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arXiv:1706.01256 · Physical Review A

Single atoms coupled to a near-concentric cavity

Chi Huan Nguyen, Adrian Nugraha Utama, Nick Lewty, Kadir Durak, Gleb Maslennikov, Stanislav S. Straupe, Matthias Steiner, Christian Kurtsiefer

Published Sep 13, 2017• 10 citations
arXivHTMLPDF

Abstract

Concentric cavities can lead to strong photon-atom coupling without a need for high finesse or small physical-cavity volume. In this proof-of-principle experiment we demonstrate coupling of single Rb atoms to an 11-mm-long near-concentric cavity with a finesse $F=138(2)$. Operating the cavity $1.7(1)\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ shorter than the critical length, we observe an atom-cavity coupling constant ${g}_{0}=2\ensuremath{\pi}\ifmmode\times\else\texttimes\fi{}5.0(2)$ MHz which exceeds the natural dipole decay rate $\ensuremath{\gamma}$ by a factor of ${g}_{0}/\ensuremath{\gamma}=1.7(1)$.

DOI
10.1103/physreva.96.031802

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Concentric cavities can lead to strong photon-atom coupling without a need for high finesse or small physical-cavity volume. In this proof-of-principle experiment we demonstrate coupling of single Rb atoms to an 11-mm-long near-concentric cavity with a finesse F=138(2)F=138(2)F=138(2). Operating the cavity 1.7(1)\ensuremathμm1.7(1)\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}1.7(1)\ensuremathμm shorter than the critical length, we observe an atom-cavity coupling constant g0=2\ensuremathπ\ifmmode×\else\texttimes\fi5.0(2){g}_{0}=2\ensuremath{\pi}\ifmmode\times\else\texttimes\fi{}5.0(2)g0​=2\ensuremathπ\ifmmode×\else\texttimes\fi5.0(2) MHz which exceeds the natural dipole decay rate \ensuremathγ\ensuremath{\gamma}\ensuremathγ by a factor of g0/\ensuremathγ=1.7(1){g}_{0}/\ensuremath{\gamma}=1.7(1)g0​/\ensuremathγ=1.7(1).
Summary form only given. Cavity Quantum Electrodynamics (CQED) has been at the forefront of modem atomic physics and quantum optics. Of particular importance of CQED is the strong coupling regime whereby the coupling rate between atoms and cavity field dominates other dissipative rates [1]. To date, the requirements to obtain such significant interaction have been presumed to have micro-cavities with a high finesse and small mode volumes. Though having demonstrated remarkable results and progress, such cavities with limited optical access, high reflectivity coating and close distance between mirrors pose a challenge for scaling up and embedding other systems with CQED such as ion traps.In this work, we take an alternative approach of employing cavity designs with strong focusing modes [2]. Central to our experiment is a cavity in the near concentric point with a finesse of 150. The cavity has a length of 11mm, a radius of curvature of 5.5mm and accommodates a focusing mode of approximately 5um beam waist. We trapped a single Rb87 atom at the centre of the cavity by an intra-cavity dipole trap. We observed the lineshape splitting in the transmission spectrum of cavity under the present of the trapped single atoms. The cooperativity of 0.1 can be deduced from Fig 1 and place our system in the intermediate coupling regime. This first observation of lineshape splitting with centimetre long and low finesse cavity promises the broader use of CQED and new exploration of CQED physics at strong focusing regime, where all optical cavity modes become degenerate.
In the near-concentric regime, the transverse modes of an optical cavity have small mode volumes with a beam waist on the order of the atomic cross section. This shows potential for strong interaction between single atoms and light, which is a key component in distributed quantum computing architectures [1] . Furthermore, with centimetresized near-concentric cavities, the frequency spacing of the transverse modes ranges from tens of MHz to a GHz, matching well with the hyperfine or the Zeeman level splitting of an atom [2] . This near-degeneracy in resonant frequencies close to the critical point allows to explore the physics of atomic nonlinearities with multiple photonic modes strongly coupled to different hyperfine or magnetic energy levels of a quantum emitter.
