Superconducting triplet pairing in Ni--Ga-bilayer junctions

Ni--Ga bilayers are a versatile platform for exploring the competition between strongly antagonistic ferromagnetic and superconducting phases. We characterize the impact of this competition on the transport properties of highly-ballistic Al/Al$_2$O$_3$(/EuS)/Ni--Ga tunnel junctions from both experimental and theoretical points of view. While the conductance spectra of junctions comprising Ni (3 nm)--Ga (60 nm) bilayers can be well understood within the framework of earlier results, which associate the emerging main conductance maxima with the junction films' superconducting gaps, thinner Ni (1.6 nm)--Ga (30 nm) bilayers entail completely different physics, giving rise to novel conductance-peak subseries that we term conductance shoulders. Detecting the paramagnetic Meissner response in Ga from polarized neutron reflectometry provides the essential experimental hint that these conductance shoulders are caused by superconducting triplet pairings that Ni's ferromagnetic exchange interaction induces near thin Ni--Ga bilayers' interfaces -- most likely owing to inhomogeneously magnetized interface domains. We further substantiate our findings by means of a phenomenological theoretical model, clarifying that induced superconducting triplet pairings around the interface of Ni--Ga bilayers can indeed manifest themselves in the observed conductance shoulders. Arranging our work in a broader context demonstrates that Ni--Ga-bilayer junctions have a strong potential for efficient triplet-pairing engineering in superconducting-spintronics applications.Comment: 24 pages, 10 figures, 1 tabl

Electrical Manipulation of Telecom Color Centers in Silicon

Silicon color centers have recently emerged as promising candidates for commercial quantum technology, yet their interaction with electric fields has yet to be investigated. In this paper, we demonstrate electrical manipulation of telecom silicon color centers by fabricating lateral electrical diodes with an integrated G center ensemble in a commercial silicon on insulator wafer. The ensemble optical response is characterized under application of a reverse-biased DC electric field, observing both 100% modulation of fluorescence signal, and wavelength redshift of approximately 1.4 GHz/V above a threshold voltage. Finally, we use G center fluorescence to directly image the electric field distribution within the devices, obtaining insight into the spatial and voltage-dependent variation of the junction depletion region and the associated mediating effects on the ensemble. Strong correlation between emitter-field coupling and generated photocurrent is observed. Our demonstration enables electrical control and stabilization of semiconductor quantum emitters

Hyperfine Spectroscopy of Isotopically Engineered Group-IV Color Centers in Diamond

A quantum register coupled to a spin-photon interface is a key component in quantum communication and information processing. Group-IV color centers in diamond (SiV, GeV, and SnV) are promising candidates for this application, comprising an electronic spin with optical transitions coupled to a nuclear spin as the quantum register. However, the creation of a quantum register for these color centers with deterministic and strong coupling to the spin-photon interface remains challenging. Here, we make first-principles predictions of the hyperfine parameters of the group-IV color centers, which we verify experimentally with a comprehensive comparison between the spectra of spin active and spin neutral intrinsic dopant nuclei in single GeV and SnV emitters. In line with the theoretical predictions, detailed spectroscopy on large sample sizes reveals that hyperfine coupling causes a splitting of the optical transition of SnV an order of magnitude larger than the optical linewidth and provides a magnetic-field insensitive transition. This strong coupling provides access to a new regime for quantum registers in diamond color centers, opening avenues for novel spin-photon entanglement and quantum sensing schemes for these well-studied emitters

Development of a Boston-area 50-km fiber quantum network testbed

Distributing quantum information between remote systems will necessitate the integration of emerging quantum components with existing communication infrastructure. This requires understanding the channel-induced degradations of the transmitted quantum signals, beyond the typical characterization methods for classical communication systems. Here we report on a comprehensive characterization of a Boston-Area Quantum Network (BARQNET) telecom fiber testbed, measuring the time-of-flight, polarization, and phase noise imparted on transmitted signals. We further design and demonstrate a compensation system that is both resilient to these noise sources and compatible with integration of emerging quantum memory components on the deployed link. These results have utility for future work on the BARQNET as well as other quantum network testbeds in development, enabling near-term quantum networking demonstrations and informing what areas of technology development will be most impactful in advancing future system capabilities.Comment: 9 pages, 5 figures + Supplemental Material

Telecom networking with a diamond quantum memory

Practical quantum networks require interfacing quantum memories with existing channels and systems that operate in the telecom band. Here we demonstrate low-noise, bidirectional quantum frequency conversion that enables a solid-state quantum memory to directly interface with telecom-band systems. In particular, we demonstrate conversion of visible-band single photons emitted from a silicon-vacancy (SiV) center in diamond to the telecom O-band, maintaining low noise ($g^2(0)<0.1$) and high indistinguishability ($V=89\pm8\%$). We further demonstrate the utility of this system for quantum networking by converting telecom-band time-bin pulses, sent across a lossy and noisy 50 km deployed fiber link, to the visible band and mapping their quantum states onto a diamond quantum memory with fidelity $\mathcal{F}=87\pm 2.5 \%$. These results demonstrate the viability of SiV quantum memories integrated with telecom-band systems for scalable quantum networking applications.Comment: 9 pages, 5 figures + Supplemental Material

