arXiv subjects

Jakob Reichel

Publications and source records attributed to Jakob Reichel.

48 records · Page 3Linked to original sources

arXiv2010

Coherent manipulation of Bose-Einstein condensates with state-dependent microwave potentials on an atom chip

Entanglement-based technologies, such as quantum information processing, quantum simulations, and quantum-enhanced metrology, have the potential to revolutionise our way of computing and measuring and help clarifying the puzzling concept of entanglement itself. Ultracold atoms on atom chips are attractive for their implementation, as they provide control over quantum systems in compact, robust, and scalable setups. An important tool in this system is a potential depending on the internal atomic state. Coherent dynamics in this potential combined with collisional interactions allows entanglement generation both for individual atoms and ensembles. Here, we demonstrate coherent manipulation of Bose-condensed atoms in such a potential, generated in a novel way with microwave near-fields on an atom chip. We reversibly entangle atomic internal and motional states, realizing a trapped-atom interferometer with internal-state labelling. Our system provides control over collisions in mesoscopic condensates, paving the way for on-chip generation of many-particle entanglement and quantum-enhanced metrology with spin-squeezed states.

quant-ph↗

arXiv2010

Spin self-rephasing and very long coherence times in a trapped atomic ensemble

We perform Ramsey spectroscopy on the ground state of ultra-cold 87Rb atoms magnetically trapped on a chip in the Knudsen regime. Field inhomogeneities over the sample should limit the 1/e contrast decay time to about 3 s, while decay times of 58 s are actually observed. We explain this surprising result by a spin self-rephasing mechanism induced by the identical spin rotation effect originating from particle indistinguishability. We propose a theory of this synchronization mechanism and obtain good agreement with the experimental observations. The effect is general and susceptible to appear in other physical systems.

quant-ph↗

arXiv2010

Enhanced and reduced atom number fluctuations in a BEC splitter

We measure atom number statistics after splitting a gas of ultracold 87Rb atoms in a purely magnetic double-well potential created on an atom chip. Well below the critical temperature for Bose-Einstein condensation T_c, we observe reduced fluctuations down to -4.9dB below the atom shot noise level. Fluctuations rise to more than +3.8dB close to T_c, before reaching the shot noise level for higher temperatures. We use two-mode and classical field simulations to model these results. This allows us to confirm that the super-shot noise fluctuations directly originate from quantum statistics.

quant-ph↗

arXiv2010

Fiber Fabry-Perot cavity with high finesse

We have realized a fiber-based Fabry-Perot cavity with CO2 laser-machined mirrors. It combines very small size, high finesse F>=130000, small waist and mode volume, and good mode matching between the fiber and cavity modes. This combination of features is a major advance for cavity quantum electrodynamics (CQED), as shown in recent CQED experiments with Bose-Einstein condensates enabled by this cavity [Y. Colombe et al., Nature 450, 272 (2007)]. It should also be suitable for a wide range of other applications, including coupling to solid-state emitters, gas detection at the single-particle level, fiber-coupled single-photon sources and high-resolution optical filters with large stopband.

physics.optics↗

arXiv2010

Cavity-based single atom preparation and high-fidelity hyperfine state readout

We prepare and detect the hyperfine state of a single 87Rb atom coupled to a fiber-based high finesse cavity on an atom chip. The atom is extracted from a Bose-Einstein condensate and trapped at the maximum of the cavity field, resulting in a reproducibly strong atom-cavity coupling. We use the cavity reflection and transmission signal to infer the atomic hyperfine state with a fidelity exceeding 99.92% in a read-out time of 100 microseconds. The atom is still trapped after detection.

quant-ph↗

arXiv2008

Spin squeezing in a bimodal condensate: spatial dynamics and particle losses

We propose an analytical method to study the entangled spatial and spin dynamics of interacting bimodal Bose-Einstein condensates. We show that at particular times during the evolution spatial and spin dynamics disentangle and the spin squeezing can be predicted by a simple two-mode model. We calculate the maximum spin squeezing achievable in experimentally relevant situations with Sodium or Rubidium bimodal condensates, including the effect of the dynamics and of one, two and three-body losses.

quant-ph↗

arXiv2007

Strong atom-field coupling for Bose-Einstein condensates in an optical cavity on a chip

