Ultracold Samples Research Articles

Ultra-Cold Cryogenic TEM Sample Holder with Liquid Helium and High Stability

Ultra-Cold Cryogenic TEM Sample Holder with Liquid Helium and High Stability

Field deployable atomics package for an optical lattice clock

An atomics package is the heart of any atom based quantum sensing device. Here we report on the realisation of a field deployable atomics package for alkaline earth atoms, e.g. Sr or Yb. In terms of size (∼121 L), weight (<75 kg) and power (∼320 W), it is the smallest package to date which is designed to load Sr atoms into an optical lattice. It consists of an ultra-high vacuum assembly (<4 L), lasers, magnetic field coils & optics required for cooling & trapping as well as a module for imaging & detection. The package can routinely produce ultra cold and dense samples of 1.6 × 105 88Sr atoms trapped in a 1D optical lattice in less than a second. Its robustness has been demonstrated by conducting two transportation campaigns within out-of-the-lab environments. This advancement will have impact not only on transportable optical clock development but also will influence the wider areas of quantum science and technologies, particularly requiring field deployable cold atom based quantum sensors.

Reducing Number Fluctuations in an Ultracold Atomic Sample Using Faraday Rotation and Iterative Feedback

We demonstrate a method to reduce number fluctuations in an ultracold atomic sample using real-time feedback. By measuring the Faraday rotation of an off-resonant probe laser beam with a pair of avalanche photodetectors in a polarimetric setup, we produce a proxy for the number of atoms in the sample. We iteratively remove a fraction of the excess atoms from the sample to converge on a target proxy value in a way that is insensitive to environmental perturbations and robust to errors in light polarization. Using absorption imaging for out-of-loop verification, we demonstrate a reduction in the number fluctuations from $3\mathrm{%}$ to $0.45\mathrm{%}$ for samples at a temperature of $16.4\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{K}$ over the time scale of several hours, which is limited by temperature fluctuations, beam-pointing noise, and photon shot noise.

Characterization of the lowest electronically excited-state ro-vibrational level of 23Na87Rb

Starting from an ultracold sample of ground-state 23Na87Rb molecules, we investigate the lowest ro-vibrational level of the b 3Π state with high resolution laser spectroscopy. This electronic spin-forbidden X 1Σ+ ↔ b 3Π transition features a nearly diagonal Franck–Condon factor and has been proposed useful for probing and manipulating the ultracold molecular gas. We measure the transition strength directly by probing the ac Stark shift induced by near resonance light and determine the total excited-state spontaneous emission rate by observing the loss of molecules. From the extracted branching ratio and the theoretical modeling, we find that the leakage to the continuum of the a 3Σ+ state plays the dominant role in the total transition linewidth. Based on these results, we show that it is feasible to create optical trapping potentials for maximizing the rotational coherence with laser light tuned close to this transition.

Simultaneous Zeeman deceleration of polyatomic free radical with lithium atoms

Chemistry in the ultracold regime enables fully quantum-controlled interactions between atoms and molecules, leading to the discovery of the hidden mechanisms in chemical reactions which are usually curtained by thermal averaging in the high temperature. Recently a couple of diatomic molecules have been cooled to ultracold regime based on laser cooling techniques, but the chemistry associated with these simple molecules is highly limited. In comparison, free radicals play a major role in many important chemical reactions, but yet to be cooled to submillikelvin temperature. Here we propose a novel method of decelerating \ce{CH3}, the simplest polyatomic free radical, with lithium atoms simultaneously by travelling wave magnetic decelerator. This scheme paves the way towards co-trapping \ce{CH3} and lithium, so that sympathetical cooling can be used to preparing ultracold free radical sample.

Efficient generation of heralded narrowband color-entangled states.

We show that narrowband two-color entangled single Stokes photons can be generated in a ultra-cold atoms sample via selective excitation of two spontaneous four-wave mixing (SFWM) processes. Under certain circumstances, the generation, heralded by the respective common anti-Stokes photon, is robust against losses and phase-mismatching and is remarkably efficient owing to balanced resonant enhancement of the two four-wave mixing processes in a regime of combined induced transparency. Maximally color-entangled states can be easily attained by adjusting the detunings of the external couplings and driving fields, even when these are quite weak.

Observation of Magnetic Solitons in Two-Component Bose-Einstein Condensates.

