Characterization and reduction of microfabrication-induced decoherence in superconducting quantum circuits

Chow J.M., Gambetta J.M., Magesan E., Abraham D.W., Cross A.W., Johnson B.R., Masluk N.A., Ryan C.A., Smolin J.A., Srinivasan S.J., Steffen M.

With favourable error thresholds and requiring only nearest-neighbour interactions on a lattice, the surface code is an error-correcting code that has garnered considerable attention. At the heart of this code is the ability to perform a low-weight parity measurement of local code qubits. Here we demonstrate high-fidelity parity detection of two code qubits via measurement of a third syndrome qubit. With high-fidelity gates, we generate entanglement distributed across three superconducting qubits in a lattice where each code qubit is coupled to two bus resonators. Via high-fidelity measurement of the syndrome qubit, we deterministically entangle the code qubits in either an even or odd parity Bell state, conditioned on the syndrome qubit state. Finally, to fully characterize this parity readout, we develop a measurement tomography protocol. The lattice presented naturally extends to larger networks of qubits, outlining a path towards fault-tolerant quantum computing. Quantum error correction protocols aim at protecting quantum information from corruption due to decoherence and imperfect control. Using three superconducting transmon qubits, Chow et al. demonstrate necessary elements for the implementation of the surface error correction code on a two-dimensional lattice.

Barends R., Kelly J., Megrant A., Veitia A., Sank D., Jeffrey E., White T.C., Mutus J., Fowler A.G., Campbell B., Chen Y., Chen Z., Chiaro B., Dunsworth A., Neill C., et. al.

A universal set of logic gates in a superconducting quantum circuit is shown to have gate fidelities at the threshold for fault-tolerant quantum computing by the surface code approach, in which the quantum bits are distributed in an array of planar topology and have only nearest-neighbour couplings. Quantum computers can only work in practice if, like conventional computers, they are fault-tolerant. This means that a system has to be in place to detect any errors and correct them. For quantum error correction such a system involves entangling several quantum bits (qubits) with each other. In the so-called surface code error-correction architecture, qubits are placed in a lattice and are entangled with four nearest neighbours. Rami Barends et al. report the construction of such a surface code system with five qubits in a row made from superconducting devices. This system performs with fidelity that is at the threshold for quantum error correction, suggesting that error-free quantum computing should be possible. The platform lends itself to scaling up to larger numbers of qubits and two-dimensional architecture. A quantum computer can solve hard problems, such as prime factoring1,2, database searching3,4 and quantum simulation5, at the cost of needing to protect fragile quantum states from error. Quantum error correction6 provides this protection by distributing a logical state among many physical quantum bits (qubits) by means of quantum entanglement. Superconductivity is a useful phenomenon in this regard, because it allows the construction of large quantum circuits and is compatible with microfabrication. For superconducting qubits, the surface code approach to quantum computing7 is a natural choice for error correction, because it uses only nearest-neighbour coupling and rapidly cycled entangling gates. The gate fidelity requirements are modest: the per-step fidelity threshold is only about 99 per cent. Here we demonstrate a universal set of logic gates in a superconducting multi-qubit processor, achieving an average single-qubit gate fidelity of 99.92 per cent and a two-qubit gate fidelity of up to 99.4 per cent. This places Josephson quantum computing at the fault-tolerance threshold for surface code error correction. Our quantum processor is a first step towards the surface code, using five qubits arranged in a linear array with nearest-neighbour coupling. As a further demonstration, we construct a five-qubit Greenberger–Horne–Zeilinger state8,9 using the complete circuit and full set of gates. The results demonstrate that Josephson quantum computing is a high-fidelity technology, with a clear path to scaling up to large-scale, fault-tolerant quantum circuits.

Deng C., Otto M., Lupascu A.

