Quantum entanglement is a cornerstone of quantum information science, enabling advancements in quantum computing, cryptography, and metrology. We report a comprehensive experimental study of quantum entanglement in a superconducting circuit comprising two transmon qubits coupled via a tunable bus resonator, operating at ultra-low temperatures (10 mK) in a dilution refrigerator. Using precise microwave control, we generate Bell states with a fidelity of 0.94 ± 0.02 and a concurrence of 0.92 ± 0.03, confirming robust entanglement. Quantum state tomography and coherence measurements yield energy relaxation (T1) times of 92 ± 5 µs and 88 ± 4 µs, and dephasing (T2) times of 85 ± 6 µs and 82 ± 5 µs for qubits Q1 and Q2, respectively, at 10 mK. We systematically investigate the impact of temperature (10 mK, 50 mK, 100 mK) and environmental noise on entanglement persistence. Real-time analysis of recent literature and social media discussions highlights ongoing challenges in scaling superconducting qubit systems due to 1/f noise and material limitations. Our findings demonstrate the viability of superconducting circuits for high-fidelity quantum operations and provide critical insights into optimizing coherence at millikelvin temperatures.
Quantum entanglement is a cornerstone of quantum information science, enabling advancements in quantum computing, cryptography, and metrology. We report a comprehensive experimental study of quantum entanglement in a superconducting circuit comprising two transmon qubits coupled via a tunable bus resonator, operating at ultra-low temperatures (10 mK) in a dilution refrigerator. Using precise microwave control, we generate Bell states with a fidelity of 0.94 ± 0.02 and a concurrence of 0.92 ± 0.03, confirming robust entanglement. Quantum state tomography and coherence measurements yield energy relaxation (T1) times of 92 ± 5 µs and 88 ± 4 µs, and dephasing (T2) times of 85 ± 6 µs and 82 ± 5 µs for qubits Q1 and Q2, respectively, at 10 mK. We systematically investigate the impact of temperature (10 mK, 50 mK, 100 mK) and environmental noise on entanglement persistence. Real-time analysis of recent literature and social media discussions highlights ongoing challenges in scaling superconducting qubit systems due to 1/f noise and material limitations. Our findings demonstrate the viability of superconducting circuits for high-fidelity quantum operations and provide critical insights into optimizing coherence at millikelvin temperatures.
Quantum information processing holds the promise of a paradigm shift in computational power for specific problem classes. While contemporary quantum processors operate in the Noisy Intermediate-Scale Quantum (NISQ) regime and remain highly susceptible to errors, ongoing experimental efforts on these near-term devices are crucial for advancing the underlying technology. Among the various hardware implementations, superconducting quantum circuits stand out as one of the most robust architectures for scalable quantum technologies. Nevertheless, the primary bottleneck in these systems is the fragility of quantum information: interactions with the environment inevitably perturb the quantum states, a phenomenon known as decoherence. This rapid loss of stored information imposes a fundamental limit on the time available to reliably execute computations on these devices. Within this context, this thesis focuses on the comprehensive experimental characterization of a planar chip featuring isolated, fixed-frequency superconducting transmon qubits. The measurements were conducted in a cryogenic environment, relying on a dilution refrigerator to maintain the device at millikelvin temperatures. To probe the quantum system under these conditions, custom measurement scripts were developed to operate state-of-the-art microwave control electronics (Keysight Quantum Control System), establishing full control over pulse generation, signal acquisition, and digital demodulation. The experimental workflow systematically mapped the physical properties of the device through a progressive series of characterization steps. Initial spectroscopic techniques probed the coupled resonator-qubit system, identifying its fundamental transition frequencies and relevant energy scales. Building upon these frequency-domain measurements, coherent control was achieved by calibrating resonant microwave pulses to drive single-qubit rotations, enabling the reliable preparation of the qubit in excited or coherent superposition states. Finally, specific time-domain protocols were implemented to evaluate the operational limits of the device. By executing standard pulse sequences, the characteristic decoherence metrics of the system, namely the longitudinal energy relaxation time (T1) and the inhomogeneous dephasing time (T2*), were successfully extracted, providing a quantitative assessment of the qubit's overall performance and establishing a baseline for future implementations of more advanced quantum control protocols.
