels of ^{40}K.

Contact and Momentum Distribution of the Unitary Fermi Gas

“…(1) is that in a unit volume, the number of pairs of fermions separated by a distance smaller than s is C s/(4π) in the s→0 limit. Hence C controls the (anomalously high) density of pairs with vanishing interparticle distance [9,11,13].A large variety of experimentally studied observables are directly expressible in terms of the contact: the population of the closed channel molecular state measured by laser molecular spectroscopy [14,15], the largemomentum tail of the static structure factor measured by Bragg spectroscopy [16][17][18], the tail of the momentum distribution measured by non-interacting time-offlight or by momentum-resolved radiofrequency spectroscopy [19], the derivative of the energy with respect to the inverse scattering length [10] extracted from the pressure equation of state measured by in-situ imaging [20], the large-frequency tail in radiofrequency spectroscopy [19,[21][22][23], and the short-distance densitydensity correlation function extracted from the threebody loss rate in presence of a bosonic cloud [24].The experimental study [22] is particularly important because it is spatially resolved and for the first time yields the temperature dependence of the contact for a homogeneous system. Recently, two other experimental groups have presented preliminary data for the temperaturedependent homogeneous contact [25].…”

In situ momentum-distribution measurement of a quantum degenerate Fermi gas using Raman spectroscopy

“…In this paper, we demonstrate an in situ measurement of the momentum distribution of a degenerate weaklyinteracting Fermi gas using Raman spectroscopy and a sensitive fluorescence detection scheme [30]. There are four main advantages to this approach.…”

“…In Raman spectroscopy we measure the momentum-dependent transition rate Γ(δ) from state |2 to the state |3 , which is initially unoccupied and is weakly-interacting with atoms in other states, in the range of magnetic fields applied in these experiments. We employ a sensitive fluorescence detection scheme to count the Raman-coupled atoms in state |3 [30]. First, we use MW adiabatic sweep to transfer these atoms to state |F = 7/2, mF = −3/2 , which is magnetically trappable.…”

“…The experiments are conducted with a quantum degenerate gas of fermionic 40 K atoms. The gas is prepared in a balanced incoherent mixture of the states |1 = |F = 9/2, m F = −9/2 and |2 = |F = 9/2, m F = −7/2 in the 4 2 S 1/2 manifold, whose interaction, charac-terized by the s-wave scattering length a, can be tuned by an external magnetic field near a Feshbach resonance at B 0 = 202.14G [30]. The experimental apparatus and cooling sequence are the same as described in Refs.…”

“…The experimental apparatus and cooling sequence are the same as described in Refs. [30,31]. Two counter-propagating Raman beams overlap with the atomic cloud (see figure 3a and 3b).…”