We report on the characterization of microwave loss of thin aluminum oxide films at low temperatures using superconducting lumped resonators. The oxide films are fabricated using plasma oxidation of aluminum and have a thickness of 5 nm. We measure the dielectric loss versus microwave power for resonators with frequencies in the GHz range at temperatures from 54 to 303 mK. The power and temperature dependence of the loss are consistent with the tunneling two-level system theory. These results are relevant to understanding decoherence in superconducting quantum devices. The obtained oxide films are thin and robust, making them suitable for capacitors in compact microwave resonators.

Khalil M.S., Stoutimore M.J., Gladchenko S., Holder A.M., Musgrave C.B., Kozen A.C., Rubloff G., Liu Y.Q., Gordon R.G., Yum J.H., Banerjee S.K., Lobb C.J., Osborn K.D.

Two-level system (TLS) defects in dielectrics are known to limit the performance of electronic devices. We study TLS using millikelvin microwave (6.4 GHz) loss measurements of three atomic layer deposited (ALD) oxide films–crystalline BeO (c-BeO), amorphous Al2O3 (a–Al2O3), and amorphous LaAlO3 (a–LaAlO3)–and interpret them with room temperature characterization measurements. We find that the bulk loss tangent in the crystalline film is 6 times higher than in the amorphous films. In addition, its power saturation agrees with an amorphous distribution of TLS. Secondary ion mass spectrometry (SIMS) impurity analysis of the c-BeO film showed excess surface carbon (C) impurities and a uniform hydrogen (H) impurity distribution, which coupled with the analysis of loss tangent strongly suggests H limited loss. Impurity analysis of the amorphous films reveals that they have excess H impurities at the ambient-exposed surface, and we extract the associated H-based surface loss tangent. We compare two a–Al2O3 films with drastically different C impurity concentrations and similar H impurity concentrations and conclude that H rather than C is the likely source of loss in the amorphous films and we find the loss per H concentration in a–Al2O3 to be KH =3×10−24 cm3.

Minev Z.K., Pop I.M., Devoret M.H.

We introduce a microwave circuit architecture for quantum signal processing combining design principles borrowed from high-Q 3D resonators in the quantum regime and from planar structures fabricated with standard lithography. The resulting “2.5D” whispering-gallery mode resonators store 98% of their energy in vacuum. We have measured internal quality factors above 3 × 106 at the single photon level and have used the device as a materials’ characterization platform to place an upper bound on the surface resistance of thin film aluminum of less than 250 nΩ.

Barends R., Kelly J., Megrant A., Sank D., Jeffrey E., Chen Y., Yin Y., Chiaro B., Mutus J., Neill C., O’Malley P., Roushan P., Wenner J., White T.C., Cleland A.N., et. al.

We demonstrate a planar, tunable superconducting qubit with energy relaxation times up to 44 μs. This is achieved by using a geometry designed to both minimize radiative loss and reduce coupling to materials-related defects. At these levels of coherence, we find a fine structure in the qubit energy lifetime as a function of frequency, indicating the presence of a sparse population of incoherent, weakly coupled two-level defects. We elucidate this defect physics by experimentally varying the geometry and by a model analysis. Our "Xmon" qubit combines facile fabrication, straightforward connectivity, fast control, and long coherence, opening a viable route to constructing a chip-based quantum computer.

Neill C., Megrant A., Barends R., Chen Y., Chiaro B., Kelly J., Mutus J.Y., O'Malley P.J., Sank D., Wenner J., White T.C., Yin Y., Cleland A.N., Martinis J.M.

Superconducting resonators, used in astronomy and quantum computation, couple strongly to microscopic two-level defects. We monitor the microwave response of superconducting resonators and observe fluctuations in dissipation and resonance frequency. We present a unified model where the observed dissipative and dispersive effects can be explained as originating from a bath of fluctuating two-level systems. From these measurements, we quantify the number and distribution of the defects.

Holder A.M., Osborn K.D., Lobb C.J., Musgrave C.B.

