Building scalable, innovative quantum systems | Rigetti Computing

Building quantum computers combines advances in engineering, physics, computer science, and manufacturing. Integrating all these specialties under one roof and in one technology stack allows us to move further, faster.
Quantum processor chips are the foundation of our technology stack. Manufacturing these chips begins with the ability to design high quality quantum-coherent superconducting microwave devices. We leverage advanced modeling and simulation tools to design linear and nonlinear chip components, accurately predict performance behavior of large scale integrated quantum circuits (QuICs), and produce masksets to be fabbed in our manufacturing facility, Fab-1.Fab-1 is a captive quantum integrated circuit foundry. We combine modern silicon semiconductor and MEMS processing technologies with novel manufacturing methods to produce state-of-the-art superconducting qubits and device layers for microwave circuitry. Our process flows leverage superconducting materials such as aluminum, indium, and niobium in a series of subtractive patterning, etching, lithography, and deposition processes that result in ultra-low-loss superconducting devices. We incorporate best practices from industrial and research fabs to maintain a stable process of record while also running continuous shortloop development tests to optimize for fast learning cycles.Scalable performance in a quantum integrated circuit architecture requires both 3D integration to support I/O requirements and careful engineering of the electromagnetic environment in which the qubits operate. Rigetti processors employ through-silicon vias and a bonded superconducting cap to isolate the components on our chips, maximize coherence, minimize crosstalk, and enable advanced packaging. Our packaging capabilities allow for scalable processor architectures that reliably interface to dense signal delivery wiring without tradeoffs in quantum performance.Continuous improvement in processor performance requires high throughput test capabilities to gather statistically significant data for each design iteration or fab process change. Rigetti Fab-1 engineers use dedicated cryogenic systems with custom measurement automation software to efficiently characterize dozens of qubits simultaneously. This reduces learning time and widens the range of tests engineers can run.
Superconducting Qubits
Rigetti systems are powered by superconducting qubit-based quantum processors. As a lithographically defined chip-based technology, superconducting qubits are intrinsically highly scalable. They also offer fast gate times, low-latency conditional logic, and fast program execution times, making them ideally suited to NISQ era applications and the requirements for quantum error correction and fault-tolerant quantum computing.At the chip level, each superconducting qubit consists of a non-linear Josephson inductance in parallel with an ultra-low-loss capacitor to create a resonant structure in the 3-6GHz range. Qubits are coupled to a linear superconducting resonator for readout. The combination of the qubit, the linear readout resonator, and the associated wiring provides a general purpose quantum circuit element capable of reliably encoding, manipulating, and reading out quantum information. Rigetti processors use arrays of qubits coupled to one another with on-chip capacitances. Single and multi-qubit logic operations are implemented through the application of microwave or DC pulses.Scalable Quantum Processors
Reliably building large planar arrays of qubits requires sustained innovation in design and manufacturing capabilities. Rigetti processors leverage our distinctive Fab-1 capabilities to achieve both performance and scalability, including superconducting through-silicon vias and superconducting flip-chip cap bonding. Each of these plays a dual role in the qubit and processor design, providing an electromagnetic housing for each qubit while also enabling targeted connections to other qubits and to signal delivery wiring. Further scalability enhancements, such as multiplexed readout, are created through on-chip design features that allow single input/output lines to address a plurality of qubits.
Turning a quantum processor into a fully functional computer requires the ability to control the qubits in a reliable and programmable way. In a superconducting quantum computer, QuICs are packaged and housed in cryogenic dilution refrigerators. Room temperature microwave control electronics generate qubit control and readout signals that are then delivered to and received from the quantum processor through a cryogenically compatible interconnect system.Our custom FPGA-based control hardware includes a powerful classical CPU and memory system. This integrated system is designed and built for the low-latency architecture necessary for hybrid quantum-classical computing and implementation of high-throughput quantum programs. These instruments drive the quantum processors, calibrate and operate single and multi-qubit gates, and read out qubit states at the end of a computation. The system is designed to maintain phase coherence, control signal stability, and ease of calibration to ensure consistent QPU performance.Traditional interconnect systems use coaxial cables to carry signals to and from the QPU. With multiple transmission lines associated with each qubit, this design does not scale well for larger quantum processors. We’ve developed and integrated high-density flexible circuits, each of which replace as many as 10 coax lines for higher signal density. These highly compact assemblies have low loss and low thermal heat load. They incorporate passive components like attenuators, RF filters, bias Ts, and amplifiers that optimize signal processing and protect the qubits from noise.From there, signals interface with the QuIC package where they are routed vertically to the chip. Compared with routing signals to the perimeter of the chip, this three-dimensional signaling ar