The Electronic Control System of a Trapped-Ion Quantum Processor: A Systematic Literature Review

[Background] Ions in an ion trap are among the most promising technologies to implement a quantum processor. This machine is governed by a classic electronic control system, for which we found no systematic electronic system-level approach for its conception, design, implementation, and/or verification. The trapped-ion quantum processor cannot advance its roadmap without an appropriate and fitting electronic control system. To fully enable further advancements in the field, an understanding of the electronic control system as a system of its own and the conception of its electronic system-level description, are due needed. [Objective] Therefore, we want to address the electronic control system of a trapped-ion quantum processor as a system of its own by first identifying its published literature. [Method] For that purpose, we conducted a systematic literature review abiding by the APISSER methodology for systematic literature reviews in engineering, which describes and delimits the reach of this study. [Results] In this paper, we report the results of the study, classify them, and present possible research directions and considerations to be taken in the conception of this system. [Conclusion] This study lays a introductory foundation to grasp the requirements and environment of the electronic control system of a trapped-ion quantum processor. A foundation much needed to leverage further research on this topic by the engineering community. Furthermore, it serves as a checkpoint in time: listing and synthesizing, the existing published body-of-knowledge of this topic within the defined boundaries of our method.

INDEX TERMS Quantum computing, processor, trapped-ion, electronic control system, APISSER. This paper reports the systematic literature review (SLR) conducted to identify the published literature of the electronic control system (ECS) of trapped-ion quantum processors (TIQPs). This SLR as well as this report, abide by the method and reporting items recommended by the APISSER methodology for SLRs in engineering [1]. The target audience of this paper is the engineering community, whose involvement in the conception and realization of the ECS of TIQPs is due-needed. The paper is organized as follows: After a brief introduction to the topic, the rationale and research questions (RQs) for this SLR are presented in Section I. The background of the topic is presented in Section II. The method used to conduct this study is described in Section III. The The associate editor coordinating the review of this manuscript and approving it for publication was Siddhartha Bhattacharyya . findings of the SLR are classified and listed in Section IV. Section V provides an answer to our initial RQs, describing characteristics to consider for future research in the field. Finally Section VI concludes the study and points to future work.

I. INTRODUCTION
Topic. Quantum computation is one of four domains of the quantum technologies (QT) [2]. Many future applications of this domain are expected to have a substantial impact on society [3]. Quantum computation is the type of computation that takes advantage of the quantum properties of objects in nature, explained by the laws of quantum mechanics, to speed the computation of certain algorithms [4]. A quantum processor is the computing device that executes quantum computations. One of the most promising technologies for implementing a quantum processor is the ions in an ion trap, in which case the processor is called a TIQP. At its core is an ion trap governed by a classic ECS.
In the past years, the design approach for the ECS of a TIQP has been the embedded-system one. The latter is an electronic system formed by the integration of various off-the-shelf electronic components on a board or the interconnection of many of these boards via a bus-based system. These type of systems are actively interacting with the environment in real-time and its system architecture lays upon a fine tradeoff of hardware-software co-design. In contrast, integrated circuit (IC) electronics is the design approach by the assembly of application-specific intellectualproperty cores (IP-cores), fabricated to function as a single unit and integrated in a single silicon substrate. Within this context, the electronic system-level (ESL) design is an emerging electronic design methodology that focuses on a higher abstraction level of understanding for the conception, design, development and verification of an electronic system [5]. The ESL method is an established approach for IC system-on-chip (SoC) design, and is increasingly being used for the top-down methodology approach for system design. Therefore, given this contrast of design methodologies, we want to bring forward the concepts behind the ESL IC design methodology to benefit the conception of the ECS of a TIQP.
Rationale. The realization of a TIQP has been foreseeable for approximately two decades. During this time, the main focus has been on the development of the ion trap itself to experimentally allow the execution of quantum operations with the ions. However, to fully enable a TIQP to perform algorithmic computations, we must merge this quantum world with the existing classical computing stack. The ECS is the gateway between the classic and quantum worlds; other QTs have referred to the ECS as the "electronic interface" of the quantum processor [6]. Currently, the bottleneck of scalability in a TIQP resides on physically managing the growing number of ions in the trap, while still allowing quantum computation with them. A few scalable architectures have been proposed, in which the ECS plays an important role to enable the required mechanisms.
