


Welcome to the Quantum Matter Group!
We are performing experiments to explore many body quantum physics with ultracold atoms and molecules. We are located at the Centre for Quantum Technologies (CQT) hosted by the National University of Singapore.
We have open postdoc and PhD positions.

Make one laser lock get one for free
We report on a transferlock laser frequency stabilization that utilizes a frequency comb (FC) and a radio frequency counter referenced to a GPS frequency standard to compensate for the frequency drifts of two lasers, which are locked to a single passive FabryPerot resonator (FPR). The method requires only one optical phase lock with the FC and allows transfer locking of lasers at wavelengths beyond the usable range of the FC. To attain a large frequency tuning range for the lasers, we implement optical serrodyning. We further demonstrate an efficient scheme to suppress residual amplitude modulation, thereby improving the stability of the PoundDreverHall lock used in this case. The absolute frequency stability was found to be better than 2x1013 on timescales up to 300s. Hence, together with the frequency stability on short timescales provided by the FPR, this scheme facilitates coherent Raman spectroscopy as needed for an example for the production of ultracold dipolar heteronuclear molecules.



ICOLS 2015 Proceedings published
The 22nd International Conference on Laser Spectroscopy (ICOLS) took place in Singapore on June 28  Jul 3.
ICOLS features the latest developments in the area of laser spectroscopy and related topics in atomic, molecular, and optical physics and other disciplines. The talks covered a broad range of exciting physics, such as precision tests of fundamental symmetries with atoms and molecules, atomic clocks, quantum manybody physics with ultracold atoms, atom interferometry, quantum information science with photons and ions, quantum optics, and ultrafast atomic and molecular dynamics.



PhD thesis defense
Congratualtions to Sambit Pal!
Molecular Spectroscopy of Ultracold 6lithium and 40potassium molecules: Towards STIRAP Transfer to Absolute Ground State, Sambit B. Pal, Centre for Quantum Technologies, National University of Singapore, (2016).



PhD thesis defense
Congratualtions to Christian Gross!
Atomic 2S 1/2 to 3P 3/2 Transition for Production and Investigation of a Fermionic Lithium Quantum Gas, Christian Gross, Centre for Quantum Technologies, National University of Singapore, (2016). Thesis.



Ultracold dipolar molecules feature in roadmap on quantum systems
This roadmap bundles fast developing topics in experimental optical quantum sciences,
addressing current challenges as well as potential advances in future research. We have
focused on three main areas: quantum assisted high precision measurements, quantum
information/simulation, and quantum gases. Quantum assisted high precision
measurements are discussed in the first three sections, which review optical clocks, atom
interferometry, and optical magnetometry. These fields are already successfully utilized in
various applied areas. We will discuss approaches to extend this impact even further. In the
quantum information/ simulation section, we start with the traditionally successful
employed systems based on neutral atoms and ions. In addition, the marvelous
demonstrations of systems suitable for quantum information is not progressing, unsolved
challenges remain and will be discussed. We will also review, as an alternative approach,
the utilization of hybrid quantum systems based on superconducting quantum devices and
ultracold atoms. Novel developments in atomtronics promise unique access in exploring
solidstate systems with ultracold gases and are investigated in depth. The sections
discussing the continuously fastdeveloping quantum gases include a review on dipolar
heteronuclear diatomic gases, Rydberg gases, and ultracold plasma. Overall, we have
accomplished a roadmap of selected areas undergoing rapid progress in quantum optics,
highlighting current advances and future challenges. These exciting developments and vast
advances will shape the field of quantum optics in the future.



6Li molecular quantum gas in a large volume transport trap
We report on an efficient production scheme for a large quantum degenerate sample of fermionic lithium. The approach is based on our previous work on narrowline 2S1/2→3P3/2 laser cooling resulting in a high phasespace density of up to 3x104. This allows utilizing a large volume crossed optical dipole trap with a total power of 45 W, leading to high loading efficiency and 8x106 trapped atoms. The same optical trapping configuration is used for rapid adiabatic transport over a distance of 25 cm in 0.9 s, and subsequent evaporative cooling. With optimized evaporation we achieve a degenerate Fermi gas with 1.7x106 atoms at a temperature of 60 nK, corresponding to T/TF=0.16(2). Furthermore, the performance is demonstrated by evaporation near a broad Feshbach resonance creating a molecular BoseEinstein condensate of 3x105 lithium dimers.
Alloptical production and transport of a large 6Li quantum gas in a crossed optical dipole trap , Ch. Gross, H. C. J. Gan, and K. Dieckmann, Phys. Rev. A, 93, 053424 (2016).



