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In many chemical reactions, the transformation from reactants to products is mediated by transient intermediate complexes. For gas-phase reactions involving molecules with a few atoms, these complexes typically live on the order of 10 ps or less before dissociating, and are therefore rarely influenced by external processes. Here, we demonstrate that the transient intermediate complex K2Rb2*, formed from collisions between ultracold KRb molecules, undergoes rapid photo-excitation in the presence of a continuous-wave laser source at 1,064 nm, a wavelength commonly used to confine ultracold molecules. These excitations are facilitated by the exceptionally long lifetime of the complex under ultracold conditions. Indeed, by monitoring the change in the complex population after the sudden removal of the excitation light, we directly measure the lifetime of the complex to be 360 ± 30 ns, in agreement with our calculations based on the Rice–Ramsperger–Kassel–Marcus (RRKM) statistical theory. Our results shed light on the origin of the two-body loss widely observed in ultracold molecule experiments. Additionally, the long complex lifetime, coupled with the observed photo-excitation pathway, opens up the possibility to spectroscopically probe the structure of the complex with high resolution, thus elucidating the reaction dynamics. A transient intermediate complex in a chemical reaction—formed from collisions between molecules with a few atoms—is observed under ultracold conditions. Its lifetime can be directly measured after suppression of the photo-excitation process.

We thank Bo Gao and Paul Julienne for discussion and Robert Moszynski for providing theoretical transition dipole moments of NaCs. This work is supported by the NSF (PHY-1806595), the AFOSR (FA9550-19-1-0089), ARO DURIP (W911NF1810194), and the Arnold and Mabel Beckman foundation. J. T. Z. is supported by a National Defense Science and Engineering Graduate Fellowship. W. B. C. is supported by a Max PlanckHarvard Research Center for Quantum Optics fellowship. K. W. is supported by an NSF GRFP fellowship. J. M. H. is supported by the UK Engineering and Physical Sciences Research Council (EPSRC) Grants No. EP/N007085/1, No. EP/P008275/1, and No. EP/P01058X/1. R. G.-F. acknowledges financial support of the Spanish Project FIS2017-89349-P (MINECO) and the Andalusian research group FQM-207., We report coherent association of atoms into a single weakly bound NaCs molecule in an optical tweezer through an optical Raman transition. The Raman technique uses a deeply bound electronic excited intermediate state to achieve a large transition dipole moment while reducing photon scattering. Starting fromtwo atoms in their relative motional ground state, we achieve an optical transfer efficiency of 69%. The molecules have a binding energy of 770.2 MHz at 8.83(2) G. This technique does not rely on Feshbach resonances or narrow excited-state lines and may allow a wide range of molecular species to be assembled atom by atom., National Science Foundation (NSF) PHY-1806595, United States Department of Defense, Air Force Office of Scientific Research (AFOSR) FA9550-19-1-0089, ARO DURIP W911NF1810194, Arnold and Mabel Beckman foundation, National Defense Science and Engineering Graduate Fellowship, Max PlanckHarvard Research Center for Quantum Optics fellowship, National Science Foundation (NSF), NSF - Office of the Director (OD), UK Research & Innovation (UKRI), Engineering & Physical Sciences Research Council (EPSRC) EP/N007085/1 EP/P008275/1 EP/P01058X/1, MINECO FIS2017-89349-P, Andalusian research group FQM-207

Qubit coherence times are critical to the performance of any robust quantum computing platform. For quantum information processing using arrays of polar molecules, a key performance parameter is the molecular rotational coherence time. We report a 93(7) ms coherence time for rotational state qubits of laser cooled CaF molecules in optical tweezer traps, over an order of magnitude longer than previous systems. Inhomogeneous broadening due to the differential polarizability between the qubit states is suppressed by tuning the tweezer polarization and applied magnetic field to a ``magic'' angle. The coherence time is limited by the residual differential polarizability, implying improvement with further cooling. A single spin-echo pulse is able to extend the coherence time to nearly half a second. The measured coherence times demonstrate the potential of polar molecules as high fidelity qubits.

