2Q gate fidelity 99.8-, IQM for fast and high fidelity CZ gates

Superconductivity and ion traps are the two main physical implementations of quantum computing, but superconductivity and ion traps have advantages and disadvantages, for example, ion traps have higher fidelity and superconductivity has a significant advantage in gate operation speed. So is there a double quantum bit gate with both fast and high fidelity? Recently, IQM, a European superconducting quantum computing company, has realized a controlled Z gate (CZ gate) with both fast and high fidelity - a common double quantum bit gate - by introducing a long-range transmon coupler.

Tunable couplers of superconducting quantum bits are important for isolated gate operation in scalable quantum processor architectures. In this study, IQM researchers demonstrate a tunable quantum bit-quantum bit coupler based on a floating transmon device that can space quantum bits at least 2 mm apart from each other while maintaining coupling between the coupler and the quantum bits over 50 MHz. Using their proposed flexible and scalable new architecture, a CZ gate with (99.81 ± 0.02)% fidelity is demonstrated.

In the introduced tunable coupler design, both quantum bit-quantum bit and quantum bit-coupler coupling are mediated by two waveguides rather than relying on direct capacitive coupling between the components, reducing the effect of quantum bit-quantum bit distance on the coupling. This therefore leaves room for each quantum bit to have the separate readout resonators and Purcell filters required for fast high-fidelity readout. In addition, the large quantum bit-quantum bit distance reduces unwanted non-nearest neighbor coupling and allows multiple control lines to cross the structure with minimal crosstalk.

A related paper, "Long-range transmon coupler with CZ gate fidelity above 99.8%," has been submitted to the arXiv.org website [1].

01New design for fast and high fidelity CZ gates

Implementing high-fidelity dual quantum bit gates is a key requirement for scalable quantum processors. The performance of quantum gates relying only on static quantum bit-quantum bit coupling is usually limited by the spurious ZZ interactions between quantum bits, resulting in long gate times due to poorer on/off of the coupling. To solve this problem, different increasingly complex designs of tunable couplers have been proposed.

An important step towards tunable couplers with high on/off ratios and minimal impact on quantum bit coherence was the observation that a coupler-mediated tunable interaction could be designed to cancel the static quantum bit-quantum bit coupling at a specific coupler off frequency above the quantum bit frequency. Since then, such couplers have been successfully applied in several experiments.

Despite the great success of ZZ interaction-free tunable coupler designs in achieving high-fidelity two-qubit quantum gates, the static quantum bit-quantum bit coupling arises from direct coupling between quantum bits and is mainly controlled by the quantum bit-quantum bit distance. As a result, this imposes severe restrictions on the placement of quantum bits and limits the space in the square quantum bit lattice, allowing only the most basic components to be placed between the lattice locations.

In the study, IQM introduces and experimentally demonstrates an extended floating coupler that allows us to increase the physical distance between the quantum bits, thus providing space on the chip for readout resonators with independent Purcell filters for high-fidelity readout and reduced parasitic non-nearest-neighbor coupling.

Furthermore, if flip-chip technology is utilized, the long quantum bit-quantum bit distance allows for multiple control lines with low crosstalk paths on the quantum bit-coupler-quantum bit structure. Here, the ability to have long quantum bit-quantum bit distances is achieved using two waveguide expanders that regulate direct quantum bit-quantum bit coupling, as well as coupling between quantum bits and couplers. These couplings mainly come from the cross-finger capacitor between the two waveguide extenders and the coupler, which makes it possible to maintain coupling over a wide range of quantum bit-quantum bit distances.

(a) Circuit diagram of the seemingly aggregate element coupled through a tunable coupling structure consisting of waveguide extenders (black) and floating coupler quantum bits (red) (blue and orange). The gray line shows the effective lumped-element model of the coupling capacitance induced by the black waveguide extenders C and F. (b) Simulation of the effective coupling strength between quantum bit 1 and coupler g1c, quantum bit 2 and coupler g2c, and quantum bit 1 and quantum bits 2 and g12 for different quantum bit-quantum bit distances dqq (solid line). For the device with dqq = 1960 m (dashed line), the measured coupling values (points with 68% confidence interval) are shown (c) Schematic of a simplified flip-chip architecture, where the quantum bits and couplers are connected by a waveguide expander (light gray) on the bottom chip and a long vertical transmission line (black) runs through the quantum bit-coupler structure on the top chip. (d) Simulated voltage coupling ratios from each component to the transmission line (same color as in (c)) at various crossover locations of the transmission line xcross, total coupling (black dashed line), and low-crosstalk crossover region (blue shaded).

The IQM researchers were trying to demonstrate that the coupling between the quantum bits and the coupler is high enough for quantum bit-quantum bit distances exceeding 1 mm. They implemented a fast and high-fidelity CZ gate with a duration of 33 ns and fidelity of (99.81 ± 0.02)% for a quantum bit-quantum bit distance of 1.96 mm, a distance four times longer than typical tunable coupler designs. IQM says this coupler design is easily scalable to square quantum bit lattices, making it an attractive building block for scalable high-fidelity quantum processors. IQM says this coupler design easily scales to a square quantum bit grid, making it an attractive building block for scalable high-fidelity quantum processors.

02Optical interface developed for scaling superconducting quantum processors

In addition to announcing its latest results, IQM also announced the development of an optical interface for scaling superconducting quantum processors with QphoX, a Dutch quantum modem company [2].

QphoX has acquired the first quantum processor chip from IQM to begin developing new optical interconnects for superconducting quantum computers.

One of the major hurdles facing today's quantum processors is that microwave quantum processors must operate in harsh cryogenic environments while controlled by microwave lines and cryogenic amplifiers that generate a lot of heat, limiting the size of the processors, said an IQM press release. As manufacturers move toward larger chips, it is critical to find scalable methods that will eventually allow computers with hundreds of thousands of quantum bits.

By leveraging our unique microwave-to-optical conversion technology, signals can be routed through the cryostat via optical fiber," said Frederick Hijazi, COO and co-founder of QphoX. As a result, both the space and thermal load constraints placed on the cryostat will be reduced, allowing larger processors to be built in a single cryostat. We are very excited about starting this new partnership. We have been using IQM's processors for the past few months and have been very impressed with the quality and performance."

IQM COO and co-founder Dr. Juha Vartiainen said, "Future large-scale quantum computers will require either optical communication or cryogenic signal generation technology, or both. qphoX's expertise and technology plan is a promising alternative to using optical fiber to transmit the control and readout signals from quantum computers to a quantum bit chip. With simplified cabling and new product innovations, this collaboration will be a driving force for systems with more than 1,000 quantum bits."

Reference links：

[1]https://arxiv.org/abs/2208.09460

[2]https://www.meetiqm.com/articles/press-releases/iqm-and-qphox-to-develop-optical-interface/