ICOLS 2015, Singapore
Thank you for travelling to Singapore and your contributions to a successful conference!



Narrowline cooling of fermionic lithium
We report an experimental study of peak and phasespace density of a twostage magnetooptical trap (MOT) of 6Li atoms, which exploits the narrower 2S1/2→3P3/2 ultraviolet (UV) transition at 323nm following trapping and cooling on the more common D2 transition at 671nm. The UV MOT is loaded from a red MOT and is compressed to give a high phasespace density up to 3x104. Temperatures as low as 33μK are achieved on the UV transition. We study the density limiting factors and in particular find a value for the lightassisted collisional loss coefficient of 1.3(±0.4)x1010cm3/s for low repumping intensity.
Twostage magnetooptical trapping and narrowline cooling of 6Li atoms to high phasespace density , J. Sebastian, Ch. Gross, Ke Li, H. C. J. Gan, Wenhui Li, and K. Dieckmann, Phys. Rev. A, 90, 033417 (2014).



Guest Lecture Series by Prof. J.T.M. Walraven
Prof. Walraven, University of Amsterdam, is visiting our group from Jan until May, 2014. Throughout the semester he will teach this fully creditable module:
PC5239: "Quantum gases collisions and statistics"
This course introduces basic concepts of the physics of ultracold quantum gases  lowdensity gases of neutral atoms studied at (sub)microkelvin temperatures. Quantum gases are important both from the fundamental point of view and for their potential application in quantum information processing. The course is focused on quantum collisions and quantum statistics as these phenomena provide the underpinning for the very existence of the field. A systematic introduction is given into the quantum mechanics of lowenergy collisions and the consequences of the quantum statistical nature of the collision partners for the behavior of the gas. The students will learn to distinguish between varieties of collisional phenomena and understand their consequences both from the kinetic and the thermodynamic point of view.
Module flyer.



ICOLS2015 in Singapore



Streetview of our labs
CQT´s picture wizard Daniel Oi photorgraphed one of our labs with a wide angle lens, stitched the shots together, and made them available in streetview style. Zoom into our lab here.



Megahertz precision wavelength measurement
In an industry collaboration with TEM Messtechnik GmbH, Hannover, Germany, we report on a calibration procedure that enhances the precision of an interferometer based frequency stabilization by several orders of magnitude. For this purpose, the frequency deviations of the stabilization are measured precisely by means of a frequency comb. This allows us to implement several calibration steps that compensate different systematic errors. The resulting frequency deviation is shown to be less than 5.7 MHz (rms 1.6 MHz) in the whole wavelength interval 750795 nm. Wide tuning of a stabilized laser at this exceptional precision is demonstrated.



Magic wavelengths for mass and spinimbalanced
mixtures in 1D optical lattices
In a collaborative theoretical study by researchers from Munich, Innsbruck, Bologna, Lyon, Wyoming, Santa Barbara, and Singapore we present a systematic investigation of attractive binary mixtures in the presence of both spin and massimbalance
in onedimensional setups described by the Hubbard model. After discussing typical cold atomic
experimental realizations and the relation between microscopic and effective parameters, we study several
manybody features of trapped FermiFermi and BoseBose mixtures such as density profiles, momentum
distributions, and correlation functions by means of densitymatrixrenormalizationgroup and quantum Monte
Carlo simulations. In particular, we focus on the stability of FuldeFerrellLarkinOvchinnikov, dimer, and trimer
fluids in inhomogeneous situations, as typically realized in cold gas experiments due to the harmonic confinement.
We finally consider possible experimental signatures of these phases both in the presence of a finite polarization
and of a finite temperature.
Dimer, trimer, and FuldeFerrellLarkinOvchinnikov liquids in mass and spinimbalanced
trapped binary mixtures in one dimension,
M. Dalmonte, K. Dieckmann, T. Roscilde, C. Hartl, A. E. Feiguin, U. Schollwck, and F. HeidrichMeisner,
Phys. Rev. A 85, 063608, (2012).


Image © Daniel Oi. 
Shared frequency comb system started operation
For our experiments with ultracold atoms and molecules we recently installed a commercial frequency comb system that can serve as a frequency reference for multiple application lasers. The system is operating in our lab and sharing the frequency comb with two other laboratories in CQT. We are using a subkiloherz linewidth diode laser stabilized to a highly stable optical reference resonator (design  courtesy by MaxPlanckInstitute for Quantum Optics, Munich, Germany.) in order to improve the short time stability of the comb system.



