There has been a great deal of effort and progress made towards building a fully scalable quantum computer but there are still significant engineering challenges to overcome. Trapped ions currently represent the best fundamental technology towards achieving this goal and so they form the basis for the work in this thesis. In this thesis I describe the work towards creating a closed cycle cryogenic vacuum system driven by a Gifford McMahon cryogenic cooler. I will show the equipment required to trap a Ytterbium 174/171 ion on a cryogenic surface ion trap. The primary goal is to create a fast turn-around system for ion trap research. Room temperature systems suffer from long baking times and as such ion traps are rarely changed, this means that there is little room for testing and comparing of trap designs and predicting optimum trapping parameters as each trap is usually not changed unless there is a fault. In the cryogenic system, every effort is made to make the system generic enough to allow for any trap design to be tested and to be replaced within 24 hours. To create reliable trapping parameters we require a toolkit that can numerically simulate the required RF and DC voltages used to trap the ions above the surface of the ion trap. There are many numerical methods that can be used (FEM, FVM, BEM, FTDI, etc) and in this thesis I describe multiple methods and what their relative benefits are. Since we want to create a full toolkit to allow for chip design and optimisation, most commercial programs suffer from inadequate API interfaces or they dictate which programming languages you can interface with. This would limit our abilities to develop and optimise trapping parameters, especially in a cryogenic system where the goal is rapid deployment. To this end I created a new toolkit based on the Scuff BEM solver engine that allows us to go from AutoCAD layout to field solver and gives us accurate field potentials that match micromotion compensated voltages to within 1%. I describe a novel cryogenic resonator built out of superconducting wire based on an auto-transformer topology, I compare its strengths and weaknesses compared to other technologies already in use for trapped ion research. I also describe a novel DSUB design with integrated modular filtering which allows for 50 connections per DSUB. This DSUB is used to connect two sets of 50 wires to a PCB for wirebonding to the surface trap. This allows me to save space within the 4k shield where other PCB designs would not fit. Also it is modular in design which allows for different cutoff frequencies to be swapped out as desired. I also describe different chip mounting technologies including epoxy based, physical clamping and indium foil based. After extensive testing the indium foil diebonded approach is deemed as being the most reliable and fastest non-destructive method. I also describe a novel cryogenic coil design for generating 12T m-1 field gradients for microwave based ion gates with Ytterbium 171. I then describe the creation of a combined RF and microwave antenna for delivering coherent RF and 12.6GHz microwave radiation inside the cryogenic environment. This design is much smaller than typical waveguide typologies used. I finish by showing the first trapped ions in a cryogenic surface ion trap in the UK and heating rate tests performed on two different linear surface traps. The first trap was a sapphire based, 150µm ion height linear trap with a measured heating rate of Se(1MHz) = 5.93(10) * 10-14(V /m)2. The second ion trap is a 50k Silicon based, 100µm ion height linear trap with a measured heating rate of Se(1MHz) = 2.45(10)*10-13(V /m)2
There has been a great deal of effort and progress made towards building a fully scalable quantum computer but there are still significant engineering challenges to overcome. Trapped ions currently represent the best fundamental technology towards achieving this goal and so they form the basis for the work in this thesis. In this thesis I describe the work towards creating a closed cycle cryogenic vacuum system driven by a Gifford McMahon cryogenic cooler. I will show the equipment required to trap a Ytterbium 174/171 ion on a cryogenic surface ion trap. The primary goal is to create a fast turn-around system for ion trap research. Room temperature systems suffer from long baking times and as such ion traps are rarely changed, this means that there is little room for testing and comparing of trap designs and predicting optimum trapping parameters as each trap is usually not changed unless there is a fault. In the cryogenic system, every effort is made to make the system generic enough to allow for any trap design to be tested and to be replaced within 24 hours. To create reliable trapping parameters we require a toolkit that can numerically simulate the required RF and DC voltages used to trap the ions above the surface of the ion trap. There are many numerical methods that can be used (FEM, FVM, BEM, FTDI, etc) and in this thesis I describe multiple methods and what their relative benefits are. Since we want to create a full toolkit to allow for chip design and optimisation, most commercial programs suffer from inadequate API interfaces