Browsing by Author "Kim, Jungsang"

Ion traps are a leading candidate for scaling quantum computers. The component technologies can be difficult to integrate and manufacture. Experimental systems are also subject to mechanical drift creating a large maintenance overhead. A full system redesign with stability and scalability in mind is presented. The center of our approach is a compact cryogenic ion trap package (trap cryopackage). A surface trap is mounted to a modified ceramic pin grid array (CPGA) this is enclosed using a copper lid. The differentially pumped trap cryopackage has all necessary optical feedthroughs and an ion source (ablation target). The lid pressure is held at ultra-high vacuum (UHV) by cryogenic sorption pumping using carbon getter. We install this cryopackage into a commercial low-vibration closed-cycle cryostat which sits inside a custom monolithic enclosure. The system is tested and trapped ions are found to have common mode heating rate on the order of 10 quanta/s. The modular optical setup provides for a couterpropagating single qubit coherence time of 527 ms. We survey a population of FM two-qubit gates (gate times 120 μs - 450 μs) and find an average gate fidelity of 98\%. We study the gate survey with quantum Monte Carlo simulation and find that our two-qubit gate fidelity is limited by low frequency (30 Hz - 3 kHz) coherent electrical noise on our motional modes.

The challenge of building scalable quantum computer lies in striking appropriate balance between designing a reliable system architecture from large number of faulty computational resources and improving the physical quality of system components. The detailed investigation of performance variation with physics of the components and the system architecture requires adequate performance simulation tool. In this thesis we demonstrate a software tool capable of (1) mapping and scheduling the quantum circuit on a realistic quantum hardware architecture with physical resource constraints, (2) evaluating the performance metrics such as the execution time and the success probability of the algorithm execution, and (3) analyzing the constituents of these metrics and visualizing resource utilization to identify system components which crucially define the overall performance.

Using this versatile tool, we explore vast design space for modular quantum computer architecture based on trapped ions. We find that while success probability is uniformly determined by the fidelity of physical quantum operation, the execution time is a function of system resources invested at various layers of design hierarchy. At physical level, the number of lasers performing quantum gates, impact the latency of the fault-tolerant circuit blocks execution. When these blocks are used to construct meaningful arithmetic circuit such as quantum adders, the number of ancilla qubits for complicated non-clifford gates and entanglement resources to establish long-distance communication channels, become major performance limiting factors. Next, in order to factorize large integers, these adders are assembled into modular exponentiation circuit comprising bulk of Shor's algorithm. At this stage, the overall scaling of resource-constraint performance with the size of problem, describes the effectiveness of chosen design. By matching the resource investment with the pace of advancement in hardware technology, we find optimal designs for different types of quantum adders. Conclusively, we show that 2,048-bit Shor's algorithm can be reliably executed within the resource budget of 1.5 million qubits.

The visible light photon counter (VLPC) is a high quantum efficiency (QE), Si-based, single-photon detector with high gain, low-noise multiplication, low timing jitter, and photon number resolution. While the VLPC has high QE in the visible wavelengths, the QE in the ultraviolet and infrared is low due to minimal absorption within the active layers of the device. In the ultraviolet, the absorption coefficient of Si is high and most of the incident photons are absorbed within the top contact of the device, whereas, in the infrared, Si is practically transparent. A number of applications in quantum information science would benefit from use of the VLPC if the QE was improved in the ultraviolet (e.g., state detection of trapped ions) and the infrared (e.g., long-distance quantum cryptography). This thesis describes the development of the ultraviolet photon counter (UVPC) and the infrared photon counter (IRPC), which are modified versions of the VLPC with increased QE in the ultraviolet and infrared wavelengths, respectively. The UVPC has a transparent metal Schottky contact to reduce absorption within the top contact of the VLPC, resulting in an increase in the QE in the ultraviolet by several orders of magnitude. The IRPC is a proposed device that has an InGaAs absorption layer that is wafer-fusion bonded to the VLPC. The band alignment of the resulting InGaAs/Si heterojunction is measured and shows a large discontinuity in the valence band that impedes carrier transport at the interface. A ultra-high vacuum wafer-bonding system was developed to understand the impact of the surface chemistry of the bonded wafers on the band alignment of the InGaAs/Si heterojunction of the IRPC.

