In many quantum information applications quantum bits (qubits) are stored in single trapped atoms or ions. In particular, single ions held in radiofrequency electromagnetic traps are preferred because of their long achievable decoherence time and record-breaking quantum gate fidelity [1]. However, for scalable quantum information processing many such stationary qubits need to be connected. A preferred way of doing this would be via optical fibres, i.e., by transferring quantum information between stationary qubits with the help of “flying” single-photon qubits. Efficient operation of the quantum network thus requires (i) high-fidelity transfer of an ion qubit onto a single photon, (ii) low-loss coupling of the photon into a single-mode optical fibre, and (iii) achieving conditions (i) and (ii) without disturbing the ion trap.
In many quantum information applications quantum bits (qubits) are stored in single trapped atoms or ions. In particular, single ions held in radiofrequency electromagnetic traps are preferred because of their long achievable decoherence time and record-breaking quantum gate fidelity [1]. However, for scalable quantum information processing many such stationary qubits need to be connected. A preferred way of doing this would be via optical fibres, i.e., by transferring quantum information between stationary qubits with the help of “flying” single-photon qubits. Efficient operation of the quantum network thus requires (i) high-fidelity transfer of an ion qubit onto a single photon, (ii) low-loss coupling of the photon into a single-mode optical fibre, and (iii) achieving conditions (i) and (ii) without disturbing the ion trap.
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.
Constructing a quantum network comprising of matter-based stationary qubits and photonic flying qubits is a key goal towards scalable quantum computing. Individually trapped neutral atoms are amongst the most promising candidates for stationary qubits, while single photons emitted from optical cavities appears to be the best approach to forming flying qubits. This thesis presents a fast and versatile method to trap and transport an array of neutral atoms, with the aid of a spatial light modulator (SLM). We demonstrate trapped 87Rb atoms adapting to the arbitrary and dynamic potential landscapes imposed by the SLM. On top of this we also investigate the inclusion of fibre-tip cavities into the experiment. In these microcavities, the mirrors are replaced by ablated fibre-tip faces, coated in a highly reflective dielectric stack. These have the benefit of small geometries, increasing their scalability; which in turns leads to extremely small mode volumes, allowing for strong coupling with atoms inside the cavity modes. We construct and characterise a high finesse ℱ = 120, 000 fibre-tip cavity constructed from two single mode fibres.
In recent years there has been rapid progress into realising a working universal quantum computer, in particular with the development of chip-based radio frequency ion traps. The next significant leap will come with successfully integrating optical cavities into these ion traps to allow for interaction between remote ions via photons as required for more efficient and scalable quantum networking schemes. Fibre-tip cavities are especially interesting for such applications as they enable highly efficient coupling of photons from the cavity into optical fibres for onward transmission. Here we analyse theoretically and numerically the effects of parallel off-axial misalignment in millimetre scale optical Fabry-Perot cavities. While near-concentric cavity configurations produce the smallest mode waist and thus strongest coupling to a trapped ion, their mode is extremely sensitive to misalignment. Shorter cavities exhibit more robust modes, but at the cost of larger mode waists. For example, for typical experimental parameters (mirror radius of curvature 0.7 mm, mirror diameter 0.140 mm, operation wavelength 850 nm) we find that the cavity lifetime is reduced by a factor 1/e for a misalignment of 0.95 nm for a beam waist of 2.91 um (cavity length of 1.397 mm), which increases to 11.0 nm for a waist of 4.33 um (length of 1.386 mm), and 3.12 um for a waist of 7.38 um (length of 1.273 mm). In the parameter regimes of interest, we derive a simple relation between cavity length, mirror radius, and misalignment sensitivity. Finally, we also consider the effect of mode matching of the misaligned cavity mode with the optical mode of the fibre for efficient cavity to fibre coupling. In conclusion, our model allows us to optimise photon-ion coupling in fibre-tip resonators for quantum information processing in the presence of finite fabrication and alignment tolerances.
The distribution of quantum information as well as the utilization of non-locality are at the heart of quantum networks, which show great promise for future applications like quantum communication, distributed quantum computing and quantum metrology. Their practical realization however is a formidable challenge. On the one hand, the state of the respective quantum system has to be under perfect control, ideally in all degrees of freedom. This requires low decoherence, i.e. minimal interaction with the environment. But at the same time, strong, tailored interactions are required to enable all envisioned processing tasks. In this respect, single atoms and photons are the ideal building blocks of a quantum network. Atoms can act as stationary nodes as they are long-lived and their interaction with the environment is weak, while their external and internal degree of freedom can precisely be controlled and manipulated. Photons can be transmitted over larger distances using existing fiber technology and do not mutually interact.
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.