Laser cooling and magneto-optical trapping of molecules analyzed using optical Bloch equations and the Fokker-Planck-Kramers equation J.A. Devlin and M.R. Tarbutt

Phys. Rev. A. 98, 063415 (2018)

Synopsis: We develop a sophisticated model of laser cooling and magneto-optical trapping of molecules and use it to establish the principle mechanisms at work. We find that the balance between Doppler forces and Sisyphus forces play a critical role in determining temperature and other properties, and that transient dark states are important to the Sisyphus mechanism and in determining the photon scattering rate. Most of the predictions of our model are in good quantitative agreement with experimental results.


Contributes to objectives: 2 – we develop an improved understanding of the cooling and trapping mechanisms. 11 – understanding the cooling mechanisms is important for efficient tweezer loading.

Characteristics of unconventional Rb magneto-optical traps K.N Jarvis, B.E Sauer and M.R Tarbutt

Phys. Rev. A 98, 043432 (2018)

Synopsis: We characterise in detail several unconventional magneto-optical trapping (MOT) arrangements. Some use red-detuned light and others blue-detuned light. This work brings useful insights into the range of possibilities for making molecular MOTs.


Contributes to objectives: 9 – Unconventional MOTs may be needed to load molecules into chip traps. 11 – Unconventional MOTs may offer good ways to load molecules into tweezer traps.

Quantum probe spectroscopy for cold atomic systems
A. Usui, B. Buča and J. Mur-Petit 

New J. Phys. 20, 103006 (2018)

In this work we studied a two-level impurity coupled locally to a quantum gas on an optical lattice. For state-dependent interactions  between the impurity and the gas, we show that its evolution encodes information on the local excitation spectrum of the gas at the coupling site. Based on this, we design a nondestructive method to probe the system’s excitations in a broad range of energies by measuring the state of the probe using standard atom optics methods. We illustrate our findings with numerical simulations for quantum lattice systems, including realistic dephasing noise on the quantum probe, and discuss practical limits on the probe dephasing rate to fully resolve both regular and chaotic spectra.

Contributes to objectives:
21, 24 – The proposed protocol could be applied to investigate the many-body phases, and quantify the correlations, in a cold-molecule quantum system.  25 – An understanding of how a contact-interacting probe interacts with a many-body system paves the way to design methods to exploit the long-range character of the dipole-dipole interaction for quantum probing and sensing applications.

Revealing missing charges with generalised quantum fluctuation relations
J Mur-Petit, A. Relano, R.A. Molina, D Jaksch
Nature Communications, 9. 2006 (2018)

Synopsis:  The increasing miniaturisation of electronic chips is enabling a technological revolution, exemplified in a growing number of applications, from powerful mobile phones to the internet of things. But as the components of our electronic devices become smaller and smaller, we need to use the unintuitive laws of quantum physics to understand their behaviour and predict, for instance, how much heat a small quantum machine will generate when driven at maximum power. Things can get event stranger when dealing with quantum systems that feature some additional conserved charges (say, spin or the number of photons). In general, it is very difficult to know how many of these ‘charges’ a quantum system has, which makes predicting their dynamics a difficult task.

The theory team at Oxford, in collaboration with colleagues at Complutense University in Madrid and the Institute for the Structure of Matter of the Spanish National Research Council (CSIC), has developed a set of mathematical laws –known as quantum fluctuation relations– that enable to reveal conserved charges in a quantum system. This will enable to understand how energy or current flow and fluctuate in such small quantum systems, and thus will contribute to a better modelling and improved design of new micro- and nano-meter sized devices where the interplay of thermal and quantum effects is paramount.

We accompany our derivations with computer simulations that highlight the importance of these results to get accurate temperature measurements of strongly-interacting quantum systems, which is a difficult task facing the field of quantum simulations. Because of this, we expect their findings will lead to new insights into long-standing questions on the relaxation and thermalisation of quantum systems.

Contributes to objectives:  22, 24, 25 – This work provides new ways to assess quantum simulation platforms, including cold molecule quantum simulators.

Hyperfine structure of 2Σ molecules containing alkaline-earth atoms
J. Aldegunde and J. M. Hutson
Phys. Rev. A 97 042505 (2018)

Synopsis: (detail to follow)
Contributes to objectives:

Magnetic Trapping and Coherent Control of Laser-Cooled Molecules
H.J. Williams, L. Caldwell, N.J. Fitch, S. Truppe, J. Rodewald, E.A. Hinds, B.E. Sauer, M.R. Tarbutt
Phys. Rev. Lett. 120 163201 (2018)

Synopsis: Quantum science with ultracold molecules requires control over the quantum state of molecules and confinement in a trap that preserves that state. To meet these requirements, we transfer ultracold CaF molecules into a single state, coherently control their rotational, hyperfine and magnetic state, and then trap them magnetically for several seconds.


