A split Cooper pair is a natural source for entangled electrons which is a basic ingredient for quantum information in the solid state. We report an experiment on a superconductor-graphene double quantum dot (QD) system, in which we observe Cooper pair splitting (CPS) up to a CPS efficiency of ∼10%. With bias on both QDs, we are able to detect a positive conductance correlation across the two distinctly decoupled QDs. Furthermore, with bias only on one QD, CPS and elastic cotunneling can be distinguished by tuning the energy levels of the QDs to be asymmetric or symmetric with respect to the Fermi level in the superconductor.A split Cooper pair is a natural source for entangled electrons which is a basic ingredient for quantum information in the solid state. We report an experiment on a superconductor-graphene double quantum dot (QD) system, in which we observe Cooper pair splitting (CPS) up to a CPS efficiency of ∼10%. With bias on both QDs, we are able to detect a positive conductance correlation across the two distinctly decoupled QDs. Furthermore, with bias only on one QD, CPS and elastic cotunneling can be distinguished by tuning the energy levels of the QDs to be asymmetric or symmetric with respect to the Fermi level in the superconductor.

arXiv:1412.8451Physical Review LettersMar 4, 2015129 citations

Quantum processing with graphene

Nature IndiaNov 26, 20120 citations

Quantum Materials to the Pioneering Future of Computing and Communication

Nakamura Shuto, Akari Chiyo, Himari Ken +1 more

Quantum materials, characterized by their unique electronic, magnetic, and optical properties, are poised to revolutionize the future of computing and communication. These materials, including topological insulators, superconductors, and two-dimensional materials like graphene, exhibit quantum mechanical effects at macroscopic scales, offering unprecedented capabilities for technological advancement. In computing, quantum materials enable the development of quantum bits (qubits) that can perform complex computations at speeds unattainable by classical computers, significantly enhancing processing power and efficiency. For communication, they facilitate ultra-secure quantum communication networks through quantum entanglement and superposition, ensuring data integrity and privacy. Additionally, quantum materials support the creation of novel electronic devices with superior performance and energy efficiency. As research progresses, integrating quantum materials into practical applications will address current limitations and open new horizons in information technology, marking a pivotal shift toward a pioneering future in computing and communication.Quantum materials, characterized by their unique electronic, magnetic, and optical properties, are poised to revolutionize the future of computing and communication. These materials, including topological insulators, superconductors, and two-dimensional materials like graphene, exhibit quantum mechanical effects at macroscopic scales, offering unprecedented capabilities for technological advancement. In computing, quantum materials enable the development of quantum bits (qubits) that can perform complex computations at speeds unattainable by classical computers, significantly enhancing processing power and efficiency. For communication, they facilitate ultra-secure quantum communication networks through quantum entanglement and superposition, ensuring data integrity and privacy. Additionally, quantum materials support the creation of novel electronic devices with superior performance and energy efficiency. As research progresses, integrating quantum materials into practical applications will address current limitations and open new horizons in information technology, marking a pivotal shift toward a pioneering future in computing and communication.

Fusion of Multidisciplinary Research An International JournalFeb 15, 202013 citations

Nonequilibrium-induced metal-superconductor quantum phase transition in graphene

So Takei, Yong Baek Kim

We study the effects of dissipation and time-independent nonequilibrium drive on an open superconducting graphene. In particular, we investigate how dissipation and nonequilibrium effects modify the semi-metal-BCS quantum phase transition that occurs at half filling in equilibrium graphene with attractive interactions. Our system consists of a graphene sheet sandwiched by two semi-infinite three-dimensional Fermi-liquid reservoirs, which act both as a particle pump/sink and a source of decoherence. A steady-state charge current is established in the system by equilibrating the two reservoirs at different but constant chemical potentials. The graphene sheet is described using the attractive Hubbard model in which the interaction is decoupled in the $s$ -wave channel. The nonequilibrium BCS superconductivity in graphene is formulated using the Keldysh path-integral formalism, and we obtain generalized gap and number density equations valid for both zero and finite voltages. The behavior of the gap is discussed as a function of both attractive interaction strength and electron densities for various graphene-reservoir couplings and voltages. We discuss how tracing out the dissipative environment (with or without voltage) leads to decoherence of Cooper pairs in the graphene sheet, hence, to a general suppression of the gap order parameter at all densities. For weak enough attractive interactions we show that the gap vanishes even for electron densities away from half filling and illustrate the possibility of a dissipation-induced metal-superconductor quantum phase transition. We find that the application of small voltages does not alter the essential features of the gap as compared to the case when the system is subject to dissipation alone (i.e., zero voltage). The possibility of tuning the system through the metal-superconductor quantum critical point using voltage is presented.We study the effects of dissipation and time-independent nonequilibrium drive on an open superconducting graphene. In particular, we investigate how dissipation and nonequilibrium effects modify the semi-metal-BCS quantum phase transition that occurs at half filling in equilibrium graphene with attractive interactions. Our system consists of a graphene sheet sandwiched by two semi-infinite three-dimensional Fermi-liquid reservoirs, which act both as a particle pump/sink and a source of decoherence. A steady-state charge current is established in the system by equilibrating the two reservoirs at different but constant chemical potentials. The graphene sheet is described using the attractive Hubbard model in which the interaction is decoupled in the $s$-wave channel. The nonequilibrium BCS superconductivity in graphene is formulated using the Keldysh path-integral formalism, and we obtain generalized gap and number density equations valid for both zero and finite voltages. The behavior of the gap is discussed as a function of both attractive interaction strength and electron densities for various graphene-reservoir couplings and voltages. We discuss how tracing out the dissipative environment (with or without voltage) leads to decoherence of Cooper pairs in the graphene sheet, hence, to a general suppression of the gap order parameter at all densities. For weak enough attractive interactions we show that the gap vanishes even for electron densities away from half filling and illustrate the possibility of a dissipation-induced metal-superconductor quantum phase transition. We find that the application of small voltages does not alter the essential features of the gap as compared to the case when the system is subject to dissipation alone (i.e., zero voltage). The possibility of tuning the system through the metal-superconductor quantum critical point using voltage is presented.

arXiv:0712.1043Physical Review BOct 2, 200815 citations

s

-wave channel. The nonequilibrium BCS superconductivity in graphene is formulated using the Keldysh path-integral formalism, and we obtain generalized gap and number density equations valid for both zero and finite voltages. The behavior of the gap is discussed as a function of both attractive interaction strength and electron densities for various graphene-reservoir couplings and voltages. We discuss how tracing out the dissipative environment (with or without voltage) leads to decoherence of Cooper pairs in the graphene sheet, hence, to a general suppression of the gap order parameter at all densities. For weak enough attractive interactions we show that the gap vanishes even for electron densities away from half filling and illustrate the possibility of a dissipation-induced metal-superconductor quantum phase transition. We find that the application of small voltages does not alter the essential features of the gap as compared to the case when the system is subject to dissipation alone (i.e., zero voltage). The possibility of tuning the system through the metal-superconductor quantum critical point using voltage is presented.