This thesis describes the first experimentally achieved coupling of a single atomic ion to an optical fibre-cavity as well as the first photonic interaction between a single ion and a single semiconductor quantum dot. The newly developed apparatus stores a single Yb+ ion in a radio-frequency Paultrap. There the ion is overlapped in-situ with the field of a miniature Fabry-Perot cavity which is mounted on a 3D-translation stage. The key elements of the setup are the cavity mirrors which are microfabricated onto the front facets of optical fibres. The small size of these mirrors are ideal for integration with an electrode structure of similar dimensions. Moreover these cavities are intrinsically fibre coupled and thereby well suited for distributed networks based on photonic interaction. We focus in our studies of the cavity-ion interaction on the application of the cavity as a converter between atomic and photonic excitations. To this end, we investigate the probability of the ion to emit photons into the cavity mode and find that this rate exceeds the natural spontaneous emission rate of the transition. We confirm that the emitted light field consists of a stream of single photons by measuring the second order correlation function. Furthermore, when prepared in a single Zeeman state, the emission process correlates the polarisation of the photon with the spin state of the ion. Remarkably, swapping the roles of ion and light field does not affect the interaction strength, i.e. we observe strongly enhanced absorption by the ion when a faint probe beam is injected into the cavity mode. The novel cavity-ion geometry advances the prospects to reach the strong-coupling regime with single ions and the feasibility of a distributed ion-based quantum network. Finally we employ the cavity-enhanced absorption to demonstrate photonic coupling between an InAs/GaAs quantum dot and a Yb+ ion. The setups for both systems are spatially separated by 20 m and optically linked by a single mode fibre. Quantum dot photons are guided into the cavity and the absorption by the ion is investigated for various excitation parameters. Our results mark the first steps towards hybrid networks and set the stage for further studies of the interaction between this unequal couple.
This thesis describes the first experimentally achieved coupling of a single atomic ion to an optical fibre-cavity as well as the first photonic interaction between a single ion and a single semiconductor quantum dot. The newly developed apparatus stores a single Yb+ ion in a radio-frequency Paultrap. There the ion is overlapped in-situ with the field of a miniature Fabry-Perot cavity which is mounted on a 3D-translation stage. The key elements of the setup are the cavity mirrors which are microfabricated onto the front facets of optical fibres. The small size of these mirrors are ideal for integration with an electrode structure of similar dimensions. Moreover these cavities are intrinsically fibre coupled and thereby well suited for distributed networks based on photonic interaction. We focus in our studies of the cavity-ion interaction on the application of the cavity as a converter between atomic and photonic excitations. To this end, we investigate the probability of the ion to emit photons into the cavity mode and find that this rate exceeds the natural spontaneous emission rate of the transition. We confirm that the emitted light field consists of a stream of single photons by measuring the second order correlation function. Furthermore, when prepared in a single Zeeman state, the emission process correlates the polarisation of the photon with the spin state of the ion. Remarkably, swapping the roles of ion and light field does not affect the interaction strength, i.e. we observe strongly enhanced absorption by the ion when a faint probe beam is injected into the cavity mode. The novel cavity-ion geometry advances the prospects to reach the strong-coupling regime with single ions and the feasibility of a distributed ion-based quantum network. Finally we employ the cavity-enhanced absorption to demonstrate photonic coupling between an InAs/GaAs quantum dot and a Yb+ ion. The setups for both systems are spatially separated by 20 m and optically linked by a single mode fibre. Quantum dot photons are guided into the cavity and the absorption by the ion is investigated for various excitation parameters. Our results mark the first steps towards hybrid networks and set the stage for further studies of the interaction between this unequal couple.
We investigate the coupling of a single trapped ion to a miniature optical cavity operating in the ultraviolet. Our cavity provides a source of single photons at a high rate into a single spatial mode. Using our apparatus, we have demonstrated the highest atom-cavity coupling rate achieved with a single ion by an order of magnitude. When the ion is continuously excited, we observe phase-sensitive correlations between emission into free-space and into the cavity mode, which can be explained by a cavity induced back-action effect on a driven dipole. We demonstrate coherent manipulation of a hyperfine qubit and ultra-short optical π rotations, which are essential tools for creation and detection of spin-photon entanglement. To this end, we have developed optical fibre-based Fabry-Perot cavities in the ultraviolet spectral range. These cavities operate near the primary dipole transition of Yb at 370 nm, and allow us to couple a pure atomic two-level system offered by a single trapped ion to the cavity mode. A new Paul trap apparatus in an ultra-high vacuum chamber has been built which allows for the integration of these cavities at very small ion-mirror separations. In order for independent operation of the trap, a compact system of diode lasers has been built which are stabilised to low-drift optical reference cavities. Coherent control of the hyperfine qubit in Yb 171 is achieved through application of microwave radiation, and ultra-short optical π rotations are performed with resonant light pulses derived from a frequency-doubled mode-locked titanium-sapphire laser. The experiment is controlled through a system of hardware and software which has been developed in a modular fashion and will allow for efficient control on the nanosecond time-scale when several such systems are interconnected. The success of our system opens the door to future experiments with trapped ions which will reach the strong coupling regime with a single ion. Furthermore, when operated in the fast-cavity regime, systems based on our approach will enable high-efficiency collection of photons from the ion into the single mode of an optical fibre. These systems will allow for the generation of distributed entanglement and will prove ideal as nodes in a larger quantum network of trapped ions.