Quantum communication protocols, such as quantum key distribution (QKD), are practically important in the dawning of a new quantum information age where quantum computers can perform efficient prime factorization to render public key cryptosystems obsolete. QKD is a communication scheme that utilizes the quantum state of a single photons to transmit information, such as a cryptographic key, that is robust against adversaries including those with a quantum computer. In this thesis I describe the contributions that I have made to the development of high-rate, free-space quantum communication systems.

My effort is focused on building a robust quantum receiver for a high-dimensional time-phase QKD protocol where the data is encoded and secured using a single photon's timing and phase degrees of freedom. This type of communication protocol can encode information in a high-dimensional state, allowing the transmission of $>1$ bit per photon. To realize a successful implementation of the protocol a high-performance single-photon detection system must be constructed. My contribution to the field begins with the development of low-noise, low-power cryogenic amplifiers for a detection system using superconducting nanowire single-photon detectors (SNSPDs). Detector characteristics such as maximum count rate and timing resolution are heavily influenced by the design of the read-out circuits that sense and amplify the detection signal. I demonstrate a read-out system with a maximum count rate $>20\,$million counts-per-second and timing resolution as high as $35$\,ps. These results are achieved while maintaining a low power dissipation $<3$\,mW at 4\,K operation, enabling a scalable read-out circuit strategy.

A second contribution I make to the development of detection systems utilizing SNSPDs is extending the superb performance of these detectors to include photon number resolving capabilities. I demonstrate that SNSPDs exhibit multi-photon detection up to four photons where the absorbed photo number is encoded in the rise time of the electrical waveform generated by the detector. Additionally, our experiment agrees well with the predictions of a universal model for turn-on dynamics of SNSPDs. A feature our multi-photon detection system demonstrates high resolution between $n=1$ and $n>1$ photons with a bit-error-rate (BER) of $4.2\times10^{-4}$.

Finally, I extend the utility of the time-phase QKD protocol to free-space applications. Atmospheric turbulences cause spatial mode scrambling of the optical beam during transmission. Therefore, the quantum receiver, and most importantly the time-delay interferometer needed for the measurement of a phase encoding of a single photon, must support many spatial modes. I construct and characterize an interferometer with a 5\,GHz free spectral range that has a wide field-of-view and is passively a-thermal. The results of interferometer characterization are highlighted by a $>99\,\%$ single-mode, and $>98\,\%$ multi-mode interference visibility with negligible dependence on the spatial mode structure of the input beam and modest temperature fluctuations. Additionally, the interferometer displays a small path-length shift of 130\,nm/$^{\,\circ}$C, allowing for great thermal stability with modest temperature control.

The trapped atomic ion qubits feature desirable properties for use in a quantum computer such as long coherence times, high qubit readout fidelity, and universal logic gates. While these essential properties have been demonstrated, the ability to scale a trapped ion quantum system has not yet been shown. The challenge of scaling the system calls for methods to realize high-fidelity logic gates in scalable trap structures. Surface electrode ion traps, that are microfabricated from a silicon substrate, provide a scalable platform for trapping ion qubits only if high-fidelity operations are achievable in these structures. Here, we present a system for trapping and manipulating ions in a scalable surface trap. Trapping times exceeding 20 minutes without laser cooling, and heating rates as low as 0.8 quanta/ms indicate stable trapping conditions in these microtraps. Coherence times of more than one second verify adequate qubit and control field stability. We demonstrate low-error single-qubit gates performed using stimulated Raman transitions driven by lasers that are tightly focused on the ion qubit. Digital feedback loops are implemented to control the driving field's amplitude and frequency. Gate errors are measured using a randomized benchmarking protocol for single qubit gates, where residual amplitude error in the control beam is compensated using various pulse sequence techniques. Using pulse compensation, we demonstrate single qubit gates with an average error per randomized Clifford group gate of $3.6(3)\times10^{-4}$, which is below the fault-tolerant threshold for some error-correction schemes.