This themed collection includes a selection of articles on quantum computing and quantum information storage.

Harnessing the potential wide-ranging quantum science applications of molecules will require control of their interactions. Here, we used microwave radiation to directly engineer and tune the interaction potentials between ultracold calcium monofluoride (CaF) molecules. By merging two optical tweezers, each containing a single molecule, we probed collisions in three dimensions. The correct combination of microwave frequency and power created an effective repulsive shield, which suppressed the inelastic loss rate by a factor of six, in agreement with theoretical calculations. The demonstrated microwave shielding shows a general route to the creation of long-lived, dense samples of ultracold polar molecules and evaporative cooling.

Chemical reactions represent a class of quantum problems that challenge both the current theoretical understanding and computational capabilities. Reactions that occur at ultralow temperatures provide an ideal testing ground for quantum chemistry and scattering theories, as they can be experimentally studied with unprecedented control, yet display dynamics that are highly complex. Here, we report the full product state distribution for the reaction 2KRb $\rightarrow$ K$_2$ + Rb$_2$. Ultracold preparation of the reactants grants complete control over their initial quantum degrees of freedom, while state-resolved, coincident detection of both products enables the measurement of scattering probabilities into all 57 allowed rotational state-pairs. Our results show an overall agreement with a state-counting model based on statistical theory, but also reveal several deviating state-pairs. In particular, we observe a strong suppression of population in the state-pair closest to the exoergicity limit, which we precisely determine to be $9.7711^{+0.0007}_{-0.0005}$ cm$^{-1}$, as a result of the long-range potential inhibiting the escape of products. The completeness of our measurements provides a valuable benchmark for quantum dynamics calculations beyond the current state-of-the-art., Comment: 31 pages, 6 figures

Based on a theoretical model where the nuclear spins remain unchanged during a collision, we provide an analytical and general expression for the nuclear spin state-to-state distribution of an ultracold chemical reaction in a magnetic field, for given rotational transitions of the molecules. It simply requires knowledge of the field-dependent eigenfunctions of the molecular reactants and products of the chemical reaction. The final state-to-state distribution drastically changes with the magnetic field. When the distribution is summed over all the final products, a simplified expression is found where only the knowledge of the eigenfunctions of the molecular reactants is required. The present theoretical formalism has been successfully used to explain the magnetic field behavior of the product-state distribution in chemical reactions of ultracold KRb molecules [Hu et al., Nat. Chem. 13, 435 (2021)].

Fully internal and motional state controlled and individually manipulable polar molecules are desirable for many quantum science applications leveraging the rich state space and intrinsic interactions of molecules. While prior efforts at assembling molecules from their constituent atoms individually trapped in optical tweezers achieved such a goal for exactly one molecule (Zhang J T et al 2020 Phys. Rev. Lett. 124 253401; Cairncross W B et al 2021 Phys. Rev. Lett. 126 123402; He X et al 2020 Science 370 331–5), here we extend the technique to an array of five molecules, unlocking the ability to study molecular interactions. We detail the technical challenges and solutions inherent in scaling this system up. With parallel preparation and control of multiple molecules in hand, this platform now serves as a starting point to harness the vast resources and long-range dipolar interactions of molecules.

Advances in atomic, molecular, and optical (AMO) physics techniques allowed the cooling of simple molecules down to the ultracold regime ($\lesssim$ 1 mK), and opened the opportunities to study chemical reactions with unprecedented levels of control. This review covers recent developments in studying bimolecular chemistry at ultralow temperatures. We begin with a brief overview of methods for producing, manipulating, and detecting ultracold molecules. We then survey experimental works that exploit the controllability of ultracold molecules to probe and modify their long-range interactions. Further combining the use of physical chemistry techniques, such as mass spectrometry and ion imaging, significantly improved the detection of ultracold reactions and enabled explorations of their dynamics in the short-range. We discuss a series of studies on the reaction KRb + KRb $\rightarrow$ K$_2$ + Rb$_2$ initiated below 1 $\mu$K, including the direct observation of a long-lived complex, the demonstration of product rotational state control via conserved nuclear spins, and a test of the statistical model using the complete quantum state distribution of the products., Comment: 25 pages, 9 figures, submitted to Annual Review of Physical Chemistry on 06/15/2021

