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arXiv:2512.03007 · Review of Scientific Instruments

Generation of strong ultralow-phase-noise microwave fields with tunable ellipticity for ultracold polar molecules

Shrestha Biswas, Sebastian Eppelt, Christian Buchberger, Xing-Yan Chen, Andreas Schindewolf, Michael Hani, Erwin M. Biebl, Immanuel Bloch, Xinyu Luo

Published Apr 1, 2026• 2 citations
arXivHTMLPDF

Abstract

Microwave (MW) fields with strong field strength, ultralow phase-noise, and tunable polarization are crucial for stabilizing and manipulating ultracold polar molecules, which have emerged as a promising platform for quantum science. In this article, we present the design, characterization, and performance of a robust MW setup tailored for precise control of molecular states. This setup achieves a high electric field intensity of 6.9 kV/m in the near-field from a dual-feed waveguide antenna, enabling a Rabi frequency as high as 71 MHz for the rotational transition of sodium-potassium molecules. In addition, the low noise signal source and controlled electronics provide ultralow phase-noise and dynamically tunable polarization. Narrowband filters within the MW circuitry further reduce phase noise by more than 20 dB at 20 MHz offset frequency, ensuring prolonged one-body molecular lifetimes up to 10 s. We also show practical methods to measure the MW field strength and polarization using a simple homemade dipole probe and to characterize phase-noise down to -170 dBc/Hz using a commercial spectrum analyzer and a notch filter. Those capabilities allowed us to evaporatively cool our molecular sample to deep quantum degeneracy. Furthermore, the polarization tunability enabled the observation of field-linked resonances and facilitated the creation of field-linked tetramers. These techniques advance the study of ultracold polar molecules and broaden the potential applications of MW tools in other platforms of quantum science.