We experimentally investigate the dynamics of spin solitary waves (magnetic solitons) in a harmonically trapped, binary superfluid mixture. We measure the insitu density of each pseudospin component and their relative local phase via an interferometric technique we developed and as such, fully characterize the magnetic solitons while they undergo oscillatory motion in the trap. Magnetic solitons exhibit nondispersive, dissipationless longtime dynamics. By imprinting multiple magnetic solitons in our ultracold gas sample, we engineer binary collisions between solitons of either the same or opposite magnetization and map out their trajectories.

Probing ultracold chemistry using ion spectrometry.

Rapid progress in atomic, molecular, and optical (AMO) physics techniques enabled the creation of ultracold samples of molecular species and opened opportunities to explore chemistry in the ultralow temperature regime. In particular, both the external and internal quantum degrees of freedom of the reactant atoms and molecules are controlled, allowing studies that explored the role of the long-range potential in ultracold reactions. The kinetics of these reactions have typically been determined using the loss of reactants as proxies. To extend such studies into the short-range, we developed an experimental apparatus that combines the production of quantum-state-selected ultracold KRb molecules with ion mass and kinetic energy spectrometry, and directly observed KRb + KRb reaction intermediates and products [M.-G. Hu and Y. Liu, et al., Science, 2019, 366, 1111]. Here, we present the apparatus in detail. For future studies that aim for detecting the quantum states of the reaction products, we demonstrate a photodissociation based scheme to calibrate the ion kinetic energy spectrometer at low energies.

Ultracold mercury–alkali-metal molecules for electron-electric-dipole-moment searches

Heavy polar diatomic molecules are the leading candidates in searches for the permanent electric dipole moment of the electron (eEDM). Next-generation eEDM search experiments ideally require extremely large coherence times, in large ensembles of trapped molecules that have a high sensitivity to the eEDM. We consider a family of molecules, mercury-alkali diatomics, that can be feasibly produced from ultracold atoms. We present calculations of the effective electric fields experienced by the electron in these molecules. The combination of reasonably large effective electric fields, and a straightforward path to obtaining trapped ultracold samples, lead us to identify these molecules as favorable candidates for eEDM search experiments.

High-sensitivity measurement of Rydberg population via two-photon excitation in atomic vapour using optical heterodyne detection technique

We demonstrate a technique based on optical heterodyne detection to measure the Rydberg population in the thermal atomic vapour. The technique used a probe beam far off-resonant to the D2 line of rubidium along with a reference beam with frequency offset by 800 MHz in the presence of a coupling laser that couples to Rydberg state via two-photon resonance. The polarisation of the probe, reference and coupling beams are suitably chosen such that only the probe beam goes through a nonlinear phase shift due to the two-photon process which is measured relative to the phase shift of the reference beam using optical heterodyne detection technique. We show that the technique has a sensitivity to measure the minimum phase shift of the order of a few $$\mu $$ rad. We have used a suitable model of two-photon excitation of a three-level atom to show that the minimum phase shift measured in our experiment corresponds to the Rydberg population of the order of $$10^{-5}$$ . The corresponding probe absorption for the given laser parameters is of the order of $$10^{-7}$$ . We demonstrate that this technique is insensitive to polarisation impurity or fluctuations in the beams. The technique is particularly useful in measuring the Rydberg population via two-photon excitation in thermal vapour where microchannel plates (MCP) could be relatively difficult to implement. It can also be used in the ultracold atomic sample with suitable laser parameters.

Velocimetry of cold atoms by matter-wave interferometry

We present an elegant application of matterwave interferometry to the velocimetry of cold atoms whereby, in analogy to Fourier transform spectroscopy, the 1-D velocity distribution is manifest in the frequency domain of the interferometer output. By using stimulated Raman transitions between hyperfine ground states to perform a three-pulse interferometer sequence, we have measured the velocity distributions of clouds of freely-expanding $^{85}$Rb atoms with temperatures of 33 $\mu$K and 17 $\mu$K. Quadrature measurement of the interferometer output as a function of the temporal asymmetry yields velocity distributions with excellent fidelity. Our technique, which is particularly suited to ultracold samples, compares favourably with conventional Doppler and time-of-flight techniques, and reveals artefacts in standard Raman Doppler methods. The technique is related to, and provides a conceptual foundation of, interferometric matterwave accelerometry, gravimetry and rotation sensing.

Relativistic Measurement Backaction in the Quantum Dirac Oscillator.