No comment; International audience; Quantum-state control of reactive systems has enabled microscopic probes of underlying interaction potentials and the alteration of reaction rates using quantum statistics. However, extending such control to the quantum states of reaction outcomes remains challenging. Here, we realize this goal by utilizing the conservation of nuclear spins throughout the reaction. Using resonance-enhanced multiphoton ionization spectroscopy to investigate the products formed in bimolecular reactions between ultracold KRb molecules we find that the system retains a near-perfect memory of the reactants’ nuclear spins, manifested as a strong parity preference for the rotational states of the products. We leverage this effect to alter the occupation of these product states by changing the coherent superposition of initial nuclear spin states with an external magnetic field. In this way, we are able to control both the inputs and outputs of a reaction with quantum-state resolution. The techniques demonstrated here open up the possibilities to study quantum entanglement between reaction products and ultracold reaction dynamics at the state-to-state level.

Tweezing cold molecules Arrays of optical tweezers have been used to trap atoms, but trapping and laser-cooling molecules in this setting is tricky. Such an approach would, however, be generalizable to many molecular species. Anderegg et al. created an optical tweezer array of calcium monofluoride molecules, which were laser cooled to their ground state (see the Perspective by Kotochigova). By distinguishing between single and multiple molecules in the tweezers, the researchers were able to observe molecular collisions. Boasting exquisite control over individual molecules, the optical tweezer array platform holds much promise for extending the applications of ultracold molecules. Science , this issue p. 1156 ; see also p. 1079

We demonstrate the formation of a single NaCs molecule in an optical tweezer by magnetoassociation through an s-wave Feshbach resonance at 864.11(5) G. Starting from single atoms cooled to their motional ground states, we achieve conversion efficiencies of 47(1)%, and measure a molecular lifetime of 4.7(7) ms. By construction, the single molecules are predominantly [77(5)%] in the center-of-mass motional ground state of the tweezer. Furthermore, we produce a single p-wave molecule near 807 G by first preparing one of the atoms with one quantum of motional excitation. Our creation of a single weakly bound molecule in a designated internal state in the motional ground state of an optical tweezer is a crucial step towards coherent control of single molecules in optical tweezer arrays.

We measure inelastic collisions between ultracold CaF molecules by combining two optical tweezers, each containing a single molecule. We observe collisions between $^2\Sigma$ CaF molecules in the absolute ground state $|X,v=0, N=0,F=0\rangle$, and in excited hyperfine and rotational states. In the absolute ground state, we find a two-body loss rate of $7(4) \times 10^{-11} \text{cm}^{3}/\text{s}$, which is below, but close to the predicted universal loss rate., Comment: 5 pages, 4 figures

We reduce the intensity noise of laser light by using an electro-optic modulator and a cousto-optic modulator in series. The electro-optic modulator reduces noise at high frequency(10 kHz to 1 MHz), while the acousto-optic modulator sets the average power of the light and reduces noise at low frequency (up to 10 kHz). The light is then used to trap single sodium atoms in an optical tweezer, where the lifetime of the atoms is limited by parametric heating due to laser noise at twice the trapping frequency. With our noise eater, the noise is reduced by up to 15 dB at these frequencies, and the lifetime is increased by an order of magnitude to around 6 seconds. Our technique is general and acts directly on the laser beam, expanding laser options for sensitive optical trapping applications., Comment: 7 pages, 4 figures, submitted to Optics Express

