Opportunities for fundamental physics research with radioactive molecules.

Molecules containing short-lived, radioactive nuclei are uniquely positioned to enable a wide range of scientific discoveries in the areas of fundamental symmetries, astrophysics, nuclear structure, and chemistry. Recent advances in the ability to create, cool, and control complex molecules down to the quantum level, along with recent and upcoming advances in radioactive species production at several facilities around the world, create a compelling opportunity to coordinate and combine these efforts to bring precision measurement and control to molecules containing extreme nuclei. In this manuscript, we review the scientific case for studying radioactive molecules, discuss recent atomic, molecular, nuclear, astrophysical, and chemical advances which provide the foundation for their study, describe the facilities where these species are and will be produced, and provide an outlook for the future of this nascent field.

High Density Loading and Collisional Loss of Laser-Cooled Molecules in an Optical Trap.

We report optical trapping of laser-cooled molecules at sufficient density to observe molecule-molecule collisions for the first time in a bulk gas. SrF molecules from a red-detuned magneto-optical trap (MOT) are compressed and cooled in a blue-detuned MOT. Roughly 30% of these molecules are loaded into an optical dipole trap with peak number density n_{0}≈3×10^{10} cm^{-3} and temperature T≈40 µK. We observe two-body loss with rate coefficient ß=2.7_{-0.8}^{+1.2}×10^{-10} cm^{3} s^{-1}. Achieving this density and temperature opens a path to evaporative cooling towards quantum degeneracy of laser-cooled molecules.

Relativistic Exact Two-Component Coupled-Cluster Study of Molecular Sensitivity Factors for Nuclear Schiff Moments.

Relativistic exact two-component coupled-cluster calculations of molecular sensitivity factors for nuclear Schiff moments (NSMs) are reported. We focus on molecules containing heavy nuclei, especially octupole-deformed nuclei. Analytic relativistic coupled-cluster gradient techniques are used and serve as useful tools for identifying candidate molecules that sensitively probe for physics beyond the Standard Model in the hadronic sector. Notably, these tools enable straightforward "black-box" calculations. Two competing chemical mechanisms that contribute to the NSM are analyzed, illuminating the physics of ligand effects on NSM sensitivity factors.

High-sensitivity low-noise photodetector using a large-area silicon photomultiplier.

The application of silicon photomultiplier (SiPM) technology for weak-light detection at a single photon level has expanded thanks to its better photon detection efficiency in comparison to a conventional photomultiplier tube (PMT). SiPMs with large detection area have recently become commercially available, enabling applications where the photon flux is low both temporarily and spatially. On the other hand, several drawbacks exist in the usage of SiPMs such as a higher dark count rate, many readout channels, slow response time, and optical crosstalk; therefore, users need to carefully consider the trade-offs. This work presents a SiPM-embedded compact large-area photon detection module. Various techniques are adopted to overcome the disadvantages of SiPMs so that it can be generally utilized as an upgrade from a PMT. A simple cooling component and recently developed optical crosstalk suppression method are adopted to reduce the noise which is more serious for larger-area SiPMs. A dedicated readout circuit increases the response frequency and reduces the number of readout channels. We favorably compare this design with a conventional PMT and obtain both higher photon detection efficiency and larger-area acceptance.

Frequency-doubled Nd:YAG MOPA laser system with programmable rectangular pulses up to 200 microseconds.

A compact frequency-doubled diode-pumped Nd:YAG master-oscillator power-amplifier laser system with programmable microsecond pulse length has been developed. Analog pulse shaping of the output from a single-frequency continuous-wave Nd:YAG oscillator, and subsequent amplification, allowed the generation of rectangular pulses with pulse lengths on the order of the Nd:YAG fluorescence lifetime. Temporally flat-top pulses of 1064 nm light with 520 mJ pulse energy, 2.6 kW peak power, and 200 µs duration, with linewidth below 10 kHz, were obtained at a repetition rate of 2 Hz. Second harmonic generation in a LBO crystal yielded pulses of 262 mJ and 1.3 kW peak power at 532 nm. The peak power can be maintained within 2.9% over the duration of the laser pulse, and long-term intensity stability of 1.1% was observed. The spatially flat-top beam at 1064 nm used in the amplifier is converted to a Gaussian beam at 532 nm with beam quality factor M2 = 1.41(14) during the second harmonic generation. This system has potential as a pump source for Ti:sapphire, dye, or optical parametric amplifiers to generate tunable high-power single-frequency radiation for applications in precision measurements and laser slowing.

Suppression of the optical crosstalk in a multi-channel silicon photomultiplier array.

We propose and study a method of optical crosstalk suppression for silicon photomultipliers (SiPMs) using optical filters. We demonstrate that attaching absorptive visible bandpass filters to the SiPM can substantially reduce the optical crosstalk. Measurements suggest that the absorption of near infrared light is important to achieve this suppression. The proposed technique can be easily applied to suppress the optical crosstalk in SiPMs in cases where filtering near infrared light is compatible with the application.

