Quantum Operations in Transmon Qubit Research

The US—nor any other country or continent—has a moat when it comes to Quantum Computing. China’s 105-qubit superconducting quantum processor, 𝗭𝘂𝗰𝗵𝗼𝗻𝗴𝘇𝗵𝗶 3.0, makes it strikingly clear why. As a physicist, I admire how transparently the Chinese research team has shared the details of their hardware innovations. 𝗨𝗻𝗱𝗲𝗿 𝘁𝗵𝗲 𝗛𝗼𝗼𝗱: - 105 transmon qubits arranged in a 15x7 lattice, connected by 182 flux-tunable couplers for dynamic gate operations. - Modular flip-chip design using indium bump bonds, with superconducting films of tantalum and aluminum to reduce dielectric losses. - Qubit relaxation times (T₁) of 72 µs and dephasing times (T₂, CPMG) at 58 µs—matching Google's Willow chip performance. 𝗦𝘁𝗮𝗻𝗱𝗼𝘂𝘁 𝗧𝗲𝗰𝗵𝗻𝗶𝗰𝗮𝗹 𝗙𝗲𝗮𝘁𝘂𝗿𝗲𝘀: - High-fidelity gate operations with 99.90% for single-qubit gates and 99.62% for two-qubit gates. - Readout error rates as low as 0.82%, achieved through high coupling strength (~130 MHz) and optimized readout resonator linewidths (~10 MHz). - Clever signal management: Modular cryogenic wiring with just 332 cables and robust readout using TWPAs, HEMTs, and bandpass filters. - A smart "4-patch" calibration method allows dynamic gate tuning and accurate error estimation, even for large circuits. 𝗧𝗵𝗲 𝗘𝘅𝗽𝗲𝗿𝗶𝗺𝗲𝗻𝘁: Zuchongzhi 3.0 showcased its capabilities through large-scale random circuit sampling, running circuits with up to 83 qubits and 32 cycles deep. They used structured gate sequences (patterns ABCD-CDAB) to maintain coherence and minimize errors. 𝗧𝗵𝗲 𝗥𝗲𝘀𝘂𝗹𝘁: They performed tasks that would take Frontier, one of the world’s fastest supercomputers, approximately 6.4 billion years to replicate classically. The world tends to overlook China’s quantum progress until it becomes impossible to ignore. Yet here it is, openly shared—beautifully showcasing what superconducting quantum processors can achieve today. 📸 Credits: Dongxin Gao et al. (2025) IBM Google Microsoft Alibaba Group IQM Quantum Computers

My team qcrew SG did a series of studies on how to extract information from quantum harmonic oscillators as effectively and robustly as possible. The most recent result is now out at Physical Review Research https://lnkd.in/gdw8bCPf!! 🥳 Tanjung Krisnanda Fernando Valadares Here, we developed a numerical technique to map any target observables of the oscillator to an auxiliary transmon qubit. We think this is a very handy and convenient tool that allows us to ask very specific questions about our oscillator state in a single measurement, e.g. is it in 0+4 photons? If you'd like to try, all the code is available online and linked in the article! Of course, we would still need to do full tomography if we'd like to know all the details about the oscillator state. Our work on using photon number counting does this very efficiently, even if you happen to have a bad auxiliary qubit! More information can be found in our PRX Quantum paper https://lnkd.in/g-imMnr5

New preprint out today with my PhD student Simon Pettersson Fors and researcher Jorge Fernández Pendás at the Wallenberg Centre for Quantum Technology (WACQT) at Chalmers tekniska högskola: ”Comprehensive explanation of ZZ coupling in superconducting qubits” https://lnkd.in/dyqWUn6X We have now become so good at protecting superconducting quantum computers from decoherence due to unwanted couplings to their surroundings that unwanted couplings between qubits within the quantum computer are emerging as a major challenge for scaling up. All such couplings between qubits can manifest as a ZZ coupling, a shift of the energy of one qubit conditioned on the state of another qubit. This coupling can be used as a two-qubit controlled-Z (CZ) gate if it’s strong, but when doing other operations on the quantum computer we want it to be zero or close to zero, to not introduce errors. There have been many setups suggested in the past few years to suppress or cancel the ZZ coupling between two superconducting qubits. To guide the search in this large parameter space of setups and coupling strengths, it is important to have a good understanding of the mechanisms giving rise to the ZZ coupling. In our paper, we present extensive analytical and numerical results for the ZZ coupling in a setup with two fixed-frequency transmon qubits and a flux-tunable transmon coupler. We introduce a diagrammatic perturbation theory to clarify the mechanisms behind the ZZ coupling to a greater extent than has been done before. To support our approximations in the perturbation theory, and the results emerging from it, we perform careful numerical modeling, which considers the Hamiltonian for the transmon qubits from a low level and leverages an improved algorithm to identify eigenstates in the system. We find that the qubit frequencies, anharmonicities, and coupling strengths in our considered system can be chosen to create three types of parameter regions with zero or near-zero ZZ coupling that can be accessed by current technology without major redesigns. Through our diagrammatic perturbation theory we are able to explain the underlying mechanisms (both level repulsions and some higher-order mechanisms) for the existence of all these regions. Our results thus open up both for improving gate speeds for CPHASE and CZ gates, and for improving fidelities of other gates, which are negatively affected by ZZ coupling. Through the analytical and numerical methods we introduce, system parameters and architectures can be constrained to a more manageable search space. Our methods are not limited to the three-transmon setup we study here as an example; we expect them to find applications in investigations of larger systems (including ZZZ and higher-order couplings), in setups with other types of superconducting qubits, possibly also in other quantum-computing systems where ZZ coupling constitutes a challenge, e.g., semiconductor qubits.

