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.
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.
Achieving strong coupling between a single ion and a cavity is an important condition for cavity quantum electrodynamics, mainly for its applications in quantum networks, quantum computing and quantum interfaces. While strong coupling has been achieved in various physical systems, so far it remained elusive for single atomic ions. In this thesis I present the first observation of strong coupling between a single ion and an optical cavity. Our system is a hybrid system where a fibre based Fabry-Pérot cavity was incorporated to a 3D Paul trap. The position of the ion, relative to the cavity mode, was adjusted by applying additional rf signals on the radial electrodes of the trap, in order to maximise the coupling between the ion and the cavity. The coupling strength was measured to be g = 2p × (12.3±0.1) MHz, which exceeds both the atomic decay rate, ? = 2p × 11.5 MHz, and the cavity decay rate, ? = 2p × (4.1 ± 0.1) MHz, placing the ion-cavity coupling in the strong coupling regime. Cavity assisted Raman spectroscopy was used 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. Due to geometric constraints in our trap, Doppler cooling was not optimal along the cavity axis, hindering the localisation of the ion and thus its coupling to the cavity. To improve the localisation of the ion, cavity cooling was used to efficiently cool the ion along the cavity axis. By using cavity cooling, we obtain an enhanced ion-cavity coupling of g0 = 2p × (16.7±0.1) MHz, compared with g0 = 2p × (15.2±0.1) MHz when using only Doppler cooling.
We present the realization of a combined trapped-ion and optical cavity system, in which a single Yb(+) ion is confined by a micron-scale ion trap inside a 230 μm-long optical fiber cavity. We characterize the spatial ion-cavity coupling and measure the ion-cavity coupling strength using a cavity-stimulated Λ transition. Owing to the small mode volume of the fiber resonator, the coherent coupling strength between the ion and a single photon exceeds the natural decay rate of the dipole moment. This system can be integrated into ion-photon quantum networks and is a step towards cavity quantum electrodynamics based quantum information processing with trapped ions.
We investigate the possibility of achieving the strong coupling regime of cavity quantum electrodynamics using rare-earth-metal-ions as impurities in monolithic optical resonators. We conclude that due to the weak oscillator strengths of the rare-earth-metals, it may be possible but difficult to reach the regime where the single photon Rabi frequency is large compared to both the cavity and atom decay rates. However, reaching the regime where the saturation photon and atom numbers are less than one should be much more achievable. We show that in this ``bad cavity'' regime, transfer of quantum states and an optical phase shift conditional on the state of the atom is still possible and suggest a method for coherent detection of single dopants.
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.
Precise control over internal states of atomic ions in ion traps has been possible for many years enabling high precision spectroscopy of atomic transitions. Trapped ions are suitable candidates for quantum information processing due to their long storage times, low decoherence rates and high readout efficiency. Recent advances in micro-machining fibre optic mirrors and integrated fibre optic-ion trap systems for cavity QED allows one to realise strong coupling between an ion and a cavity mode by reducing the mode volume. For strong ion-cavity coupling, deterministic transfer of quantum states between ions and photons is possible. We have designed and constructed a miniaturised linear Paul trap with an integrated optical cavity, intended for coupling multiple ions to the cavity mode. The RF-electrode separation is 50μm, while the endcap separation is 350μm. The endcap electrodes are narrow tubes of inner diameter 300μm allowing optical fibers through the tubes to form the cavity. The trap depth we calculate to be 650meV for a 10V applied potential at 20MHz. The 4P1⁄2→ 3D3⁄2 transition of the atomic ion 40Ca+ with Γ = 24π × 22MHz can be coupled to an optical fibre Fabry-Perot cavity with length of 380µm, finesse of 33,000 and cavity linewidth k = 2π ×11MHz.The ion-cavity coherent coupling g = 2π × 19:5MHz of this system will put it in single-ion strong coupling regime with cooperativity, C=1.5.
Incorporating optical cavities in ion traps is becoming increasingly important in the development of photonic quantum networks. However, the presence of the cavity can hamper efficient laser cooling of ions because of geometric constraints that the cavity imposes and an unfavourable Purcell effect that can modify the cooling dynamics substantially. On the other hand the coupling of the ion to the cavity can also be exploited to provide a mechanism to efficiently cool the ion. In this paper we demonstrate experimentally how cavity cooling can be implemented to improve the localisation of the ion and thus its coupling to the cavity. By using cavity cooling we obtain an enhanced ion-cavity coupling of 2π × (16.7 ± 0.1) MHz, compared with 2π × (15.2 ± 0.1) MHz when using only Doppler cooling.