Matthias Lettner

Remote entanglement between a single atom and a Bose-Einstein condensate

Entanglement has been recognised as a puzzling yet central element of quantum physics. While photons serve as flying qubits to distribute entanglement, the entanglement of stationary qubits at remote sites is a key resource for envisioned applications like distributed quantum computing [1]. In our experiment we create remote entanglement between a single atom located inside a high-finesse optical cavity and a Bose-Einstein condensate (BEC). To this end we generate a single photon in the atom-cavity system, entangling the photon polarisation with the atomic Zeeman state [2,3]. The photon is transported to a different laboratory in an optical fiber, where it is stored in a BEC employing electromagnetically induced transparency (EIT) [4–6]. This converts the atom-photon entanglement into remote matter-matter entanglement. Subsequently we map the matter-matter entanglement onto photon-photon entanglement. The experimental setup is sketched in Fig. 1.

Bose-Einstein condensate as a quantum memory for a photonic polarization qubit

A scheme based on electromagnetically induced transparency is used to store light in a Bose-Einstein condensate. In this process, a photonic polarization qubit is stored in atomic Zeeman states. The performance of the storage process is characterized and optimized. The average process fidelity is 1.000 +/- 0.004. For long storage times, temporal fluctuations of the magnetic field reduce this value, yielding a lifetime of the fidelity of 1.1 +/- 0.2 ms. The write-read efficiency of the pulse energy can reach 0.53 +/- 0.05.

Dissipation-induced hard-core boson gas in an optical lattice

We present a theoretical investigation of a lattice Tonks-Girardeau gas that is created by inelastic, instead of elastic interactions. An analytical calculation shows that in the limit of strong two-body losses, the dynamics of the system is effectively that of a hard-core boson gas. We also derive an analytic expression for the effective loss rate. We find good agreement between these analytical results and results from a rigorous numerical calculation. The hard- core character of the particles is visible both in a reduced effective loss rate and in the momentum distribution of the gas.

Lieb-Liniger model of a dissipation-induced Tonks-Girardeau gas

We show that strong inelastic interactions between bosons in one dimension create a Tonks-Girardeau gas, much as in the case of elastic interactions. We derive a Markovian master equation that describes the loss caused by the inelastic collisions. This yields a loss rate equation and a dissipative Lieb-Liniger model for short times. We obtain an analytic expression for the pair correlation function in the limit of strong dissipation. Numerical calculations show how a diverging dissipation strength leads to a vanishing of the actual loss rate and renders an additional elastic part of the interaction irrelevant.

Combination of a magnetic Feshbach resonance and an optical bound-to-bound transition

We use laser light near resonant with an optical bound-to-bound transition to shift the magnetic field at which a Feshbach resonance occurs. We operate in a regime of large detuning and large laser intensity. This reduces the light-induced atom-loss rate by one order of magnitude compared to our previous experiments [D.M. Bauer et al. Nature Phys. 5, 339 (2009)]. The experiments are performed in an optical lattice and include high-resolution spectroscopy of excited molecular states, reported here. In addition, we give a detailed account of a theoretical model that describes our experimental data.

Observation of nonadditive mixed-state phases with polarized neutrons.

In a neutron polarimetry experiment the mixed-state relative phases between spin eigenstates are determined from the maxima and minima of measured intensity oscillations. We consider evolutions leading to purely geometric, purely dynamical, and combined phases. It is experimentally demonstrated that the sum of the individually determined geometric and dynamical phases is not equal to the associated total phase which is obtained from a single measurement, unless the system is in a pure state.

Atom-molecule Rabi oscillations in a Mott insulator.

We observe large-amplitude Rabi oscillations between an atomic and a molecular state near a Feshbach resonance. The experiment uses 87Rb in an optical lattice and a Feshbach resonance near 414 G. The frequency and amplitude of the oscillations depend on the magnetic field in a way that is well described by a two-level model. The observed density dependence of the oscillation frequency agrees with theoretical expectations. We confirmed that the state produced after a half-cycle contains exactly one molecule at each lattice site. In addition, we show that, for energies in a gap of the lattice band structure, the molecules cannot dissociate.

New aspects of geometric phases in experiments with polarized neutrons

Geometric phase phenomena have been observed with single neutrons in polarimeter and interferometer experiments. Interacting with static and time-dependent magnetic fields, the state vectors acquire a geometric phase tied to the evolution within spin subspace. In a polarimeter experiment the non-additivity of quantum phases for mixed spin input states is observed. In a Si perfect-crystal interferometer experiment appearance of geometric phases, induced by interaction with an oscillating magnetic field, is verified. The total system is characterized by an entangled state, consisting of neutron and radiation fields, governed by a Jaynes–Cummings Hamiltonian. In addition, the influence of the geometric phase on a Bell measurement, expressed by the Clauser–Horne–Shimony–Holt (CHSH) inequality, is studied. It is demonstrated that the effect of the geometric phase can be balanced by an appropriate change of Bell angles.

Nonadditive Mixed State Phases in Neutron Optics

In a neutron polarimetry experiment mixed neutron spin phases are determined. We consider evolutions leading to purely geometric, purely dynamical and combined phases. It is experimentally demonstrated that the sum of the geometric and dynamical phases—both obtained in separate measurements—is not equal to the associated total phase as obtained from a third measurement, unless the system is in a pure state. In this sense, mixed state phases are not additive.

A Dissipative Tonks-Girardeau Gas of Molecules

Strongly correlated states in many-body systems are traditionally created using elastic interparticle interactions. Here we show that inelastic interactions between particles can also drive a system into the strongly correlated regime. This is shown by an experimental realization of a specific strongly correlated system, namely a one-dimensional molecular Tonks-Girardeau gas.