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Novel Computing Architecture on Arrays of Josephson Persistent Current Bits

Jie Han, Pieter P. Jonker

Published Apr 22, 2002• 3 citations
OpenAlex

Abstract

A superconducting qubit (or quantum bit), which consists of a micrometer-sized loop with three Josephson junctions, has two persistent currents of opposite direction as its two states. The states of the qubit can be brought into quantum coherence to perform quantum computing. Classical bits can also be obtained from these superconducting loops, making it possible to base a classical computer architecture. We study a novel computing structure based on these Josephson Persistent Current (PC) Bits, starting from elementary logic gates to a Random Access Memory (RAM). The investigation shows that the Josephson PC Bit technology would not surpass semiconductor technology in term of the device density, while it is a promising candidate for unltra-fast memory, which can be integrated with other technologies. The classical computer might also serve as pre and post processor for the quantum computing performed in the heart of the array. The Josephson PC circuits, therefore, seems a good vehicle for the study of the quantum computer paradigm.

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Quantum information processing systems can be used in a variety of applications. Among proposed realizations of quantum bits nano-electronic circuits appear most promising for integration in electronic circuits and large-scale applications. We discuss Josephson junction circuits in two regimes where they can be used for quantum computing. In one limit, two logic states of the quantum bit differ by a Cooper pair charge, 2e, on a superconducting island. In the other limit different magnetic fluxes penetrate a superconducting loop in two states. Single-qubit and two-qubit logic operations can be performed by means of voltage or current pulses. Weak coupling to the environment allows a series of these elementary steps before the phase coherence is lost. To read out the result of the computation a quantum measurement is needed. For charge qubits this can be accomplished by coupling a single-electron transistor (SET) capacitively to the qubit. Monitoring the current in the transistor one can extract the information about the state of the qubit. We discuss recent experiments and present a suitable set of circuit parameters available with the present-day technology.

IEEE International Symposium on Circuits and Systems proceedings•Nov 7, 2002•4 citations
A superconducting qubit (or quantum bit), which consists of a micrometer-sized loop with three Josephson junctions, has two persistent currents of opposite direction as its two states. The states of the qubit can be brought into quantum coherence to perform quantum computing. Classical bits can also be obtained from these superconducting loops, making it possible to base a classical computer architecture. We study a novel computing structure based on these Josephson Persistent Current (PC) Bits, starting from elementary logic gates to a Random Access Memory (RAM). The investigation shows that the Josephson PC Bit technology would not surpass semiconductor technology in term of the device density, while it is a promising candidate for unltra-fast memory, which can be integrated with other technologies. The classical computer might also serve as pre and post processor for the quantum computing performed in the heart of the array. The Josephson PC circuits, therefore, seems a good vehicle for the study of the quantum computer paradigm.
A superconducting qubit (or quantum bit), which consists of a micrometer-sized loop with three or four Josephson junctions, has two persistent currents of opposite direction as its two states. The states of the qubits, manipulated with magnetic fields and measured with a SQUID, can be brought into quantum coherence to perform quantum computing. Classical bits can also be obtained from these superconducting loops by increasing its critical current, making it possible to base a processor array architecture on these cubits (quantum bits used in a classical way). Such a classical computer might also serve as pre and post processor for the quantum computing performed in the heart of the array. Because classical and quantum computing based on the same device can be studied now simultaneously, architecture of arrays of qubits and cubits seems a good vehicle for studying the quantum computer paradigm, independently from the question whether superconducting loops will be the ultimate implementation vehicle. Other (e.g. spin-based) devices may be more successful. In this paper, the architectural issues of a heterogeneous quantum/classical computer for an implementation of Shor's factoring algorithm based on arrays of qubits and cubits, are presented.
A qubit was designed that can be fabricated with conventional electron beam lithography and is suited for integration into a large quantum computer. The qubit consists of a micrometer-sized loop with three or four Josephson junctions; the two qubit states have persistent currents of opposite direction. Quantum superpositions of these states are obtained by pulsed microwave modulation of the enclosed magnetic flux by currents in control lines. A superconducting flux transporter allows for controlled transfer between qubits of the flux that is generated by the persistent currents, leading to entanglement of qubit information.
A qubit was designed that can be fabricated with conventional electron beam lithography and is suited for integration into a large quantum computer. The qubit consists of a micrometer-sized loop with three or four Josephson junctions; the two qubit states have persistent currents of opposite direction. Quantum superpositions of these states are obtained by pulsed microwave modulation of the enclosed magnetic flux by currents in control lines. A superconducting flux transporter allows for controlled transfer between qubits of the flux that is generated by the persistent currents, leading to entanglement of qubit information.
Quantum computing is a novel paradigm for information processing that promises computational speedup for certain computational tasks while not being driven by Moore's law. Superconducting circuits based on Josephson junctions are one of the leading approaches for implementing the quantum bits, or qubits, in a quantum processor. Most superconducting qubit architectures involve circuit quantum electrodynamics, where the qubits are analogous to artificial atoms that are coupled to superconducting linear resonant cavities. This scheme allows for the readout of individual qubits, the manipulation of single-qubit states, and the generation of entanglement between qubits. The performance of current state-of-the-art systems is limited by decoherence of the qubits and associated gate errors. Nonetheless, experiments have been performed that already demonstrate a significant computational speedup compared to classical processing. Future implementations of fault-tolerant quantum computers will require the development of quantum error correction. Initial tests of elements of error correction in superconducting qubit arrays are underway.
The superconducting qubit quantum computer is one of the most promising quantum computing architectures for large-scale integration due to its maturity and close proximity to the well-established semiconductor manufacturing infrastructure. From an education perspective, it also bridges classical microwave electronics and quantum electrodynamics. In this paper, we will review the basics of quantum computers, superconductivity, and Josephson junctions. We then introduce important technologies and concepts related to DiVincenzo's criteria, which are the necessary conditions for the superconducting qubits to work as a useful quantum computer. Firstly, we will discuss various types of superconducting qubits formed with Josephson junctions, from which we will understand the trade-off across multiple design parameters, including their noise immunity. Secondly, we will discuss different schemes to achieve entanglement gate operations, which are a major bottleneck in achieving more efficient fault-tolerant quantum computing. Thirdly, we will review readout engineering, including the implementations of the Purcell filters and quantum-limited amplifiers. Finally, we will discuss the nature and review the studies of two-level system defects, which are currently the limiting factor of qubit coherence time. DiVincenzo's criteria are only the necessary conditions for a technology to be eligible for quantum computing. To have a useful quantum computer, large-scale integration is required. We will review proposals and developments for the large-scale integration of superconducting qubit devices. By comparing with the application of electronic design automation (EDA) in semiconductors, we will also review the use of EDA in superconducting qubit quantum computer design, which is necessary for its large-scale integration.
Quantum information processing systems can be used in a variety of applications. Among proposed realizations of quantum bits nano-electronic circuits appear most promising for integration in electronic circuits and large-scale applications. We discuss Josephson junction circuits in two regimes where they can be used for quantum computing. In one limit, two logic states of the quantum bit differ by a Cooper pair charge, 2e, on a superconducting island. In the other limit different magnetic fluxes penetrate a superconducting loop in two states. Single-qubit and two-qubit logic operations can be performed by means of voltage or current pulses. Weak coupling to the environment allows a series of these elementary steps before the phase coherence is lost. To read out the result of the computation a quantum measurement is needed. For charge qubits this can be accomplished by coupling a single-electron transistor (SET) capacitively to the qubit. Monitoring the current in the transistor one can extract the information about the state of the qubit. We discuss recent experiments and present a suitable set of circuit parameters available with the present-day technology.