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Science

Optically manipulating superconductors

J. Stajic

Published Sep 4, 2014• 0 citations
Publisher

Abstract

Superconductivity In superconductors, electrons of opposite momenta pair to form a highly correlated state that manages to flow without encountering any resistance. Matsunaga et al. manipulated the wavefunction of these pairs in the superconductor NbN with an electromagnetic pulse that they transmitted through a thin layer of the material (see the Perspective by Pashkin and Leitenstorfer). The superconducting gap, which is the energy needed to break the pairs apart, oscillated at twice the frequency of the pulse's electric field. When they matched this frequency to half the gap, the authors excited a collective mode in the superconductor called the Higgs mode, a relative of the Higgs boson in particle physics. Science , this issue p. [1145][1]; see also p. [1121][2] [1]: /lookup/doi/10.1126/science.1254697 [2]: /lookup/doi/10.1126/science.1257302

DOI
10.1126/science.345.6201.1131-l

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Superconductivity
In superconductors, electrons of opposite momenta pair to form a highly correlated state that manages to flow without encountering any resistance. Matsunaga et al. manipulated the wavefunction of these pairs in the superconductor NbN with an electromagnetic pulse that they transmitted through a thin layer of the material (see the Perspective by Pashkin and Leitenstorfer). The superconducting gap, which is the energy needed to break the pairs apart, oscillated at twice the frequency of the pulse's electric field. When they matched this frequency to half the gap, the authors excited a collective mode in the superconductor called the Higgs mode, a relative of the Higgs boson in particle physics.
Science , this issue p. 1145; see also p. 1121
The Josephson junction is a device and constructed by separating two superconductors with an insulating barrier, which is thin enough so that super-electrons or Cooper pairs, composed of two electrons with opposite momenta and spins, formed by the electron-phonon interaction, and described as probability waves, can quantummechanically tunnel through the barrier and cause superposition and interferences. Due to the quantum e↵ect, a Josephson junction can generate and sustain an indefinitely long direct electric current or potential di↵erence, which becomes alternating one when an external voltage is applied. This paper investigates the Josephson pulse including its generation, property, and control. The results obtained from this study indicate that perturbations of the superconductors can generate strong superconducting pulses in the Josephson junction. The electric potential and field in the barrier oscillate when the two superconductors have di↵erent physical properties including the number densities of super-electrons, the phases of superconductors, and their rates of variations. The oscillations can be harmonic, combined, and purely pulse-like. The strength of the pulse increases with the di↵erences in the rates of densities and phases between the two superconductors. The pulse can be stronger in strength and narrower in width when one superconductor has lower density but higher rates of density and phase than another superconductor. Light or electromagnetic waves can perturb the superconductivities of superconductors and hence can be applied to control the pulses of Josephson junctions.
Mesoscopic Physics In superconductors, single particles cannot have energies smaller than the superconducting gap. Yet when two superconductors are separated by a thin nonsuperconducting bridge (the “weak link”), quasi-particles can occupy states that are inside the gap, the so-called Andreev bound states (ABSs). Janvier et al. fabricated such a structure out of superconducting aluminum and manipulated the occupation of a pair of ABSs. They observed oscillations in population between two of the energy levels, forming a type of qubit, which they dubbed the Andreev qubit. The results may lead to applications in quantum information processing. Science , this issue p. [1199][1] [1]: /lookup/doi/10.1126/science.aab2179
The author discusses superconductor-light interactions. After a brief introduction to the theory of superconductivity concentrates on aspects of particular interest to optical scientists. From the theoretical point of view the coherence of the superconductor wave function leads to interference effects. From an experimental point of view, various possibilities exist for optically controlling, and even creating, devices such as switches and Josephson junctions. The author's objective is to present rapidly the theoretical background of superconductivity and light-superconductor interactions and go on to discuss various related experiments and devices.
Superconductivity In conventional superconductors, the electron pairs responsible for superconductivity are large and overlapping. Starting from this so-called Bardeen-Cooper-Schrieffer (BCS) limit, increasing interactions can set the system on a path of crossover to the opposite limit of small, tightly bound electron pairs that undergo Bose-Einstein condensation (BEC). Nakagawa et al. intercalated lithium ions into the insulating material zirconium nitride chloride, varying the carrier density across a large range (see the Perspective by Randeria). This induced superconductivity and enabled the system to enter the crossover regime between the BCS and BEC limits. Science , this issue p. [190][1]; see also p. [132][2] [1]: /lookup/doi/10.1126/science.abb9860 [2]: /lookup/doi/10.1126/science.abg1752
Quantum Simulation The concept of broken symmetry has relevance across all branches of physics, including particle and condensed matter physics. In many cases, the shape of the potential energy resembles a Mexican hat, and the symmetry of the system is broken when it chooses a particular spot along the “trough” of the hat. Out of this minimum-energy state, the system can undergo collective excitations either along the trough or perpendicular to it. Leonard et al. detected these so-called Goldstone and Higgs modes in a supersolid Bose-condensed atomic gas held in two crossed optical cavities. By monitoring the dynamics of the light field in each cavity, the oscillations of the order parameter associated with both modes were observed in real time. Science , this issue p. [1415][1] [1]: /lookup/doi/10.1126/science.aan2608