We perform ab initio calculations of hydrogen-based tunneling defects in alumina to identify deleterious two-level systems (TLS) in superconducting qubits. The defects analyzed include bulk hydrogenated Al vacancies, bulk hydrogen interstitial defects, and a surface OH rotor. The formation energies of the defects are first computed for an Al- and O-rich environment to give the likelihood of defect occurrence during growth. The potential energy surfaces are then computed and the corresponding dipole moments are evaluated to determine the coupling of the defects to an electric field. Finally, the tunneling energy is computed for the hydrogen defect and the analogous deuterium defect, providing an estimate of the TLS energy and the corresponding frequency for photon absorption. We predict that hydrogenated cation vacancy defects will form a significant density of GHz-frequency TLSs in alumina.

Chang J.B., Vissers M.R., Córcoles A.D., Sandberg M., Gao J., Abraham D.W., Chow J.M., Gambetta J.M., Beth Rothwell M., Keefe G.A., Steffen M., Pappas D.P.

We demonstrate enhanced relaxation and dephasing times of transmon qubits, up to ∼60 μs, by fabricating the interdigitated shunting capacitors using titanium nitride (TiN). Compared to qubits made with lift-off aluminum deposited simultaneously with the Josephson junction, this represents as much as a six-fold improvement and provides evidence that surface losses from two-level system (TLS) defects residing at or near interfaces contribute to decoherence. Concurrently, we observe an anomalous temperature dependent frequency shift of TiN resonators, which is inconsistent with the predicted TLS model.

Fowler A.G., Mariantoni M., Martinis J.M., Cleland A.N.

This article provides an introduction to surface code quantum computing. We first estimate the size and speed of a surface code quantum computer. We then introduce the concept of the stabilizer, using two qubits, and extend this concept to stabilizers acting on a two-dimensional array of physical qubits, on which we implement the surface code. We next describe how logical qubits are formed in the surface code array and give numerical estimates of their fault-tolerance. We outline how logical qubits are physically moved on the array, how qubit braid transformations are constructed, and how a braid between two logical qubits is equivalent to a controlled-NOT. We then describe the single-qubit Hadamard, S and T operators, completing the set of required gates for a universal quantum computer. We conclude by briefly discussing physical implementations of the surface code. We include a number of appendices in which we provide supplementary information to the main text.

Chow J.M., Gambetta J.M., Córcoles A.D., Merkel S.T., Smolin J.A., Rigetti C., Poletto S., Keefe G.A., Rothwell M.B., Rozen J.R., Ketchen M.B., Steffen M.

We use quantum process tomography to characterize a full universal set of all-microwave gates on two superconducting single-frequency single-junction transmon qubits. All extracted gate fidelities, including those for Clifford group generators, single-qubit pi/4 and pi/8 rotations, and a two-qubit controlled-NOT, exceed 95% (98%), without (with) accounting for state preparation and measurement errors. Furthermore, we introduce a process map representation in the Pauli basis which is visually efficient and informative. This high-fidelity gate set serves as another critical building block towards scalable architectures of superconducting qubits for error correction schemes.

Geerlings K., Shankar S., Edwards E., Frunzio L., Schoelkopf R.J., Devoret M.H.

Applications in quantum information processing and photon detectors are stimulating a race to produce the highest possible quality factor on-chip superconducting microwave resonators. We have tested the surface-dominated loss hypothesis by systematically studying the role of geometrical parameters on the internal quality factors of compact resonators patterned in Nb on sapphire. Their single-photon internal quality factors were found to increase with the distance between capacitor fingers, the width of the capacitor fingers, and the resonator impedance. Quality factors were improved from 210 000 to 500 000 at T = 200 mK. All of these results are consistent with our starting hypothesis.

Megrant A., Neill C., Barends R., Chiaro B., Chen Y., Feigl L., Kelly J., Lucero E., Mariantoni M., O’Malley P.J., Sank D., Vainsencher A., Wenner J., White T.C., Yin Y., et. al.