or they dictate which programming languages you can interface with. This would limit our abilities to develop and optimise trapping parameters, especially in a cryogenic system where the goal is rapid deployment. To this end I created a new toolkit based on the Scuff BEM solver engine that allows us to go from AutoCAD layout to field solver and gives us accurate field potentials that match micromotion compensated voltages to within 1%. I describe a novel cryogenic resonator built out of superconducting wire based on an auto-transformer topology, I compare its strengths and weaknesses compared to other technologies already in use for trapped ion research. I also describe a novel DSUB design with integrated modular filtering which allows for 50 connections per DSUB. This DSUB is used to connect two sets of 50 wires to a PCB for wirebonding to the surface trap. This allows me to save space within the 4k shield where other PCB designs would not fit. Also it is modular in design which allows for different cutoff frequencies to be swapped out as desired. I also describe different chip mounting technologies including epoxy based, physical clamping and indium foil based. After extensive testing the indium foil diebonded approach is deemed as being the most reliable and fastest non-destructive method. I also describe a novel cryogenic coil design for generating 12T m-1 field gradients for microwave based ion gates with Ytterbium 171. I then describe the creation of a combined RF and microwave antenna for delivering coherent RF and 12.6GHz microwave radiation inside the cryogenic environment. This design is much smaller than typical waveguide typologies used. I finish by showing the first trapped ions in a cryogenic surface ion trap in the UK and heating rate tests performed on two different linear surface traps. The first trap was a sapphire based, 150µm ion height linear trap with a measured heating rate of Se(1MHz) = 5.93(10) * 10-14(V /m)2. The second ion trap is a 50k Silicon based, 100µm ion height linear trap with a measured heating rate of Se(1MHz) = 2.45(10)*10-13(V /m)2
Quantum technology has made great strides in the last two decades with trapped ions demonstrating all the necessary building blocks for a quantum computer. While these proof of principle experiments have been demonstrated, it still remains a challenging task to scale these experiments down to smaller systems. In this thesis I describe the development of technology towards scalable cryogenic ion trapping and quantum hybrid systems. I first discuss the fundamentals of ion trapping along with the demonstration of ion trapping on a novel surface electrode ion trap with a ring shaped architecture. I then present the development of a cryogenic vacuum system for ion trapping at ~4 K, which utilizes a closed cycle Gifford McMahon cryocooler with a helium gas buffered ultra-low vibration interface to mechanically decouple a ultra-high vacuum system. Ancillary technologies are also presented, including a novel in-vacuum superconducting rf resonator, low power dissipation ceramic based atomic source oven and an adaptable in-vacuum permanent magnet system for long-wavelength based quantum logic. The design and fabrication of microfabricated surface ion traps toward quantum hybrid technologies are then presented. A superconducting ion trap with an integrated high quality factor microwave cavity and vertical ion shuttling capabilities is described. The experimental demonstration of the cavity is also presented with quality factors of Q6~6000 and Q~15000 for superconducting niobium nitride and gold based cavities respectively, which are the highest demonstrated for microwave cavities integrated within ion trapping electrode architectures. An ion trap with a multipole electrode geometry is then presented, which is capable of trapping a large number of ions simultaneously. The homogeneity of five individual linear trapping regions are optimized and the design for the principle axis rotation of each linear region is presented. An overview of microfabrication techniques used for fabricating surface electrode ion traps is then presented. This includes the detailed microfabrication procedure for ion traps designed within this thesis. A scheme for the integration of ion trapping and superconducting qubit systems as a step towards the realization of a quantum hybrid system is then presented. This scheme addresses two key diffculties in realizing such a system; a combined microfabricated ion trap and superconducting qubit architecture, and the experimental infrastructure to facilitate both technologies. Solutions that can be immediately implemented using current technology are presented. Finally, as a step towards scalability and hybrid quantum systems, the interaction between a single ion and a microwaves field produced from an on chip microwave cavity is explored. The interaction is described for the high-Q microwave cavity designed in this thesis and a 171Yb+ion. A description of the observable transmission from the cavity is described and it is shown that the presence of a single ion can indeed be observed in the emission spectrum of high-Q microwave cavity even in the weak coupling regime.
