arXiv subjects

Stephan Ritter

Publications and source records attributed to Stephan Ritter.

30 records · Page 2Linked to original sources

arXiv2011

A Single-Atom Quantum Memory

The faithful storage of a quantum bit of light is essential for long-distance quantum communication, quantum networking and distributed quantum computing. The required optical quantum memory must, first, be able to receive and recreate the photonic qubit and, second, store an unknown quantum state of light better than any classical device. These two requirements have so far been met only by ensembles of material particles storing the information in collective excitations. Recent developments, however, have paved the way for a new approach in which the information exchange happens between single quanta of light and matter. This single-particle approach allows one to address the material qubit and thus has fundamental advantages for realistic implementations: First, to combat inevitable losses and finite efficiencies, it enables a heralding mechanism that signals the successful storage of a photon by means of state detection. Second, it allows for individual qubit manipulations, opening up avenues for in situ processing of the stored quantum information. Here we demonstrate the most fundamental implementation of such a quantum memory by mapping arbitrary polarization states of light into and out of a single atom trapped inside an optical cavity. The memory performance is analyzed through full quantum process tomography. The average fidelity is measured to be 93% and low decoherence rates result in storage times exceeding 180μs. This makes our system a versatile quantum node with excellent perspectives for optical quantum gates and quantum repeaters.

quant-ph↗

arXiv2010

Lossless State Detection of Single Neutral Atoms

We introduce lossless state detection of trapped neutral atoms based on cavity-enhanced fluorescence. In an experiment with a single 87-Rb atom, a hyperfine-state-detection fidelity of 99.4% is achieved in 85 microseconds. The quantum bit is interrogated many hundreds of times without loss of the atom while a result is obtained in every readout attempt. The fidelity proves robust against atomic frequency shifts induced by the trapping potential. Our scheme does not require strong coupling between the atom and cavity and can be generalized to other systems with an optically accessible quantum bit.

quant-ph↗

arXiv2010

Electromagnetically induced transparency with single atoms in a cavity

Optical nonlinearities offer unique possibilities for the control of light with light. A prominent example is electromagnetically induced transparency (EIT) where the transmission of a probe beam through an optically dense medium is manipulated by means of a control beam. Scaling such experiments into the quantum domain with one, or just a few particles of both light and matter will allow for the implementation of quantum computing protocols with atoms and photons or the realisation of strongly interacting photon gases exhibiting quantum phase transitions of light. Reaching these aims is challenging and requires an enhanced matter-light interaction as provided by cavity quantum electrodynamics (QED). Here we demonstrate EIT with a single atom quasi-permanently trapped inside a high-finesse optical cavity. The atom acts as a quantum-optical transistor with the ability to coherently control the transmission of light through the cavity. We furthermore investigate the scaling of EIT when the atom number is increased one by one. The measured spectra are in excellent agreement with a theoretical model. Merging EIT with cavity QED and single quanta of matter is likely to become the cornerstone for novel applications, e.g. the dynamic control of the photon statistics of propagating light fields or the engineering of Fock-state superpositions of flying light pulses.

quant-ph↗

arXiv2009

Dynamical Coupling between a Bose-Einstein Condensate and a Cavity Optical Lattice

A Bose-Einstein condensate is dispersively coupled to a single mode of an ultra-high finesse optical cavity. The system is governed by strong interactions between the atomic motion and the light field even at the level of single quanta. While coherently pumping the cavity mode the condensate is subject to the cavity optical lattice potential whose depth depends nonlinearly on the atomic density distribution. We observe bistability already below the single photon level and strong back-action dynamics which tunes the system periodically out of resonance.

quant-ph↗

arXiv2008

Cavity Opto-Mechanics with a Bose-Einstein Condensate

Cavity opto-mechanics studies the coupling between a mechanical oscillator and a cavity field, with the aim to shed light on the border between classical and quantum physics. Here we report on a cavity opto-mechanical system in which a collective density excitation of a Bose-Einstein condensate is shown to serve as the mechanical oscillator coupled to the cavity field. We observe that a few photons inside the ultrahigh-finesse cavity trigger a strongly driven back-action dynamics, in quantitative agreement with a cavity opto-mechanical model. With this experiment we approach the strong coupling regime of cavity opto-mechanics, where a single excitation of the mechanical oscillator significantly influences the cavity field. The work opens up new directions to investigate mechanical oscillators in the quantum regime and quantum gases with non-local coupling.