Proposed molecular quantum gate takes advantage of internal coherence and resonant electric dipolar interaction with high fidelity and optical scalability., We propose a two-qubit gate based on dipolar exchange interactions between individually addressable ultracold polar molecules in an array of optical dipole traps. Our proposal treats the full Hamiltonian of the 1Σ+ molecule NaCs, utilizing a pair of nuclear spin states as storage qubits. A third rotationally excited state with rotation-hyperfine coupling enables switchable electric dipolar exchange interactions between two molecules to generate an iSWAP gate. All three states are insensitive to external magnetic and electric fields. Impacts on gate fidelity due to coupling to other molecular states, imperfect ground-state cooling, blackbody radiation and vacuum spontaneous emission are small, leading to potential fidelity above 99.99% in a coherent quantum system that can be scaled by purely optical means.

We demonstrate full quantum state control of two species of single atoms using optical tweezers and assemble the atoms into a molecule. Our demonstration includes 3D ground-state cooling of a single atom (Cs) in an optical tweezer, transport by several microns with minimal heating, and merging with a single Na atom. Subsequently, both atoms occupy the simultaneous motional ground state with 61(4)\% probability. This realizes a sample of exactly two co-trapped atoms near the phase-space-density limit of one, and allows for efficient stimulated-Raman transfer of a pair of atoms into a molecular bound state of the triplet electronic ground potential $a^3\Sigma^+$. The results are key steps toward coherent creation of single ultracold molecules, for future exploration of quantum simulation and quantum information processing.

Quantum simulators are a promising technology on the spectrum of quantum devices from specialized quantum experiments to universal quantum computers. These quantum devices utilize entanglement and many-particle behaviors to explore and solve hard scientific, engineering, and computational problems. Rapid development over the last two decades has produced more than 300 quantum simulators in operation worldwide using a wide variety of experimental platforms. Recent advances in several physical architectures promise a golden age of quantum simulators ranging from highly optimized special purpose simulators to flexible programmable devices. These developments have enabled a convergence of ideas drawn from fundamental physics, computer science, and device engineering. They have strong potential to address problems of societal importance, ranging from understanding vital chemical processes, to enabling the design of new materials with enhanced performance, to solving complex computational problems. It is the position of the community, as represented by participants of the NSF workshop on "Programmable Quantum Simulators," that investment in a national quantum simulator program is a high priority in order to accelerate the progress in this field and to result in the first practical applications of quantum machines. Such a program should address two areas of emphasis: (1) support for creating quantum simulator prototypes usable by the broader scientific community, complementary to the present universal quantum computer effort in industry; and (2) support for fundamental research carried out by a blend of multi-investigator, multi-disciplinary collaborations with resources for quantum simulator software, hardware, and education., Comment: 41 pages -- references and acknowledgments added in v2

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 [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., Comment: 10 pages, 9 figures

The multichannel Na-Cs interactions are characterized by a series of measurements using two atoms in an optical tweezer, along with a multichannel quantum defect theory (MQDT). The triplet and singlet scattering lengths are measured by performing Raman spectroscopy of the Na-Cs motional states and least-bound molecular state in the tweezer. Magnetic Feshbach resonances are observed for only two atoms at fields which agree well with the MQDT. Our methodology, which promotes the idea of an effective theory of interaction, can be a key step towards the understanding and the description of more complex interactions. The tweezer-based measurements in particular will be an important tool for atom-molecule and molecule-molecule interactions, where high densities are experimentally challenging and where the interactions can be dominated by intra-species processes.

Femtochemistry techniques have been instrumental in accessing the short time scales necessary to probe transient intermediates in chemical reactions. Here we take the contrasting approach of prolonging the lifetime of an intermediate by preparing reactant molecules in their lowest ro-vibronic quantum state at ultralow temperatures, thereby drastically reducing the number of exit channels accessible upon their mutual collision. Using ionization spectroscopy and velocity-map imaging of a trapped gas of potassium-rubidium molecules at a temperature of 500~nK, we directly observe reactants, intermediates, and products of the reaction $^{40}$K$^{87}$Rb + $^{40}$K$^{87}$Rb $\rightarrow$ K$_2$Rb$^*_2$ $\rightarrow$ K$_2$ + Rb$_2$. Beyond observation of a long-lived energy-rich intermediate complex, this technique opens the door to further studies of quantum-state resolved reaction dynamics in the ultracold regime., Comment: 4 figures in main, 4 figures in SM, 2 table in SM

