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Acta Physica Sinica

Three-dimensional transmon coherence measurement method based on network analyser

Xiao-Yu Wu, Hu Zhao, Zhi Li

Published Jan 1, 2020• 1 citation
Publisher

Abstract

Three-dimensional(3D) transmon is a kind of superconducting qubit with long decoherence time, which has important applications in superconducting quantum computation, quantum optics, cavity quantum electrodynamics, et al. Rabi oscillation is a vital method to characterize the decoherence time of quantum system, and it is also a basic experiment to demonstrate the energy level evolution of quantum system. In order to test the Rabi oscillation of 3D transmon, strict timing control is necessary, and the process of testing and debugging is complicated. In this paper, 3D transmon samples are fabricated and their basic parameters EC = 348.74 MHz and EJ = 11.556 GHz are tested and characterized. The coupling coefficient g2/Δ between qubit and the 3D cavity is 43 MHz, which is located in the dispersive regime. The qubit’s first transition frequency f01 = 9.2709 GHz, and the second transition frequency f12 = 9.0100 GHz. The 3D resonator is fabricated by the material 6061T6 aluminum, the loaded quality factor is 4.8 × 105, and the bare frequency of the resonator is 8.108 GHz. Through comparison, it is found that the Rabi oscillation time obtained by the proposed method is shorter than by the Jaynes-Cummings method. The main reasons are as follows. First, the measurement of network analyzer is a continuous measurement, and the test signal always affects the decoherence process of 3D transmon. Second, the quantum bit is in the ground state after decoherence, and the ground state measured by the network analyzer accounts for a relatively high proportion, which causes the curve measured by the network analyzer to be one-sided attenuation oscillation. Third, the dispersive readout method is related to the quality factor of the superconducting cavity. The storage time of microwave photons in the superconducting cavity is longer than the decoherence time of 3D transmon, so the quantum information is partially decohered before leaving the superconducting cavity, which will shorten the Rabi oscillation time. An innovative Rabi oscillation test method based on network analyzer is presented. The test system based on this method is simple to build and can be used as a new way to quickly verify whether 3D transmon has quantum characteristics. This method can also be extended to other quantum systems for preliminarily verifying the time domain characteristics.

