Superconducting Quantum Devices Research Articles

Dynamical parity selection in superconducting weak links

Excess quasiparticles play a crucial role in superconducting quantum devices ranging from qubits to quantum sensors. In this work we analyze their dynamics for phase-biased finite-length weak links with several Andreev subgap states, where the coupling to a microwave resonator allows for parity state (even/odd) readout. Our theory shows that almost perfect dynamical polarization in a given parity sector is achievable by applying a microwave pulse matching a transition in the opposite parity sector. Our results qualitatively explain key features of recent experiments on hybrid semiconducting nanowire Josephson junctions and provide theoretical guidelines for efficiently controlling the parity state of Andreev qubits.

Variable ansatz applied to spectral operator decomposition in a physical superconducting quantum device

Variable ansatz applied to spectral operator decomposition in a physical superconducting quantum device

Tunable directional photon scattering from a pair of superconducting qubits

The ability to control the direction of scattered light is crucial to provide flexibility and scalability for a wide range of on-chip applications, such as integrated photonics, quantum information processing, and nonlinear optics. Tunable directionality can be achieved by applying external magnetic fields that modify optical selection rules, by using nonlinear effects, or interactions with vibrations. However, these approaches are less suitable to control microwave photon propagation inside integrated superconducting quantum devices. Here, we demonstrate on-demand tunable directional scattering based on two periodically modulated transmon qubits coupled to a transmission line at a fixed distance. By changing the relative phase between the modulation tones, we realize unidirectional forward or backward photon scattering. Such an in-situ switchable mirror represents a versatile tool for intra- and inter-chip microwave photonic processors. In the future, a lattice of qubits can be used to realize topological circuits that exhibit strong nonreciprocity or chirality.

Near-Field Localization of the Boson Peak on Tantalum Films for Superconducting Quantum Devices.

Superconducting circuits are among the most advanced quantum computing technologies; however, their performance is limited by losses found in surface oxides and disordered materials. In this work, we demonstrate the identification and spatial localization of a near-field signature of loss centers on tantalum films using terahertz scattering-type scanning near-field optical microscopy. By utilizing terahertz nanospectroscopy, we observe a localized excess vibrational mode around 0.5 THz and identify this resonance as the boson peak, a signature of amorphous materials. Grazing-incidence wide-angle X-ray scattering reveals that oxides on freshly solvent-cleaned samples are amorphous, whereas crystalline phases emerge after aging in air. Through nanoscale localization of defect centers, our findings provide valuable insights for the optimization of fabrication procedures for new low-loss superconducting circuits.

Unconventional Giant "Magnetoresistance" in Bosonic Semiconducting Diamond Nanorings.

The emergence of superconductivity in doped insulators such as cuprates and pnictides coincides with their doping-driven insulator-metal transitions. Above the critical doping threshold, a metallic state sets in at high temperatures, while superconductivity sets in at low temperatures. An unanswered question is whether the formation of Cooper pairsin a well-established metal will inevitably transform the host material into a superconductor, as manifested by a resistance drop. Here, this question is addressed by investigating the electrical transport in nanoscale rings (full loops) and half loops manufactured from heavily boron-doped diamond. It is shown that in contrast to the diamond half-loops (DHLs) exhibiting a metal-superconductor transition, the diamond nanorings (DNRs) demonstrate a sharp resistance increase up to 430% and a giant negative "magnetoresistance" below the superconducting transition temperature of the starting material. The finding of the unconventional giant negative "magnetoresistance", as distinct from existing categories of magnetoresistance, that is, the conventional giant magnetoresistancein magnetic multilayers, the colossal magnetoresistancein perovskites, and the geometric magnetoresistance in semiconductor-metal hybrids, reveals the transformation of the DNRs from metals to bosonic semiconductors upon the formation of Cooper pairs. DNRs like these could be used to manipulate Cooper pairs in superconducting quantum devices.

