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arXiv:1308.3230 · Physical Review B

Singlet-triplet superconducting quantum magnetometer

Mohammad Alidoust, Klaus Halterman, Jacob Linder

Published Aug 27, 2013• 16 citations
arXivHTMLPDF

Abstract

Motivated by the recent experimental realization of a quantum interference transistor based on the superconducting proximity effect, we here demonstrate that the inclusion of a textured ferromagnet both strongly enhances the flux sensitivity of such a device and additionally allows for singlet-triplet switching by tuning a bias voltage. This functionality makes explicit use of the induced spin-triplet correlations due to the magnetic texture. Whereas the existence of such triplet correlations is well known, our finding demonstrates how spin-triplet superconductivity may be utilized for concrete technology, namely, to improve the functionality of ultrasensitive magnetometers.

DOI
10.1103/physrevb.88.075435

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Motivated by the recent experimental realization of a quantum interference transistor based on the superconducting proximity effect, we here demonstrate that the inclusion of a textured ferromagnet both strongly enhances the flux sensitivity of such a device and additionally allows for singlet-triplet switching by tuning a bias voltage. This functionality makes explicit use of the induced spin-triplet correlations due to the magnetic texture. Whereas the existence of such triplet correlations is well known, our finding demonstrates how spin-triplet superconductivity may be utilized for concrete technology, namely, to improve the functionality of ultrasensitive magnetometers.
A superconducting quantum magnetometer based on magnetic flux-driven modulation of the density of states of a proximized metallic nanowire is theoretically analyzed. With optimized geometrical and material parameters transfer functions up to a few mV$/{\ensuremath{\Phi}}_{0}$ and intrinsic flux noise $\ensuremath{\sim}{10}^{\ensuremath{-}9}{\ensuremath{\Phi}}_{0}/\sqrt{\text{Hz}}$ below 1 K are achievable. The opportunity to access single-spin detection joined with limited dissipation (of the order of $\ensuremath{\sim}{10}^{\ensuremath{-}14}$ W) make this magnetometer interesting for the investigation of the switching dynamics of molecules or individual magnetic nanoparticles.
Hybrid systems comprising superconducting and semiconducting materials are promising architectures for quantum computing. Superconductors induce long-range interactions between the spin degrees of freedom of semiconducting quantum dots. These interactions are widely anisotropic when the semiconductor material has strong spin-orbit interactions. We show that this anisotropy is tunable and enables fast and high-fidelity two-qubit gates between singlet-triplet (ST) spin qubits. Our design is immune to leakage of the quantum information into noncomputational states and removes always-on interactions between the qubits, thus resolving key open challenges for these architectures. Our ST qubits do not require additional technologically demanding components nor fine-tuning of parameters. They operate at low magnetic fields of a few millitesla and are fully compatible with superconductors. By suppressing systematic errors in realistic devices, we estimate infidelities below ${10}^{\ensuremath{-}3}$, which could pave the way toward large-scale hybrid superconducting-semiconducting quantum processors.
Hybrid devices integrating quantum dots with Josephson junctions are gaining interest because they combine spin-based quantum computing with circuit quantum electrodynamics (circuit QED) methods. In particular, Andreev spin qubits have shown significant experimental progress including strong two-qubit coupling, and are predicted to exhibit all-to-all connectivity. Here we propose superconducting singlet-triplet (SST) qubits that rely on parallel-aligned double quantum dots in Josephson junctions. While Andreev spin qubits require spin-orbit interaction to unlock the spin degree-of-freedom, SST qubits do not require spin-orbit interaction, making the advantages of hybrid devices available to a wider range of materials. Similar to Andreev spin qubits, the qubit states couple to the superconducting phase across the junction, which allows for control and readout using circuit QED, and supports all-to-all connectivity. Only NNN flux lines are required to perform any single- and two-qubit gate among NNN qubits, and thus the overhead of control lines is small. Finally, linear protection from charge or flux noise makes these qubits interesting candidates for a future quantum processor.
Superconductivity serves as a unique solid-state platform for electron interference at a device-relevant lengthscale, which is essential for quantum information and sensing technologies. As opposed to semiconducting transistors that are operated by voltage biasing at the nanometer scale, superconductive quantum devices cannot sustain voltage and are operated with magnetic fields, which impose a large device footprint, hindering miniaturization and scalability. Here, we introduce a system of superconducting materials and devices that have a common interface with a ferroelectric layer. An amorphous superconductor was chosen for reducing substrate-induced misfit strain and for allowing low-temperature growth. The common quantum pseudowavefunction of the superconducting electrons was controlled by the nonvolatile switchable polarization of the ferroelectric by means of voltage biasing. A controllable change of 21% in the critical temperature was demonstrated for a continuous film geometry. Moreover, a controllable change of 54% in the switching current of a superconducting quantum interference device was demonstrated. The ability to voltage bias superconducting devices together with the nonvolatile nature of this system paves the way to quantum-based memory devices.
A superconducting condensate is characterized by the emergence of macroscopic and collective order, established between its constituent electrons. The degree of correlation at a given spatial position is a complex scalar: it is characterized both by an amplitude (proportional to the minimal energy to generate a fundamental excitation) and a complex phase. Through its gradient, the latter enables the flow of the condensate, demonstrating the quantum fingerprint of superconductivity. Superconductors exhibit strong coupling to electromagnetic fields, so that phase-dependent dissipationless transport through “weak link” circuital elements is easily manipulated by applying voltage or magnetic flux bias to superconducting terminals and loops, respectively. For these reasons, superconducting electronics is nowadays a core technology to enable robust access and manipulation of the fundamental degrees of freedom in quantum devices, from ultrasensitive electromagnetic sensors to superconducting qubits. In this work, we explore different designs of micro-magnetometers based on superconducting interferometers. Differently from conventional designs based on Superconductor, Insulator, Superconductor (SIS) tunnel junctions, here the core elements are nanoscale diffusive metal wires acting as superconducting weak links. These consist in circuital dishomogeneities that can be fabricated over scales much smaller than typical superconductor coherence lengths, typically yielding unique response properties. On the other hand, their intrinsic transparency is usually associated with strong supercurrent concentration, which can severely limit their practicality due to superconducting depairing and thermal-driven hysteresis upon switching to the dissipative regime. In this thesis we demonstrate how a judicious use of nanofabricated designs makes it possible to counter these drawbacks and achieve complete phase polarization in interferometers based on diffusive weak links. The resulting micro-magnetometers are characterized by extremely high magnetic responsivity.
The design for an inductive superconducting quantum interference proximity transistor with enhanced performance, the L-SQUIPT, is presented and analyzed. The interferometer is based on a double-loop structure, where each ring comprises a superconductor--normal-metal--superconductor mesoscopic Josephson weak link and the readout electrode is implemented in the form of a superconducting tunnel probe. Our design allows us to both improve the coupling of the transistor to the external magnetic field and increase the characteristic magnetic flux transfer functions, thereby leading to an improved ultrasensitive quantum limited magnetometer. The L-SQUIPT behavior is analyzed in both the dissipative and the dissipationless Josephson-like operation modes, in the latter case by exploiting both an inductive and a dispersive readout scheme. The improved performance makes the L-SQUIPT promising for magnetic flux detection as well as for specific applications in quantum technology, where a responsive dispersive magnetometry at millikelvin temperatures is required.