Lighting the way to molecules, one by one When chemists run reactions, what they are really doing is mixing up an enormous number of reacting partners and then hoping that they collide productively. It is possible to manipulate atoms more deliberately with a scanning tunneling microscope tip, but the process is then confined to a surface. Liu et al. directly manipulated individual atoms with light to form single molecules in isolation (see the Perspective by Narevicius). They used optical tweezers of two different colors to selectively steer ultracold sodium (Na) and cesium (Cs) atoms together. A subsequent optical excitation formed NaCs. Science , this issue p. 900 ; see also p. 855

While Bose–Einstein condensates of atoms were achieved in the mid-1990s, extending the regime of quantum degeneracy to polar molecules took another two decades of dedicated work. The researchers that contributed to this achievement span many generations of students in many different laboratories around the world.

Microscopically controlled neutral atoms in optical tweezers and lattices have led to exciting advances in the study of quantum information and quantum many-body systems. The light shifts of atomic levels from the trapping potential in these systems can result in detrimental effects such as fluctuating dipole force heating, inhomogeneous detunings, and inhibition of laser cooling, which limits the atomic species that can be manipulated. In particular, these light shifts can be large enough to prevent loading into optical tweezers directly from a magneto-optical trap. We implement a general solution to these limitations by loading, as well as cooling and imaging the atoms with temporally alternating beams, and present an analysis of the role of heating and required cooling for single atom tweezer loading. Because this technique does not depend on any specific spectral properties, it should enable the optical tweezer platform to be extended to nearly any atomic or molecular species that can be laser cooled and optically trapped.

We report Raman sideband cooling of a single sodium atom to its three-dimensional motional ground state in an optical tweezer. Despite a large Lamb-Dicke parameter, high initial temperature, and large differential light shifts between the excited state and the ground state, we achieve a ground state population of $93.5(7)$% after $53$ ms of cooling. Our technique includes addressing high-order sidebands, where several motional quanta are removed by a single laser pulse, and fast modulation of the optical tweezer intensity. We demonstrate that Raman sideband cooling to the 3D motional ground state is possible, even without tight confinement and low initial temperature., Comment: 5 pages, 4 figures

Deborah Jin, a pioneer in modern atomic and molecular physics, passed away on September 15, 2016, at age 47, after a courageous fight with cancer. She was a world renowned scientist at JILA (formerly known as the Joint Institute for Laboratory Astrophysics) in Boulder, Colorado, a joint institute of the National Institute for Standards and Technology (NIST) and the University of Colorado. Her research field was the world of atoms and molecules and their quantum mechanical behavior at temperatures close to absolute zero. Although she was a pioneer in her field, Jin was also humble and modest. She was a role model to many physicists worldwide through her enthusiastic and methodical approach to science.

We use (1+1$'$) resonance-enhanced multiphoton photodissociation (REMPD) to detect the population in individual rovibronic states of trapped HfF$^+$ with a single-shot absolute efficiency of 18%, which is over 200 times better than that obtained with fluorescence detection. The first photon excites a specific rotational level to an intermediate vibronic band at 35,000-36,500 cm$^{-1}$, and the second photon, at 37,594 cm$^{-1}$ (266 nm), dissociates HfF$^+$ into Hf$^+$ and F. Mass-resolved time-of-flight ion detection then yields the number of state-selectively dissociated ions. Using this method, we observe rotational-state heating of trapped HfF$^+$ ions from collisions with neutral Ar atoms. Furthermore, we measure the lifetime of the $^3\Delta_1$ $v=0,\, J=1$ state to be 2.1(2) s. This state will be used for a search for a permanent electric dipole moment of the electron., Comment: 4 pages, 5 figures