The scientific community has been called to align to a roadmap which sets goals for the future development of the QT field [7]. To that end, we consider the development of the ECS as a key milestone to fully enable further research and development of the TIQP. Therefore, in this study, we take a step towards understanding the ECS of a TIQP as a system of its own and from an ESL perspective by first identifying its published work. We aim to understand the use cases, requirements, architectures, strategies, and technologies that have been utilized in published ECSs of TIQPs at the time of the execution of this SLR. 1 To this end, a SLR was conducted by using a defined, auditable, and reproducible method. The reach and limits of this study are clearly described by our 1 mid 2021, for detailed dates refer to Appendix- Table 11. method. The SLR as a research genre, is a proven method for the synthesis of existing research in a given topic [8].
Research Questions. This SLR addresses the following RQs for published ECSs of TIQPs. RQ0. What is the use-case of the TIQP? RQ1. What are the physical requirements of the ECS? RQ2. What are the electrical requirements for the ECS? RQ3. What are the system architectures and technologies used in the ECS?

II. BACKGROUND
Quantum computation with ions. The ions in an ion trap are the objects under control by the ECS of a TIQP. Ion traps are chambers that confine charged particles (ions) to specific locations in space by creating electromagnetic wells. These were invented by Paul et al. in 1958 [9] and Dehmelt in 1967 [10], for which they received the Nobel Prize in Physics in 1989 [11]. Within the quantum computing (QC) paradigm, ions are plausible physical objects for the physical implementation of quantum-bits (qubits). The envisioned processor that executes quantum operations using ions in an ion trap as qubit, is a TIQP. In quantum computation, a generic quantum algorithm maps to a quantum circuit composed of an initial qubit state and a sequence of gate operations to be applied to the qubit. In the case of a TIQP, the quantum algorithm maps to gate operations (computations) to be executed on ions. Computation of quantum gates is executed by shining lasers to the ions with a specific frequency, phase, and for a specific period of time. Measurement is performed by shining lasers onto the ions and detecting emitted photons either with a photo multiplier tube (PMT) or a camera. With this idea in mind, Cirac and Zoller from the University of Innsbruck (UIBK) proposed the first theoretical implementation scheme of a control-NOT quantum gate in 1995 [12], making the concept of a TIQP a possibility. In the same year, a team at the National institute of standards and technology (NIST) lead by Monroe et al. experimentally demonstrated this [13], making the concept of a TIQP a foreseeable reality.
Types of ion traps. A TIQP uses an ion trap as the physical environment to contain ions used as computational qubits. Ion traps can be classified by their working principle as either Paul or Penning. The community primarily employs Paul traps for experimentation, because they store ions at a fixed point in space, which is a significant benefit for qubit readout and manipulation. Penning traps have the disadvantage that trapped ions are not stationary but rather rotate continuously. Therefore, it seems the Paul trap is a better choice for implementing a TIQP. These can be further classified by their electrode arrangement as either 3D or surface traps (1D or 2D) [14], classified by the nature of the RF trap either dot or linear trap, and classified by their fabrication technology either macroscopic or micro-fabricated [15]. To perform major quantum computations, a reasonable initial requirement is that the ions in the trap scale to 1000 [16]. Macroscopic traps are not easily scalable given their large size. A dot trap can hold only one ion, whereas a linear trap has demonstrated to hold up to 24 computational-ions [17]. Therefore, surface linear traps have better scalability and their micro-fabrication processes will enable their industrial production. The ion trap is operated within a ultra-high vacuum (UHV) chamber that achieves a "good-enough" vacuum (< 10 −9 Torr) environment. The process to reach that vacuum level can take weeks. A faster method to achieve this vacuum level is to place the device within a vacuum and a cryogenic chamber operating at temperatures as low as 4K.
