Latest research highlights:
SA IV — Quantum information and entanglement click titles to expand/contract each highlight
Direct measurement of the van der Waals interaction between two Rydberg atoms  June 2013 
Palaiseau, France 
The van der Waals interaction that exists between all polarizable particles underlies many important effects in physics, chemistry and biology. To date, direct measurements of this interaction have only been performed either between macroscopic bodies, or between an atom and a macroscopic body. Such approaches have the drawback of being only indirectly linked to the underlying interatomic interaction.
We have directly measured this interaction U_{vdW} = C_6/R_6 between two Rydberg atoms, excited from a pair of single atoms trapped in two microscopic optical tweezers separated by a controlled distance R (Fig. A below). For that, we use collective coherent dynamics in the atom pair as a probe of the interaction. Indeed, in a regime of partial blockade (i.e. when the van der Waals interaction U_{vdW} and the Rabi coupling are comparable), the dynamics of the population of the doubly excited state is very sensitive to U_{vdW}. Our results (Fig. B below) are in very good agreement with theoretical calculations and show a very fast increase of the C_6 coefficient with the principal quantum number n. This allows us to observe the effect of interactions between two single atoms separated by as much as 20 µm!
Our results show that an unprecedented experimental control over a system of a few Rydberg atoms can be reached. This is a prerequisite in view of the application of such systems for quantum information processing and quantum simulation of longrange interacting systems.
 Reference: L. Béguin, A. Vernier, R. Chicireanu, T. Lahaye, and A. Browaeys, Direct measurement of the van der Waals interaction between two Rydberg atoms, Phys. Rev. Lett. 110, 263201 (2013) This article was also highlighted as a Physics Viewpoint, M. Weidemüller, Atomic Interactions at a Distance, or see our full list of publications

Trapping atoms on a magnetic lattice atom chip  May 2013 
Amsterdam, The Netherlands 
Cold rubidium atoms were trapped on a new magneticfilm atom chip. The pattern of microtraps consists of two lattices, of hexagonal and square geometries respectively, joining in an interface. With a lattice spacing of 10 µm this is an excellent starting point to investigate Rydberg dipole blockade in and between these microtraps.


SubPoissonian statistics of interacting darkstate polaritons  May 2013 
Heidelberg, Germany 
Electromagneticallyinduced transparency (EIT) and the associated appearance of hybrid quasiparticles (darkstate polaritons) in ultracold Rydberg gases opens the intriguing perspective to create atomlight interfaces operating at the quantum level. For the first time we give a complete picture of interacting dark state polaritons by probing both the photonic and atomic degrees of freedom in a single experiment. This field has recently become a hot topic with several groups worldwide recently showing Rydberg EIT can be used to generate nonclassical photon correlations. Using our complementary approach, we show that interactions between darkstate polaritons also result in nonclassical statistics for the polariton number distribution.
 Reference: C. S. Hofmann, G. Günter, H. Schempp, M. RobertdeSaintVincent, M. Gärttner, J. Evers, S. Whitlock and M. Weidemüller, SubPoissonian statistics of Rydberginteracting darkstate polaritons, Phys. Rev. Lett. 110, 203601 (2013), or see our full list of publications

Measurement of 87Rb Rydbergstate hyperfine splitting in a roomtemperature vapor cell  April 2013 
Amsterdam, The Netherlands 
Hyperfine splittings of Rydberg sstates in 87Rb were measured using electromagnetically induced transparency (EIT) spectroscopy in a roomtemperature vapor cell. In spite of Doppler broadening, an accuracy of 100 kHz was achieved. The top figure shows the splitting in a Stark map measurement for the 20s state. The bottom figure shows the differential hyperfine splitting (without the common Stark shift) of the F=2 state.
 Reference: A. Tauschinsky, R. Newell, H. B van Linden van den Heuvell, R. J. C Spreeuw, Measurement of 87Rb Rydbergstate hyperfine splitting in a roomtemperature vapor cell, Phys. Rev. A, 87, 042522, or see our full list of publications