DOI
10.1063/5.0314943

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Microwave (MW) fields with strong field strength, ultralow phase-noise, and tunable polarization are crucial for stabilizing and manipulating ultracold polar molecules, which have emerged as a promising platform for quantum science. In this article, we present the design, characterization, and performance of a robust MW setup tailored for precise control of molecular states. This setup achieves a high electric field intensity of 6.9 kV/m in the near-field from a dual-feed waveguide antenna, enabling a Rabi frequency as high as 71 MHz for the rotational transition of sodium-potassium molecules. In addition, the low noise signal source and controlled electronics provide ultralow phase-noise and dynamically tunable polarization. Narrowband filters within the MW circuitry further reduce phase noise by more than 20 dB at 20 MHz offset frequency, ensuring prolonged one-body molecular lifetimes up to 10 s. We also show practical methods to measure the MW field strength and polarization using a simple homemade dipole probe and to characterize phase-noise down to -170 dBc/Hz using a commercial spectrum analyzer and a notch filter. Those capabilities allowed us to evaporatively cool our molecular sample to deep quantum degeneracy. Furthermore, the polarization tunability enabled the observation of field-linked resonances and facilitated the creation of field-linked tetramers. These techniques advance the study of ultracold polar molecules and broaden the potential applications of MW tools in other platforms of quantum science.
We report an experimental demonstration of microwave-assisted coherent control of ultracold 85^{85}85Rb133^{133}133Cs molecules in a ladder-type configuration of rotational-state double resonances, following an observation of Rabi oscillations of the related neighbouring rotational states. The population of middle rotational states, which is coupled with a probe microwave (MW) field, can be reduced by a control MW field. Broadening of spectral splitting and shift of central frequency are observed and both are attributed to the control MW field. Applying Akaike's information criterion, we conclude that our observed coherent spectra happen near the crossover range of electromagnetically induced transparency and Aulter-Townes splitting. Our work is a significant development in microwave-assisted quantum control of ultracold polar molecules with multilevel configuration, and also offers a great potential in quantum information based on ultracold molecules.
Ultracold polar molecules present a rich platform for experimental quantum science leveraging their unique properties. Of particular relevance is the intrinsic electric dipole moment of the molecule, which results in strong coupling to microwave fields and can be used to engineer long-range, anisotropic, and tunable dipole-dipole interactions. Such interactions can generate entanglement between molecules and enable the study of novel quantum many-body phenomena in parameter regimes inaccessible in other systems. Modern experiments in optical tweezers and optical lattices have demonstrated controlled interactions between molecules in coherent quantum states, and recently bulk gases of molecules have been cooled to quantum degeneracy. Motivated by these landmark advances, we present this article to explain many of the key concepts relevant to current research in the field.
We demonstrate microwave-assisted coherent control of ultracold 85Rb133Cs molecules in a ladder-type configuration of rotational states. Specifically, we use a probe and a control MW field to address the transitions between the J = 1 → 2 and J = 2 → 3 rotational states of the X1Σ+(v = 0) vibrational level, respectively, and use the control field to modify the response of the probe MW transition by coherently reducing the population of the intermediate J = 2 state. We observe that an increased Rabi frequency of the control field leads to broadening of the probe spectrum splitting and a shift of the central frequency. We apply Akaike's information criterion (AIC) to conclude that the observed coherent spectral response appears across the crossover regime between electromagnetically induced transparency and Aulter-Townes splitting. Our work is a significant development in microwave-assisted quantum control of ultracold polar molecules with multilevel configuration.
We report on the design and characterization of a compact microwave antenna for atomic and molecular physics experiments. The antenna is comprised of four loop antennas arranged in a cloverleaf shape, allowing for precise adjustment of polarization by tuning the relative phase of the loops. We optimize the antenna for left-circularly polarized microwaves at 3.5 GHz and characterize its near-field performance using ultracold NaCs molecules as a precise quantum sensor. Observing an unusually high Rabi frequency of 2π × 46.1(2) MHz, we extract an electric field amplitude of 33(2) V/cm at 22 mm distance from the antenna. The polarization ellipticity is 2.3(4)°, corresponding to a 24 dB suppression of right-circular polarization. The cloverleaf antenna is planar and provides large optical access, making it highly suitable for quantum control of atoms and molecules and potentially other quantum systems that operate in the microwave regime.
This thesis presents the development of a toolbox for the coherent control of ultracold polar molecules. Such systems of molecules promise the creation of long-lived, highly dipolar quantum gases with applications spanning the fields of quantum state controlled chemistry, quantum information, quantum simulation, and precision measurement. However, the addition of vibrational and rotational degrees of freedom leads to molecular systems being significantly more complex than their widely used atomic counterparts. In this work we demonstrate full control of the quantum state down to the hyperfine level of an optically trapped sample of ultracold bosonic 87Rb133 Cs molecules, and exploit that control to begin an investigation into the collision processes which take place in an ultracold molecular gas. We create a sample of up to ∼ 4000 optically trapped molecules in their rovibronic and hyperfine ground state. We characterise the molecules by measuring their temperature, binding energy, and molecule-frame electric dipole moment. We perform spectroscopy of the first rotationally excited state with hyperfine state resolution using microwaves to determine accurate values of rotational and hyperfine coupling constants. We use coherent π pulses to perform complete transfer population between selected hyperfine levels of the ground, first-excited, and second-excited rotational states. We investigate the effect of the off-resonant light of our optical dipole trap on the rotational and hyperfine structure of the molecules. Through a combination of high-resolution microwave spectroscopy and parametric heating measurements, we characterise the polarisability of the 87Rb133Cs molecule. We demonstrate that coupling between neighbouring hyperfine states manifests in rich structure with many avoided crossings in any rotational state other than the ground state. This coupling may be tuned by rotating the polarisation of the linearly polarised trapping light. Finally, we study the lifetime of polar bosonic 87Rb133Cs molecules in our 3D optical dipole trap. We examine the lifetime of the molecules as a function of dipole trap intensity, magnetic field, and hyperfine and rotational state. Despite the chemical stability of the 87Rb133 Cs molecule, we observe lifetimes of ∼1 s corresponding to 2-body decay rates close to the universal limit.
We report on the creation of bosonic NaCs molecules in their absolute rovibrational ground state via stimulated Raman adiabatic passage. We create ultracold gases with up to 22 000 dipolar NaCs molecules at a temperature of 300(50) nK and a peak density of 1.0(4)×10^{12} cm^{-3}. We demonstrate comprehensive quantum state control by preparing the molecules in a specific electronic, vibrational, rotational, and hyperfine state. We measure the ground state ac polarizability at 1064 nm along with the two-body loss rate, which we find to be universal. Employing the tunability and strength of the permanent electric dipole moment of NaCs, we induce dipole moments of up to 2.6 D at a dc electric field of 2.1(2) kV/cm and demonstrate strong microwave coupling between the two lowest rotational states with a Rabi frequency of 2π×45 MHz. A large electric dipole moment, accessible at relatively small electric fields, makes ultracold gases of NaCs molecules well suited for the exploration of strongly interacting phases of dipolar quantum matter.