Scalability. For trapped-ion QC, the one requirement of the DiVincenzo criteria [18] which has not yet been experimentally demonstrated is scalability [7]. As we think past a noisy intermediate-scale quantum (NISQ) processor technology (50 to few-100s qubits) [3], solutions to manage the next level of 1000 qubits need to be considered. Being able to control only a limited number of computational-ions in a linear trap, an approach to up-scale the number of ions available for computation in a TIQP is to increase the number of traps and interconnect them [19]. Architectures that allow this type of scalability have been proposed, the latest is the modular universal scalable ion-trap quantum computer (MUSIQC) [20] in 2011, and in 2002 the quantum chargecoupled device (QCCD) [21]. Both architectures up-scale the number of ions by having many ion traps in the system; however, the difference resides in the interconnections across them. The MUSIQC architecture connects the several ion traps via an optical network, whereas the QCCD architecture moves ions between individual trapping areas through a mechanism called ion-transport. There are four types of ion-transport operations: shuttle, merge/split, and SWAP. Our particular research focus is on a TIQP with a QCCD architecture. The scalability of ions in a TIQP poses a challenge to its ECS. The QCCD architecture requires the transport of ions between regions, and the latter requires the control of many voltages to drive direct current (DC) electrodes in the trap. One of the main challenges is that the number of DC electrodes rapidly increases with the number of ions, and the method to manage and feed all these signals into the vacuum/cryogenic chamber is one of the main engineering bottlenecks to enable the scalability of a QCCD-TIQP. The ECS plays a key role in steering the transport of ions by applying a sequence of voltages to each of the DC electrodes in the segmented ion-trap.
System-level functionality. The ECS of a TIQP needs to seamlessly and precisely integrate all tasks that enable the computation of quantum operations on ions, as well as the measurement of the results. It also needs to be considered that these machines exhibit intrinsic environmental disturbances. Therefore, we must address the mitigation and correction of errors, which requires the introduction of quantum error correction (QEC) protocols [22]. The latter requires integration of a feedback mechanism into the ECS. Consequently, the ECS of a TIQP executes the following main tasks. (1) Traps (confines) ions by controlling the radio frequency (RF) and DC electrodes in the trap. (2) Controls electromagnetic wave (EMW) generators (i.e. lasers) to perform computations on the ions by feeding RF signals to acousto-optic modulators (AOMs), where the lasers shine.
(3) Measures emitted photons to determine the qubits state. (4) Incorporates feedback to enable QEC protocols. And within the context of a QCCD-TIQP, the ECS also requires to (5) move (transport) ions between trap regions by driving the DC electrodes in the trap. Therefore, a QCCD-TIQP has a functional block diagram as depicted in Figure 1. The input to the TIQP is an instruction of the quantum instruction set architecture (QISA). A summary of the types of signals controlled by the ECS of a TIQP, and their main use is shown in Table 1.

III. METHODS
In this study we applied the APISSER methodology for SLRs in engineering [1]. In this section, we present the plan items of the publications evaluated in each phase of the method.
The (P1) study characterization is described as follows. The reach of the study is to identify publications that describe a TIQP with at lest a minimal description of its ECS design, implementation, and/or verification. We limit our study to Paul-trap based TIQPs with a QCCD architecture which require ion-transport mechanism. The keywords that drive our study are: "quantum, computer, control, electronics, ion trap, ion-transport, cryogenic, and vacuum". The types of publications we evaluate are journal articles and conference proceedings. We have also included key-relevant university thesis publications in the field. The online data base (DB) we used to execute our search queries is the WOS [23]. The (P2) eligibility criteria is composed of the inclusion criteria (IN) shown in Table 2, and the exclusion criteria (EXC) shown in Appendix- Table 10, which also shows the defined tags, their category, and color-code. This literature review methodologically covers the published ECSs for TIQPs until the time of execution of this SLR 1 . Since the concept of a TIQP was first made possible in 1995, we consider the approximate two decades to be an appropriate time lapse to look back at. The (P3) data items to extract are linked to a RQ and are listed in Table 3. The DB (P4) search strategy we employ was to manually execute search queries in the previously defined DB until a reasonable logical combination of all keywords with some selected IN and EXC terms was achieved. The DB search queries, performed in the WOS DB, are listed in Appendix- Table 11. The (P5) selection process is shown in Table 4, as well as the number of publications that made it trough each phase of the method. As for the (P6) software tools needed, we developed two graphical user interfaces (GUIs) in Python [24]. One to support the "screen & select" phase and the other to support the "data extract" phase. According to the steps in the selection process, the data management proceeds as follows. From each search query, a full record of all identified publications was exported from the WOS DB in a comma-separated value (CSV) format. Using scripts, the data were parsed, imported, and organized in tables of a SQLite DB [25]. We extended each item (publication record) in the database to add the extended fields as listed in the APISSER methodology. This extended DB is our starting data pool and is referred to as local data base (L-DB), from which the GUIs read and write data. The L-DB was placed under a global information tracker (GIT) open-source version-control system [26]. Through scripts, we recognized repeated publications and mark the repeated field in the L-DB accordingly to only evaluate the repeated publication once. We used the "screen & select" GUI to fill the inclusion criteria and the reviewers annotations in the L-DB. Publications that comply with all IN are marked as selected publications in the L-DB. The "data extract" GUI was then used to fill the data items and the category fields in the L-DB for each selected publication. Using scripts, we accessed the filled DB and generate a report of all selected publications. The SLR (P7) protocol was gathered all previously described information.