Adiabatic passage for quantum gates in mesoscopic ensembles  December 2012 
Open University, UK in collaboration with Wisconsin, USA 
We demonstrated a protocol for adiabatic passage that allows for geometric phase compensation when applied to atomic ensembles of unknown number of atoms. This opens the way to high fidelity logic operation with mesoqubits.
 Reference: I. Beterov, M. Saffman, E. A. Yakshina, D. B. Tretyakov, V. M. Entin,I. I. Ryabtsev, C. W. Mansell, C. MacCormick, S. Bergamini, and V. P. Zhukov, Adiabatic passage for quantum gates in mesoscopic atomic ensembles, Preprint arXiv:1212.1138, or see our full list of publications

Laser cooling of Holmium  December 2012 
Wisconsin, USA 
In recent years laser cooling techniques have been extended to many different atomic species. During the first year of the Coherence project the Saffman research group at University of Wisconsin Madison has demonstrated laser cooling of the rare earth element Holmium (Ho) for the first time. Among all the known elements Ho has the distinction of having the largest number of stable ground states. We are working to use the 128 ground states of Ho to encode a quantum register. The cold sample of Ho atoms shown in the Figure is an important step towards this goal. A publication is in preparation


Generation of entanglement via laser driven Rydberg atoms  December 2012 
Aarhus, Denmark 
An atom excited by a laser field decays back to its ground state in a few tens of nanoseconds. The Aarhus COHERENCE team has shown that as a consequence of the short lifetimes of their optically excited states, pairs of atoms with strong Rydberg excited state interactions will quickly decay into “dark states” which do not absorb laser light and which are strongly entangled superposition states. Since the rapidly decaying state is coupled resonantly we achieve 95% overlap with the entangled state within few microseconds. The state preparation relies on the decay of the unwanted state components, and the dark state protection does not crucially depend on the strengths of the Rydberg interaction.
Figure: Population of maximally entangled state, cf., solution of the master equation for two atoms with ground states 0>,1>, optically excited state p> and Rydberg excited state r>). The transition 1> p>r> is strongly driven, off the intermediate resonance. of Insert shows that even when the blockade interaction Δ is much smaller than the Rabi frequency Ω, one can still obtain the entangled state with above 90% fidelity.


Deterministic quantum computation with Rydberg interactions  October 2012 
Open University, UK 
Deterministic quantum computation with one quantum bit (DQC1) is a model of quantum computation with an exponential speedup compared to known classical algorithms. Rather than entanglement, quantum discord is thought to be responsible for this speedup. We have identified a protocol to experimentally implement the DQC1 algorithm and quantify a geometric measure of quantum discord with ensembles of ultracold atoms exploiting Rydberg interactions. This system provides large Hilbert space for realistic numbers of atoms in the ensemble. This will allow to experimentally investigate the physics and the computational power of quantum discord for a specific algorithm and extend the validity of the protocol to high dimension Hilbert spaces.


Interacting Fibonacci anyons in a Rydberg gas  October 2012 
Nottingham, UK 
Fibonacci anyons are particles which neither obey fermionic nor bosonic exchange statistics. These nonAbelian anyons are conjectured to occur as quasiparticles in certain fractional quantum Hall states. In this paper we have shown that a Rydberg lattice gas constitutes an analogue quantum simulator for Fibonacci anyons. The underlying insight is that the Rydberg blockade and the resulting exclusion principle direct links to the socalled fusion paths that determine the nature of an ensemble of Fibonacci anyons. By this one can show that the Hilbert space structure of a Rydberg lattice gas is equivalent to that of an ensemble of Fibonacci anyons. Interactions between the anyons can be straightforwardly implemented and anyonic observables can be measured. This demonstrates that a Rydberg gas is a wellsuited platform for the study of exotic quantum states of matter such as topological quantum liquids.
Left: Fibonacci anyons occur in two types, 1 and τ. Each of these types can be related to the internal state of atom. The figure shows a socalled fusion path and the corresponding representation by atoms. Along this path two τ's must not appear next to each other which is ensured by the Rydberg blockade, i.e., the fact that consecutive atoms cannot be excited simultaneously to the Rydberg state.
 Reference: I. Lesanovsky and H. Katsura, Interacting Fibonacci anyons in a Rydberg gas, Phys. Rev. A 86, 041601 (2012), or see our full list of publications