Trapped ions provide a promising platform to build a practical quantum computer. Scaling the high performance of small systems to longer ion chains is a technical endeavor that benefits from both better hardware system design and gate-level control techniques. In this thesis, I discuss our work on building a small-scale trapped-ion quantum computing system that features stable laser beam control, low-crosstalk individual addressing and capability to implement high-fidelity multi-qubit gates.

We develop control techniques to extend the pack-leading fidelity of entangling gates in two-ion systems to longer chains. A major error source limiting entangling gate fidelities in ion chains is crosstalk between target and neighboring spectator qubits. We propose and demonstrate a crosstalk suppression scheme that eliminates all first-order crosstalk utilizing only local control of target qubits, as opposed to an existing scheme which requires control over all neighboring qubits. Using the scheme, we achieve a $99.5\%$ gate fidelity in a 5-ion chain. Complex quantum circuits can benefit from native multi-qubit gates such as the $N$-Toffoli gate, which substantially reduce the overhead cost from performing universal decomposition into single- and two-qubit gates. We take advantage of novel performance benefits of long ion chains to realize scalable Cirac-Zoller gates, which uses a simple pulse sequence to efficiently implement $N$-Toffoli gates. We demonstrate the Cirac-Zoller 3- and 4-Toffoli gates in a five-ion chain with higher fidelities than previous results using trapped ions. We also present the first experimental realization of a 5-Toffoli gate.

Trapped ion systems are the leading candidate for quantum information processing because many of the critical components have already been demonstrated. Scaling trapped ion systems to large numbers of ions is currently believed possible, but much work remains to prove it. Microfabricated surface ion traps are increasing in popularity for their ease of mass production and their ability to manipulate individual ions and interact arbitrary pairs of ions. Even with the advent of scalable ion traps, detection of an individual ion trapped in a high vacuum poses a challenge. The internal state of the ion chosen for a quantum bit can be measured via exposure to a probe beam that causes one state to scatter light (a "bright" state), but not the other state (a "dark" state). In free space, a single ion acts like a point source that emits in all directions; a standard two inch lens system can only collect about 2% of the light emitted by the ion. Poor light collection results in a high error rate and slow determination of the internal state of the ion. Fast, high fidelity state detection is necessary for quantum error correction and loophole-free Bell experiments at short (less than 100\,km) distances, and high efficiency collection is necessary to rapidly interconnect separate quantum computers. We demonstrate state detection fidelities of 99%, 99.856(8)% and 99.915(7) % which correspond to detection times of 10.5, 28.1 and 99.8 us, respectively.

This dissertation addresses challenges in quantum computing with trapped ions, including system integration of the trapped ion hardware, heating-induced decoherence, and efficient cooling mechanisms, by investigating multi-species or isotope ion chains with sympathetic cooling techniques. The study explores loading ion chains with 174Yb and 138Ba species, using an Nd:YAG ablation laser for isotope-selective trapping. A detail of the design of a compact room-temperature trapped ion system is presented, comprising an ultra-high vacuum (UHV) package, a micro-fabricated surface trap, and a small form-factor ion pump, demonstrating trapping of 174Yb+ ions and achieving a suitable vacuum level. A detailed ion chain cooling model based on sympathetic cooling addresses the heating problem of 171Yb+ qubits on a two-dimensional surface trap, revealing the crucial role of the mass ratio between cooling and qubit ions in the sympathetic cooling dynamics. The dissertation further delves into the implementation of sympathetic cooling in trapped ion systems, successfully cooling qubit 171Yb+ ions using 172Yb+ or 174Yb+ ions as cooling ions, reducing motional heating and decoherence without direct interaction with qubits. In summary, this dissertation contributes to developing and optimizing compact trapped ion systems for quantum computing, providing insights on multi-species ion chains, sympathetic cooling techniques, and ion chain cooling models to enhance trapped ion quantum computing performance and fidelity.