We describe the fabrication and measurement of microwave coplanar waveguide resonators with internal quality factors above 10 million at high microwave powers and over 1 million at low powers, with the best low power results approaching 2 million, corresponding to ~1 photon in the resonator. These quality factors are achieved by controllably producing very smooth and clean interfaces between the resonators' aluminum metallization and the underlying single crystal sapphire substrate. Additionally, we describe a method for analyzing the resonator microwave response, with which we can directly determine the internal quality factor and frequency of a resonator embedded in an imperfect measurement circuit.

Pop I.M., Fournier T., Crozes T., Lecocq F., Matei I., Pannetier B., Buisson O., Guichard W.

The authors have performed a detailed study of the time stability and reproducibility of submicron Al/AlOx/Al tunnel junctions, fabricated using standard double angle shadow evaporations. The authors have found that by aggressively cleaning the substrate before the evaporations; thus preventing any contamination of the junction, they obtained perfectly stable oxide barriers. The authors also present measurements on large ensembles of junctions which prove the reproducibility of the fabrication process. The measured tunnel resistance variance in large ensembles of identically fabricated junctions is in the range of only a few percent. Finally, the authors have studied the effect of different thermal treatments on the junction barrier. This is especially important for multiple step fabrication processes which imply annealing the junction.

Paik H., Schuster D.I., Bishop L.S., Kirchmair G., Catelani G., Sears A.P., Johnson B.R., Reagor M.J., Frunzio L., Glazman L.I., Girvin S.M., Devoret M.H., Schoelkopf R.J.

Superconducting quantum circuits based on Josephson junctions have made rapid progress in demonstrating quantum behavior and scalability. However, the future prospects ultimately depend upon the intrinsic coherence of Josephson junctions, and whether superconducting qubits can be adequately isolated from their environment. We introduce a new architecture for superconducting quantum circuits employing a three-dimensional resonator that suppresses qubit decoherence while maintaining sufficient coupling to the control signal. With the new architecture, we demonstrate that Josephson junction qubits are highly coherent, with T2 ∼ 10 to 20  μs without the use of spin echo, and highly stable, showing no evidence for 1/f critical current noise. These results suggest that the overall quality of Josephson junctions in these qubits will allow error rates of a few 10(-4), approaching the error correction threshold.

Colao Zanuz D., Ficheux Q., Michaud L., Orekhov A., Hanke K., Flasby A., Bahrami Panah M., Norris G.J., Kerschbaum M., Remm A., Swiadek F., Hellings C., Lazăr S., Scarato C., Lacroix N., et. al.

Chang R.D., Shumiya N., McLellan R.A., Zhang Y., Bland M.P., Bahrami F., Mun J., Zhou C., Kisslinger K., Cheng G., Smitham B.M., Pakpour-Tabrizi A.C., Yao N., Zhu Y., Liu M., et. al.

Kreidel M., Chu X., Balgley J., Antony A., Verma N., Ingham J., Ranzani L., Queiroz R., Westervelt R.M., Hone J., Fong K.C.

The discovery of van der Waals superconductors in recent years has generated a lot of excitement for their potentially novel pairing mechanisms. However, their typical atomic-scale thickness and micrometer-scale lateral dimensions impose severe challenges to investigations of pairing symmetry by conventional methods. We demonstrate an improved technique that employs high-quality-factor superconducting resonators to measure the kinetic inductance—up to one part per million—and loss of a van der Waals superconductor. We analyze the equivalent circuit model to extract the kinetic inductance, superfluid stiffness, penetration depth, and ratio of imaginary and real parts of the complex conductivity. We validate the technique by measuring aluminum and finding excellent agreement in both the zero-temperature superconducting gap as well as the complex conductivity data when compared with BCS theory. We then demonstrate the utility of the technique by measuring the kinetic inductance of multilayered niobium diselenide and discuss the limits to the accuracy of our technique when the transition temperature of the sample, NbSe2 at 7.06 K, approaches our Nb probe resonator at 8.59 K. Our method will be useful for practitioners in the growing fields of superconducting physics, materials science, and quantum sensing, as a means of characterizing superconducting circuit components and studying pairing mechanisms of the novel superconducting states which arise in layered two-dimensional materials and heterostructures. Published by the American Physical Society 2024

Kopas C.J., Goronzy D.P., Pham T., Torres Castanedo C.G., Cheng M., Cochrane R., Nast P., Lachman E., Zhelev N.Z., Vallières A., Murthy A.A., Oh J., Zhou L., Kramer M.J., Cansizoglu H., et. al.

