Considering the hurdles of experiments with more than one atomic species, the temptation arises of rephrasing Arthur L. Schawlow: "Double-species Bose-Einstein condensates are condensates with one species too many". We think otherwise. Quantum mixtures allow the investigation of a wealth of genuinely quantum phenomena: mixed phases of superfluids and Mott insulators, impurities and polarons, chemistry at zero-temperature.

We have experimentally and theoretically studied how the interactions affect the interference pattern of two expanding 87Rb condensates. Our analysis shows that the condensate phase is modified by the mutual interaction only in the region where the wave packets overlap. This result proves that the general assumption of phase rigidity has to be abandoned for an accurate description of matter-wave interference.

A. Burchianti et al.
Effect of interactions in the interference pattern of Bose-Einstein condensates
Phys. Rev. A 102, 043314 (2020)

We show that the Lee-Huang-Yang (LHY) energy functional for a heteronuclear Bose mixture can be accurately approximated by an expression that has the same functional form as in the homonuclear case. It is characterized by two exponents, which can be treated as fitting parameters. We demonstrate that the values of these parameters which preserve the invariance under permutation of the two atomic species are exactly those of the homonuclear case. Deviations from the actual expression of LHY energy functional are discussed quantitatively.

F. Minardi et al.
Effective expression of the Lee-Huang-Yang energy functional for heteronuclear mixtures
Phys. Rev. A 100 063636 (2019)

We report on the formation of heteronuclear quantum droplets in an attractive bosonic mixture of 41K and 87Rb. We observe long-lived self-bound states, both in free space and in an optical waveguide. In the latter case, the dynamics under the effect of a species-dependent force confirms their bound nature. By tuning the interactions from the weakly to the strongly attractive regime, we study the transition from expanding to localized states, in both geometries. We compare the experimental results with numerical simulations and we find a good agreement in the full range of explored interactions.

C. D'Errico et al.
Observation of Quantum Droplets in a Heteronuclear Bosonic Mixture
Phys. Rev. Research 1, 033155 (2019)

We report on the production of a 41K−87Rb dual-species Bose-Einstein condensate in a hybrid trap, consisting of a magnetic quadrupole and an optical dipole potential. After loading both atomic species in the trap, we cool down 87Rb first by magnetic and then by optical evaporation, while 41K is sympathetically cooled by elastic collisions with 87Rb. We eventually produce two-component condensates with more than 105 atoms and tunable species population imbalance. We observe the immiscibility of the quantum mixture by measuring the density profile of each species after releasing them from the trap.

A. Burchianti, et al.
Dual-species Bose-Einstein condensate of 41K and 87Rb in a hybrid trap
Phys. Rev. A 98, 063616 (2018)

We have realized a double-species Bose-Einstein Condensate of 87Rb-41K both in the F=2, mF=2 hyperfine states. The preparation of the superfluid mixtures involves different cooling stages. After a double-MOT phase we transfer the mixture in a magnetic quadrupole field where Rb is evaporated by a microwave radiation resonant on the (F=2, mF=2) - (F=1, mF=1) transition while K is sympathetically cooled by thermal contact with Rb. When the temperature is low enough, we transfer the mixture in a crossed optical trap through an intermediate stage of a hybrid magneto-optical trapping potential. The last stage of cooling is performed by pure optical evaporation in the crossed optical potential. At the end of our typical experimental runs we produce pure condensates of 6×104 atoms for both atomic species.

Laser cooling based on dark states, i.e. states decoupled from light, has proven to be effective to increase the phasespace density of cold trapped atoms. Dark-states cooling requires open atomic transitions, in contrast to the ordinary laser cooling used for example in magneto-optical traps (MOTs), which operate on closed atomic transitions. For alkali atoms, dark-states cooling is therefore commonly operated on the D1 transition nS1/2 → nP1/2. We show that, for 87Rb, thanks to the large hyperfine structure separations the use of this transition is not strictly necessary and that “quasi-dark state” cooling is efficient also on the D2 line, 5S1/2 → 5P1/2. We report temperatures as low as (4.0 ± 0.3) μK and an increase of almost an order of magnitude in the phase space density with respect to ordinary laser sub-Doppler cooling.

S. Rosi, et al.
Λ -enhanced grey molasses on the D2 transition of Rubidium-87 atoms
Sci. Rep. 8, 1301 (2018)

We achieved a 87Rb condensate of 4⨯105 atoms in the F=2, mF=2 state. We use a hybrid trap consisting of a single focused laser beam at 1064nm (dimple) in the horizontal direction and a quadrupole magnetic field. The dimple is vertically shifted with respect to the quadrupole center to avoid Majorana spin-flips. A first evaporation ramp with a microwave driving the (2,2) to (1,1) transition, is followed by an optical evaporation.

In an array of one-dimensional traps, we create impurities of K atoms immersed in reservoir of Rb atoms. The impurities are first localized by an external species-selective potential and then suddenly freed: their subsequent dynamics exhibits "breathing" oscillations, due to a weaker confining potential. We find that the amplitude of these oscillations is reduced when increasing the strength of the impurity-reservoir interaction, irrespective of its sign. We interpret our data with a polaric mass shift model derived following Feynman variational approach.

J. Catani et al.
Quantum dynamics of impurities in a one-dimensional Bose gas
Phys. Rev. A 85, 023623 (2012)

K/Rb people

Luca Cavicchioli
PhD student
Lorenzo Marconi
Postdoctoral fellow
Alessia Burchianti
Scientific staff
Chiara Fort
Scientific staff
Massimo Inguscio
Scientific staff
Francesco Minardi
Scientific staff
Former members:
Giovanni Barontini
Jacopo Catani
Stefano Conclave
Chiara D'Errico
Luigi De Sarlo
Giacomo Lamporesi
Paolo Maioli
Sara Rosi
Devang S. Naik
Gregor Thalhammer

K/Rb contacts

For further information, request of material, job opportunities, please contact:

Francesco Minardi

K/Rb funding

EU Quantum Flagship
Qombs (2018-2021)
  Fondazione Cassa di Risparmio Firenze
SUPERACI (2018-2020)
  Progetto Premiale 2013 ABnanotech