Emerging qubit systems: Guest editorial

Over the past few years, the field of quantum information science has seen tremendous progress toward realizing large-scale quantum computers. With demonstrations of quantum computers outperforming classical computers for a select range of problems,^1–3 we have finally entered the noisy, intermediate-scale quantum (NISQ) computing era. While the quantum computers of today are technological marvels, they are not yet error corrected, and it is unclear whether any system will scale beyond a few hundred logical qubits without significant changes to architecture and control schemes. Today's quantum systems are analogous to the ENIAC (Electronic Numerical Integrator And Computer) and EDVAC (Electronic Discrete Variable Automatic Computer) systems of the 1940s, which ran on vacuum tubes. These machines were built on a solid, nominally scalable architecture and when they were developed, nobody could have predicted the development of the transistor and the impact of the resulting semiconductor industry. Simply put, the computers of today are nothing like the early computers of the 1940s. We believe that the qubits of future fault-tolerant quantum systems will look quite different from the qubits of the NISQ machines in operation today.

This Special Topic issue is devoted to new and emerging quantum systems with a focus on enabling technologies that can eventually lead to the quantum analog to the transistor. We have solicited both research^4–18 and perspective articles^19–21 to discuss new and emerging qubit systems with a focus on novel materials, encodings, and architectures. We are proud to present a collection that touches on a wide range of technologies including superconductors,^7–13,21 semiconductors,^15–17,19 and individual atomic qubits.¹⁸ 

Superconducting qubits are presently one of the most studied quantum systems. Today's largest operational quantum processors are built using superconducting qubits, and they were the first systems to achieve “quantum supremacy,”¹ that is the solving of some specific, classically intractable problem. Most of the large-scale systems being developed today are built on superconducting charge qubits comprising capacitively shunted Josephson junctions (i.e., the transmon or gatemon),²² but the field is seeing a resurgence in the fabrication of qubits based on quantized magnetic flux in a superconducting ring.^23–25 Encoding the qubit in the magnetic flux offers a charge-noise insensitive qubit with a high anharmonicity (mitigating excited state leakage), and there are device encodings that offer topological protection.²⁶ In this Special Topic, we are happy to include a perspective piece²¹ discussing the progress, opportunities, and challenges of this technology.

We are also excited to present research papers delving into both superconducting flux qubit and charge qubit design, operation, and readout. These papers include a description of new flux-qubit coupling schemes¹¹ as well as a study on ionizing radiation on fluxon dynamics.⁵ We, furthermore, include a demonstration of combined microwave and flux bias control⁹ and theoretical works describing the use of SINIS (superconductor-insulator-normal metal-insulator-superconductor) junctions for high-fidelity state preparation⁸ and the implementation of a high-fidelity i-Toffoli gate.⁶ 

Semiconducting qubits generally encode information in some state of either a confined electron or hole. The most common devices in this field are based on either lithographically defined quantum dots²⁷ or electrons bound to donors and defects.²⁸ Until recently, quantum dot and donor systems have offered high fidelity single-qubit operation,²⁹ but two-qubit fidelities have been low,^30–35 largely due to not fully controlling the dynamics of electrons evolving under the exchange interaction. Within the past year, several groups have finally achieved high-fidelity multi-qubit operation,^36–39 but there are many outstanding challenges, including controlling disorder, engineering larger valley, and orbital energy splittings, and scaling to larger systems.

There are several articles in this collection that address some of these key challenges. One intriguing direction is to replace the standard Si/SiGe or GaAs/AlGaAs heterostructures with 2D transition metal dichalcogenides (TMDs),¹⁵ which offer strong spin–orbit interactions (enabling electric field spin control) and a direct bandgap that could enable fully optical control.

This collection also features an intriguing study on the application of engineered strain to isolated nuclear spins in silicon.¹⁶ Such qubits offer the longest coherence times in the solid state,^40,41 and application of strain can perturb the quadrupole moment of high-spin nuclei.⁴² There have been recent demonstrations of electric-field control of such nuclei,^43,44 but here the authors propose the use of piezoelectric films to drive nuclear spins acoustically. This may eventually open the path to coupling between highly coherent nuclear spin qubits and mechanical systems.

Cold atom-based systems are at the forefront of quantum science research with trapped ions⁴⁵ and neutral atoms⁴⁶ as leading quantum computing platforms. This collection includes a proposal for an exciting new architecture for quantum computing with trapped ions.¹⁸ Alkaline earth(-like) ions have a rich internal structure that can host several qubit encodings. Zeeman qubits are based on the electron spin with a typical energy in the MHz range, hyperfine qubits are based on the hyperfine splitting, which is typically in the GHz range, and optical qubits use a metastable optically excited state that is hundreds of THz above the ground state. Each encoding has unique advantages and disadvantages. This new proposed architecture¹⁸ provides an efficient way to leverage their strengths while obviating their weaknesses. Due to the use of qubits encoded in both the ground “g” and metastable “m” states (Zeeman or hyperfine) as well as qubits encoded optically “o,” this architecture has been branded as “omg.” Following this work, similar architectures have already been proposed for neutral alkaline earth(-like) atoms.^47,48

Efficient state preparation and measurement is an outstanding challenge for nearly every quantum system and is becoming increasingly important as systems are scaled up. We have, therefore, included several papers that discuss quantum state preparation and readout. Included is a perspective article outlining the use of traveling wave parametric amplifiers,²⁰ which are used for high bandwidth qubit readout not only throughout the superconducting qubit community⁴⁹ but also for semiconductor spin qubit devices.⁵⁰ 

Even in systems with state-of-the-art amplifiers, when measurement bandwidths are high, state identification can be a challenge. Here, we present an article demonstrating efficient state classification using neural networks.¹⁴ 

Many of the challenges associated with quantum computing boil down to short coherence and relaxation times, which are often linked to materials properties. The oxidation states in Nb, for example, are suspected to have non-insulating species that can limit qubit T₁ times, and simply switching to Ta-based devices has led to a major improvement in coherence.⁵¹ Likewise, for spin qubits, isotopically enriching quantum wells has led to a significant increase in coherence²⁹ and corresponding fidelities. While the last two decades have led to major improvements in the qubit materials, there is still work to do and we describe several new techniques to help probe material properties.

First, we feature an article describing the use of THz scanning probe microscopy to elucidate the nanoscale dielectric properties and carrier concentration in the proximity of superconducting devices.⁷ Such a technique gives critical information that can be used to feedback into the fabrication process to improve device performance. Also featured is a detailed study of the oxidation states in Nb as well as an identification of magnetic impurities in the sample through x-ray absorption measurements.¹³ 

It is our hope that this Special Topic issue will not only highlight some new research directions in the quantum information sciences but also will inspire new ideas. As the quantum engineering community moves from few-qubit demonstrations to NISQ devices, a new set of engineering challenges is emerging, and it is becoming increasingly important to develop new materials, encodings, and architectures. It is an exciting time for quantum information science and engineering, and we believe that the best is yet to come.

We would like to thank all of the authors who contributed to this Special Topic. We would also like to thank the editorial team at APL including Jessica Trudeau, Emma Van Burns, Martin Weides, and Lesley Cohen.

The authors have no conflicts to disclose.

Data sharing is not applicable to this article as no new data were created or analyzed in this study.