Abstract The performance of superconducting qubits is often limited by dissipation and two-level systems (TLS) losses. The dominant sources of these losses are believed to originate from amorphous materials and defects at interfaces and surfaces, likely as a result of fabrication processes or ambient exposure. Here, we explore a novel wet chemical surface treatment at the Josephson junction-substrate and the substrate-air interfaces by replacing a buffered oxide etch (BOE) cleaning process with one that uses hydrofluoric acid followed by aqueous ammonium fluoride. We show that the ammonium fluoride etch process results in a statistically significant improvement in median T 1 by ∼ 22 % (p = 0.002), and a reduction in the number of strongly-coupled TLS in the tunable frequency range. Microwave resonator measurements on samples treated with the ammonium fluoride etch after niobium deposition and etching also show ∼ 33 % lower TLS-induced loss tangent compared to the BOE treated samples. As the chemical treatment primarily modifies the Josephson junction-substrate interface and substrate-air interface, we perform targeted chemical and structural characterizations to examine materials differences at these interfaces and identify multiple microscopic changes that could contribute to decreased TLS losses.

Tao H., Zhang C., Du L., Yang X., Guo L., Chen Y., Zhang H., Jia Z., Kong W., Duan P., Guo G.

Airbridges are extensively employed in superconducting quantum circuits to suppress parasitic slotline modes in coplanar waveguide and minimize crosstalk between control lines. Here, we introduce a fabrication technique for airbridges, leveraging niobium as the bridge layer and aluminum as the sacrificial layer to preclude the introduction of lossy dielectrics or residues upon release. Additionally, we utilize a triangular evaporation method to significantly bolster the structural integrity of the airbridges. Our experimental evaluation, focused on resonators equipped with these airbridges, reveals that the resultant additional loss per bridge is minimal, quantified at (5.0±2.8)×10−9 in the single-photon regime and (6.3±0.9)×10−9 at high drive powers. This advancement underscores the potential of niobium airbridges in facilitating the development of large-scale and high-performance superconducting quantum circuits.

Cleland A.Y., Wollack E.A., Safavi-Naeini A.H.

AbstractNanomechanical oscillators offer numerous advantages for quantum technologies. Their integration with superconducting qubits shows promise for hardware-efficient quantum error-correction protocols involving superpositions of mechanical coherent states. Limitations of this approach include mechanical decoherence processes, particularly two-level system (TLS) defects, which have been widely studied using classical fields and detectors. In this manuscript, we use a superconducting qubit as a quantum sensor to perform phonon number-resolved measurements on a piezoelectrically coupled phononic crystal cavity. This enables a high-resolution study of mechanical dissipation and dephasing in coherent states of variable size ($$\bar{n}\simeq 1-10$$ n ¯ ≃ 1 − 10 phonons). We observe nonexponential relaxation and state size-dependent reduction of the dephasing rate, which we attribute to TLS. Using a numerical model, we reproduce the dissipation signatures (and to a lesser extent, the dephasing signatures) via emission into a small ensemble (N = 5) of rapidly dephasing TLS. Our findings comprise a detailed examination of TLS-induced phonon decoherence in the quantum regime.

Ganjam S., Wang Y., Lu Y., Banerjee A., Lei C.U., Krayzman L., Kisslinger K., Zhou C., Li R., Jia Y., Liu M., Frunzio L., Schoelkopf R.J.

