From Chips to Qubits: Inside IBM’s Quantum System Two

Key Concepts

• IBM Quantum System Two: A modular and scalable quantum computing infrastructure designed for data centers.
• Cryogenic Infrastructure: The system responsible for cooling quantum processors to extremely low temperatures.
• Qubits: The basic unit of quantum information, analogous to bits in classical computing.
• Heron Processors: IBM's previous generation of quantum processors, with 156 qubits each.
• Nighthawk Quantum Chip: IBM's latest quantum chip, featuring 120 qubits arranged in a more connected square lattice.
• Dilution Refrigerator: A specialized refrigerator that uses a mixture of helium-3 and helium-4 to achieve temperatures as low as 15 millikelvin.
• FPGA (Field-Programmable Gate Array): A type of integrated circuit that can be programmed after manufacturing, used for classical control electronics.
• Quantum Links: Technology for connecting quantum chips in a network, enabling scaling.
• Error Correction: Strategies for mitigating errors in quantum computations, which involve different qubit arrangements rather than simply adding more chips.
• Lithography: A process used in semiconductor manufacturing to etch patterns onto silicon wafers, similar to standard CMOS processes.
• Kelvin (K): A unit of thermodynamic temperature. Absolute zero is 0 K (-273.15 °C).
• Millikelvin (mK): One-thousandth of a Kelvin.
• Helium-3: A rare isotope of helium used in dilution refrigerators.

IBM Quantum System Two: A Modular Approach to Quantum Computing

The video provides an in-depth look at IBM's Quantum System Two, a new infrastructure designed for quantum computing, emphasizing its modularity and scalability for data center deployment. Jerry Chow of IBM guides the tour at the Thomas J. Watson Center.

Cryogenic Infrastructure and Processor Cooling

• Core Functionality: The system's cryogenic component is central to its operation. It houses a cryogenic system that cools down the quantum processors.
• Current Configuration: The current setup includes three Heron processors, each containing 156 qubits, housed within this cryogenic infrastructure.
• Scalability: The design prioritizes scalability, allowing for marginal expansion of the cryogenic infrastructure and its footprint over time.
• Classical Control Electronics: Classical control electronics, responsible for generating signals to manipulate and read out qubits, are located adjacent to the cryogenic system. This proximity is crucial for minimizing control latency and ensuring short timescales. These electronics are also designed to scale with the growing footprint of the cryogenic system.
• Data Center Analogy: The classical control electronics are described as resembling racks of electronics found in traditional data centers, utilizing FPGA components.

The Nighthawk Quantum Chip and its Integration

• Nighthawk Chip Details: The latest quantum chip, Nighthawk, is presented. It features 120 qubits arranged in a square lattice, offering greater connectivity compared to the previous Heron processors.
• Physical Structure: The chip is mounted onto a printed circuit board (PCB) that includes connectors for signal transmission to manipulate and read qubit states.
• Operating Temperature: This assembly is bolted and mounted at the bottom of the dilution refrigeration systems, operating at an extremely low temperature of 15 millikelvin (mK).
• Chip-to-Chandelier Connection: A single Nighthawk chip is integrated into one of the "chandeliers" (referring to the suspended cryogenic components). The system can accommodate a varying number of these chips within the cryogenic infrastructure.
• Scaling Challenges and Solutions: A significant challenge in scaling is the size of the wiring and cabling. IBM's strategy involves miniaturizing wiring, cabling, and other components to enable more control for a larger number of chips within the same cryogenic footprint.

Scaling Strategies: Modularity and Interconnectivity

• Parallel Chip Integration: The system supports placing multiple chips in parallel, analogous to how classical electronics are rigged.
• Cryogenic Quantum Links: IBM is developing and plans to implement cryogenic quantum links within the next year. This technology, previously demonstrated with their "Flamingo" architecture, allows for quantum networking and scaling of chips in a quantum manner.
• Classical Communication Networks: In addition to quantum links, classical communication networks within the control electronics facilitate communication between chips. This enables real-time decision-making and dynamic circuit execution, where measurements on one chip can inform operations on another.

Error Correction vs. Scaling with Multiple Chips

• Distinct Approaches: The video clarifies that adding more chips is not the primary strategy for error correction. Error correction relies on fundamentally different qubit arrangements on a single device.
• Benefits of Multiple Chips: The use of multiple chips primarily enables the execution of a wider variety of quantum circuits and algorithms by leveraging the tight coupling of different chips.
• Chip Stacking: The Nighthawk chip can be stacked, with two chips placed back-to-back, forming a unit that mounts onto the control board. The system can accommodate "many Nighthawks."

Manufacturing and Testing Processes

• Silicon Fabrication: The chips are manufactured on 300-millimeter silicon wafers using lithography techniques familiar to standard CMOS process users.
• Quantum Test and Characterization Lab: The video shows the inside of a testing fridge within IBM's quantum test and characterization lab.
• Qualification Steps: Processed chips (Heron or Nighthawk) undergo rigorous qualification steps after fabrication. These include room-temperature tests and cryogenic tests to characterize their performance before deployment in a data center.
• Environment Recreation: The lab recreates the operational environment, including the extreme cold, to thoroughly assess device characteristics and ensure quality.

Achieving Extreme Cold: The Dilution Refrigerator

• Target Temperature: The system operates at 15 millikelvin (mK), which is 0.015 Kelvin.
• Comparison to Space: This temperature is orders of magnitude colder than space, which is around several Kelvin (e.g., the cosmic microwave background radiation).
• Cooling Stages: Achieving 15 mK involves a two-stage cooling process:
  1. Pulse Tube Cooler: This stage, using liquid helium, brings the infrastructure down to approximately 4 Kelvin.
  2. Dilution Refrigerator: This closed-loop refrigerator uses a mixture of helium-4 and the rare isotope helium-3. Helium-3 has two protons and one neutron, differing from the more common helium-4. This mixture is essential for reaching the 15 mK temperature. The noise heard in the lab is from the compressed circulation of helium in the pulse tube cooler.