IV. RESULTS
A comprehensive story cannot be told from the development of the ECS of TIQPs over the past years, as mostly single standalone systems have been developed to satisfy a specific experimental need and they do not necessarily build upon each other (with a few exceptions). In all publications, the ECSs for the TIQPs were used for scientific experimentation in a laboratory environment. Some ECSs were reused within the community, which shows the demand for these systems. We classified the ECSs into four categories: publications that describe the full system, those that describe only the transmit (TX) or receive (RX) paths, and finally other relevant publications. 2 A summary of the included papers is shown in Table 5, a graphical category distribution of the publications is shown in Figure 2, and their chronological distribution in Figure 3. Full systems integrate laser control, RF/DC voltage control, and have at least the possibility to integrate an RX path. All systems are rack-based and personal computer (PC)driven systems. A comparison of the full ECSs is presented in Table 6. The first ECS found in literature dates to 2005 by Pham [29], which then was adapted in 2008 to the ion trap  setup at the UIBK by Schindler [30]. The programmable pulse generator was designed to generate precisely timed digital and RF pulses. The use of this ECS was explicitly reported in [79], yet it was extended and used for many years at the UIBK for research purposes. Different groups at the NIST developed in 2006 (Langer [31]) and 2010 (Rosenband [32]) systems to control trapped-ion quantum computing experiments. In 2014 Graham et al. [33], facing the problem of having no electronics to feed numerous DC voltages into their trap, developed a network distributed control system for a surface ion trap and demonstrated the trapping, cooling, and shuttling of ions. The first commercially available ECS was released in 2017 by Bourdeauducq et al. who developed the Sinara hardware for the already existing Artiq software [34], [35], [36]. Since then, this open-source project has been further developed and maintained by the M-Labs company. In 2018, Negnevitsky designed and developed the M-ACTION ECS [37]. This is a rack-board based control system for a TIQP, which includes both hardware and partially supporting software. Publications that directly report the use of this ECS are [80] and [81].
TX systems incorporate only the actuators path, namely RF and/or DC voltage generation. Most of these systems are published as arbitrary waveform generators (AWGs) and aim shuttling, as that has been the main interest behind the advanced design-efforts of the ECS of TIQPs. A comparison of the TX ECSs is presented in Table 7. In 2013, both Bowler et al. [38] and Baig et al. [39] developed custom-made multichannel AWGs for ion-shuttling in TIQPs. The use of Bowler's ECS is reported in [82] and Baig's device was first described in [83] and patented in [84]. A first attempt to integrate the electronics within the vacuum chamber was reported in 2014 by Guise et al. [40] who incorporated a standard commercial digital-analog converter (DAC) into the vacuum chamber of an ion trap system and demonstrated the trapping and transport of ions. One of the first cryogenic environments at the UIBK was reported in 2016 by Brandl et al. [41] who developed a cryogenic setup for a TIQP. In the publication they demonstrated its operation by trapping ions inside a vacuum chamber at 4K. Its ECS successfully integrated many already existent TX systems placed at room-temperature and special cryogenic woven loom wires were used for its integration to the cryogenic environment. An ECS developed for a MUSIQC architecture was reported in 2016 by Mount et al. [42]. In 2017, Beev et al. [43] presented yet another custom-made multichannel DC voltage generator. The primary design motivation was to meet the required resolution, stability, and low noise of the output voltages. An ECS aimed to control a hybrid QC system was published in 2018 by Perego et al. [44]. This system aims to control a hybrid system of trapped ions and quantum gas of neutral atoms. Therefore, the flexibility demanded from the ECS is greater, as it targets the control of a hybrid quantum system, each at two different time scales. A key architectural design-feature of this system is that they developed only two types of boards. The first attempt to bring the electronics into the cryogenic chamber was reported in 2019 by Stuart et al. [45] who published the cryogenic operation of an ion trap that incorporated a monolithically integrated on-chip highvoltage 180 nm complementary metal-oxide semiconductor (CMOS) DAC. They trapped ions operating the trap in a cryogenic (4K) vacuum apparatus and characterized the electronics. One of the most re-used system for ion-transport is the one developed in 2020 by Kaushal et al. [46] who published an ECS for the control of ion-shuttling. The device was designed specifically for the ion trap at the University of Mainz, but has been adapted to operate in other setups. Publications that report the use of this system are [85], [86], and [87]. In 2021, Lee et al. [47] developed yet another an ECS for shuttling. In their experiments, basic ion operations (cooling, transition) were also performed.