Excitation of single Rydberg atoms in a new apparatus  October 2012 
Palaiseau, France 
We have built a new experimental setup (below, left) dedicated to the trapping and Rydberg excitation of single atoms in an array of micronsized optical tweezers. Our ability to cancel stray electric fields using eight independent electrodes around the traps has allowed us to obtain highquality Rabi oscillations between the ground and Rydberg states of a single atom (below, right).
As a first step towards the entanglement of several atoms, we have observed the first signature of Rydberg blockade between two atoms in the new setup (right). When shining excitation lasers on two atoms separated by a small distance, such that they interact strongly, we observe a very strong reduction of the probability to excite both atoms to the Rydberg state (black dots), while the probability to excite only one of the two atoms (blue triangles) oscillates √2 times as fast as in the case of only one atom (orange diamonds).


A new magneticfilm atom chip  October 2012 
Amsterdam, The Netherlands 
A new magneticfilm atom chip was fabricated and installed in our vacuum system. The chip hosts square and triangular lattices of magnetic microtraps, spaced by 10 µm. These will be used to trap ensembles of 10100 atoms and, using the Rydberg dipole blockade, create entangled states.


Quantum state control of stored Rydberg polaritons  October 2012 
Durham, UK 
Using a weak signal and strong control light fields, we have achieved storage of single photons as Rydberg polaritons. Using a microwave field to drive oscillations of the stored polaritons between neighbouring Rydberg states, we can induce and control longranged dipoledipole interactions between the stored photons. For weak microwave powers, the range of the interactions is increased beyond the separation between the polaritons which leads to dephasing observed as a suppression of the retrieved photon number. The controllable interactions between the polaritons could be exploited to perform quantum operations on photonic qubits, e.g. to implement a controlled phase gate.
Above left: The retrieved photon signal shows a signature of nonclassicality imposed by the Rydberg blockade mechanism, namely the suppression of photon coincidences at zero time delay (red bar). Above right: collective oscillations of the retrieved photon number.
 Reference: D. Maxwell, D. J. Szwer, D. ParedesBarato, H. Busche, J. D. Pritchard, A. Gauguet, K. J. Weatherill, M. P. A. Jones, and C. S. Adams, Storage and Control of Optical Photons Using Rydberg Polaritons, Phys. Rev. Lett. 110, 103001 (2013), or see our full list of publications

Optically resolving Schrödinger's cat with Rydberg atoms  October 2012 
Dresden, Germany 
In his famous thought experiment, Schroedinger contemplated the existence of macroscopic objects (cats) in quantum superposition states. Since then, experiments have created ever larger superposition states, to probe the boundary of the classical and quantum realms. We propose a Schroedinger's cat state made of a pair of ultracold atom clouds with about 20 atoms each. The clouds are briefly subjected to intercloud interactions that crucially involve Rydberg atoms. Since Rydberg interactions intimately link the overall quantum state and the character of mechanical motion, this allows an initial spin cat to be converted into a spatial cat. We then have a state where each atom cloud is quantum mechanically at two different positions at once. For the first time, the spatial separation in the superposition would be large enough to be visualized in direct absorption images.


Four wave mixing in a thermal gas involving Rydberg states  June 2012 
Stuttgart, Germany 
Fourwave mixing is a spectroscopy technique to control the light emission of atoms in a coherent way. The combination of fourwave mixing with the strong interaction of Rydberg atoms allows for the generation of single photons, as it has been shown with a sample of ultracold atoms (Kuzmich et.al., Science 336, 887, 2012). Our plan is to convert this result to a thermal gas of atoms to build a room temperature single photon source. The observation of a fourwave mixing signal is a major milestone towards this goal.
 Reference: A. Koelle, G. Epple, H. Kuebler, R. Löw, and T. Pfau, Fourwave mixing involving Rydberg states in a thermal vapor cell, Phys. Rev A 85, 063821 (2012), or see our full list of publications

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