Toward Valley‐Coupled Spin Qubits

Abstract

The bid for scalable physical qubits has attracted many possible candidate platforms. In particular, spin‐based qubits in solid‐state form factors are attractive as they could potentially benefit from processes similar to those used for conventional semiconductor processing. However, material control is a significant challenge for solid‐state spin qubits as residual spins from substrate, dielectric, electrodes, or contaminants from processing contribute to spin decoherence. In the recent decade, valleytronics has seen a revival due to the discovery of valley‐coupled spins in monolayer transition metal dichalcogenides. Such valley‐coupled spins are protected by inversion asymmetry and time reversal symmetry and are promising candidates for robust qubits. In this report, the progress toward building such qubits is presented. Following an introduction to the key attractions in fabricating such qubits, an up‐to‐date brief is provided for the status of each key step, highlighting advancements made and/or outstanding work to be done. This report concludes with a perspective on future development highlighting major remaining milestones toward scalable spin‐valley qubits.

Gate‐Defined Quantum Confinement in CVD 2D WS2

“…[ 2,7–10,24–27 ] Stringent conditions such as cryogenic‐temperature operation are required for applications in quantum information processing in order to preserve the fragile quantum states, which necessitate a firm understanding of low‐temperature scattering mechanisms. [ 14,15,28,29 ] Furthermore, realizing gated TMDC quantum dot devices is essential for quantum computation applications with tunable electrical control while minimizing undesirable edge states from physical etching, and requires suitable dielectrics for effective carrier confinement at nanoscale dimensions. [ 30–35 ]…”

“…[2,[7][8][9][10][24][25][26][27] Stringent conditions such as cryogenic-temperature operation are required for applications in quantum information processing in order to preserve the fragile quantum states, which necessitate a firm understanding of low-temperature scattering mechanisms. [14,15,28,29] Furthermore, realizing gated TMDC quantum dot devices is essential for quantum computation applications with tunable electrical control while minimizing undesirable edge states from physical etching, and requires suitable dielectrics for effective carrier confinement at nanoscale dimensions. [30][31][32][33][34][35] Experiments hoping to shed light on the influence of dielectric deposition on low-temperature carrier transport in TMDCs typically face two challenges: 1) poor contacts at cryogenic temperatures with large Schottky barrier heights (SBHs); and 2) significant device-to-device variations.…”

“…[4][5][6][7][8][9][10] 2D TMDCs are an important class of atomically thin materials with novel properties such as tunable layer-dependent direct bandgaps, strong photoluminescence, large spin-orbit coupling, and unique spin-valley coupling. [11][12][13][14][15] These exceptional properties are attractive for applications in electronics, optoelectronic and quantum information processing. 2D TMDCs were first isolated with the "Scotch tape" technique, where adhesive tapes "peel" thin flakes from bulk crystals that are then repeatedly exfoliated to a single atomic layer.…”

Gate‐Controlled Quantum Dots Based on 2D Materials

“…QPCs based on TMDs have also been realized, in which the quantized conductance is observed and can be electrostatically pinched off [202][203][204][205] . Moreover, the coupled spin-valley degrees of freedom in TMDs provide possibilities for encoding information as well [73] . Besides the common K valleys, which are similar to those studied in graphene, carriers from other valleys can also dominate the transport behavior in TMDs, due to the unique band structure and its layer-sensitive feature.…”

“…As a consequence, the electron intervalley scattering is expected to be weak, leading to a long valley lifetime. Therefore, the valley degree of freedom offers a promising quantum degree of freedom that can be used to encode, process, and store information, serving as qubits for realizing quantum computation [71][72][73] . In order to fully unlock this potential, manipulating the valley degree of freedom at single-particle level is essential.…”