Atomic ions trapped in microfabricated surface traps can be utilized as a physical platform with which to build a quantum computer. They possess many of the desirable characteristics of such a device, including high fidelity state preparation and readout, universal logic gates, and long coherence times, and can be readily entangled with each other through photonic interconnects. The use of optical cavities integrated with trapped ion qubits as a photonic interface presents the possibility for order of magnitude improvements in performance in several key areas for their use in quantum computation. The first part of this thesis describes the design and fabrication of a novel surface trap for integration with an optical cavity. The trap is custom made on a highly reflective mirror surface and includes the capability of moving the ion trap location along all three trap axes with nanometer scale precision. The second part of this thesis demonstrates the suitability of small microcavities formed from laser ablated, fused silica substrates with radii of curvature in the 300-500 micron range for use with the mirror trap as part of an integrated ion trap cavity system. Quantum computing applications for such a system include dramatic improvements in the photon entanglement rate of up to 10 kHz, the qubit measurement time down to 1 microsecond, and the qubit measurement error rate down to the 1e-5 range. The final part of this thesis describes a performance simulator for exploring the physical resource requirements and performance demands to scale a quantum computer to sizes capable of implementing quantum algorithms beyond the limits of classical computation.

Electronic focal plane arrays (FPA) such as CMOS and CCD sensors have dramatically improved to the point that digital cameras have essentially phased out film (except in very niche applications such as hobby photography and cinema). However, the traditional method of mating a single lens assembly to a single detector plane, as required for film cameras, is still the dominant design used in cameras today. The use of electronic sensors and their ability to capture digital signals that can be processed and manipulated post acquisition offers much more freedom of design at system levels and opens up many interesting possibilities for the next generation of computational imaging systems.

The AWARE gigapixel scale camera is one such computational imaging system. By utilizing a multiscale optical design, in which a large aperture objective lens is mated with an array of smaller, well corrected relay lenses, we are able to build an optically simple system that is capable of capturing gigapixel scale images via post acquisition stitching of the individual pictures from the array. Properly shaping the array of digital cameras allows us to form an effectively continuous focal surface using off the shelf (OTS) flat sensor technology.

This dissertation details developments and physical implementations of the AWARE system architecture. It illustrates the optomechanical design principles and system integration strategies we have developed through the course of the project by summarizing the results of the two design phases for AWARE: AWARE-2 and AWARE-10. These systems represent significant advancements in the pursuit of scalable, commercially viable snapshot gigapixel imaging systems and should serve as a foundation for future development of such systems.

Quantum key distribution systems represent a proven method to obtain fundamentally secure communication channels. However, the loss of entangled photons which create the encryption key limit the distance over which the systems are useful. Quantum repeaters have been used to increase the distance over which entangled photons can be transmitted. The most successful quantum repeaters entangle information between trapped ions and single photons and through the use of entanglement swapping protocols, relay the encryption keys over large distances. A requirement of these quantum repeaters is the ability to work in the telecom wavelengths (1310 nm or 1550 nm), where standard silica fibers exhibit low loss. With the majority of ions emitting in the ultraviolet and visible spectrum, and ytterbium in particular emitting at 369.5 nm, a conversion process is then needed that can translate the photons from 369.5 nm to the telecom band while preserving the entangled state. The experiment presented here details the first stage of a nonlinear frequency conversion setup that uses strong pump light to enable the conversion of 369.5 nm photons to 708 nm. The efficiency of the conversion process is analyzed to determine the optimum system parameters, with an initial single pass conversion efficiency of 0.20% achieved. Noise introduced during the nonlinear conversion process is examined to determine the effect it presents on the communication channel.

Among the various physical systems considered for scalable quantum information processing (QIP), individually trapped ions or neutral atoms have emerged as promising candidates. Recent experiments using these systems have demonstrated the basic building blocks required for a useful quantum computer. In many of these experiments, precisely tuned lasers control and manipulate the quantum bit (qubit) represented in the electronic energy levels of the ion or atom. Scaling these systems to the necessary number of qubits needed for meaningful calculations, requires the development of scalable optical technology capable of delivering laser resources across an array of ions or atoms. That scalable technology is currently not available.

In this dissertation, I will report on the development, design, characterization, and implementation of an optical beam steering system utilizing microelectromechanical systems (MEMS) technology. Highly optimized micromirrors enable fast reconfiguration of multiple laser beam paths which can accommodate a range of wavelengths. Employing micromirrors with a broadband metallic coating, our system has the flexibility to simultaneously control multiple beams covering a wide range of wavelengths.