RX systems describe only the sensors path, these are [48] and [49], and their characteristics are listed in Table 8. In 2018, Schwegler [48] explored different methods for qubit readout: electron-multiplying charge coupled device (EMCCD) vs. CMOS camera. In this work, the development of a standalone system for qubit readout using an EMCCD camera was reported. In 2019, Ding et al. [49] proposed the incorporation of machine-learning methods to implement fast and high-fidelity readouts of qubits using a PMT.
Other relevant publications [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], listed in VOLUME 11, 2023 Table 9, used a commercially available AWG or developed one to steer the ion trap, but provide only basic information about the electronics design. The focus of the research was mainly on the physics of the TIQP rather than its ECS. Yet, these publications show that an ECS is required for further research advancements in the field of the TIQP. Other publications [64], [65], [66], [67], [68], [69], [70], [71], [72], [73], [74], [75], [76], [77], [78] address topics that are relevant to be considered in the development of the ECS of a TIQP. In 2008, Kielpinski [64] in his review, shows the requirement for classic electronics to be integrated into the trap chambers in a TIQP. In 2009, Leibrandt et al. [65] described the fabrication, operation, and testing of a scalable multiplexed ion trap. Although the focus was on the trap itself and not on electronics, they performed extensive testing of the trap at different temperatures, ion species, RF drives, trap geometries, and pressures. The RF voltage ranges used by the ECS are dependent with the ion species, and most trap failures were caused by electrical shortcuts. In 2016, Brown et al. [66] published a review in which they explicitly defined the main tasks of the ECS for a TIQP. They provided a road map back then, which is yet to be fulfilled, and argued that the practical scalability of the TIQP may be highly limited by the scalability of its ECS. In 2019, Franke et al. [67] analyzed the scalability of quantum computation by defining the quantum rent exponent based on Rent's rule [68] to quantify the progress in overcoming this challenge at different levels throughout the QC stack. In particular, the electronics used for QEC should be integrated on-chip to reduce latencies and facilitate trivial communication with room-temperature equipment. In other quantum computation technologies, cryogenic electronics integrated within the vacuum/cryogenic chamber has been reported. In 2019 Homulle [69] presented a study showing a vast exploration of cryogenic electronics for the control of solid-state qubits. They reported initial results in [70] and [71], where the electronics was fully placed at cryogenic temperatures (4K). In [72] they addressed the need for cryo-CMOS ECS for solid-state quantum computations. Furthermore, a characterization of 0.16µm and 40nm CMOS electronics was carried out at deep-cryogenic temperatures (4, 1, 100µK), demonstrating their reliable operation [88]. In 2019, Bruzewicz et al. [74] published a complete review about trapped-ion QC. They stated that the usual approach to integrating control electronics into a TIQP is to house electronics in a remotely located rack and feed these signals through the vacuum chamber to reach the trap. Yet, it is emphasized that this approach will become unmanageable when upscaling the TIQP. In 2020, Romaszko et al. [73] presented a vast technical review of the micro-fabrication of ion traps. They emphasized the need to integrate passive and active components into the trap itself. The bare-die form IC is more suitable for UHV environments and a large-gate transistors is preferred for ICs electronics. In 2021, Dubielzig et al. [75] reported the challenges and considerations of inserting 8 high-frequency and 100 lowfrequency electric lines for ion control into a cryogenic ion trap apparatus. In 2021, Brown et al. [76] published a review of the influence and requirements of materials on the development of hardware for TIQPs. Particularly relevant to ECSs integrated within the chambers is the emission of the materials used in the devices for the integration of ion control and measurement in the trap. Finally, the problem of defining a QISA for a TIQP was addressed in the following publications [77] and [78].

