Session W48: Architectures & Infrastructure for Superconducting Quantum Systems

Gate performance statistics from quantum processing units with tens of qubits

We present an overview of design and fabrication solutions for our scalable superconducting quantum processing units (QPUs). We review the building blocks used in our QPUs including the qubits, tunable coupling elements, readout circuits, and inter-layer connections. Building on the presented solutions, we report the latest performance metrics for simultaneous single- and two-qubit gates and compare the results of standard benchmarks. Finally, we introduce our QPU scaling roadmap from 20 to 54 and 150 data qubits corresponding to more than 400 tunable elements.   

Frequency allocation strategy in parametric-gate-based fixed-frequency qubit systems

The parametric gates implemented with flux-tunable transmons as couplers offer flexibility and access to larger native gate sets for fixed-frequency qubits. However, frequency crowding becomes an increasingly complicated issue to handle in scaled-up devices with more qubits. In this talk, we discuss a possible frequency-allocation scheme to avoid crowding in a square lattice of qubits. The design is based on grouping the qubits in two sublattices, such that the frequencies and anharmonicities of each sublattice are centered around different values. This allows us to scale up to square lattices of arbitrary size, provided that crosstalk between physically distant transmons is minimal. Furthermore, we discuss a simplified model of the gate dynamics that allows us to quantitatively set constraints on the frequencies and anharmonicities of the qubits, and determine how robust the scheme is to the impact of crosstalk and frequency variations of the qubits.   

Projecting requirements for superconducting qubit systems at utility scale applications

How big would a quantum computer need to be to solve problems that are intractable on classical hardware? We present a method, built on the measurement-based quantum computing paradigm, to estimate the resources required to execute fault-tolerant applications at a scale large enough to solve useful problems. When provided with an algorithm and a specific superconducting qubit architecture, the framework produces an estimate of required resources to execute that algorithm. The architecture considered consists of individual modules filled with a square lattice of transmon qubits connected to each other via a coherent quantum interconnect. We use our method to estimate hardware resource requirements for a range of quantum signal processing applications, up to problem sizes known to be challenging for classical tensor-network methods. We can also quantify the bottlenecks and tradeoffs in chip design, module interconnectivity, and thermal management and measure their impact on the infrastructure and time required to execute a given application.   

Addressing infrastructure requirements for the coherent control of large superconducting quantum systems

Superconducting-qubit systems are a promising platform for the implementation of error-corrected, fault-tolerant quantum computers, for which the surface code is a prominent approach. We consider an architecture where a repeatable tile of flux-tunable qubits, coupled through capacitive tunable couplers, is repeated to create lattices of qubits that can be controlled with minimal footprint and overhead, in a way amenable to the implementation of the surface code. Focusing on problems arising from the need to extend such platforms to relevant system sizes, while ensuring their coherent control, we propose various methods for the enhancement of control infrastructure required for 3D-integrated qubit designs. We do so at various levels of abstraction, from the qubit chip to the control software required for system calibration, including cryogenic package assemblies, signal delivery and room-temperature control electronics. Preliminary characterization of these will be presented.   

Parity-Dependent State Transfer and Many-Qubit Entanglement Generation on a Superconducting Qubit Chain

Superconducting qubit devices have recently demonstrated high-fidelity operations, high coherence times and improved scalability, making them a leading platform for quantum computing. However, practical applications require the efficient generation of many-qubit entangled states, incurring large overheads in single- and two-qubit gates as qubit connectivity is generally limited to nearest-neighbor pairs. Nonetheless, by evolving the system under simultaneous local interactions, one can realize effective non-local multi-qubit operations, efficiently generating entanglement. In this work, we operate a circuit of six fixed-frequency transmons with tunable couplers and control the couplings via simultaneous parametric drives. We engineer the drive amplitudes and frequencies in order to implement a quantum state transfer protocol, in which excitations are coherently transferred between distant qubits. We observe the parity-dependent property of the transfer, where the number of excitations within the chain controls the phase of the transferred state. Finally, we utilize this property to prepare multi-qubit GHZ states with Hadamard gates and a single transfer operation, demonstrating efficient entanglement generation.   

A minimal and integrable device for routing arbitrary microwave quantum states in waveguide quantum electrodynamics

Routing traveling photons in a controlled directional manner is essential for operating a quantum network. Communicating information between arbitrary quantum nodes using itinerant photons requires controllable directionality, high-fidelity signal processing, and loss resilience. Implementing such a network in the microwave domain is currently limited by losses due to commercially available directional devices such as circulators and isolators [1-3]. Recent efforts have addressed this challenge and have demonstrated controlled directional emissions of flying qubit states in the 0/1 Fock state basis [4, 5]. Here we present the design and realization of a device that extends this functionality to arbitrary quantum states, such as error-correctable bosonic states. Our device features an integrable on-chip design with three SNAIL (Superconducting Nonlinear Asymmetric Inductive eLements) modes. By controlling the interference between these modes with external microwave drives, we can realize in-situ tunable directionality with low loss. We present experimental data from an experiment in which we have realized a parametrically controllable isolator and gyrator, along with analytical and numerical modeling. From our analysis, we infer that emission and absorption of arbitrary quantum states with high fidelity is achievable. This result will enable routing arbitrary quantum states with in-situ control, which will be an enabling component for remote entanglement distribution and state transfer in error-corrected modular quantum networks.