Contributes to objectives: 10 – We develop the methods for microwave control over the rotational states of molecules. 11 – Quantum state control is needed for preparing molecules in tweezer traps. 14 – Microwave-induced dipoles can be used to entangle molecules and make quantum gates.

Laser Cooled YbF Molecules for Measuring the Electron’s Electric Dipole Moment
J. Lim, J.R. Almond, M.A. Trigatzis, J.A. Devlin, N.J. Fitch, B.E. Sauer, M.R. Tarbutt, E.A. Hinds
Phys. Rev. Lett. 120 123201 (2018)

Synopsis: Ultracold molecules can be used to test fundamental physics. An excellent example is the measurement of the electron’s electric dipole moment, which is an exceptional test of physics beyond the Standard Model of particle physics. Cooling the molecules to low temperature increases the coherence time in these experiments, which makes them more precise. We demonstrate sub-Doppler laser cooling of a beam of YbF molecules to 100 μK. This is a key step towards a measurement of the electron’s electric dipole moment using ultracold molecules.


Contributes to objectives:  2 – We investigate the effectiveness of two different mechanisms of sub-Doppler laser cooling. General – We advance the application of ultracold molecules to tests of fundamental physics

Blue-Detuned Magneto-Optical Trap
K.N. Jarvis, J.A. Devlin, T.E. Wall, B.E. Sauer, M.R. Tarbutt
Phys. Rev. Lett. 120 083201 (2018)

Synopsis: Magneto-optical traps (MOTs) for atoms and molecules with dark states suffer from low phase-space density. This is particularly problematic for cooling and trapping molecules, where dark states are always present. We solve this problem, demonstrating a new MOT that increases the phase-space density a million-fold. Our MOT uses blue detuned light, overturning the conventional wisdom that MOTs have to use red-detuned light.


Contributes to objectives: 9 – The high-phase-space density MOT may be a good way to load molecules into chip traps. 11 – The high-phase-space density MOT may be a good way to load molecules into tweezer traps.

Characteristics of a magneto-optical trap of molecules
H.J. Williams, S. Truppe, M. Hambach, L. Caldwell, N. Fitch, E.A. Hinds, B.E. Sauer, M.R. Tarbutt
New J. Phys. 19 113035 (2017)

Synopsis: Magneto-optical traps (MOTs) for molecules are new, so a thorough characterisation of their properties is called for. This paper presents a detailed investigation of the properties of a MOT of CaF molecules. We first study how molecules load into the MOT from a decelerated molecular beam. Then we measure how the number of molecules, the photon scattering rate, the oscillation frequency, damping constant, temperature, cloud size and lifetime depend on the key parameters of the MOT, especially the intensity and the detuning of the light. By comparing our results to analytical and numerical models we shed light on the mechanisms at work in molecule MOTs. We hope this paper will be a useful resource to all groups wishing to build and characterize their own molecular MOTs.

Contributes to objectives: 2 –  The MOT is the first step in cooling below the Doppler limit. 9 – A chip-based MOT may be a good way to load molecules into microscopic chip traps. 11 – The MOT is the starting point for loading single molecules into tweezer traps.

A buffer gas beam source for short, intense and slow molecular pulses
S. Truppe, M. Hambach, S. Skoff, N. Bulleid, J. Bumby, R.J. Hendricks, E.A. Hinds, B.E. Sauer, M.R. Tarbutt
Journal of Modern Optics, 65:3 246-254 (2018)

Synopsis: Experiments with directly-cooled molecules usually begin with a source. This paper describes a pulsed source of molecules with characteristics that are ideal for many experiments – the pulses have high flux, low speed, low temperature and short duration. We make CaF molecules by laser ablation of a Ca target in the presence of SF6 gas. The molecules are caught in a flow of 4K helium, which cools them and produces a directed beam. We present the design of the source, give practical details about how to build it, and characterize its performance.