The reconfiguration of two independent beams at different wavelengths (780 and 635 nm) across a common 5x5 array of target sites is reported along with micromirror switching times as fast as 4 us. The optical design of the system minimizes residual intensity at neighboring sites to less than 40 dB below the peak intensity. Integration of a similar system into a neutral atom QIP experiment is reported where 5 individually trapped atoms are selectively manipulated through single qubit rotations with a single laser source. This demonstration represents the first application of MEMS technology in scalable QIP laser addressing.

Over the past decade, the study of zinc oxide (ZnO) II-VI semiconducting nanostructures has been a burgeoning research area because of this material's unique electrical and optical properties. Despite the promise of its characteristics for numerous applications, usage of ZnO in the fabrication of nanoscale devices on a commercial scale remains a challenge because of our lack of knowledge of the underlying physics and chemistry of nanostructures. Sustainable progress in nanowire manufacturing techniques requires that we first undertake basic studies to address these poorly understood underlying concepts before we embark on applied engineering. If these fundamental studies prove successful, then characterization, fabrication, and large-scale integration of nanostructures that use ZnO could be applied to a range of engineering fields. This doctoral dissertation is primarily concerned with the synthesis and doping required for the creation of novel ZnO nanostructures and the growth mechanisms of such structures. Numerous studies have been made of various kinds of ZnO nanostructures. However, no studies have been reported of systematic theoretical modeling that uses both density functional theory and as-synthesized nanostructures to explain the growth mechanisms involved in these devices. First, sulfur-doped ZnO nanostars, synthesized through a hydrothermal method, will be discussed. This section uses ab initio simulations in discussing the synthesis of novel ZnO nanostructures and their proposed growth mechanisms. Moreover, this discussion also addresses the optical properties of ZnO structures that cause sulfur doping to enhance their emission of green light. The next section introduces a novel synthetic methodology to reliably produce well-aligned vertical ZnO nanowire arrays on amorphous substrates. Vertical alignment of nanowires significantly improves the performance of devices like LEDs and solar cells. Because these vertically aligned arrays have historically been made using sapphire substrates that hinder their commercialization, substantial effort has been invested in using ZnO nanocrystal seeds to grow vertically aligned ZnO nanowires on silicon substrates. Well-known synthetic methods, such as zinc acetate dissolved in methanol or zinc acetate combined with sodium hydroxide (or potassium hydroxide), have typically been used in pursuit of this goal without a detailed understanding of the mechanisms of seed creation. The consequence of this lack of knowledge has been inconsistent reproducibility in growing vertically aligned nanowires on silicon substrates. This discussion includes the details of mechanisms that explain the why and how of creation of vertical/misoriented ZnO nanocrystal seeds on silicon substrates. In addition, a preferential c-axis-oriented ZnO nanocrystal seed has been successfully synthesized using a solution composed of ammonium hydroxide (NH4OH) and zinc acetate (Zn(O2CCH3)2). Lastly, the synthesis of sea urchin-like microstructures known as ZnO sea urchins will be introduced. Among the various kinds ZnO structures, the ZnO sea urchin is a integrated structure composed of a 3-D microsphere and 1-D nanowires. Dye-sensitized solar cells (DSSCs) made of ZnO sea urchins have shown a higher power conversion efficiency than planar nanowires. This is because ZnO sea urchins have a higher surface area per unit of volume than planar nanowire arrays. This larger surface area allows larger amounts of dye to access the semiconducting nanowires. We have synthesized the sea urchin structures composed of ZnOxPy microspheres, a mixed of zinc phosphide (Zn3P2) and ZnO phase, encapsulated in an array of ZnO nanowires. Synthesis of these interesting structures was achieved without resorting to the prefabricated 3-D microsphere templates that other groups used in previous studies. This new approach to the synthesis of ZnO sea urchin structures was accomplished by simply adding Zn3P2 powder to the C (graphite) and ZnO source powders in a chemical vapor transport method. The ZnO sea urchin's material properties and growth mechanism will be characterized and discussed in detail.