An elegant method to circumvent quantum measurement backaction is the use of quantum mechanics free subsystems (QMFS), with one approach involving the use of two oscillators with effective masses of opposite signs. Since negative energies, and hence masses, are a characteristic of relativistic systems a natural question is to what extent QMFS can be realized in this context. Using the example of a one-dimensional Dirac oscillator we investigate conditions under which this can be achieved, and identify Zitterbewegung or virtual pair creation as the physical mechanism that fundamentally limits the feasibility of the scheme. We propose a tabletop implementation of a Dirac oscillator system based on a spin-orbit coupled ultracold atomic sample that allows for a direct observation of the corresponding analog of virtual pair creation on quantum measurement backaction.

Testing Lorentz symmetry violation with an invariant minimum speed

This work presents an experimental test of Lorentz invariance violation in the infrared (IR) regime by means of an invariant minimum speed in spacetime and its effects on the time when an atomic clock given by a certain radioactive single-atom (e.g. isotope Na[Formula: see text]) is a thermometer for an ultracold gas like the dipolar gas Na[Formula: see text]K[Formula: see text]. So, according to a Deformed Special Relativity (DSR) so-called Symmetrical Special Relativity (SSR), where there emerges an invariant minimum speed V in the subatomic world, one expects that the proper time of such a clock moving close to V in thermal equilibrium with the ultracold gas is dilated with respect to the improper time given in lab, i.e. the proper time at ultracold systems elapses faster than the improper one for an observer in the lab, thus leading to the so-called proper time dilation so that the atomic decay rate of an ultracold radioactive sample (e.g. Na[Formula: see text]) becomes larger than the decay rate of the same sample at room temperature. This means a suppression of the half-life time of a radioactive sample thermalized with an ultracold cloud of dipolar gas to be investigated by NASA in the Cold Atom Lab (CAL).

Interspecies thermalization in an ultracold mixture of Cs and Yb in an optical trap

We present measurements of interspecies thermalization between ultracold samples of $^{133}$Cs and either $^{174}$Yb or $^{170}$Yb. The two species are trapped in a far-off-resonance optical dipole trap and $^{133}$Cs is sympathetically cooled by Yb. We extract effective interspecies thermalization cross sections by fitting the thermalization measurements to a rate equation model, giving $\sigma_{\mathrm{Cs^{174}Yb}} = \left(5 \pm 2\right) \times 10^{-13} \, \mathrm{cm^{2}}$ and $\sigma_{\mathrm{Cs^{170}Yb}} = \left(18 \pm 8\right) \times 10^{-13} \, \mathrm{cm^{2}}$. We perform quantum scattering calculations of the thermalization cross sections and optimize the CsYb interaction potential to reproduce the measurements. We predict scattering lengths for all isotopic combinations of Cs and Yb. We also demonstrate the independent production of $^{174}$Yb and $^{133}$Cs Bose-Einstein condensates using the same optical dipole trap, an important step towards the realization of a quantum-degenerate mixture of the two species.

Quantum Engineering of a Low-Entropy Gas of Heteronuclear Bosonic Molecules in an Optical Lattice

We demonstrate a generally applicable technique for mixing two-species quantum degenerate bosonic samples in the presence of an optical lattice, and we employ it to produce low-entropy samples of ultracold ^{87}Rb^{133}Cs Feshbach molecules with a lattice filling fraction exceeding 30%. Starting from two spatially separated Bose-Einstein condensates of Rb and Cs atoms, Rb-Cs atom pairs are efficiently produced by using the superfluid-to-Mott insulator quantum phase transition twice, first for the Cs sample, then for the Rb sample, after nulling the Rb-Cs interaction at a Feshbach resonance's zero crossing. We form molecules out of atom pairs and characterize the mixing process in terms of sample overlap and mixing speed. The dense and ultracold sample of more than 5000 RbCs molecules is an ideal starting point for experiments in the context of quantum many-body physics with long-range dipolar interactions.

Theoretical research on an efficient population transfer based on two different laser pulse sequences