We report on the design and characterization of a low-temperature external cavity diode laser (ECDL) system for broad wavelength tuning. The performance achieved with multiple diode models addresses the scarcity of commercial red laser diodes below 633 nm, which is a wavelength range relevant to spectroscopy of many molecules and ions. Using a combination of multiple-stage thermoelectric cooling and water cooling, the operating temperature of a laser diode is lowered to -64{\deg}C, more than 85{\deg}C below the ambient temperature. The laser system integrates temperature and diffraction grating feedback tunability for coarse and fine wavelength adjustments, respectively. For two different diode models, single-mode operation was achieved with 38 mW output power at 616.8 nm and 69 mW at 622.6 nm, more than 15 nm below their ambient temperature free-running wavelengths. The ECDL design can be used for diodes of any available wavelength, allowing individual diodes to be tuned continuously over tens of nanometers and extending the wavelength coverage of commercial laser diodes., Comment: 6 pages, 6 figures

In optical dipole traps, the excited rotational states of a molecule may experience a very different light shift than the ground state. For particles with two polarizability components (parallel and perpendicular), such as linear $^1\Sigma$ molecules, the differential shift can be nulled by choice of elliptical polarization. When one component of the polarization vector is $\pm i\sqrt{2}$ times the orthogonal component, the light shift for a sublevel of excited rotational states approaches that of the ground state at high optical intensity. In this case, fluctuating trap intensity need not limit coherence between ground and excited rotational states.

A rich internal structure and long-range interactions between them make molecules with non-vanishing dipole moments interesting for many applications. An experiment demonstrating the efficient transfer of loosely bound heteronuclear molecules into more deeply bound energy levels indicates a route towards producing dense ensembles of cold polar molecules.

We have fabricated submicron giant magnetoresistive (GMR) structures and evaluated their sensitivity for biomagnetic applications. GMR devices were fabricated using electron beam lithography, with minimum dimensions below 100 nm. We developed a new characterization technique for these sensors, using a scanned nanoscale magnetic probe and monitoring the resulting response of the sensors. The magnetic field from the scanned probe is similar to that generated by the magnetic particles used to tag bioanalytes. The devices demonstrated extremely high magnetic field resolution. Noise measurements, combined with a local field sensitivity from the scanned probe measurements, predict a sensitivity of 2 × 1 0 − 16 emu/Hz 1/2 for a magnetic particle 100 nm above the sensor surface. This corresponds to detection of single 100 nm commercially available magnetic labels, which are the lowest size scale of labels now used in biological studies, with a signal-to-noise of unity. Additionally, we predict detection of single 200 nm magnetic labels with a position sensitivity of 93 nm/Hz 1/2 , allowing proximity detection for particles not directly bound to the sensor surface, with a corresponding signal-to-noise of 10.

Polar molecules are desirable systems for quantum simulations and cold chemistry. Molecular ions are easily trapped, but a bias electric field applied to polarize them tends to accelerate them out of the trap. We present a general solution to this issue by rotating the bias field slowly enough for the molecular polarization axis to follow but rapidly enough for the ions to stay trapped. We demonstrate Ramsey spectroscopy between Stark-Zeeman sublevels in 180Hf19F+ with a coherence time of 100 ms. Frequency shifts arising from well-controlled topological (Berry) phases are used to determine magnetic g-factors. The rotating-bias-field technique may enable using trapped polar molecules for precision measurement and quantum information science, including the search for an electron electric dipole moment., Comment: Accepted to Science

Rapid advances are being made toward optically cooling a single mode of a micro-mechanical system to its quantum ground state and observing quantum behavior at macroscopic scales. Reaching this regime in room-temperature environments requires a stringent condition on the mechanical quality factor $Q_m$ and frequency $f_m$, $Q_{m}f_{m}{\gtrsim}k_{B}T_{{bath}}/h$, which so far has been marginally satisfied only in a small number of systems. Here we propose and analyze a new class of systems that should enable unprecedented $Q_{m}f_m$ values. The technique is based upon using optical forces to "trap" and stiffen the motion of a tethered mechanical structure, thereby freeing the resultant mechanical frequencies and decoherence rates from underlying material properties., Comment: 23 pages, 5 figures