The research undertaken unites two distinct areas of quantum information processing: single ions stored in radio-frequency traps and single photons in optical fibres. Strings of ions are presently the most successful implementation of quantum computing, with elementary quantum algorithm and quantum simulations realised. The principal challenge in the field is to enhance the quantum processing power by scaling up current devices to larger systems. We pursue one of the most promising strategies: distributed quantum computation in which multiple small-scale ion processors are interlinked by exchanging photonic quantum bits via optical fibres. This requires a coherent quantum interface between ions and photons, mapping ionic to photonic quantum states and vice versa. To maximise fidelity and the success rate of the scheme, the interaction of ions and photons must take place in a microscopic optical cavity with high finesse. To this end, single 40Ca+ were trapped in a radio-frequency ion trap whose trapping electrodes are hollow cylinders. Optical fibres with mirrors machined on the facets are inserted into the electrodes to form a Fabry-Pérot cavity. Because the fibres are shielded by the electrodes the detrimental distortion of the trapping field due to their presence is suppressed and ions can be trapped for several hours. 40Ca+ has a -type energy level scheme wherein the ion is cooled on the 42P1/2 ? 42S1/2 transition and the cavity is tuned to the 42P1/2 ? 32D3/2 transition. This thesis reports the successful coupling of single ions to a high finesse optical fibre based cavity, with coupling strength g = 2p · 4:6 MHz. The cavity has length 367 µm, finesse of 40,000 and linewidth 2k = 2p · 9:4 MHz. In this coupling regime, the enhancement of the ion's emission rate through the Purcell effect was observed. Further, anti-correlation was observed in the emission rates between the P1/2 ? D3/2 and P1/2 ? S1/2 transitions with an effective emission rate suppression of up to 60% in the latter transition. The built system offers greater promises. Once the position in the cavity mode has been optimised we expect to reach the long-sought after strong coupling regime with (g, k, y) = 2p · (12:2; 4:7; 11:2) MHz.
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.
The controlled emission and absorption of single photons is an important enabling technology in the fields of quantum communication, cryptography and computing. We have realised a novel single photon source, based on a miniature ‘endcap’ ion trap with integrated optical fibres. To minimise distortion of the trapping field the fibres are tightly integrated and recessed within the two hollow cylindrical rf electrodes of the trap. This allows us to bring the fibres to within approximately 300 µm of the trapped ion. With the fibres in place we are able to collect the ion’s fluorescence using no further optics. In this thesis the ion trap is fully characterised, and the quantum nature of the light demonstrated in the results of a variant on the Hanbury Brown-Twiss photon-correlation experiment. The scheme will ultimately be extended to implement a coherent ion-photon interface through strong coupling cavity-QED. Towards this end, an ultra-high-finesse cavity has been designed and fabricated by laser-machining and coating surfaces with a range of radii of curvature on the end facets of the fibres. To improve the stability and precision manipulation of the cavity in-vacuum, an entirely new trap has been designed and built. Finally, the current status of the fibre cavity and the outlook for the experiment are presented.
One of the most exciting prospects of quantum physics today is to simulate large quantum systems, intractable by classical computation, by connecting many smaller systems together. A cavity-based quantum network of atoms and photons constitutes one such approach, where each cavity interfaces between stationary atomic qubits in a node and photonic qubits travelling along optical fibre channels. For this scheme to work well, the cavity-based interface must be faithful and well-controlled. In this thesis, we construct a fibre-tip Fabry-Pérot cavity by aligning two optical fibre-tips against each other, where each has a concave, mirror-coated surface. Three properties make it highly suitable for interfacing: a small mode volume for strong atom-photon coupling, directly fibre-coupled cavity light, and optical access for trapped atoms. For the latter, we design a scheme that allows for a 2D reconfigurable array of atoms, held in the cavity mode by holographically-generated optical tweezers. Our particular design will allow us to move atoms independently of one another. We find the atom-cavity system parameters for optimal single photon generation, absorption and remote state transfer using V-STIRAP (vacuum-stimulated emission by Raman adiabatic passage). Using master equations and the Nelder-Mead algorithm, we optimise both the parameters and V-STIRAP pulse shapes to do this. We assemble the first fibre cavity formed of two single-mode fibres in ultra-high vacuum. Aligning two single-mode fibres is particularly demanding; we overcome this with an assembly method that gives permanent sub-micron alignment precision, unlike other similar cavities that need realigning using a translation stage. We fully characterise the cavity's parameters, then test it by probabilistically loading atoms from a magneto-optical trap, measuring the Purcell-enhanced fluorescence from the cavity. Altogether, our advances facilitate deterministically controlled, strongly-coupled and networked quantum systems.