Chemical mechanical planarization for Ta-based superconducting quantum devices

We report on the development of a chemical mechanical planarization (CMP) process for thick damascene Ta structures with pattern feature sizes down to 100 nm. This CMP process is the core of the fabrication sequence for scalable superconducting integrated circuits at a 300 mm wafer scale. This work has established the elements of various CMP-related design rules that can be followed by a designer for the layout of circuits that include Ta-based coplanar waveguide resonators, capacitors, and interconnects for tantalum-based qubits and single flux quantum circuits. The fabrication of these structures utilizes a 193 nm optical lithography along with 300 mm process tools for dielectric deposition, reactive ion etch, wet-clean, CMP, and in-line metrology—all tools typical for a 300 mm wafer CMOS foundry. Theprocess development was guided by measurements of the physical and electrical characteristics of the planarized structures. Physical characterization such as atomic force microscopy across the 300 mm wafer surface showed that local topography was less than 5 nm. Electrical characterization confirmed low leakage at room temperature, and less than 12% within wafer sheet resistance variation for damascene Ta line widths ranging from 100 nm to 3 μm. Run-to-run reproducibility was also evaluated. Effects of process integration choices including the deposited thickness of Ta are discussed.

Atmospheric Neutron-Induced Fault Generation and Propagation in Quantum Bits and Quantum Circuits

Quantum Computing, by exploiting the quantum properties of quantum bits, significantly improves the performance and efficiency of computation. Unfortunately, quantum devices are very susceptible to external perturbation, including natural radiation. In this paper we measure, through GEANT4 simulations, the charge deposited by impinging neutrons in superconducting quantum devices. As we show, most atmospheric neutrons deposit sufficient energy to break Cooper pairs and, thus, can potentially modify the qubit state. Then, with a dedicated circuit-level fault-injector, we track the transient fault propagation in real quantum circuits and measure its impact on the output probabilities distribution. We discuss the fault model for quantum bits and show how to measure faults impact on the circuit output.

Biomolecular and quantum algorithms for the dominating set problem in arbitrary networks

A dominating set of a graph G = (V, E) is a subset U of its vertices V, such that any vertex of G is either in U, or has a neighbor in U. The dominating-set problem is to find a minimum dominating set in G. Dominating sets are of critical importance for various types of networks/graphs, and find therefore potential applications in many fields. Particularly, in the area of communication, dominating sets are prominently used in the efficient organization of large-scale wireless ad hoc and sensor networks. However, the dominating set problem is also a hard optimization problem and thus currently is not efficiently solvable on classical computers. Here, we propose a biomolecular and a quantum algorithm for this problem, where the quantum algorithm provides a quadratic speedup over any classical algorithm. We show that the dominating set problem can be solved in O(2^{n/2}) queries by our proposed quantum algorithm, where n is the number of vertices in G. We also demonstrate that our quantum algorithm is the best known procedure to date for this problem. We confirm the correctness of our algorithm by executing it on IBM Quantum’s qasm simulator and the Brooklyn superconducting quantum device. And lastly, we show that molecular solutions obtained from solving the dominating set problem are represented in terms of a unit vector in a finite-dimensional Hilbert space.

Entanglement of Signal Paths via Noisy Superconducting Quantum Devices.

Quantum routers will provide for important functionality in emerging quantum networks, and the deployment of quantum routing in real networks will initially be realized on low-complexity (few-qubit) noisy quantum devices. A true working quantum router will represent a new application for quantum entanglement-the coherent superposition of multiple communication paths traversed by the same quantum signal. Most end-user benefits of this application are yet to be discovered, but a few important use-cases are now known. In this work, we investigate the deployment of quantum routing on low-complexity superconducting quantum devices. In such devices, we verify the quantum nature of the routing process as well as the preservation of the routed quantum signal. We also implement quantum random access memory, a key application of quantum routing, on these same devices. Our experiments then embed a five-qubit quantum error-correcting code within the router, outlining the pathway for error-corrected quantum routing. We detail the importance of the qubit-coupling map for a superconducting quantum device that hopes to act as a quantum router, and experimentally verify that optimizing the number of controlled-X gates decreases hardware errors that impact routing performance. Our results indicate that near-term realization of quantum routing using noisy superconducting quantum devices within real-world quantum networks is possible.

Building compact superconducting microwave resonators with Hilbert space-filling curves

Superconducting quantum computing is currently one of the most promising platforms for universal quantum information processing. The readout resonator is an essential integral part of a superconducting qubit, while its size is much larger compared to the Josephson junction. We propose and realize a new readout resonator using space-filling curves, specifically Hilbert space-filling curves. We introduce the frequency analysis method and demonstrate a qubit sample, in which the Hilbert-space-filling-curves resonator (HSFCR) is used to read out the qubit states. We also propose to fabricate the HSFCRs and Josephson junctions simultaneously in the same processes of E-beam lithography and E-beam evaporation. Our design reduces the resonator area sufficiently and, thus, will help to improve the integration of superconducting qubits, as well as to design other superconducting quantum devices.