Contributes to objectives: 2-  The source makes the CaF, ready for laser cooling

Characterizing Feshbach resonances in ultracold scattering calculations
M.D. Frye, J.M. Hutson
Phys. Rev. A 96, 042705 (2017)

Hyperfine structure of alkali-metal diatomic molecules
J. Aldegunde, J.M. Hutson
Phys. Rev. A 96, 042506 (2017)

Inelastic losses in radio-frequency-dressed traps for ultracold atoms
D.J. Owens, J.M. Hutson
Phys. Rev. A 96, 042707 (2017)

Atomic clock measurements of quantum scattering phase shifts spanning Feshbach resonances at ultralow fields
A. Bennett, K. Gibble, S. Kokkelmans, J.M. Hutson
Phys. Rev. Lett. 119, 113401 (2017)

Molecules cooled below the Doppler limit
S. Truppe, H.J. Williams, M. Hambach, L. Caldwell, N. Fitch, E.A. Hinds, B. Sauer, M.R. Tarbutt
Nature Physics 13 1173-1176 (2017)

Synopsis: The magneto-optical trap (MOT) is the workhorse of experiments with ultracold atoms. A MOT uses a combination of laser beams and a magnetic field to trap and cool atoms. The main cooling mechanism is Doppler cooling, which is effective when the laser light is detuned to the red side of the atomic transition, but cannot cool below a certain temperature limit, known as the Doppler limit.  A molecule MOT, first demonstrated in Yale using SrF molecules, is an ideal starting point for many applications of ultracold molecules. In this paper, we demonstrate the word’s second molecule MOT, using CaF molecules. We capture about 10,000 molecules, cool them to a few millikelvin, and store them for about 100 ms. We then demonstrate that a second cooling mechanism, sub-Doppler cooling, is effective when the light is blue-detuned and the magnetic field is turned off. Using this mechanism, we cool molecules below the Doppler limit for the first time, reaching a temperature of 50 microkelvin.

Contributes to objectives: 2-  We have cooled CaF molecules to sub-Doppler temperatures. 11 – Loading into a tweezer trap requires low temperature and dissipation, which we demonstrate.

ac Stark effect in ultracold polar 87Rb133Cs molecules
P. D. Gregory, J. A. Blackmore, J. Aldegunde, J. M. Hutson, and S. L. Cornish
Phys. Rev. A 96, 021402(R) (2017)

Synopsis: To interrogate ultracold molecules for a long time it is necessary to confine them in a trap. In the case of RbCs molecules where there is no magnetic moment, this trap has to be formed using far-off resonance laser beams in a so-called optical trap. In this paper, we investigate the effect of such trapping light on the internal rotational and hyperfine states of ultracold RbCs molecules. We use high-precision microwave spectroscopy to measure the differential ac Stark shifts between hyperfine states in the ground and first excited rotational levels of the molecule that result from the interaction with the trapping light. We demonstrate through both experiment and theory that coupling between neighbouring hyperfine states manifests in rich structure with many avoided crossings. This coupling may be tuned by rotating the polarization of the linearly polarized trapping light. Using such spectroscopic measurements in combination with direct measurements of the trap frequencies, we completely characterise the trap in both the ground and first excited rotational states. This will enable quantitative studies of molecular collisions and lifetimes in the trap, as well as allowing us to design optical lattice potentials for future experiments.

Contributes to objectives: 16 – Contributes to understanding of microwave spectroscopy in the presence of the trap. 19 – Aids with the design of optical traps and lattices. 22 – Allows us to understand how optical and magnetic fields affect molecules at the level of hyperfine states.

Interspecies thermalization in an ultracold mixture of Cs and Yb in an optical trap
A. Guttridge, S. A. Hopkins, S. L. Kemp, M. D. Frye, J. M. Hutson, and S. L. Cornish
Phys. Rev. A 96, 012704 (2017)

Synopsis: The most successful method for the creation of ultracold molecules from ultracold atoms involves magnetoassociation of the atoms using  an interspecies Feshbach resonance. To be able to predict positions of CsYb Feshbach resonances we require knowledge of the interaction between Cs and Yb atoms. By confining both Cs and Yb in an optical dipole trap we observe sympathetic cooling of Cs by Yb. The rate of this cooling is related to the interspecies thermalisation cross section, which we extract from fitting our results with our kinetic model. Measuring this cross section for mixtures Cs-170Yb and Cs-174Yb we optimise the CsYb interaction potential to reproduce the measurements. This improves our knowledge of the CsYb interaction potential and allows the prediction of scattering length for all isotopologs of CsYb. We also demonstrate the production of Bose-Einstein condensates of Cs and 174Yb an important step towards a quantum-degenerate mixture of the two species.

Contributes to objectives: 1- Allows predictions of Feshbach resonances for formation of CsYb molecules