quant-ph↗

arXiv2007

Cavity QED with a Bose-Einstein condensate

Cavity quantum electrodynamics (cavity QED) describes the coherent interaction between matter and an electromagnetic field confined within a resonator structure, and is providing a useful platform for developing concepts in quantum information processing. By using high-quality resonators, a strong coupling regime can be reached experimentally in which atoms coherently exchange a photon with a single light-field mode many times before dissipation sets in. This has led to fundamental studies with both microwave and optical resonators. To meet the challenges posed by quantum state engineering and quantum information processing, recent experiments have focused on laser cooling and trapping of atoms inside an optical cavity. However, the tremendous degree of control over atomic gases achieved with Bose-Einstein condensation has so far not been used for cavity QED. Here we achieve the strong coupling of a Bose-Einstein condensate to the quantized field of an ultrahigh-finesse optical cavity and present a measurement of its eigenenergy spectrum. This is a conceptually new regime of cavity QED, in which all atoms occupy a single mode of a matter-wave field and couple identically to the light field, sharing a single excitation. This opens possibilities ranging from quantum communication to a wealth of new phenomena that can be expected in the many-body physics of quantum gases with cavity-mediated interactions.

quant-ph↗

arXiv2007

Critical Behavior of a Trapped Interacting Bose Gas

The phase transition of Bose-Einstein condensation is studied in the critical regime, when fluctuations extend far beyond the length scale of thermal de Broglie waves. Using matter-wave interference we measure the correlation length of these critical fluctuations as a function of temperature. The diverging behavior of the correlation length above the critical temperature is observed, from which we determine the critical exponent of the correlation length for a trapped, weakly interacting Bose gas to be $ν=0.67\pm 0.13$. This measurement has direct implications for the understanding of second order phase transitions.

cond-mat.stat-mech↗

arXiv2007

Observing the Formation of Long-range Order during Bose-Einstein Condensation

We have experimentally investigated the formation of off-diagonal long-range order in a gas of ultracold atoms. A magnetically trapped atomic cloud prepared in a highly nonequilibrium state thermalizes and thereby crosses the Bose-Einstein condensation phase transition. The evolution of phase coherence between different regions of the sample is constantly monitored and information on the spatial first-order correlation function is obtained. We observe the growth of the spatial coherence and the formation of long-range order in real time and compare it to the growth of the atomic density. Moreover, we study the evolution of the momentum distribution during the nonequilibrium formation of the condensate.

cond-mat.stat-mech↗

arXiv2006

Time interval distributions of atoms in atomic beams

We report on the experimental investigation of two-particle correlations between neutral atoms in a Hanbury Brown and Twiss experiment. Both an atom laser beam and a pseudo-thermal atomic beam are extracted from a Bose-Einstein condensate and the atom flux is measured with a single atom counter. We determine the conditional and the unconditional detection probabilities for the atoms in the beam and find good agreement with the theoretical predictions.

cond-mat.stat-mech↗

arXiv2006

Hybrid apparatus for Bose-Einstein condensation and cavity quantum electrodynamics: Single atom detection in quantum degenerate gases

We present and characterize an experimental system in which we achieve the integration of an ultrahigh finesse optical cavity with a Bose-Einstein condensate (BEC). The conceptually novel design of the apparatus for the production of BECs features nested vacuum chambers and an in-vacuo magnetic transport configuration. It grants large scale spatial access to the BEC for samples and probes via a modular and exchangeable "science platform". We are able to produce \87Rb condensates of five million atoms and to output couple continuous atom lasers. The cavity is mounted on the science platform on top of a vibration isolation system. The optical cavity works in the strong coupling regime of cavity quantum electrodynamics and serves as a quantum optical detector for single atoms. This system enables us to study atom optics on a single particle level and to further develop the field of quantum atom optics. We describe the technological modules and the operation of the combined BEC cavity apparatus. Its performance is characterized by single atom detection measurements for thermal and quantum degenerate atomic beams. The atom laser provides a fast and controllable supply of atoms coupling with the cavity mode and allows for an efficient study of atom field interactions in the strong coupling regime. Moreover, the high detection efficiency for quantum degenerate atoms distinguishes the cavity as a sensitive and weakly invasive probe for cold atomic clouds.

cond-mat.other↗

arXiv2005

Correlations and Counting Statistics of an Atom Laser

We demonstrate time-resolved counting of single atoms extracted from a weakly interacting Bose-Einstein condensate of $^{87}$Rb atoms. The atoms are detected with a high-finesse optical cavity and single atom transits are identified. An atom laser beam is formed by continuously output coupling atoms from the Bose-Einstein condensate. We investigate the full counting statistics of this beam and measure its second order correlation function $g^{(2)}(τ)$ in a Hanbury Brown and Twiss type experiment. For the monoenergetic atom laser we observe a constant correlation function $g^{(2)}(τ)=1.00\pm0.01$ and an atom number distribution close to a Poissonian statistics. A pseudo-thermal atomic beam shows a bunching behavior and a Bose distributed counting statistics.

cond-mat.mes-hall↗

arXiv2001

Reducing the linewidth of a diode laser below 30 Hz by stabilization to a reference cavity with finesse above 10^5

An extended cavity diode laser operating in the Littrow configuration emitting near 657 nm is stabilized via its injection current to a reference cavity with a finesse of more than 10^5 and a corresponding resonance linewidth of 14 kHz. The laser linewidth is reduced from a few MHz to a value below 30 Hz. The compact and robust setup appears ideal for a portable optical frequency standard using the Calcium intercombination line.

physics.optics↗