Cavity quantum electrodynamics (cavity QED) describes electromagnetic fields in a confined space and the radiative properties of atoms in such fields. The simplest example of such system is a single atom interacting with one mode of a high-finesse resonator. Besides observation and exploration of fundamental quantum mechanical effects, this system bears a high potential for applications quantum information science such as, e.g., quantum logic gates, quantum communication and quantum teleportation. In this thesis I present an experiment on the deterministic coupling of a single neutral atom to the mode of a high-finesse optical resonator. In Chapter 1 I describe our basic techniques for trapping and observing single cesium atoms. As a source of single atoms we use a high-gradient magneto-optical trap, which captures the atoms from background gas in a vacuum chamber and cools them down to millikelvin temperatures. The atoms are then transferred without loss into a standing-wave dipole trap, which provides a conservative potential required for experiments on atomic coherence such as quantum information processing and metrology on trapped atoms. Moreover, shifting the standing-wave pattern allows us to deterministically transport the atoms (Chapter 2). In combination with non-destructive fluorescence imaging of individual trapped atoms, this enables us to control their position with submicrometer precision over several millimeters along the dipole trap. The cavity QED system can distinctly display quantum behaviour in the so-called strong coupling regime, i.e., when the coherent atom-cavity coupling rate dominates dissipation in the system. This sets the main requirements on the resonator's properties: small mode volume and high finesse. Chapter 3 is devoted to the manufacturing, assembling, and testing of an ultra-high finesse optical Fabry-Perot resonator, stabilized to the atomic transition. In Chapter 4 I present the transportation of single atoms into the cavity and their coupling to the cavity mode. The strong coupling manifests itself in a strong reduction of the cavity transmission probed by a weak external laser. The atoms remain trapped and coupled to the cavity mode for several seconds until we move them out of the cavity for final analysis of their number and position.
We have used a fiber Fabry-Perot cavity on an atom chip to obtain strong, extremely stable atom-cavity coupling for single atoms and Bose-Einstein condensates trapped inside the cavity. The coherent atom-cavity coupling parameter, g0/2pi ~ 209 MHz, is larger than both cavity decay, kappa/2pi ~ 53 MHz and spontaneous emission gamma/2pi ~ 3 MHz. Due to the small mode volume, these values, well inside the strong coupling regime of cavity QED, are realized with a cavity finesse F ~ 36000, which is a moderate value compared to usual strong-coupling cavity QED experiments. The cavity also achieves a very high cooperativity parameter, C=g02/2kappagamma ~ 130.The use of trapped Bose-Einstein condensates (BECs) is a fundamentally new situation for cavity QED, which should enable a new class of experiments, where atoms that share a well-defined internal and external quantum state collectively interact with a quantized mode of the cavity. Besides, as the spatial extension of the BEC assumes the fundamental limit that can be attained for an atom cloud - much smaller than an optical wavelength in our case - an extremely strong, stable and uniform coupling to the cavity mode is achieved. This makes the system interesting for applications such as quantum memories. For single atoms, our combination of high resonator cooperativity with extreme positioning precision enables qubit detection with near-unit efficiency. This realizes a long-awaited milestone for quantum information processing on atom chips.
Near-concentric optical cavities of spherical mirrors can provide technical advantages over the conventional near-planar cavities in applications requiring strong atom-light interaction, as they concentrate light in a very small region of space. However, such cavities barely support stable optical modes, and thus impose practical challenges. Here, we present an experiment where we maintain a near-concentric cavity at its last resonant length for laser light at 780 nm resonant with an atomic transition. At this point, the spacing of two spherical mirror surfaces is 207(13)nm207(13)\phantom{\rule{0.16em}{0ex}}\mathrm{nm}207(13)nm shorter than the critical concentric point, corresponding to a stability parameter $g=\ensuremath{-}0.999962(2)$ and a cavity beam waist of $2.4\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$.
We show that a low-finesse cavity can be efficient for detecting neutral atoms. The low finesse can be compensated for by decreasing the mode waist of the cavity. We have used a near-concentric resonator with a beam waist of 12 microm and a finesse of only 1100 to detect magnetically guided Rb atoms with a detection sensitivity of 0.1 atom in the mode volume. For future experiments on single-atom detection and cavity QED applications, it should be beneficial to use miniaturized optical resonators integrated on atom chips.