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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Lineshape splitting with single atoms in concentric cavities

Chi Huan Nguyen, Adrian Nugraha Utama, Christian Kurtsiefer

Published Jun 1, 2017• 0 citations
Publisher

Abstract

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.

DOI
10.1109/cleoe-eqec.2017.8087845

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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.
Cavity quantum electrodynamic system with strongly coupled single atoms provides a good platform for studying quantum information processing, quantum simulation, quantum network, and distributed quantum computing. Cooling and trapping single atoms is a crucial technique in the quantum technology. At present, in a high-finesse cavity with finite space, cooling and trapping single atoms is a big challenge, even though it is a mature technique for free space. Great efforts have been made to cool and trap single atoms inside a cavity, and for a trapped atom its lifetime has reached as long as tens of seconds. Developing a more flexible method of cooling and trapping single atoms in a cavity is still essential for a strongly coupled cavity quantum electrodynamic system. In this work, we demonstrate experimentally that a single cesium atom in a cavity can be trapped by utilizing a single optical tweezer settled in cavity mode, and its lifetime is (2.60 ± 0.18) s. The experiment is carried out in a Fabry-Perot cavity, which is assembled by two concave mirrors each with a curvature radius of 100 mm, and cavity length of 335 μm. The concave surfaces are highly reflective, and the cavity has a finesse of 6.1 × 104. The 1080 nm optical tweezer with a waist of 2 μm is formed by an achromatic lens group with a numerical aperture of 0.4. At first, the precooled atomic assemble released from the magneto-optical trap (MOT) is transferred into cavity mode by an optical dipole trap with a waist of 36 μm. Then, one of the successfully transferred atoms is captured by the optical tweezer with the aid of cavity cooling mechanism. A blue detuned cavity locking laser is used as a standing-wave optical trap along the cavity axis. The signal of successfully trapped one atom is obtained by recording transmission of the cavity that will decrease owing to the strong coupling induced vacuum Rabi splitting. Finally, we demonstrate the precise manipulation of the atom-cavity coupling strength, which is achieved by scanning the position of the trapped atom step by step by using a high-precision translation stage. The system realized in this work can be used to study the dynamics of single atom-photon interactions with adjustable coupling strength. In addition, the mechanism adopted in this work is compatible with constructing tweezer arrays inside cavity mode, and thus possesses more flexibility and great potentials in cavity-based quantum entanglement and quantum simulation.
The strongly coupled system composed of atoms, molecules, molecule aggregates, and semiconductor quantum dots embedded within an optical microcavity/nanocavity with high quality factor and/or low modal volume has become an excellent platform to study cavity quantum electrodynamics (CQED), where a prominent quantum effect called Rabi splitting can occur due to strong interaction of cavity-mode single-photon with the two-level atomic states. In this paper, we build a new quantum model that can describe the optical response of the strongly-coupled system under the action of an external probing light and the spectral lineshape. We take the Hamiltonian for the strongly-coupled photon–atom system as the unperturbed Hamiltonian H 0 and the interaction Hamiltonian of the probe light upon the coupled-system quantum states as the perturbed Hamiltonian V . The theory yields a double Lorentzian lineshape for the permittivity function, which agrees well with experimental observation of Rabi splitting in terms of spectral splitting. This quantum theory will pave the way to construct a complete understanding for the microscopic strongly-coupled system that will become an important element for quantum information processing, nano-optical integrated circuits, and polariton chemistry.
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 $1.7(1)\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ 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 ${g}_{0}/\ensuremath{\gamma}=1.7(1)$.
The combination of an ion trap with a high finesse optical cavity is an ideal system for the investigation of strong coupling cavity quantum electrodynamics, and allows the observation of a number of interesting quantum phenomena. To achieve the small mode volumes required without impairing the ion trapping small traps with a short ion electrode distance are needed. Two microscopic linear rf ion traps have been developed and built to accommodate experimental cavities of lengths of several 100 microns. The first trap design, the 'sandwich' trap, was successfully used to trap 40Ca+ - ions for several hours. It was characterised extensively including a measurement of the heating rates of the ions in the trap. Spectroscopy measurements of the cooling transition, as well as the two repumping transitions were carried out. The second trap design, the 'alumina' trap, also successfully trapped 40Ca+ - ions, and a full characterisation of this trap was made. The experimental cavity was installed at a preliminary cavity length distance of 3.7 mm. The cavity characteristics were examined. Finally the trapped ions were overlapped with the cavity mode by adjusting the trap minimum position along the trap axis via dc voltages and the vertical position of the cavity. To progress further a locking scheme for the cavity length as well as a single - photon detection setup are necessary. To achieve strong coupling a reduction of the cavity length will have to be made.
Strong coupling between an atom and an electromagnetic resonator is an important condition in cavity quantum electrodynamics. While strong coupling in various physical systems has been achieved so far, it remained elusive for single atomic ions. Here, we achieve a coupling strength of 2π×(12.3±0.1) MHz between a single ^{40}Ca^{+} ion and an optical cavity, exceeding both atomic and cavity decay rates which are 2π×11.5 and 2π×(4.1±0.1) MHz, respectively. We use cavity assisted Raman spectroscopy to precisely characterize the ion-cavity coupling strength and observe a spectrum featuring the normal mode splitting in the cavity transmission due to the ion-cavity interaction. Our work paves the way towards new applications of cavity quantum electrodynamics utilizing single trapped ions in the strong coupling regime for quantum optics and quantum technologies.
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.