AbstractThe performance of superconducting quantum circuits for quantum computing has advanced tremendously in recent decades; however, a comprehensive understanding of relaxation mechanisms does not yet exist. In this work, we utilize a multimode approach to characterizing energy losses in superconducting quantum circuits, with the goals of predicting device performance and improving coherence through materials, process, and circuit design optimization. Using this approach, we measure significant reductions in surface and bulk dielectric losses by employing a tantalum-based materials platform and annealed sapphire substrates. With this knowledge we predict the relaxation times of aluminum- and tantalum-based transmon qubits, and find that they are consistent with experimental results. We additionally optimize device geometry to maximize coherence within a coaxial tunnel architecture, and realize on-chip quantum memories with single-photon Ramsey times of 2.0 − 2.7 ms, limited by their energy relaxation times of 1.0 − 1.4 ms. These results demonstrate an advancement towards a more modular and compact coaxial circuit architecture for bosonic qubits with reproducibly high coherence.

Smirnov N.S., Krivko E.A., Solovyova A.A., Ivanov A.I., Rodionov I.A.

AbstractQuantum processors using superconducting qubits suffer from dielectric loss leading to noise and dissipation. Qubits are usually designed as large capacitor pads connected to a non-linear Josephson junction (or SQUID) by a superconducting thin metal wiring. Here, we report on finite-element simulation and experimental results confirming that more than 50% of surface loss in transmon qubits can originate from Josephson junctions wiring and can limit qubit relaxation time. We experimentally extracted dielectric loss tangents of qubit elements and showed that dominant surface loss of wiring can occur for real qubits designs. Finally, we experimentally demonstrate up to 20% improvement in qubit quality factor by wiring design optimization.

Crowley K.D., McLellan R.A., Dutta A., Shumiya N., Place A.P., Le X.H., Gang Y., Madhavan T., Bland M.P., Chang R., Khedkar N., Feng Y.C., Umbarkar E.A., Gui X., Rodgers L.V., et. al.

Tantalum-based superconducting qubits have shown great promise in extending qubit lifetimes. New systematic measurements identify the key sources of loss and noise in this material system.

Zikiy E.V., Ivanov A.I., Smirnov N.S., Moskalev D.O., Polozov V.I., Matanin A.R., Malevannaya E.I., Echeistov V.V., Konstantinova T.G., Rodionov I.A.

AbstractDielectric losses are one of the key factors limiting the coherence of superconducting qubits. The impact of materials and fabrication steps on dielectric losses can be evaluated using coplanar waveguide (CPW) microwave resonators. Here, we report on superconducting CPW microwave resonators with internal quality factors systematically exceeding 5 × 106 at high powers and 2 × 106 (with the best value of 4.4 × 106) at low power. Such performance is demonstrated for 100-nm-thick aluminum resonators with 7–10.5 um center trace on high-resistivity silicon substrates commonly used in Josephson-junction based quantum circuit. We investigate internal quality factors of the resonators with both dry and wet aluminum etching, as well as deep and isotropic reactive ion etching of silicon substrate. Josephson junction compatible CPW resonators fabrication process with both airbridges and silicon substrate etching is proposed. Finally, we demonstrate the effect of airbridges’ positions and extra process steps on the overall dielectric losses. The best quality factors are obtained for the wet etched aluminum resonators and isotropically removed substrate with the proposed ultrasonic metal edge microcutting.

Lei C.U., Ganjam S., Krayzman L., Banerjee A., Kisslinger K., Hwang S., Frunzio L., Schoelkopf R.J.

Understanding the loss mechanisms in materials is crucial to improving coherence in superconducting quantum circuits. The authors present a technique based on multimode superconducting resonators that distinguishes and quantifies all loss channels in relevant materials. Applying this technique reveals that both chemical etching and diamond turning reduce surface losses in high-purity aluminum, while coating diamond-turned surfaces with thin-film aluminum significantly improves joint quality. This method can be used to design on-chip superconducting devices to characterize microwave losses, as well as to quantify the effects of fabrication processes.