V. DISCUSSION
The use-case of the TIQP. The functional diagram of the ECS for TIQP shown in Figure 1 was generated as a result of the analysis of the use-cases of the publications included in this study and is the first contribution of this work. The study showed that publications that describe a TIQP with a comprehensive description of its ECS, report trapping, shuttling, and/or readout of qubits as stand-alone operations, and not the execution of an algorithm. The latter shows that we are still at the stage of the TIQP-roadmap [2], where the focus is on building the machine itself. We are still in a phase in which the TIQP is still science, instead for it to do science [89]. Towards the speed-up of this roadmap, we expect a substantial positive impact of the definition of a full-stack system-level description for the TIQP, and of the definition of an ESL description for its ECS. Additionally, given the type of journals and authors where the publications were found, we see little involvement of system and electronic engineers, as well as computer scientists in the development of the ECS of the TIQP. Therefore, in the roadmap ahead, we hope that the present review lays a basic foundation for engineers to understand the ECS requirements, environment, and past approaches, so that further research on this topic can be pursued.
The physical requirements. In this regard, we address two topics for the ECS: first the location and second its size, these two are highly intertwined characteristics. In a TIQP, there are three locations where to physically place the ECS: outside the system at room temperature, inside the vacuum chamber, or inside the cryogenic chamber; each approach has its benefits and challenges. If the ECS is placed outside, usually in a massive rack, the problem of feeding the many electric signals through the chambers to reach the ion trap arises, limiting its scalability. Yet, the latter has been the most common approach, as scientists have mostly focused on the inner working of the TIQP, rather than its deployment. If the ECS is placed in the vacuum chamber, the cabling is reduced, but the challenge of making vacuum-compatible electronic arises. Subsequently, material emissions and temperatures come into consideration in this environment. Publications reporting such an approach are [40] and [59]. Finally, if the ECS is placed next to the trap at cryogenic temperatures, the electronics must endure and function in a cryogenic environment while also limiting its heat and material emissions. The only study that reports such an approach is [45]. Therefore, on the roadmap ahead, we must further explore the integration of micro-electronics in-vacuum and on-chip at cryogenic temperatures. The latter aligns to the medium-term goal (5-10 years) of the QT roadmap to integrate optics and control electronics into a scalable micro-fabricated ion trap [7]. Not all parts of the ECS can go inside the cryogenic, nor in the vacuum chamber, nor outside. But most likely a distributed system across these three environments needs to be considered in the ESL design of the ECS of a TIQP.
The electrical requirements. The classification of the signals managed by the ECS of a TIQP shown in Table 1, was generated as a product of this SLR. The requirements for the range and frequency of the RF and DC voltages, are ion-species and trap-dependent. Therefore, the ECS should be flexible enough to adapt to control different ion-species. The ECSs described in the publications included in this study have the following characteristics. The RF voltages range between 100-500V in the 20-50 MHz range. The DC voltages range between 10-40V, with support for up to 100 channels, and a precision of 1µV . For further details refer to Tables 6, 7, 8, and 9.
