A superconducting 180° hybrid ring coupler for circuit quantum electrodynamics

Abstract

Superconducting circuit quantum electrodynamics experiments with propagating microwaves require devices acting as beam splitters. Using niobium thin films on silicon and sapphire substrates, we fabricated superconducting 180 • microstrip hybrid ring couplers, acting as beam splitters with center frequencies of about 6 GHz. For the magnitude of the coupling and isolation we find −3.5 ± 0.5 dB and at least −15 dB, respectively, in a bandwidth of 2 GHz. We also investigate the effect of reflections at the superconductor-normal conductor contact by means of low temperature laser scanning microscopy. Our measurements show that our hybrid rings are well suited for on-chip applications in circuit quantum electrodynamics experiments.In superconducting circuit quantum electrodynamics (QED) 1-3 , intracavity microwave photons interact with solid-state artificial atoms 4-6 . Both cavity and atom are realized by superconducting quantum circuits on a chip with characteristic frequencies in the microwave regime (1-10 GHz). Recently, this field has been extended towards the study of propagating quantum microwaves. To this end, quantum optical techniques such as optical homodyne tomography 7 or photon-based quantum information processing and communication 8,9 are being adapted to the microwave regime. One key element for the transformation from the optical to the microwave regime is the implementation of a beam splitter which is understood on the quantum level 10 . This allows the use of signal recovery techniques employing two amplifier chains and eliminating the (not yet available) single microwave photon detectors 11 . Hereby, photon correlations can be accessed and all quadrature moments of propagating quantum microwaves and, simultaneously, those of the detector noise can be extracted 12 . The very same idea was recently used to characterize the black body radiation emitted by a 50 Ω load resistor 13 . Ideally, in experiments with propagating quantum microwaves a beam splitter has to be lossless. A device matching these conditions is the 180 • hybrid ring which is entirely based on interference effects. Usually, microwave beam splitters are realized as normal conductive devices. However, for superconducting circuit QED the on-chip implementation of the beam splitter and the superconducting quantum devices under investigation would be favorable, avoiding reflections between various circuit parts and minimizing interconnect losses. In this letter, we present a detailed study on low-loss superconducting hybrid rings fabricated from niobium microstrip lines on both silicon and sapphire substrates. For the magnitude of the coupling and isolation we find −3.5 ± 0.5 dB and better than −15 dB in a bandwidth of up to 2 GHz, respectively. We note that the isolation increases when reducing the bandwidth, reaching a maximum value of better than −60 dB at the center frequency. Our measurements indicate that the device performance is limited by reflections between the superconducting parts on the chip and the normal conducting microwave...

Dual-path methods for propagating quantum microwaves

“…We only assume that if the input states are vacuum states, also the output is a vacuum state. This assumption is well justified since a cold beam splitter is a passive device containing no energy sources [27,29].…”

“…[â,â † ] = 1. In the case of propagating signals in the frequency range of 1-10 GHz, hybrid ring structures [26,27] have already been used as beam splitters in several experiments.…”

Quantum simulation with a boson sampling circuit

“…All our proposals can be extended to work with superconducting quantum circuits, where microwave transformations such as beam splitters have been demonstrated [27,28]. In this case, the enhanced light-matter interaction allows reaching the ultrastrong coupling regime to the continuum [29] and we can no longer apply the Markov approximations.…”

Микроволновой Фотонный Детектор

“…Здесь многообещающими материалами для созда-ния масштабируемых твердотельных приборов являют-ся сверхпроводники. Во-первых, методики изготовления тонкопленочных линий передач с малыми потерями, высокодобротных резонаторов [5] и делителей мощности микроволнового сигнала [6,7] хорошо известны. Во-вторых, активно исследуемые в настоящее время сверх-проводниковые квантовые биты (кубиты) [8][9][10] могут, в принципе, играть роль двухуровневых квантовых систем для генерации и детектирования фотонов.…”