Narrow sWave Feshbach Resonance
We investigate swave interactions in a twospecies FermiFermi mixture of ^{6}Li and ^{40}K. We develop
for this case the method of crossdimensional relaxation and find from a kinetic model, Monte Carlo
simulations, and measurements that the individual relaxation rates differ due to the mass difference. The
method is applied to measure the elastic cross section at the Feshbach resonance that we previously used
for the production of heteronuclear molecules. Location (B_{0}=154.71(5) G) and width are determined for
this resonance. This reveals that molecules are being produced on the atomic side of the resonance within
a range related to the Fermi energies, therefore establishing the first observation of a many body effect in
the crossover regime of a narrow Feshbach resonance.
sWave Interaction in a TwoSpecies FermiFermi Mixture at a Narrow Feshbach Resonance,
L. Costa, J. Brachmann, A.C. Voigt, C. Hahn, M. Taglieber, T.W. Hänsch, and K. Dieckmann,
Phys. Rev. Lett. 105, 123201, (2010)



Transfer of Experiment from Munich to Singapore
In March 2010 we transfered the experimental setup from Munich to Singapore. The experiment had been developed in the group of Prof. Hänsch at the LudwigMaximiliansUniversity of Munich.



Stable Bosonic Heteronuclear Molecules
We report on the first creation of ultracold bosonic heteronuclear molecules of two fermionic species, ^{6}Li and ^{40}K, by a magnetic field sweep across an interspecies swave Feshbach resonance. This allows us to associate up to 4×10^{4} molecules with high efficiencies of up to 50%. Using direct imaging of the molecules, we measure increased lifetimes of the molecules close to resonance of more than 100ms in the moleculeatom mixture stored in a harmonic trap.
Ultracold Heteronuclear FermiFermi Molecules,
A.C. Voigt, M. Taglieber, L. Costa, T. Aoki, W. Wieser, T. W. Hänsch, and K. Dieckmann,
Phys. Rev. Lett. 102, 020405, (2009)
Science Editors Choice (Science, 323, (2009), p563)
Online article in ProPhysik: Ultrakalte Molekle aus ungleichen FermiAtomen



Quantum Degeneracy in FermiFermi Mixture
We report on the generation of a quantum degenerate FermiFermi mixture of two different atomic species. The quantum degenerate mixture is realized employing sympathetic cooling of fermionic ^{6}Li and ^{40}K gases by an evaporatively cooled bosonic ^{87}Rb gas. We describe the combination of trapping and cooling methods that proved crucial to successfully cool the mixture. In particular, we study the last part of the cooling process and show that the efficiency of sympathetic cooling of the ^{6}Li gas by ^{87}Rb is increased by the presence of ^{40}K through catalytic cooling. Because of the differing physical properties of the two components, the quantum degenerate ^{6}Li^{40}K FermiFermi mixture is an excellent candidate for a stable, heteronuclear system allowing the study of several so far unexplored types of quantum matter.
Quantum Degenerate TwoSpecies FermiFermi Mixture Coexisting with a BoseEinstein Condensate,
M. Taglieber, A.C. Voigt, T. Aoki, T. W. Hänsch, and K. Dieckmann,
Phys. Rev. Lett. 100, 010401, (2008)



Triple MagnetoOptical Trap
We report on the simultaneous trapping of two fermionic species, ^{6}Li and ^{40}K, and a bosonic species, ^{87}Rb
demonstrating the first threespecies magnetooptical trap: "Triple MOT". The apparatus including the atom
sources and the three laser systems is described, and the singlespecies MOTs and the triple MOT are characterized.
In triple MOT operation, typical atom numbers of 3.2×10^{7} for ^{6}Li, 1.5×10^{7} for ^{40}K, and 5.4×10^{9} for ^{87}Rb were achieved. Trap loss due to interspecies collisions was observed.We describe our way to optimize the
triple MOT and turn it into a suitable source for the goal to achieve quantum degeneracy by evaporative and
sympathetic cooling.
Quantum Degenerate TwoSpecies FermiFermi Mixture Coexisting with a BoseEinstein Condensate,
M. Taglieber, A.C. Voigt, F. Henkel, S. Fray, T. W. Hänsch, and K. Dieckmann,
Phys. Rev. A. 73, 011402(R), (2006)