Microfabricated ion traps are an important tool in the development of scalable quantum systems. Tremendous advancements towards an ion quantum computer were made in the past decade and most requirements for a quantum computer have been fulfilled in individual experiments. Incorporating all essential capabilities in a fully scalable system will require the further advancement of established quantum information technologies and development of new trap fabrication techniques. In my thesis I will discuss the theoretical background and experimental setup required for the operation of ion traps. Measurement of the important ion trap heating rate was performed in the setup and I will discuss the results in more detail. I will give a review of microfabrication processes used for the fabrication of traps, outlining advantages, disadvantages and issues inherent to the processes. Following the review I will present my work on a concept for a scalable ion trap quantum system based on microwave quantum gates and shuttling through X-junctions. Many of the required building blocks, including ion trap structures with current-carrying wires intended to create strong magnetic field gradients for microwave gates were investigated further. A novel fabrication process was developed to combine current-carrying wires with advanced multilayered ion trap structures. Several trap designs intended for proof of principle experiments of high fidelity microwave gates, advanced detection techniques and shuttling between electrically disconnected ion traps will be presented. Also the electrode geometry of an optimized X-junction design with strongly suppressed rf barrier height will be presented. Further, I developed several modifications for the experimental setup to extend the existing capabilities. A plasma source capable of performing in-situ cleans of the trap electrode surfaces, which has been demonstrated to dramatically reduce the heating rate in ion traps, was incorporated. I will also present a vacuum system modification designed to cool ion traps with current-carrying wires and transport the generated heat out of the vacuum system. In addition a novel low-noise, high-speed, multichannel voltage control system was developed by me. The device can be used in future experiments to precisely shuttle ions from one trapping zone to another and also to shuttle ions through ion trap junctions. Lastly I will outline the process optimization and microfabrication of my ion trap designs. A novel fabrication process which makes use of the extremely high thermal conductivity of diamond substrates and combines it with thick copper tracks embedded in the substrate was developed. Large currents will be passed through the wires creating a strong and controllable magnetic field gradient. Ion trap designs with isolated electrodes connected via buried wires can be placed on top of the current-carrying wires, allowing the most advanced electrode designs to be fabricated with current-carrying wires.
Over the past 15 years ion traps have demonstrated all the building blocks required of a quantum computer. Despite this success, trapping ions remains a challenging task, with the requirement for extensive laser systems and vacuum systems to perform operations on only a handful of qubits. To scale these proof of principle experiments into something that can outperform a classical computer requires an advancement in the trap technologies that will allow multiple trapping zones, junctions and utilize scalable fabrication technologies. I will discuss the construction of an ion trapping experiment, focussing on my work towards the laser stabilization and ion trap design but also covering the experimental setup as a whole. The vacuum system that I designed allows the mounting and testing of a variety of ion trap chips, with versatile optical access and a fast turn around time. I will also present the design and fabrication of a microfabricated Y junction and a 2- dimensional ion trap lattice. I achieve a suppression of barrier height and small variation of secular frequency through the Y junction, aiding to the junctions applicability to adiabatic shuttling operations. I also report the design and fabrication of a 2-D ion trap lattice. Such structures have been proposed as a means to implement quantum simulators and to my knowledge is the first microfabricated lattice trap. Electrical testing of the trap structures was undertaken to investigate the breakdown voltage of microfabricated structures with both static and radio frequency voltages. The results from these tests negate the concern over reduced rf voltage breakdown and in fact demonstrates breakdown voltages significantly above that typically required for ion trapping. This may allow ion traps to be designed to operate with higher voltages and greater ion-electrode separations, reducing anomalous heating. Lastly I present my work towards the implementation of magnetic fields gradients and microwaves on chip. This may allow coupling of the ions internal state to its motion using microwaves, thus reducing the requirements for the use of laser systems.
Trapped ions are one of the leading platforms for building scalable quantum information processors. Current ion traps run on less than 100 qubits. A useful quantum computer needs to operate thousands or even millions of qubits. If systems reach gate fidelities that allow for quantum error correction, gate errors below the physical fidelity can be reached, which is important for scaling. The most promising platform for scaling up ion-based quantum computers are surface electrode traps in connection with the so-called QCCD (Quantum Charge Coupled Device) architecture. In an evolution of the standard (laser-based) approach to quantum logic gates, gates can be performed by applying an oscillating magnetic near-field gradient to the ions. Current fidelities are about one order of magnitude away from reaching the fault tolerant threshold with no fundamental limitations in sight. Scaling QCCDs is impeded by heating rates of the ions as well as vacuum quality. Ions have to be stored for at least as long as it takes for an algorithm to run and subsequent readout. Collisions with background gas and heating can expel ions from the trap. The microwave near-field approach of driving gates benefits from ions that are trapped close to the trap surface. Unfortunately, heating becomes more