Abstract Superconducting qubits are one of the most promising physical systems for implementing quantum computers. However, executing quantum algorithms of practical computational advantage requires further improvements in the fidelities of qubit operations, which are currently limited by the energy relaxation and dephasing times of the qubits. Here, we report our measurement results of a high-coherence transmon qubit with energy relaxation and echo dephasing times surpassing those in the existing literature. We measure a qubit frequency of 2.9 GHz, an energy relaxation time T 1 with a median of 425 μs and a maximum of (666 ± 33)μs, and an echo dephasing time T2echo T 2 echo with a median of 541 μs and a maximum of (1057 ± 138)μs. We report in detail our design, fabrication process, and measurement setup to facilitate the reproduction and wide adoption of high-coherence transmon qubits in the academia and industry.
Current state-of-the-art superconducting microwave qubits are cooled to extremely low temperatures to avoid sources of decoherence. Higher qubit operating temperatures would significantly increase the cooling power available, which is desirable for scaling up the number of qubits in quantum computing architectures and integrating qubits in experiments requiring increased heat dissipation. To operate superconducting qubits at higher temperatures, it is necessary to address both quasiparticle decoherence (which becomes significant for aluminum junctions above 160 mK) and dephasing from thermal microwave photons (which are problematic above 50 mK). Using low-loss niobium-trilayer junctions, which have reduced sensitivity to quasiparticles due to the higher superconducting transition temperature of niobium, we fabricate transmons with higher frequencies than previously studied, up to 24 GHz. We measure decoherence and dephasing times of about 1 μ s , corresponding to average qubit quality factors of approximately 10 5 , and find that decoherence is unaffected by quasiparticles up to 1 K . Without relaxation from quasiparticles, we are able to explore dephasing from purely thermal sources, finding that our qubits can operate up to approximately 250 mK while maintaining similar performance. The thermal resilience of these qubits creates new options for scaling up quantum processors, enables hybrid quantum experiments with high heat-dissipation budgets, and introduces a material platform for even-higher-frequency qubits. Published by the American Physical Society 2024
Abstract We benchmark the decoherence of superconducting transmon qubits to examine the temporal stability of energy relaxation, dephasing, and qubit transition frequency. By collecting statistics during measurements spanning multiple days, we find the mean parameters T1 T 1 ¯ = 49 μs and T2∗ T 2 * ¯ = 95 μs; however, both of these quantities fluctuate, explaining the need for frequent re-calibration in qubit setups. Our main finding is that fluctuations in qubit relaxation are local to the qubit and are caused by instabilities of near-resonant two-level-systems (TLS). Through statistical analysis, we determine sub-millihertz switching rates of these TLS and observe the coherent coupling between an individual TLS and a transmon qubit. Finally, we find evidence that the qubit’s frequency stability produces a 0.8 ms limit on the pure dephasing which we also observe. These findings raise the need for performing qubit metrology to examine the reproducibility of qubit parameters, where these fluctuations could affect qubit gate fidelity.
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.
Abstract Superconducting qubits are multilevel micro-fabricated artificial atoms based on superconducting materials. They are mainly operated in the low microwave frequency range (2–8 GHz) and require to be cooled below 20 mK to maintain quantum coherence. They enable precise engineering of transition frequencies and coupling strengths, facilitating advancements in quantum information processing and light-matter interactions. The field has evolved from early devices, characterized by short coherence times, to modern architectures with improved noise resilience, scalability, and fidelity. Better performances have been achieved through innovations in materials, fabrication, control electronics, and circuit design, enabling qubits with millisecond coherence times and gate fidelities exceeding 99.9%. However, challenges remain in mitigating two-level system defects, scaling devices, and addressing the cryogenic and infrastructure needs of larger quantum processing units. Current research focuses on modular and scalable architectures, error correction, and application. The field’s progress highlights the potential for superconducting qubits to drive advancements in quantum computation.