Polarization Enhanced Deep Optical Dipole Trapping of Λ-Cooled Polar Molecules.

We demonstrate loading of SrF molecules into an optical dipole trap (ODT) via in-trap Λ-enhanced gray molasses cooling. We find that this cooling can be optimized by a proper choice of relative ODT and cooling beam polarizations. In this optimized configuration, we observe molecules with temperatures as low as 14(1) µK in traps with depths up to 570 µK. With optimized parameters, we transfer ∼5% of molecules from our radio-frequency magneto-optical trap into the ODT, at a density of ∼2×10^{9} cm^{-3}, a phase space density of ∼2×10^{-7}, and with a trap lifetime of ∼1 s.

Demonstration of a Sensitive Method to Measure Nuclear-Spin-Dependent Parity Violation.

Nuclear-spin-dependent parity violation (NSD-PV) effects in atoms and molecules arise from Z^{0} boson exchange between electrons and the nucleus, and from the magnetic interaction between electrons and the parity-violating nuclear anapole moment. We demonstrate measurements of NSD-PV that use an enhancement of the effect in diatomic molecules, here using the test system ^{138}Ba^{19}F. Our sensitivity surpasses that of any previous atomic parity violation measurement. We show that systematic errors can be suppressed to at least the level of the present statistical sensitivity. We measure the matrix element W of the NSD-PV interaction with total uncertainty δW/(2π)<0.7 Hz, for each of two configurations where W must have different signs. This sensitivity would be sufficient to measure NSD-PV effects of the size anticipated across a wide range of nuclei including ^{137}Ba in ^{137}BaF, where |W|/(2π)≈5 Hz is expected.

Improved Radio-Frequency Magneto-Optical Trap of SrF Molecules.

We report the production of ultracold, trapped strontium monofluoride (SrF) molecules with number density and phase-space density significantly higher than previously achieved. These improvements are enabled by three distinct changes to our recently-demonstrated scheme for radio-frequency magneto-optical trapping of SrF: modification of the slowing laser beam geometry, addition of an optical pumping laser, and incorporation of a compression stage to the magneto-optical trap. With these improvements, we observe a trapped sample of SrF molecules at density 2.5×105  cm-3 and phase-space density 6×10-14 , each a factor of 4 greater than in previous work. Under different experimental conditions, we observe trapping of up to 104 molecules, a factor of 5 greater than in previous work. Finally, by reducing the intensity of the applied trapping light, we observe molecular temperatures as low as 250 µK.

Production of RbCs Molecules in the Rovibronic Ground State via Short-Range Photoassociation to the 2 1 Π1 , 2 3 Π1 , and 3 3 Σ1+ States.

We report the production of ultracold 85 Rb133 Cs molecules in their rovibronic ground state X 1 Σ+ (v=0; J=0), by short-range photoassociation (PA) to the 2 1 Π1 , 2 3 Π1 , and 3 3 Σ1+ states. In the PA frequency range from 11650 to 12150 cm-1 (corresponding to energy levels 15500-16000 cm-1 above the bottom of the X potential), we have observed 40 sets of new PA lines. For selected PA states, we investigate vibrational branching, rotational branching, and saturation behavior. Among these 40 new PA lines, the 3 3 Σ1+ (v=3) state has the highest molecule production rate of 2 x 103  molecules s-1 into the rovibronic ground state.

A cryogenic beam of refractory, chemically reactive molecules with expansion cooling.

Cryogenically cooled buffer gas beam sources of the molecule thorium monoxide (ThO) are optimized and characterized. Both helium and neon buffer gas sources are shown to produce ThO beams with high flux, low divergence, low forward velocity, and cold internal temperature for a variety of stagnation densities and nozzle diameters. The beam operates with a buffer gas stagnation density of ∼10(15)-10(16) cm(-3) (Reynolds number ∼1-100), resulting in expansion cooling of the internal temperature of the ThO to as low as 2 K. For the neon (helium) based source, this represents cooling by a factor of about 10 (2) from the initial nozzle temperature of about 20 K (4 K). These sources deliver ∼10(11) ThO molecules in a single quantum state within a 1-3 ms long pulse at 10 Hz repetition rate. Under conditions optimized for a future precision spectroscopy application [A. C. Vutha et al., J. Phys. B: At., Mol. Opt. Phys., 2010, 43, 074007], the neon-based beam has the following characteristics: forward velocity of 170 m s(-1), internal temperature of 3.4 K, and brightness of 3 × 10(11) ground state molecules per steradian per pulse. Compared to typical supersonic sources, the relatively low stagnation density of this source and the fact that the cooling mechanism relies only on collisions with an inert buffer gas make it widely applicable to many atomic and molecular species, including those which are chemically reactive, such as ThO.