DOI
10.7498/aps.69.20200252

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Three-dimensional(3D) transmon is a kind of superconducting qubit with long decoherence time, which has important applications in superconducting quantum computation, quantum optics, cavity quantum electrodynamics, et al. Rabi oscillation is a vital method to characterize the decoherence time of quantum system, and it is also a basic experiment to demonstrate the energy level evolution of quantum system. In order to test the Rabi oscillation of 3D transmon, strict timing control is necessary, and the process of testing and debugging is complicated. In this paper, 3D transmon samples are fabricated and their basic parameters EC = 348.74 MHz and EJ = 11.556 GHz are tested and characterized. The coupling coefficient g2/Δ between qubit and the 3D cavity is 43 MHz, which is located in the dispersive regime. The qubit’s first transition frequency f01 = 9.2709 GHz, and the second transition frequency f12 = 9.0100 GHz. The 3D resonator is fabricated by the material 6061T6 aluminum, the loaded quality factor is 4.8 × 105, and the bare frequency of the resonator is 8.108 GHz. Through comparison, it is found that the Rabi oscillation time obtained by the proposed method is shorter than by the Jaynes-Cummings method. The main reasons are as follows. First, the measurement of network analyzer is a continuous measurement, and the test signal always affects the decoherence process of 3D transmon. Second, the quantum bit is in the ground state after decoherence, and the ground state measured by the network analyzer accounts for a relatively high proportion, which causes the curve measured by the network analyzer to be one-sided attenuation oscillation. Third, the dispersive readout method is related to the quality factor of the superconducting cavity. The storage time of microwave photons in the superconducting cavity is longer than the decoherence time of 3D transmon, so the quantum information is partially decohered before leaving the superconducting cavity, which will shorten the Rabi oscillation time. An innovative Rabi oscillation test method based on network analyzer is presented. The test system based on this method is simple to build and can be used as a new way to quickly verify whether 3D transmon has quantum characteristics. This method can also be extended to other quantum systems for preliminarily verifying the time domain characteristics.
The decoherence time of superconducting qubit is one of the main parameters that determine whether superconducting quantum computation can be realized. This paper mainly focuses on three-dimensional (3D) transmon. The sample is fabricated on SiO substrate, and measured in 10 mK temperature. By measuring Rabi oscillation, energy relaxation, Ramsey oscillation and spin echo, the decoherence time constants of 3D transmon are characterized. The results show that the decoherence time is around several hundred nanoseconds. Based on the relationship of different decoherence time constant, we have the concludes that the possible reason to diminish the decoherence time is the defect in SiO substrate, but not the low frequency noises.
Abstract Superconducting quantum circuits are a leading platform for quantum-information processing, but their development requires expertise spanning device design, microfabrication, cryogenic operation, microwave control, and quantum characterization. In this work, we report the realization of a flux-tunable superconducting transmon platform and present an integrated workflow encompassing design, fabrication, measurement, and numerical modeling. The device consists of an Al/AlOx/AlAl/AlO_x/AlAl/AlOx​/Al transmon coupled to a three-dimensional microwave cavity operating in the circuit quantum electrodynamics (cQED) dispersive regime. Two-tone spectroscopy reveals a fundamental transition frequency of ω01=5.0122 GHz\omega _{01}=5.0122~\textrm{GHz}ω01​=5.0122 GHz and an anharmonicity of α/2π=−202.4 MHz\alpha /2\pi =-202.4~\textrm{MHz}α/2π=−202.4 MHz , corresponding to EJ/EC≈82.9E_J/E_C \approx 82.9EJ​/EC​≈82.9 . Flux spectroscopy yields a cavity-qubit coupling strength of g/2π=260 MHzg/2\pi = 260~\textrm{MHz}g/2π=260 MHz , while the cavity linewidth gives a loaded quality factor of QL=6127Q_L=6127QL​=6127 and a Purcell limit of T1Purcell≈82 μsT_1^{\textrm{Purcell}}\approx 82~\mu \textrm{s}T1Purcell​≈82 μs . Time-domain measurements yield coherence times of T1=17.4 μsT_1 = 17.4~\mu \textrm{s}T1​=17.4 μs and T2∗=5.8 μsT_2^* = 5.8~\mu \textrm{s}T2∗​=5.8 μs . To investigate coherence and dissipation in the device, the experimental observations are analyzed within a Lindblad master-equation framework describing cavity photon loss and thermal excitation, qubit energy relaxation, and pure dephasing processes. Using a set of experimentally extracted parameters, the model reproduces the measured spectroscopic and time-domain observables, providing a quantitative description of the open-system dynamics of the transmon-cavity platform. The analysis shows that the measured coherence is primarily limited by pure dephasing and energy-relaxation processes, whereas thermal excitation plays a minor role under the experimental conditions. Inspired by the broader framework of quantum dissipation in macroscopic quantum systems pioneered by Caldeira and collaborators, this approach enables a quantitative analysis of relaxation and dephasing processes in superconducting quantum circuits. Beyond the characterization of a single device, the present work demonstrates the establishment of the expertise, infrastructure, and experimental workflow required for the design, fabrication, measurement, and modeling of superconducting quantum circuits, contributing to the development of quantum technologies in Brazil and related applications.
