Superconducting-silicon qubits: Using a bottom-up approach to make hybrid quantum devices

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Theorists propose a way to make superconducting quantum devices such as Josephson junctions and qubits, atom-by-atom, inside a silicon crystal. Such systems could combine the most promising aspects of silicon spin qubits with the flexibility of superconducting circuits.

High quality silicon is one of the historical foundations of modern computing. But it is also promising for quantum information technology. In particular, electron and nuclear spins in pure silicon crystals have been measured to have excellent properties as long-lived qubits, the equivalent of bits in conventional computers.

In a paper appearing this week in Nature Communications, Yun-Pil Shim and Charles Tahan from the University of Maryland and the Laboratory for Physical Sciences (on the College Park, MD campus) have shown how superconducting qubits and devices can be constructed out of silicon. Doing so can potentially combine the good quantum properties of silicon and the ubiquity of semiconductor technology with the flexibility of superconducting devices. They propose using “bottom-up” nano-fabrication techniques to construct precisely placed superconducting regions within silicon or germanium and show that such “wires” can be used to make superconducting tunnel junctions and other useful superconducting devices.

Qubits in superconductors and semiconductors

Superconducting circuits, made from superconducting metals and Josephson tunnel junctions (which allow superconducting electron pairs to tunnel between two superconductors), are exceptionally customizable and can produce devices ranging from magnetic field sensors to classical logic circuits. They are also likely to play a big role in processing quantum information, where they can be used as a platform for qubits, tiny quantum systems which reside in a superposition of quantum states.

Examples of superconducting-silicon quantum devices. (left) A superconducting loop interrupted at two points by junctions can form a superconducting flux qubit or a superconducting quantum interference device, or SQUID. Currents flowing in the loop can be used to measure the strength of a magnetic field threading the loop. The currents (flowing in either direction) can also be used to constitute a qubit. (middle) Separating the superconducting wires by an insulator, in this case pure, crystalline silicon, forms a Josephson junction. (right) Precisely placed, highly doped regions within the semiconductor form the superconducting wires.

Several types of superconducting circuits have been used to implement qubits and quantum logic gates with different properties and potential uses. For example, in one kind of circuit current can flow in either of two directions. These alternatives constitute the two superposed states needed for establishing a qubit. The two states can be labeled “0” and “1” in analogy with classical bits. Microwave pulses can drive transitions between the two levels allowing for quantum logic gates.

Spin qubits, are an In general, quantum systems are delicate objects and are susceptible to noise and other environmental factors which diminish performance. Prospective quantum circuits must preserve qubits from outside interference for as long as the quantum calculation proceeds. Despite rapid progress in the quality of superconducting qubits (qubit lifetimes can now surpass 100 microseconds), qubit gate error rates are still limited by loss in the metals, insulators, substrates, and interfaces that make up the heterogeneous superconducting devices.

example of qubits realized in a solid-state, silicon context. Spin is a quantum property of particles like an electron; physicists often think of an electron’s spin as being like a small magnet, which will naturally point along the direction of an applied magnetic field. Here the 0 and the 1 states correspond to the two possible orientations of the electron spin, either up or down. Because spin is naturally decoupled from charge in some systems (meaning the information stored in the direction of the spin will not be ruined by moving the electron or by it being shaken by electric noise), spin qubits are thought to be promising candidates for a robust qubit design. Further, the use of epitaxial semiconductor devices, and the ability to bury spin qubits deep inside a semiconductor medium, far away from noise at interfaces and surfaces, has resulted in qubits that live for seconds or even hours in some situations, much longer than superconducting qubits to date.

 The researcher’s results have now been published in Nature Communications.

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