Quantum computing circuit

Multi-junction, reconfigurable, tunable quantum qubit circuits with computational functionality and internal memory

Software & Information Technology

Image of a Josephson voltage standard chip containing approximately 300,000 microfabricated Josephson junctions. Image credit: NIST.

A quantum bit, or qubit, is the building block of a quantum computer. Qubits are different from conventional bits whereby quantum behavior enables a superposition of states (0 and 1) at the same time with varying occupancy probability.

This property enables quantum computers to perform simultaneous operations on 2^N information states. The information processing can use quantum algorithms that take full advantage of qubit properties. The end result is the ability to quickly solve hard problems with a limited amount of computational resources.

Researchers have been investigating the potential of solid-state qubits – a subset of qubits in which the quantum bit is integrated into a material such as a semiconductor. Realization of solid-state qubits will enable quantum processors with commercial utility. But to date, due to physical limitations and intrinsic material properties, such solid-state qubits have suffered from static behavior, state leakage, and have been prone to external effects that limit their reliability in large-scale computing architectures.

While state-of-the-art designs have been proposed and even reduced for achieving programmable qubits using external influences and qubit-qubit couplings, they are limited in their ability to internally reconfigure with nonvolatile behavior. They lack multifunctional operation capacity internal to the junctions or qubits. Such designs rely on qubits built from a certain microscale junction technology to be interfaced to separate memory blocks built from an alternate memory device technology.

These approaches can implement only specific types of computing that lack clean quantum behavior along with some level of programming capability. Moreover, these designs are functional in a limited window of operating conditions without a clear hardware capability for dynamic reconfiguration during the fabrication process or during operation. An alternative solution – photonic implementations – provide increased reliability in some cases, albeit lacking the capability for large-scale integration and robust routes for configurability.

Pushing the quantum computing envelope, Navy researchers have developed a novel approach for solid-state qubit implementations based on reconfigurable quantum circuits with built-in memory and improved reliability. This tunable quantum qubit circuit comprises interconnected Josephson tunneling junctions, a capacitive-coupled control gate, and independent control gates. The Josephson tunneling junctions are sculpted in-situ on-chip and each junction comprises a pair of high-temperature superconductors separated by an active region having a controlled charge density.

The capacitive-coupled control gate is connected to the Josephson tunneling junctions and is configured to simultaneously modulate energy levels of the tunneling junctions. The independent control gates are coupled to the Josephson tunneling junctions and are reconfigurable on-the-fly by an operator. Tunability is achieved by simultaneously modulating energy levels of the Josephson tunneling junctions with a capacitive-coupled control gate and dynamically reconfiguring the quantum qubit circuit via independent control gates.

The design allows for nonvolatile, field-programmable configurations where quantum states are created and reconfigured through gate-control coupling, providing increased performance, complexity, resiliency, and reduced leakage. This technology enables the study of fundamental quantum information science algorithms not implementable with conventional structures by providing for many possible nonvolatile configurations and energy profiles.

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