Scientists Achieve High-Fidelity Quantum Computing Gate Using Double-Transmon Coupler Researchers from the RIKEN Center for Quantum Computing and Toshiba have achieved a breakthrough in quantum computing by developing a high-fidelity quantum gate using a double-transmon coupler (DTC). This novel architecture has demonstrated exceptional performance, achieving 99.92% fidelity for a two-qubit CZ gate and 99.98% fidelity for a single-qubit gate. These advancements, part of the Japanese Q-LEAP project, represent a major step toward fault-tolerant quantum computation. Key Features of the Double-Transmon Coupler (DTC) 1. Innovative Design: The DTC connects qubits using two fixed-frequency transmons, coupled through a loop with an additional Josephson junction. This configuration minimizes noise and improves precision. 2. Tunable Coupling: The architecture allows for adjustable interactions between qubits, ensuring high gate fidelity while reducing errors. 3. Noise Resistance: Transmons are less sensitive to charge noise, making the device more stable for quantum operations. Impact on Quantum Computing 1. Enhanced NISQ Devices: These high-fidelity gates significantly improve the performance of today’s noisy intermediate-scale quantum (NISQ) devices, allowing for more reliable quantum calculations. 2. Fault-Tolerant Computing: The high gate fidelity achieved with the DTC is crucial for effective quantum error correction, a key milestone for scalable, fault-tolerant quantum systems. 3. Broad Applications: Reliable quantum gates are foundational for advancing applications in cryptography, optimization, material science, and artificial intelligence. Why High-Fidelity Gates Matter Quantum gates are the building blocks of quantum algorithms. Errors in gate operations can quickly propagate, degrading the accuracy of quantum computations. By achieving near-perfect fidelity: • Errors are minimized, reducing the need for extensive error correction. • Quantum processors become more efficient, requiring fewer resources for reliable performance. • Complex algorithms, such as Shor’s and Grover’s, can be executed with higher precision. Future Directions The success of the DTC paves the way for: 1. Scaling Quantum Systems: Connecting more qubits with high fidelity is essential for building larger, more powerful quantum computers. 2. Advanced Error Correction: Coupled with high-fidelity gates, error correction methods can be more effectively implemented, bringing practical quantum computing closer to reality. 3. Industry Adoption: These advancements could accelerate the deployment of quantum technologies in fields such as finance, drug discovery, and logistics. This breakthrough demonstrates that quantum hardware innovation is rapidly advancing, pushing the boundaries of what is possible in the realm of quantum computation.

Reducing Leakage of Single-Qubit Gates for Superconducting Quantum Processors Using Analytical Control Pulse Envelopes https://lnkd.in/e7ZhGXGM Abstract Improving the speed and fidelity of quantum logic gates is essential to reach quantum advantage with future quantum computers. However, fast logic gates lead to increased leakage errors in superconducting quantum processors based on qubits with low anharmonicity, such as transmons. To reduce leakage errors, we propose and experimentally demonstrate two new analytical methods, Fourier ansatz spectrum tuning derivative removal by adiabatic gate (FAST DRAG) and higher-derivative (HD) DRAG, both of which enable shaping single-qubit control pulses in the frequency domain to achieve stronger suppression of leakage transitions compared to previously demonstrated pulse shapes. Using the new methods to suppress the 𝑒⁢𝑓 transition of a transmon qubit with an anharmonicity of −212 MHz, we implement 𝑅𝑋⁢(𝜋/2) gates achieving a leakage error below 3.0×10−5 down to a gate duration of 6.25 ns without the need for iterative closed-loop optimization. The obtained leakage error represents a 20-fold reduction in leakage compared to a conventional cosine DRAG pulse. Employing the FAST DRAG method, we further achieve an error per gate of (1.56±0.07)×10−4 at a 7.9-ns gate duration, outperforming conventional pulse shapes both in terms of error and gate speed. Furthermore, we study error-amplifying measurements for the characterization of temporal microwave control-pulse distortions, and demonstrate that non-Markovian coherent errors caused by such distortions may be a significant source of error for sub-10-ns single-qubit gates unless corrected using predistortion.