Superconducting qubits are a promising technology for building a scalable quantum computer. An important architecture employed in the field is called Circuit Quantum Electrodynamics (circuit QED), where such qubits are combined with high quality microwave cavities to study the interaction between artificial atoms and single microwave photons. The ultra-strong coupling achieved in these systems allows for control and readout of the quantum state of qubits to perform quantum information processing. The work on circuit QED performed in this thesis consisted of realizing an experimental setup for qubit experiments in a new laboratory, investigating the coherence and decay of higher energy levels of superconducting transmon qubits and finally demonstrating a novel coaxial form of circuit QED. Designing and building a 3D circuit QED setup involved the following main accomplishments: producing high quality 3D cavities; designing and installing the cryogenic microwave setup as well as the room temperature amplification and data acquisition circuitry; successfully developing a recipe for the fabrication of Josephson junctions; controlling and measuring superconducting 3D transmon qubits at 10mK. Several qubits were fully characterised and have shown coherence times of several microseconds and relaxation times up to 25μs. Superconducting qubits in fact possess higher energy levels that can provide significant computational advantages in quantum information applications. In experiments performed at MIT, preparation and control of the five lowest states of a transmon qubit was demonstrated, followed by an investigation of the phase coherence and decay dynamics of these higher energy levels. The decay was found to proceed mainly sequentially with relaxation times in excess of 20μs for all transitions. A direct measurement of the charge dispersion of these levels was performed to explore their characteristics of dephasing. This experiment was also reproduced on a 3D transmon fabricated and measured in Oxford, where due to a higher effective qubit temperature a multi-level decay model including thermal excitations was developed to explain the observed relaxation dynamics. Finally, a coaxial transmon, which we name the coaxmon, is presented and measured with a coaxial LC readout resonator and input/output coupling ports placed inline along the third dimension. This novel coaxial circuit QED architecture holds great promise for developing a scalable planar grid of qubits to build a quantum computer.
Time-domain observations of coherent oscillations between quantum states in mesoscopic superconducting systems have so far been restricted to restoring the time-dependent probability distribution from the readout statistics. We propose a method for direct observation of Rabi oscillations in a phase qubit. The external source, typically in GHz range, induces transitions between the qubit levels. The resulting Rabi oscillations of supercurrent in the qubit loop induce the voltage oscillations across the coil of a high quality resonant tank circuit, inductively coupled to the phase qubit. It is the presence of these voltage oscillations in the detected signal which reveals the existence of Rabi oscillations in the qubit. A detailed calculation for zero and nonzero temperatures are made for the case of persistent current qubit. According to the estimates for decoherence and relaxation times, the effect can be detected using conventional rf circuitry, with Rabi frequency in the MHz range.
In a quantum computer, the information carriers, which are bits in ordinary computers, are implemented as devices that exhibit coherent superpositions of physical states and entanglement. Such components, known as quantum bits or qubits, can be realized with various different types of two-state quantum systems. Quantum computers will be built for computational speed, with hoped for applications especially in cryptography and in other tasks where classical computers remain inefficient. Circuit quantum electrodynamics (cQED) is a quantum-computer architecture which employs superconducting electronic components and microwave photon fields as building blocks. Compared to cavity quantum electrodynamics (CQED), where atoms are trapped in physical cavities, cQED is more attractive in that its qubits are tunable and conveniently integrable with the electronics already in use. This architecture has shown some of the most promising qubit designs, despite their coherence times reaching tens of microseconds, are still below the state of the art with spin qubits, which reach milliseconds. Coherence times are historically the most relevant parameters describing the fitness of a qubit, although these days not necessarily the limiting factor. This thesis presents a comprehensive set of theoretical and experimental methods for measuring the characteristic parameters of superconducting qubits. We especially study transmission-line-shunted plasma oscillation qubits, or transmons, and presents experimental results for a single sample. Transmon capacitively couples a superconducting quantum interference device (SQUID) with a coplanar waveguide (CPW) resonator, often with added frequency tunability utilizing an external magnet. The number of superconducting charge carriers tunnelled through a junction in the SQUID are used as qubit degrees of freedom. Readout of the qubit state is carried out by measuring transmission through the CPW. A cryogenic setup is employed with measurement and driving pulses delivered from microwave sources. Steady-state spectroscopy is employed to determine the resonance frequencies of the qubit and the resonator, qubit-resonator coupling constants, and the energy parameters of the qubit. Pulse-modulated measurements are employed to determine the coherence times of the qubit. The related analysis- and simulation programs and scripts are collected togithub.com/patomaki.
This report is an introduction to quantum computation and quantum information.\nWe present a two-part theory section followed by data analysis of a transmon qubit.\nThe ¯rst theory part, needed for the analysis, introduces fundamental properties of\nqubit states and the Bloch sphere description. The qubit manipulation, interaction\nwith a electromagnetic ¯eld, is then studied in the Rabi model (semi-classical) and the\nJaynes-Cummings model (fully quantized). The second theory part is a qualitative\npresentation of the density matrix representation, decoherence (e®ects of noise) and\nread-out, which are useful topics in more advanced analysis.\nWe also present a summary of qubit realizations, with focus on the superconduct-\ning qubits: charge, phase and transmon.\nIn the last part of the report, we analyze spectroscopic measurements on a trans-\nmon qubit in a cavity resonator, performed atMC2 Chalmers. The data was compared\nto the expected behavior from the Jaynes-Cummings Hamiltonian and then the full\nHamiltonian for the transmon. The comparison enabled us to extract the parame-\nters of the transmon. The ratio of the Josephson energy and the charge energy was\ndetermined to be EJ=EC = 34:5644 § 0:9456 GHz, which is in the transmon regime.\nFurthermore, the coupling strength between the cavity resonator and the qubit was\ndetermined to be 2¯eV 0\nrms = 0:1301 § 0:00095 GHz, with a 68% con¯dence interval.