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5 Essential Hardware Components of a Quantum Computer
Pages 113-134

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From page 113...
... More than 100 academic groups and government-affiliate laboratories worldwide are researching how to design, build, and control qubit systems, and numerous established and start-up companies are now working to commercialize quantum computers built from superconducting and trapped ion qubits. Even although reports in the popular press tend to focus on development of qubits and the number of qubits in the current prototypical quantum computing chip, any quantum computer requires an integrated hardware approach using significant conventional hardware to enable qubits to be controlled, programmed, and read out.
From page 114...
... To assist in conceptualizing the necessary hardware components for an analog or gate-based quantum computer, the hardware can be modeled in four abstract layers: the "quantum data plane," where the qubits reside; the "control and measurement plane," responsible for carrying out operations and measurements on the qubits as required; the "control processor plane," which determines the sequence of operations and measurements that the algorithm requires, potentially using measurement outcomes to inform subsequent quantum operations; and the "host processor," a classical computer that handles access to networks, large storage arrays, and user interfaces. This host processor runs a conventional operating system/user interface, which facilitates user interactions, and has a high bandwidth connection to the control processor.
From page 115...
... 5.1.2 Control and Measurement Plane The control and measurement plane converts the control processor's digital signals, which indicates what quantum operations are to be performed, to the analog control signals needed to perform the operations on the qubits in the quantum data plane. It also converts the analog output of measurements of qubits in the data plane to classical binary data that the 1 In some ways, the quantum data plane looks similar to a field programmable gate array, or FPGA.
From page 116...
... The nature of a QC's control signals depends on the underlying qubit technology. For example, systems using trapped ion qubits usually rely upon microwave or optical signals (forms of electromagnetic radiation)
From page 117...
... Finding: The speed of a quantum computer can never be faster than the time required to create the precise control signals needed to perform quantum operations. 5.1.3 Control Processor Plane and Host Processor The control processor plane identifies and triggers the proper Hamiltonian or sequence of quantum gate operations and measurements (which are subsequently carried out by the control and measurement plane on the quantum data plane)
From page 118...
... The rest of this chapter reviews the current candidate qubit technology choices upon which to base a quantum computer. For the two furthest developed quantum technologies, superconducting and trapped ion qubits, this discussion includes details of the qubit and control planes in use in prototypical computers at the time of publication of this report (2018)
From page 119...
... source that can be directed at a specific ion to affect its quantum state, another laser to "cool" and enable measurement of the ions, and a set of photon detectors to "measure" the state of the ions by detecting the photons that they scatter. Appendix B provides a technical overview of current strategies for constructing a trapped ion quantum data plane and its associated control and measurement plane.
From page 120...
... Further scaling of trapped ion quantum computers to well beyond the sizes necessary for demonstrating quantum supremacy
From page 121...
... . This protocol was indeed demonstrated first in trapped ions [31]
From page 122...
... For trapped ions, necessary technology developments toward scalable quantum computer systems include the ability to fabricate ion traps with higher levels of functionality, assemble stabilized laser systems with adequate control, deliver electromagnetic (EM) fields that drive the quantum gates (either microwave or optical)
From page 123...
... 5.3.1 Current Superconducting Quantum "Computers" In the context of digital quantum computation and quantum simulations, the present state-of-art for operational gate error rate is better than (below) 0.1 percent for single-qubit gates [44-46]
From page 124...
... In scaling to larger numbers of qubits, one needs to at least maintain qubit coherence and, ideally, increase it, as larger systems will likely aim to solve larger problems that require addi tional time, and higher fidelity enables more operations to be performed during the coherence time of the quantum processor. Of course, the fabrication variation that a number of qubits spans gets worse as the number of qubits increases, since a larger num ber of cells will include more improbable variations.
From page 125...
... While this should be sufficient for 1,000 qubit circuits, reducing the clock period is advantageous, as it translates directly to lower error rates. Scaling to Larger-Size Machines First, qubit fidelities need to be improved to provide the lower error rates needed to support practical quantum error correction.
From page 126...
... , reciprocal quantum logic (RQL) , and adiabatic quantum flux parametrons, significant research will be needed to be create these designs at scale, and then determine which approaches are able to create a local control and measurement layer that supports the needed high-fidelity qubit operations.
From page 127...
... 5.4 OTHER TECHNOLOGIES Since many technical challenges remain in scaling either trapped ion or superconducting quantum computers, a number of research groups are continuing to explore other approaches for creating qubits and quantum computers. These technologies are much less developed, and are still focused on creating single qubit and two qubit gates.
From page 128...
... Building gate-based quantum computers using this technology requires creating high-quality two-qubit operations and isolating these operations from other neighboring qubits. As of mid-2018, entanglement error rates of 3 percent have been achieved in isolated two-qubit systems [68]
From page 129...
... 5.5 FUTURE OUTLOOK Many qubit technologies have significantly improved over the past decade, leading to the small gate-based quantum computers available today. For all qubit technologies, the first major challenge is to lower qubit error rates in large systems while enabling measurements to be interspersed with qubit operations.
From page 130...
... While the component technologies and baseline protocols for realizing some of these integration strategies have already been demonstrated, system-scale demonstration with practical levels of performance remains a major challenge. As a result of the challenges facing superconducting and trapped ion quantum data planes, it is not yet clear if or when either of these technologies can scale to the level needed for a large error corrected quantum computer.
From page 131...
... Lee, and C Monroe, 2005, Spin-­ dependent forces on trapped ions for phase-stable quantum gates and entangled states of spin and motion, Physical Review Letters 94:153602.
From page 132...
... Kim, 2011, Efficient collection of single photons emitted from a trapped ion into a single-mode fiber for scalable quantum-information processing, Physical Review A 84:063423.
From page 133...
... Gambetta, and J.M. Chow, 2015, Demonstration of a quantum error detection code using a square lattice of four superconducting qubits, Nature Communications 6:6979.


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