Extraneous thermal motion can limit displacement sensitivity and radiation pressure effects, such as optical cooling, in a cavity-optomechanical system. Here we present an active noise suppression scheme and its experimental implementation. The main challenge is to selectively sense and suppress extraneous thermal noise without affecting motion of the oscillator. Our solution is to monitor two modes of the optical cavity, each with different sensitivity to the oscillator's motion but similar sensitivity to the extraneous thermal motion. This information is used to imprint "anti-noise" onto the frequency of the incident laser field. In our system, based on a nano-mechanical membrane coupled to a Fabry-P\'{e}rot cavity, simulation and experiment demonstrate that extraneous thermal noise can be selectively suppressed and that the associated limit on optical cooling can be reduced., Comment: 27 pages, 14 figures

Ultracold polar molecular quantum gases promise to open new research directions ranging from the study of ultra-cold chemistry, precision measurements to novel quantum phase transitions. Based on the preparation of high-phase space density gases of polar KRb molecules [1–3], I will discuss the control of dipolar collisions and chemical reactions of polar molecules in a regime where quantum statistics, single scattering partial waves, and quantum threshold laws play a dominant role [4]. In particular, I will point out the crucial role of electric dipole-dipole interactions [5] and external confinement [6] in determining the chemical reaction rate. Finally, I will discuss prospects of reaching quantum degeneracy in bialkali samples of polar molecules and prospects for these systems as novel dipolar quantum many-body systems.

Colliding in the Cold Chemical reactions occur through molecular collisions, which, in turn, are governed by the distributions of energy in each colliding partner. What happens when molecules are cooled so that they no longer have sufficient energy to collide? Ospelkaus et al. (p. 853 ; see the Perspective by Hutson ) explored this question by preparing a laser-cooled sample of potassium rubidium (KRb) diatomics with barely any residual energy in any form (translational, rotational, vibrational, or electronic). By monitoring heat release over time, evidence was gathered for exothermic atom exchange reactivity through quantum mechanical tunneling. As predicted by theory, these reactions were exquisitely sensitive to the molecular states, with rates changing by orders of magnitude on varying minor factors such as nuclear spin orientation.

Ultracold polar molecules offer the possibility of exploring quantum gases with interparticle interactions that are strong, long-range and spatially anisotropic. This is in stark contrast to the much studied dilute gases of ultracold atoms, which have isotropic and extremely short-range (or 'contact') interactions. Furthermore, the large electric dipole moment of polar molecules can be tuned using an external electric field; this has a range of applications such as the control of ultracold chemical reactions, the design of a platform for quantum information processing and the realization of novel quantum many-body systems. Despite intense experimental efforts aimed at observing the influence of dipoles on ultracold molecules, only recently have sufficiently high densities been achieved. Here we report the experimental observation of dipolar collisions in an ultracold molecular gas prepared close to quantum degeneracy. For modest values of an applied electric field, we observe a pronounced increase in the loss rate of fermionic potassium-rubidium molecules due to ultracold chemical reactions. We find that the loss rate has a steep power-law dependence on the induced electric dipole moment, and we show that this dependence can be understood in a relatively simple model based on quantum threshold laws for the scattering of fermionic polar molecules. In addition, we directly observe the spatial anisotropy of the dipolar interaction through measurements of the thermodynamics of the dipolar gas. These results demonstrate how the long-range dipolar interaction can be used for electric-field control of chemical reaction rates in an ultracold gas of polar molecules. Furthermore, the large loss rates in an applied electric field suggest that creating a long-lived ensemble of ultracold polar molecules may require confinement in a two-dimensional trap geometry to suppress the influence of the attractive, 'head-to-tail', dipolar interactions.