Benchmarking quantum error-correcting codes on quasi-linear and central-spin processors

We evaluate the performance of small error-correcting codes, which we tailor to hardware platforms of very different connectivity and coherence: on a superconducting processor based on transmon qubits and a spintronic quantum register consisting of a nitrogen-vacancy center in diamond. Taking the hardware-specific errors and connectivity into account, we investigate the dependence of the resulting logical error rate on the platform features such as the native gates, native connectivity, gate times, and coherence times. Using a standard error model parameterized for the given hardware, we simulate the performance and benchmark these predictions with experimental results when running the code on the superconducting quantum device. The results indicate that for small codes, the quasi-linear layout of the superconducting device is advantageous. Yet, for codes involving multi-qubit controlled operations, the central-spin connectivity of the color centers enables lower error rates.

Materials for a broadband microwave superconducting single photon detector

In this work, we develop and study superconducting materials for a broadband microwave single-photon detector for wide-range applications in superconducting quantum devices. Ideal materials of this type should have a superconducting gap of the order of 10 GHz (0.2 K), and possess a normal sheet resistance of the order of 50 . We find that Ti/Pt bilayers are good candidates: it enables to vary the superconducting transition temperature in a wide range, from 0.1 to 0.4 K, and the sheet resistance in the range from 10 to 50 . We present a proof-of-principle demonstration of a low-level microwave power detector based on a nanobridge made of a designed Ti/Pt bilayer. The response to the absorbed microwave power is consistent with the picture of the kinetic inductance detection in superconductors. The extracted response time corresponds to the recombination of quasiparticles with the emission of a photon to the microwave line.

Aluminum air bridges for superconducting quantum devices realized using a single-step electron-beam lithography process

In superconducting quantum devices, air bridges enable increased circuit complexity and density, and mitigate the risk of microwave loss arising from mode mixing. We implement aluminum air bridges using a simple process based on single-step electron-beam gradient exposure. The resulting bridges have sizes ranging from 20 µm to 100 µm, with a yield exceeding 99% for lengths up to 36 µm. When used to connect ground planes in coplanar waveguide resonators, the induced loss contributed to the system is negligible, corresponding to a loss per bridge less than 1.0×10−8. The bridge process is compatible with Josephson junctions and allows for the simultaneous creation of low loss bandages between superconducting layers.

Ternary metal oxide substrates for superconducting circuits

Substrate material imperfections and surface losses are one of the major factors limiting superconducting quantum circuitry from reaching the scale and complexity required to build a practical quantum computer. One potential path towards higher coherence of superconducting quantum devices is to explore new substrate materials with a reduced density of imperfections due to inherently different surface chemistries. Here, we examine two ternary metal oxide materials, spinel (MgAl2O4) and lanthanum aluminate (LaAlO3), with a focus on surface and interface characterization and preparation. Devices fabricated on LaAlO3 have quality factors three times higher than those of earlier devices, which we attribute to a reduction in the interfacial disorder. MgAl2O4 is a new material in superconducting quantum devices, and even in the presence of significant surface disorder, it consistently outperforms LaAlO3. Our results highlight the importance of materials exploration, substrate preparation, and characterization for identifying materials suitable for high-performance superconducting quantum circuitry.

Non-Markovian Quantum Process Tomography

Characterisation protocols have so far played a central role in the development of noisy intermediate-scale quantum (NISQ) computers capable of impressive quantum feats. This trajectory is expected to continue in building the next generation of devices: ones that can surpass classical computers for particular tasks -- but progress in characterisation must keep up with the complexities of intricate device noise. A missing piece in the zoo of characterisation procedures is tomography which can completely describe non-Markovian dynamics over a given time frame. Here, we formally introduce a generalisation of quantum process tomography, which we call process tensor tomography. We detail the experimental requirements, construct the necessary post-processing algorithms for maximum-likelihood estimation, outline the best-practice aspects for accurate results, and make the procedure efficient for low-memory processes. The characterisation is a pathway to diagnostics and informed control of correlated noise. As an example application of the hardware-agnostic technique, we show how its predictive control can be used to substantially improve multi-time circuit fidelities on superconducting quantum devices. Our methods could form the core for carefully developed software that may help hardware consistently pass the fault-tolerant noise threshold.