A quantum gas of ultracold molecules, with long-range and anisotropic interactions, will enable a series of fundamental studies in physics and chemistry. In particular, samples of ground-state molecules at ultralow temperatures and high number densities will facilitate the explorations of a large number of many-body physical phenomena and applications in quantum information processing. However, due to the lack of efficiently cooling techniques such as laser cooling for atomic gases, high number densities for ultracold molecular samples are not readily attainable. Associating ultracold atoms to weakly bound dimer molecules via Feshbach resonance and subsequently transferring them to a wanted molecular ro-vibronic ground state by a stimulated Raman adiabatic passages (STIRAP) have proved to be an effective way in producing ideal ultracold molecular samples. As a typical illustration, in a recent study (2010 Nat. Phys. 6 265) Danzl et al. experimentally realized the preparation of Cs2 molecule into its ro-vibronic ground state via two different multi-level STIRAPs:one is based on a single conversion route and the others are based on a cascade-connected route (labeled by 4p-STIRAP and s-STIRAP, respectively). In this work, we present a theoretical study for these two STIRAP schemes, focusing on the differences in physical principle and realistic performance between them. On the one hand, according to the theoretical approach of quasi-dark eigenstates, we conclude that a highly efficient population transfer is achievable in both schemes. On the other hand, by systematically studying the influences of the relevant parameters, including the spontaneous decays and the detunings from the intermediate states, and the temporal sequence and the amplitude of the laser pulses, we disclose their respective advantages and weaknesses in the realistic implementation. We theoretically predict that for both schemes their maximal conversion efficiencies each can attain 0.97 as long as the spontaneous decays from the intermediate excited states are sufficiently suppressed. Yet considering the fact that the already implemented efficiency is only around 0.6 for both schemes, there is still room for optimization, e.g. using stable Rydberg energy levels in future experiment. Furthermore, the success of these two schemes can provide a new route to the controllable entanglement preparation, opening more applications in the fields of quantum logic gate and so on.

Magnetic-field-mediated coupling and control in hybrid atomic-nanomechanical systems

Magnetically coupled hybrid quantum systems enable robust quantum state control through Landau-Zener transitions. Here, we show that an ultracold atomic sample coupled to a nanomechanical resonator via oscillating magnetic fields can be used to cool the resonator's mechanical motion, to measure the mechanical temperature, and to enable entanglement of these mesoscopic objects. We calculate the expected coupling for both permanent-magnet and current-conducting nanostring resonators and describe how this hybridization is attainable using recently developed fabrication techniques, including SiN nanostrings and atom chips.

Creation of an Ultracold Gas of Ground-State Dipolar ^{23}Na^{87}Rb Molecules.

We report the successful production of an ultracold sample of absolute ground-state ^{23}Na^{87}Rb molecules. Starting from weakly bound Feshbach molecules formed via magnetoassociation, the lowest rovibrational and hyperfine level of the electronic ground state is populated following a high-efficiency and high-resolution two-photon Raman process. The high-purity absolute ground-state samples have up to 8000 molecules and densities of over 10^{11} cm^{-3}. By measuring the Stark shifts induced by external electric fields, we determined the permanent electric dipole moment of the absolute ground-state ^{23}Na^{87}Rb and demonstrated the capability of inducing an effective dipole moment over 1D. Bimolecular reaction between ground-state ^{23}Na^{87}Rb molecules is endothermic, but we still observed a rather fast decay of the molecular sample. Our results pave the way toward investigation of ultracold molecular collisions in a fully controlled manner and possibly to quantum gases of ultracold bosonic molecules with strong dipolar interactions.

Exotic Chemistry with Ultracold Rydberg Atoms.

We review recent experiments carried out with dense (10(12) cm(-3)) ultracold (T = 40 μK) samples of Cs atoms which have the goal to characterize, by high-resolution spectroscopy, the interactions between Cs atoms, Cs(+) ions and electrons that lead to the formation of metastable long-range molecules. The types of molecules observed in these experiments and the mechanisms leading to the aggregation of atoms in weakly bound molecules are very different from those encountered in warmer samples. In particular, we present results on molecules with binding energies of less than 0.05 J/mol and discuss their properties in the context of a new category of molecular states arising from slow-electron-atom scattering and their relation to atomic and molecular Rydberg states. One of the astonishing aspects of these types of molecules is that they can still be treated in good approximation in the realm of the Born-Oppenheimer approximation despite a huge electronic-state density. Non-Born-Oppenheimer effects are revealed by the decay of the molecules into neutral and charged fragments.

A new quantum gas apparatus for ultracold mixtures of K and Cs and KCs ground-state molecules

We present a new quantum gas apparatus for ultracold mixtures of K and Cs atoms and ultracold samples of KCs ground-state molecules. We demonstrate the apparatus’ capabilities by producing Bose–Einstein condensates of and in a manner that will eventually allow sequential condensation within one experimental cycle, precise sample overlap and magnetic association of atoms into KCs molecules. The condensates are created independently without relying on sympathetic cooling. Our approach is universal and applicable to other species combinations when the two species show dramatically different behavior in terms of loss mechanisms and post laser cooling temperatures, i.e. species combinations that make parallel generation of quantum degenerate samples challenging. We give an outlook over the next experiments involving e.g. sample mixing, molecule formation and transport into a science chamber for high-resolution spatial imaging of novel quantum-many body phases based on K–Cs.