Injection-locking of a fiber-pigtailed high-power laser to an external cavity diode laser via a fiber optic circulator.

We report on a simple tunable laser injection-lock scheme for atomic physics experiments. Seed light from an external cavity diode laser is injected into a high-power fiber-pigtailed diode laser via a fiber optic circulator. High-power outputs (up to ∼600 mW) at the injected frequency have been obtained in a single-mode fiber with tuning over a wide wavelength range (∼15 nm). The scheme is simpler and more cost-effective than the traditional scheme of free-space injection-locking.

Probing the frontiers of particle physics with tabletop-scale experiments.

The field of particle physics is in a peculiar state. The standard model of particle theory successfully describes every fundamental particle and force observed in laboratories, yet fails to explain properties of the universe such as the existence of dark matter, the amount of dark energy, and the preponderance of matter over antimatter. Huge experiments, of increasing scale and cost, continue to search for new particles and forces that might explain these phenomena. However, these frontiers also are explored in certain smaller, laboratory-scale "tabletop" experiments. This approach uses precision measurement techniques and devices from atomic, quantum, and condensed-matter physics to detect tiny signals due to new particles or forces. Discoveries in fundamental physics may well come first from small-scale experiments of this type.

An Underappreciated Radiation Hazard from High Voltage Electrodes in Vacuum.

The use of high voltage (HV) electrodes in vacuum is commonplace in physics laboratories. In such systems, it has long been known that electron emission from an HV cathode can lead to bremsstrahlung x rays; indeed, this is the basic principle behind the operation of standard x-ray sources. However, in laboratory setups where x-ray production is not the goal and no electron source is deliberately introduced, field-emitted electrons accelerated by HV can produce x rays as an unintended hazardous byproduct. Both the level of hazard and the safe operating regimes for HV vacuum electrode systems are not widely appreciated, at least in university laboratories. A reinforced awareness of the radiation hazards associated with vacuum HV setups would be beneficial. The authors present a case study of a HV vacuum electrode device operated in a university atomic physics laboratory. They describe the characterization of the observed x-ray radiation, its relation to the observed leakage current in the device, the steps taken to contain and mitigate the radiation hazard, and suggested safety guidelines.

Computer modelling of thorium doping in LiCaAlF(6) and LiSrAlF(6): application to the development of solid state optical frequency devices.

This paper describes computer modelling of thorium doping in crystalline LiCaAlF(6) and LiSrAlF(6). The study has been motivated by the interest in using these materials as hosts for (229)Th nuclei, which are being investigated for use as frequency standards. The dopant sites and form of charge compensation are obtained; this information is essential for the further development and optimization of these devices.

Optical production of ultracold polar molecules.

We demonstrate the production of ultracold polar RbCs molecules in their vibronic ground state, via photoassociation of laser-cooled atoms followed by a laser-stimulated state transfer process. The resulting sample of X1Sigma+ (nu = 0) molecules has a translational temperature of approximately 100 microK and a narrow distribution of rotational states. With the method described here it should be possible to produce samples even colder in all degrees of freedom, as well as other bialkali species.

Production of ultracold, polar RbCs* molecules via photoassociation.

We have produced ultracold, polar RbCs* molecules via photoassociation in a laser-cooled mixture of Rb and Cs atoms. Using a model of the RbCs* molecular interaction which reproduces the observed rovibrational structure, we infer decay rates in our experiments into deeply bound X(1)Sigma(+) ground-state RbCs vibrational levels as high as 5 x 10(5) s(-1) per level. Population in such deeply bound levels could be efficiently transferred to the vibrational ground state using a single stimulated Raman transition, opening the possibility to create large samples of stable, ultracold polar molecules.

Production and state-selective detection of ultracold RbCs molecules.

Using resonance-enhanced two-photon ionization, we detect ultracold, metastable RbCs molecules formed in their lowest triplet state a (3)Sigma(+) via photoassociation in a laser-cooled mixture of 85Rb and 133Cs atoms. We obtain extensive bound-bound excitation spectra of these molecules, which provide detailed information about their vibrational distribution, as well as spectroscopic data on several RbCs molecular states including a (3)Sigma(+), (2) (3)Sigma(+), and (1) (1)Pi. Analysis of this data allows us to predict strong transitions from observed levels to the absolute vibronic ground state of RbCs, potentially allowing the production of stable, ultracold polar molecules at rates in excess of 10(6) s(-1).

New limit on the electron electric dipole moment.

We present the result of our most recent search for T violation in 205Tl, which is interpreted in terms of an electric dipole moment of the electron d(e). We find d(e) = (6.9 +/- 7.4)x10(-28)e cm, which yields an upper limit /d(e)/ < or = 1.6x10(-27)e cm with 90% confidence. The present apparatus is a major upgrade of the atomic beam magnetic-resonance device used to set the previous limit on d(e).