An optical cavity enhances the interaction between atoms and light, and the rate of coherent atom-photon coupling can be made larger than all decoherence rates of the system. For single atoms, this strong coupling regime of cavity quantum electrodynamics (cQED) has been the subject of spectacular experimental advances, and great efforts have been made to control the coupling rate by trapping and cooling the atom towards the motional ground state, which has been achieved in one dimension so far. For N atoms, the three-dimensional ground state of motion is routinely achieved in atomic Bose-Einstein condensates (BECs), but although first experiments combining BECs and optical cavities have been reported recently, coupling BECs to strong-coupling cavities has remained an elusive goal. Here we report such an experiment, which is made possible by combining a new type of fibre-based cavity with atom chip technology. This allows single-atom cQED experiments with a simplified setup and realizes the new situation of N atoms in a cavity each of which is identically and strongly coupled to the cavity mode. Moreover, the BEC can be positioned deterministically anywhere within the cavity and localized entirely within a single antinode of the standing-wave cavity field. This gives rise to a controlled, tunable coupling rate, as we confirm experimentally. We study the heating rate caused by a cavity transmission measurement as a function of the coupling rate and find no measurable heating for strongly coupled BECs. The spectrum of the coupled atoms-cavity system, which we map out over a wide range of atom numbers and cavity-atom detunings, shows vacuum Rabi splittings exceeding 20 gigahertz, as well as an unpredicted additional splitting which we attribute to the atomic hyperfine structure.

quant-ph↗

arXiv2007

Bose-Einstein condensate coupled to a nanomechanical resonator on an atom chip

We theoretically study the coupling of Bose-Einstein condensed atoms to the mechanical oscillations of a nanoscale cantilever with a magnetic tip. This is an experimentally viable hybrid quantum system which allows one to explore the interface of quantum optics and condensed matter physics. We propose an experiment where easily detectable atomic spin-flips are induced by the cantilever motion. This can be used to probe thermal oscillations of the cantilever with the atoms. At low cantilever temperatures, as realized in recent experiments, the backaction of the atoms onto the cantilever is significant and the system represents a mechanical analog of cavity quantum electrodynamics. With high but realistic cantilever quality factors, the strong coupling regime can be reached, either with single atoms or collectively with Bose-Einstein condensates. We discuss an implementation on an atom chip.

quant-ph↗

arXiv2006

Microwave potentials and optimal control for robust quantum gates on an atom chip

We propose a two-qubit collisional phase gate that can be implemented with available atom chip technology, and present a detailed theoretical analysis of its performance. The gate is based on earlier phase gate schemes, but uses a qubit state pair with an experimentally demonstrated, very long coherence lifetime. Microwave near-fields play a key role in our implementation as a means to realize the state-dependent potentials required for conditional dynamics. Quantum control algorithms are used to optimize gate performance. We employ circuit configurations that can be built with current fabrication processes, and extensively discuss the impact of technical noise and imperfections that characterize an actual atom chip. We find an overall infidelity compatible with requirements for fault-tolerant quantum computation.

quant-ph↗

arXiv2006

Quantum Information Processing in Optical Lattices and Magnetic Microtraps

We review our experiments on quantum information processing with neutral atoms in optical lattices and magnetic microtraps. Atoms in an optical lattice in the Mott insulator regime serve as a large qubit register. A spin-dependent lattice is used to split and delocalize the atomic wave functions in a controlled and coherent way over a defined number of lattice sites. This is used to experimentally demonstrate a massively parallel quantum gate array, which allows the creation of a highly entangled many-body cluster state through coherent collisions between atoms on neighbouring lattice sites. In magnetic microtraps on an atom chip, we demonstrate coherent manipulation of atomic qubit states and measure coherence lifetimes exceeding one second at micron-distance from the chip surface. We show that microwave near-fields on the chip can be used to create state-dependent potentials for the implementation of a quantum controlled phase gate with these robust qubit states. For single atom detection and preparation, we have developed high finesse fiber Fabry-Perot cavities and integrated them on the atom chip. We present an experiment in which we detected a very small number of cold atoms magnetically trapped in the cavity using the atom chip.

quant-ph↗

arXiv2004

Using magnetic chip traps to study Tonks-Girardeau quantum gases

We discuss the use of microfabricated magnetic traps, or "chip traps," to study quasi-one-dimensional quantum gases. In particular, we discuss the feasibility of studying the Tonks-Girardeau limit, in which the gas is strongly interacting. We review the scaling of the oscillation frequencies of a chip trap, and show that it seems feasible to attain a Tonks-Girardeau parameter as large as 200. The primary difficulty of this approach is detection, since the strongly interacting limit occurs for low densities. We propose a way to "freeze" the distribution, and then measure it with a single-atom detector. This method can also be applied to optical dipole traps.

cond-mat.soft↗

arXiv2004

Coherence in Microchip Traps

We report the coherent manipulation of internal states of neutral atoms in a magnetic microchip trap. Coherence lifetimes exceeding 1 s are observed with atoms at distances of $5-130 μ$m from the microchip surface. The coherence lifetime in the chip trap is independent of atom-surface distance within our measurement accuracy, and agrees well with the results of similar measurements in macroscopic magnetic traps. Due to the absence of surface-induced decoherence, a miniaturized atomic clock with a relative stability in the $10^{-13}$ range can be realized. For applications in quantum information processing, we propose to use microwave near-fields in the proximity of chip wires to create potentials that depend on the internal state of the atoms.

quant-ph↗