Recent experiments with superconductor-quantum dot hybrids have made significant advances in creating solid-state entanglement by engineering subgap processes in superconducting regions. Using the thermoelectric Cooper Pair Splitter (CPS) as foundation, researchers have presented a thorough analysis of the transport signals observed in these devices, clarifying their operating regimes and confirming the nonlocal, nonclassical nature of correlations arising from crossed Andreev processes.
This work connects these findings with quantum discord, identifying specific operating points where nonlocal quantum correlations emerge in CPS devices—information not accessible from transport signals alone. A key discovery is that contact-induced level broadening of quantum dots’ discrete energy spectra, along with hybridization with superconducting segments, can create shifted resonances in crossed Andreev processes and cause parity reversal in thermoelectric current.
Cooper pair splitters, which leverage coupling between superconducting ground states and spatially separated quantum dots, represent promising candidates for generating solid-state entanglement. These devices utilize crossed Andreev reflection (CAR), a subgap transport process occurring between distinct superconductor-normal interfaces, to create nonlocal spin-entangled electrons. Recent experimental progress has demonstrated CPS implementation in carbon nanotubes, SN heterostructures, semiconducting nanowires, and various quantum dot systems.
The coupling of superconducting regions with quantum dots enables spectral probing of subgap processes, where well-spaced quantum dot levels serve as probes through electrostatic gating. These systems exhibit not only CAR processes but also competing elastic co-tunneling between quantum dots and local Andreev reflections. Notably, thermoelectric setups eliminate local Andreev processes that typically overwhelm the nonlocal processes crucial to CPS signals.
Using the Keldysh non-equilibrium Green’s function framework with minimal assumptions, the team has decomposed currents into elastic co-tunneling and crossed Andreev reflection components, providing insights into spectral structures of currents and density of states. By computing quantum discord between spatially separated quantum dots, they precisely identified nonlocal correlations specifically induced by CAR processes.
While previous theoretical approaches relied on semi-classical rate equations, quantum master equations, or transmission formalism, this approach offers comprehensive understanding without treating the superconducting segment merely as an effective coupling between quantum dots. The team has demonstrated that the discreteness of energy levels, hybridization effects, and resulting energy level broadening are essential for explaining observed transport signatures.
This work ultimately provides detailed insights into controlling nonlocal quantum correlations in superconducting-hybrid Cooper pair splitters through gate voltage manipulation, revealing new possibilities for harnessing quantum correlations in solid-state systems.
npj Quantum Information, Published online: 08 March 2025; doi:10.1038/s41534-025-00966-6