Large-Scale Characterization of Quantum Emitters in High-Purity Diamond

Solid state quantum memories, such as color centers in diamond, are a leading platform for the distribution of quantum information. Quantum repeaters will require many qubit registers at every quantum network node, each with long-lived spin states and high-quality single photon emissions. Here, we present techniques for large-scale characterization of color centers in diamond. We first demonstrate automated confocal microscopy and apply it to characterize silicon vacancies in diamond overgrown via chemical vapor deposition and tin vacancies in overgrown and high pressure high temperature treated diamond, yielding narrow inhomogeneous distributions of both emitters. We then demonstrate widefield photoluminescence excitation microscopy as a tool to multiplex the characterization of color center optical properties, and apply it to measure the optical properties of silicon vacancies in a sample implanted with a focused ion beam. These techniques pave the way for future large-scale characterization efforts necessary to construct quantum memory nodes.S.M

Signatures of superconducting triplet pairing in Ni–Ga-bilayer junctions

Ni-Ga bilayers are a versatile platform for exploring the competition between strongly antagonistic ferromagnetic and superconducting phases. We characterize the impact of this competition on the transport properties of highly-ballistic Al/Al2O3(/EuS)/Ni-Ga tunnel junctions from both experimental and theoretical points of view. While the conductance spectra of junctions comprising Ni (3 nm)-Ga (60 nm) bilayers can be well understood within the framework of earlier results, which associate the emerging main conductance maxima with the junction films' superconducting gaps, thinner Ni (1.6 nm)-Ga (30 nm) bilayers entail completely different physics, and give rise to novel large-bias (when compared to the superconducting gap of the thin Al film as a reference) conductance-peak subseries that we term conductance shoulders. These conductance shoulders might attract considerable attention also in similar magnetic superconducting bilayer junctions, as we predict them to offer an experimentally well-accessible transport signature of superconducting triplet pairings that are induced around the interface of the Ni-Ga bilayer. We further substantiate this claim performing complementary polarized neutron reflectometry measurements on the bilayers, from which we deduce (1) a nonuniform magnetization structure in Ga in a several nanometer-thick area around the Ni-Ga boundary and can simultaneously (2) satisfactorily fit the obtained data only considering the paramagnetic Meissner response scenario. While the latter provides independent experimental evidence of induced triplet superconductivity inside the Ni-Ga bilayer, the former might serve as the first experimental hint of its potential microscopic physical origin. Finally, we introduce a simple phenomenological toy model to confirm also from the theoretical standpoint that superconducting triplet pairings around the Ni-Ga interface can indeed lead to the experimentally observed conductance shoulders, which convinces that our claims are robust and physically justified. Arranging our work in a broader context, we expect that Ni-Ga-bilayer junctions could have a strong potential for future superconducting-spintronics applications whenever an efficient engineering of triplet-pairing superconductivity is required

Superconducting triplet pairing in Al/Al₂O₃/Ni/Ga junctions

Ni/Ga bilayers are a versatile playground for exploring the competition of the strongly antagonistic ferromagnetic and superconducting phases. Systematically characterizing this competition’s impact on highly ballistic Al/Al₂O₃/Ni/Ga junctions’ transport properties from both the experimental and theoretical viewpoints, we identify novel conductance peak structures, which are caused by superconducting triplet pairings at the Ni/Ga interface, and which are widely adjustable through the Ni–Ga thickness ratio. We demonstrate that these conductance anomalies persist even in the presence of an in-plane magnetic field, which provides—together with the detection of the paramagnetic Meissner effect in Ga—the clear experimental evidence that the observed conductance features serve indeed as the triplet pairings’ unique transport fingerprints. Our work demonstrates that Ni/Ga bilayers have a strong potential for superconducting spintronics applications, in particular for triplet-pairing engineering

A fully packaged multi-channel cryogenic module for optical quantum memories

Realizing a quantum network will require long-lived quantum memories with optical interfaces incorporated into a scalable architecture. Color centers quantum emitters in diamond have emerged as a promising memory modality due to their optical properties and compatibility with scalable integration. However, developing a scalable color center emitter module requires significant advances in the areas of heterogeneous integration and cryogenically compatible packaging. Here we report on a cryogenically stable and network compatible quantum-emitter module for memory use. This quantum-emitter module is a significant development towards advanced quantum networking applications such as distributed sensing and processing.Comment: 10 pages, 8 figure