Characterization of ultrathin superconducting FeSe nanowires on SrTiO3 substrates

Superconducting quantum devices, due to their ultra-low power consumption, high sensitivity and high speed, have attracted great attention in recent years and put forward higher requirements for fabrication technology. Here, we report on the first superconducting ultrathin FeSe nanowires on SrTiO3 substrates successfully fabricated by electron beam lithography and Ar plasma ion beam etching. As the superconductivity of the molecular beam epitaxial ultrathin FeSe film is highly susceptible by moisture and oxygen, FeTe layers were deposited for protection. We synthesized a superconducting ∼300 nm wide, ∼1.1 nm thick ultrathin FeSe nanowire with (the critical temperature of zero resistance), as revealed by electrical transport measurements. The proper synthesis conditions of the high-quality ultrathin superconducting FeSe nanowires on SrTiO3 substrates are evaluated by analyzing the morphology and physical properties of the ∼300 nm width ultrathin FeSe nanowire. Our work may pave the way for future applications of air-sensitive iron-based superconducting films.

The field-free Josephson diode in a van der Waals heterostructure.

The superconducting analogue to the semiconducting diode, the Josephson diode, has long been sought with multiple avenues to realization being proposed by theorists1-3. Showing magnetic-field-free, single-directional superconductivity with Josephson coupling, it would serve as the building block for next-generation superconducting circuit technology. Here we realized the Josephson diode by fabricating an inversion symmetrybreaking van der Waals heterostructure of NbSe2/Nb3Br8/NbSe2. We demonstratethat even without a magnetic field, the junction can be superconducting with a positive current while being resistive with a negative current. The ΔIc behaviour (the difference between positive and negative critical currents) with magnetic field is symmetric and Josephson coupling is proved through the Fraunhofer pattern. Also, stable half-wave rectification of a square-wave excitation was achieved with a very low switching current density, high rectification ratio and high robustness. This non-reciprocal behaviour strongly violates the known Josephson relations and opens the door to discover new mechanisms and physical phenomena through integration of quantum materials with Josephson junctions, and provides new avenues for superconducting quantum devices.

Quantum interference in asymmetric superconducting nanowire loops

Macroscopic phase coherence in superconductors enables quantum interference and phase manipulation at realistic device length scales. Numerous superconducting electronic devices are based on the modulation of the supercurrent in superconducting loops. While the overall behavior of symmetric superconducting loops has been studied, the effects of asymmetries in such devices remain under-explored and poorly understood. Here we report on an experimental and theoretical study of the flux modulation of the persistent current in a doubly connected asymmetric aluminum nanowire loop. A model considering the length and electronic cross-section asymmetries in the loop provides a quantitative account of the observations. Comparison with experiments give essential parameters such as persistent and critical currents as well as the amount of asymmetry which can provide feedback into the design of superconducting quantum devices.

Noisy quantum amplitude estimation without noise estimation

Many quantum algorithms contain an important subroutine---the quantum amplitude estimation. As the name implies, this is essentially the parameter estimation problem and thus can be handled via the established statistical estimation theory. However, this problem has an intrinsic difficulty that the system, i.e., the real quantum computing device, inevitably introduces unknown noise; the probability distribution model then has to incorporate many nuisance noise parameters, resulting that the construction of an optimal estimator becomes inefficient and difficult. For this problem we apply the theory of nuisance parameters (more specifically, the parameter orthogonalization method) to precisely compute the maximum likelihood estimator for only the target amplitude parameter by removing the other nuisance noise parameters. That is, we can estimate the amplitude parameter without estimating the noise parameters. We validate the parameter orthogonalization method in a numerical simulation and study the performance of the estimator in the experiment using a real superconducting quantum device.

Pulse-Engineered Controlled-V Gate and Its Applications on Superconducting Quantum Device

In this paper, we demonstrate that, by employing OpenPulse design kit for IBM superconducting quantum devices, the controlled-V gate (CV gate) can be implemented in about half the gate time to the controlled-X (CX or CNOT gate) and consequently 65.5\% reduced gate time compared to the CX-based implementation of CV. Then, based on the theory of Cartan decomposition, we characterize the set of all two-qubit gates implemented with only two or three CV gates; using pulse-engineered CV gates enables us to implement these gates with shorter gate time and possibly better gate fidelity than the CX-based one, as actually demonstrated in two examples. Moreover, we showcase the improvement of linearly-coupled three-qubit Toffoli gate, by implementing it with the pulse-engineered CV gate, both in gate time and the averaged output-state fidelity. These results imply the importance of our CV gate implementation technique, which, as an additional option for the basis gate set design, may shorten the overall computation time and consequently improve the precision of several quantum algorithms executed on a real device.