 

[1] Campagne-Ibarcq, P., et al. Phys. Rev. Letter (2018)

[2] Axline, Christopher J., et al. Nat Phys. (2018)

[3] Kurpiers, Philipp, et al. Nature (2018)

[4] Kannan, Bharath, et al.  Nature Phys. (2023)

[5] Chaitali Joshi, et al. Phys. Rev. X (2023)   

Enhancing Connectivity in Superconducting Qubit Systems with an On-Chip Quantum Router Design

Connectivity plays a crucial role in the design of modular quantum systems and the development of large-scale distributed quantum computing and error correction architectures. Superconducting qubits, a leading platform for fault-tolerant quantum computing, face limitations in their connectivity, typically allowing connections to only a few neighboring qubits. Achieving higher connectivity typically often involves complex multi-layer packaging or relayed state transfer, which can be resource-intensive and limited in fidelity. Here, we propose an innovative on-chip quantum router design that facilitates long-range all-to-all connectivity and boasts a high on-off ratio. We will show experimental results for the transfer of quantum states between arbitrary qubit pairs and fidelity measurements in two-qubit gate operations mediated by the coupler pairs. This approach opens up promising possibilities for efficiently engineering couplings within large-scale quantum computers, and offers a pathway for implementing quantum algorithms and error correction schemes that benefit from enhanced qubit connectivity.   

High fidelity quantum gates and qubit measurements in an all-to-all connected, SNAIL-based quantum module

As quantum information processors scale to larger size and greater complexity, machines based on networks of qubits coupled only to their nearest-neighbors will suffer from the long times and many operations needed to move information across the qubit fabric. Modular architectures aim to address this by employing dense connectivity within subsets of qubits comprising individual quantum modules, which are ultimately connected together via quantum routers. Here, we present recent work on the design and characterization of a four-qubit module with all-to-all coupling realized by parametric driving of a central SNAIL mode which is compatible with our previously-realized quantum state router [1]. We perform two-qubit gates via three-wave interactions induced by driving the SNAIL at the difference frequency of the qubit modes of interest, enabling iSWAP family gates with gate times shorter than 100 ns. We will present results on improvements made toward achieving 0.99 gate fidelities among all pairs of qubits as well as 0.99 readout fidelity. We will also explore novel gates of interest for more efficient quantum algorithm transpilation, such as B and CNOT gates based on simultaneous parametric drives and explore simple algorithms on this device.   

Coherent control of a superconducting qubit using light

Quantum science and technology promise the realization of a powerful computational resource that relies on a network of quantum processors connected with low loss and low noise communication channels capable of distributing entangled states. While superconducting microwave qubits (∼3-8 GHz) operating in cryogenic environments have emerged as promising candidates for quantum processor nodes due to their strong Josephson nonlinearity and low loss, the information between spatially separated processor nodes will likely be carried at room temperature via telecommunication photons (200 THz) propagating in low loss optical fibers. Transduction of quantum information between these disparate frequencies is therefore critical to leverage the advantages of each platform by interfacing quantum resources. Here, we demonstrate coherent optical control of a superconducting qubit. We achieve this by developing a microwave-optical quantum transducer that operates with up to 1.18% conversion efficiency (1.16% cooperativity) and demonstrate optically-driven Rabi oscillations (2.27 MHz) in a superconducting qubit without impacting qubit coherence times (800 ns). Finally, we discuss outlooks towards using the transducer to network quantum processor nodes.   

Optimal control of a superconducting qutrit via a low-noise broadband optical link

Photonic links enable the direct generation of microwave control signals for superconducting devices at cryogenic temperatures, reducing the heat load impacts of the coaxial lines and removing the thermal background carried by those cables. These links also enable adoption of microwave photonic techniques that have improved signal to noise and distortion ratios compared with conventional electronic digital to analog converters. Quantum optimal control methods enable arbitrary, high-fidelity control of multilevel quantum systems through the direct implementation of complex unitaries and pre-compiled gate sequences. In this talk, I will describe how we integrated a photonic digital to analog converter (PDAC) with a microwave control system targeting a superconducting qutrit. We demonstrated basic control by using photonically-generated control pulses to measure energy- and phase- coherence, and then generated and tested an optimal-control implementation of the 0-2 SWAP gate with the PDAC, demonstrating arbitrary control signal generation for superconducting devices via photonic links.   

mm-Wave superconducting mode converter for kinetic inductance parametric amplifier

In the past decade, there has been a great attention towards using millimeter wave (mm-wave) technology in various applications among which is superconducting quantum devices. At mm-wave frequencies, quantum devices are less sensitive to thermal background noise and hence can be built at higher temperatures than quantum devices operating at microwave frequencies. Kinetic Inductance parametric amplifier (KIPA) is one crucial component in the superconducting qubit circuit system. These parametric amplifiers with their wide bandwidth, quantum-limited noise performance and high dynamic range are well suited to be used as readout amplifiers. The large kinetic inductance is achieved using a thin film of high resistivity superconducting material. However, to couple the KIPA to W-band waveguide feeds, a superconductor-based transition device is needed with low losses. In this work, we design and test a superconducting mode converter to be used as low-loss transitional component to convert the W-band waveguide mode (TE10) to the quasi-TEM mode supported by the KIPA. It uses a ridge gap waveguide design to eliminate the dielectric losses. We present field simulations as well as the experimental results of testing the superconducting mode converter at low temperature.