The coupling of a single ion to an optical cavity is a promising route towards scalable quantum technologies. This study presents the design, initial construction and simulations of two different ion traps for quantum computation with novel capabilities. The first system is an end cap system that utilises Fabry Perot cavities formed by the facets of fibres. The small mode volumes (cavity length ~ 300 µm) of the fibre cavities are ideal to achieve high cooperativity and coupling strength. This trap features the novel ability to mechanically adjust the position of the ion by perturbing/distorting the trapping potential with grounded side electrodes. A capability crucial to maximising the ion-cavity coupling. The design incorporates a number of improvements over the previous iteration by Takahashi et al. such as increased optical access, increased mode matching and improved mechanical stability of the cavity. To verify and test the design modifications; Pound-Drever-Hall cavity locking was set up. The test set up failed to lock to an error signal due to a mechanically unstable translation stage. The other design presented is a linear micro trap intended to trap strings of ions and does not use fibre based cavities. This design features multiple different regions reserved for storage, computation and communication and aims to implement a shuttling scheme that will allow the selective loading of single ions from a reservoir of ions into the desired regions. The fabrication process would utilise methods in microfabrication. Simulations of the trapping dynamics were used to optimise the electrode geometries and splitting protocol.
The coupling of a single ion to an optical cavity is a promising route towards scalable quantum technologies. This study presents the design, initial construction and simulations of two different ion traps for quantum computation with novel capabilities. The first system is an end cap system that utilises Fabry Perot cavities formed by the facets of fibres. The small mode volumes (cavity length ~ 300 µm) of the fibre cavities are ideal to achieve high cooperativity and coupling strength. This trap features the novel ability to mechanically adjust the position of the ion by perturbing/distorting the trapping potential with grounded side electrodes. A capability crucial to maximising the ion-cavity coupling. The design incorporates a number of improvements over the previous iteration by Takahashi et al. such as increased optical access, increased mode matching and improved mechanical stability of the cavity. To verify and test the design modifications; Pound-Drever-Hall cavity locking was set up. The test set up failed to lock to an error signal due to a mechanically unstable translation stage. The other design presented is a linear micro trap intended to trap strings of ions and does not use fibre based cavities. This design features multiple different regions reserved for storage, computation and communication and aims to implement a shuttling scheme that will allow the selective loading of single ions from a reservoir of ions into the desired regions. The fabrication process would utilise methods in microfabrication. Simulations of the trapping dynamics were used to optimise the electrode geometries and splitting protocol.
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.
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.
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.
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 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.
Scalable quantum information processing requires efficient coupling between stationary qubits (trapped atoms, ions, NV centres, etc.) via “flying” single-photon qubits. Here we investigate the optimisation and integration of fibre-tip microcavities into radiofrequency ion traps to achieve (i) high-fidelity transfer of ion qubits onto single photons, (ii) low-loss coupling of cavity photons into single-mode optical fibre, and (iii) achieving conditions (i) and (ii) without disturbing the ion trap. Our cavities are fabricated by laser micro-machining the ends of optical fibres and subsequent coating with a thin dielectric mirror, giving rise to cavity mirrors with a Gaussian profile. These cavities allow for small mode volume and thus large ion-photon coupling fulfilling requirement (i), while simultaneously providing intrinsic coupling to optical fibres, requirement (ii), as long as mode matching between the cavity mode and the fibre mode can be achieved at the cavity mirror. Using a semi-analytical numerical method, we show that Gaussian-shaped mirrors can in fact be used to enhance ion-photon coupling beyond what is possible with more common spherical cavity mirrors. Ion traps are notoriously sensitive to the presence of dielectric materials such as cavity mirrors near the trap centre, preventing requirement (iii) from being achieved. We performed an extensive numerical study of five different common ion traps to find the optimum configurations for integrating fibre-tip cavities with minimised effects on the trapping potentials. We discuss these results and present simple and effective design rules for realising optimum ion-cavity based quantum networks.