prominent near the surface. A cryogenic environment reduces the background pressure by several orders of magnitude and the kinetic energy of gas molecules by two orders of magnitude. Heating rates in a cryogenic trap are about two orders of magnitude lower than in a room temperature trap with otherwise equal physical properties. To operate a QCCD processor, it is required to employ phase-stable Raman laser beams. The cryogenic design of the apparatus needs to take into account the resulting requirements for ultra-low vibration amplitudes. This is particularly relevant for closed-cycle cooling systems, which are desirable from an economic point of view compared to liquid cryostats that boil off helium, but which also feature moving mechanical parts that can introduce a serious amount of vibrations. Here, we present a vibration isolated closed cycle Gifford-McMahon cryocooler with record low vibration amplitudes of 29 nm (RMS=7.8 nm) in the vertical and 51 nm (RMS=13.5 nm) in the horizontal direction. It contains a self-sustained inner vacuum chamber that houses the trap. It features 100 DC connections, 10 of which are able to carry up to 1 A of current and 8 high frequency lines. At 5 K, we can apply continuous 3.4 W of microwave power continuously at 1 GHz to the trap electrodes without heating the system. This is about 5.8 times more than state of the art near-field gates require. We also present the laser systems required to for single-shot ablation loading and Doppler cooling of Be ions: a 70 mW 313 nm Dopper cooling laser, a 1064 nm ns-pulse laser for ablation of neutral beryllium atoms and a 235 nm laser for ionizing them. The detection system features an in vacuum imaging system on a cryogenic 3D translation stage that is able to scan and image the complete surface of an ion trap. The light can be detected on a photomultiplier tube or spatially resolved on an EMCCD camera. Our trap features a meander structure for driving gates and can trap up to 5 ions of which we, so far, did not lose any due to background gas collisions. This hints at a vacuum pressure of 1e-12 mbar or lower. The demonstrated system could hold a cryogenic QCCD chip with a few 10 qubits
While quantum information processing offers a tantalizing possibility of a significant speedup in execution of certain algorithms, as well as enabling previously unmanageable simulations of large quantum systems, it remains extremely difficult to realize experimentally. Recently, fundamental building blocks of a quantum computer, including one and two qubit gates, teleportation and error correction, were demonstrated using trapped atomic ions. Scaling to a larger number of qubits requires miniaturization of the ion traps, currently limited by the sharply increasing motional state decoherence at sub-100 μm ion-electrode distances. This thesis explores the source and suppression of this decoherence at cryogenic temperatures, and demonstrates fundamental logic gates in a surface electrode ion trap.
Construction of the apparatus requires the development of a number of experimental techniques. Design, numerical simulation and implementation of a surface electrode ion trap is presented. Cryogenic cooling of the trap to near 4 K is accomplished by contact with a bath cryostat. Ions are loaded by ablation or photoionization, both of which are characterized in terms of generated stray fields and heat load.
The bulk of new experimental results deals with measurements of electric field noise at the ion's position. Upon cooling to 6 K, the measured rates are suppressed by up to 7 orders of magnitude, more than two orders of magnitude below previously published data for similarly sized traps operated at room temperature. The observed noise depends strongly on fabrication process, which suggests further improvements are possible. The measured dependence of the electric field noise on temperature is inconsistent with published models, and can be explained using a continuous spectrum of activated fluctuators.
The fabricated surface electrode traps are used to demonstrate coherent operations and the classical control required for trapped ion quantum computation. The necessary spectral properties of coherent light sources are achieved with a novel design using optical feedback to a triangular, medium finesse, cavity, followed by electronic feedback to an ultra-high finesse reference cavity. Single and two qubit operations on a single ion are demonstrated with classical fidelity in excess of 95%. Magnetic field gradient coils built into the trap allow for individual addressing of ions, a prerequisite to scaling to multiple qubits. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)
This thesis describes the experimental work towards the development of a scalable quantum computer based on microfabricated ion trap using long-wavelength radiation and magnetic field gradient. There are three key elements in implementing such a quantum computer: the junction trap to shuttle ions, the structure to generate high gradient magnetic field and the structure to induce strong microwave coupling to the ions. A new dynamic simulation tool was developed addressing the problems faced by static solvers. This tool was used to aid the design and optimisation of an X junction geometry allowing the ions to be shuttled with minimised motional heating gain. The design and fabrication techniques were reported on the structure to generate a high gradient magnetic field. A discussion was given on the design of producing microwave and maximise the coupling to the cold ions. A review was given on the far-field methods and near-field methods. A novel design was reported where the single-qubit gate is predicted to be 45 times faster than a conventional setup. Two essential topics on the microfabrication of a reliable scalable quantum computer unit were discussed: breakdown and RF loss. Investigation using numerical simulations showed that dielectric can breakdown due to high voltage and local heating which are results of impedance mismatch. Microfabrication processes were improved, high-quality films were reported to have twice as much breakdown voltages. The mechanisms of RF loss were reviewed. Novel structures and smooth electroplating technique were developed to minimise the loss. A low loss ion trap was produced and tested. An experimentally observed anomalous glow discharge phenomenon was reported and investigated.