The electronic system-level architecture and technologies. Most of the ECSs found, were designed for the TX-path specifically for shuttling purposes, little focus has been placed on the feedback RX-path, and all full-ECSs are remote-rack based with an experimental user interface. The results clearly show the re-design of the same system over and over again by an embedded-system design approach. Yet, the ECS of a TIQP is a complex device that should be addressed and understood as a system of its own: a system-of-systems. The latter would enable its partitioning into smaller/design-digestible subsystems. The design of a system in engineering, can take one of two approaches: topdown or bottom-up. From the review we can also derive that the conception of the ECS has adopted a bottom-up design approach. In the long run, a bottom-up approach to the design the ECS of a TIQP will continue to generate standalone TX or RX systems designed for a specific function, opposed to meeting the global aim of executing a quantum algorithm with the processor and scaling the system. We propose to approach the design of the ECS of a TIQP with a top-down methodology, which calls for its ESL description. In this study, we intend to lay a foundation for understanding the big picture behind the role of the ECS in a TIQP, which is much needed to initiate a top-down design approach. These two system-design approaches have already been considered for the design of the ECS of quantum processors in other technologies [90]. We need to consider that the focus in recent years has been on building the physical part of the TIQP machine; therefore, little research and focus has been placed on the ESL description of its ECS. The usual approach has been the system integration of existing technologies (embedded-system) to build a ECS that fulfills the required physics experimental need. In contrast, other quantum computation technologies have already proposed a control microarchitecture [91], [92], [93], a proposal towards a full-stack [94], [95], [96], [97], and have addressed the importance and impact of the ECS in the quantum processor [6], [98]. As we come to the end-road of the bottom-up embedded-system electronics design approach, we need to start designing application-specific microelectronics for the ECS of a TIQP. The integration of control electronics into the trap, i.e. on-chip analog and digital processing, is foreseeable [74]. The use of ICs in vacuum and on the ion trap seems to be feasible, due to field-programmable gate arrays (FPGAs) having already been proven to work at cryogenic temperatures [99], and the characterization of 0.16µm and 40nm CMOS electronics has proven to be reliable [88]. Furthermore, a TIQP is envisioned to function as a quantum co-processor/accelerator in a higher hybrid classical-QC system [16], [89]. Thus, the ESL design must consider that the full stack of a TIQP will be shared with a classical computing system. In addition, most of the ECSs found in this SLR were driven by a central PC, and in some cases, a user interface was created to ease the interaction of a human-user with the TIQP. For experimentation purposes, we agree that the centralized PC user-oriented approach is the most efficient plug-in experimental solution. Nevertheless, as we head towards a TIQP being used as a coprocessor/accelerator in a higher hybrid classical-QC system, we should aim to have a machine-driven interface to the TIQP, not a human one. That means that the input-output interface of the ECS of a TIQP should be ready to receive instructions from such a higher computational system, and not from a human-user interface. In computer science, the instruction set architecture (ISA) is a processor's vocabulary, that is, the words (instructions) that the processor knows (decodes) and can execute [100]. Correspondingly, the input to the ECS should be an instruction from the QISA, as shown in Figure 1. The QISA plays an important role as it defines the operations executed by the ECS, i.e. defines its functionality, and it is the interface for a higher computational system to communicate to the TIQP. In addition, the integration of a feedback mechanism is due needed. The systems included in this study revealed that limited efforts have been invested in the integration of the RX path to the ECS. Although it is true that the measurement of ions breaks their quantum state and therefore the qubits are immediately downgraded to classical one-zero bits, we must include feedback (RX-path) at some point to implement an efficient QEC concept.

VI. CONCLUSION
The ECS plays a key role in enabling the deployment and scalability of a TIQP. We have shown the due need to address this system as a research topic of its own and unify the research efforts towards the definition of its ESL description.
In engineering, there are two main types of literature reviews: systematic and narrative [101]. In this study, we conducted a systematic one to establish the published literature of the ECS of a TIQP. We acknowledge that the semi-automated literature review process of the APISSER method, can cause non-appropriately-classified publications in the database to be left out of the search queries and therefore from the review. The latter can be a drawback of this method in comparison to the narrative literature review (oldfashioned/human-expert), which often relies on knowledge that cannot be found readily or at all through literature search terms, but lacks of a methodology.
In this study we used a defined, auditable, and reproducible method to search the body-of-knowledge, select, classify, and synthesize publications relevant to our topic. We have also presented a vision of possible further research considerations in the field. This review lays a foundation, an initial work, that can be extended and updated [102]. We encourage others to extend this initial work by adding other types of literature, such as patents, and/or updating the SLR to include the latest years since its execution 1 . This study aims to lay a foundational understanding of the topic for engineers, so that the engineering community engages in the much-needed further research of the ECS of a TIQP.

ACKNOWLEDGMENT
This work was enhanced and has benefit from numerous discussions with physicist of the Institute for Experimental Physics at the UIBK. STEFANIE CASTILLO received the B.Sc. degree (cum laude) in electronic engineering from Universidad del Valle de Guatemala, Guatemala, in 2008, and the Master of Advanced Electronic Systems degree in microelectronics from Universidad del Pais Vasco, Spain, in 2011. Since then, her professional research focusing on ASIC and FPGA system-level digital design, development and verification, and the system integration of embedded systems. Her current research interest includes the electronic control system of trapped-ion quantum processors.