Van Damme J., Ivanov T., Favia P., Conard T., Verjauw J., Acharya R., Perez Lozano D., Raes B., Van de Vondel J., Vadiraj A.M., Mongillo M., Wan D., De Boeck J., Potočnik A., De Greve K.

The fabrication of superconducting circuits requires multiple deposition, etching, and cleaning steps, each possibly introducing material property changes and microscopic defects. In this work, we specifically investigate the process of argon milling, a potentially coherence-limiting step, using niobium and aluminum superconducting resonators as a proxy for the surface-limited behavior of qubits. We find that niobium microwave resonators exhibit an order of magnitude decrease in quality factors after surface argon milling, while aluminum resonators are resilient to the same process. Extensive analysis of the niobium surface shows no change in the suboxide composition due to argon milling, while two-tone spectroscopy measurements reveal an increase in two-level system electrical dipole moments, indicating a structurally altered niobium oxide. However, a short dry etch can fully recover the argon-milling-induced losses on niobium, offering a potential route towards state-of-the-art overlap Josephson junction qubits with niobium circuitry.

Cárdenas-López F.A., Retamal J.C., Chen X.

AbstractShortcuts to adiabaticity provide a flexible method to accelerate and improve a quantum control task beyond adiabatic criteria. However, their application to the fast generation of multi-partite quantum gates is still not optimized. Here we propose the reverse-engineering approach to design the longitudinal coupling between a set of qubits coupled to several field modes, for achieving a fast generation of multi-partite quantum gates in photonic or qubit-based architecture. We show that the enhancing generation time is at the nanosecond scale that does not scale with the number of system components. In addition, our protocol does not suffer noticeable detrimental effects due to the dissipative dynamics. Finally, the possible implementation is discussed with the state-of-the-art circuit quantum electrodynamics architecture.

Gassenq A., Nguyen H., Cleyet-merle E., Cueff S., Pereira A.

Micro-structuration by etching is commonly used in integrated optics, adding complex and costly processing steps that can also potentially damage the device performance, owing to degradation of the etched sidewalls. For diffraction grating fabrication, different strategies have been developed to avoid etching, such as layer deposition on a structured surface or grating deposition on top of active layers. However, etching remains one of the best processes for making high aspect ratio diffraction gratings. In this work, we have developed fully structured diffraction gratings (i.e., like fully etched gratings) using lift-off based processing performed in pulsed laser deposited layers, since the combination of both techniques is of great interest for making micro-structures without etching. We have first studied the influence of the lithography doses in the lift-off process, showing that (1) micrometric spatial resolution can be achieved and (2) the sidewall angle can be controlled from 50° to 150° in 0.5 µm thick layers. Using such optimizations, we have then fabricated Er-doped Y2O3 uniaxial diffraction gratings with different periods ranging from 3 to 8 µm. The fabricated devices exhibit emission and reflectivity properties as a function of the collection angle in good agreement with the modeling, with a maximum luminescence enhancement of ×15 compared with an unstructured layer at a wavelength of 1.54 µm. This work thus highlights lift-off based processing combined with pulsed laser deposition as a promising technique for etch-free practical applications, such as luminescence enhancement in Er-doped layers.

Read A.P., Chapman B.J., Lei C.U., Curtis J.C., Ganjam S., Krayzman L., Frunzio L., Schoelkopf R.J.

To better understand decoherence in superconducting qubits, the authors develop a technique to measure the loss tangent of dielectric substrates and predict the impact of dielectric loss on qubit lifetimes. This is done with no need to fabricate planar devices; the technique is independent of material platform. Measurements of sapphire in a demonstration of the approach suggest that coherence of superconducting qubits on a common form of sapphire is limited significantly by bulk dielectric loss. The same technique also shows that another form of sapphire would substantially mitigate this bulk dielectric loss and prolong qubit coherence.