Laugh, quantum computer, laugh… in IBM’s gum tree.

Popular Science discusses what could be IBM’s quantum breakthrough – a quantum processor named “Kookaburra” that’s set to start computing in 2025:

Last year, IBM debuted a 127-qubit computing chip and a structure called the IBM Quantum System Two, intended to house components like the chandelier cryostat, wiring, and electronics for these bigger chips down the line. … Today, the company is laying out its three-year-plan to reach beyond 4,000-qubits by 2025 with a processor it is calling “Kookaburra.”

To scale up its processing abilities for qubits, IBM will flesh out development on both the hardware and software components for the quantum chips. First to come is a new processor called Heron that boasts 133 qubits. In addition to having more qubits, the Heron chip has a different design from its predecessor, Eagle. “It actually allows us to get a much larger fraction of functioning 2-qubit gates. It’s using a new architecture called tunable couplers,” says Jerry Chow, director of quantum hardware system development at IBM Quantum.

Before you can understand what a qubit is, you need to understand what a bit is, and what a gate is, too. On classical computers, information is encoded as binary bits (0 or 1). Transistors are switches that control the flow of electrons. Transistors are connected to several electrodes, including a gate electrode. Changing the electrical charge on the gate electrode controls whether the transistor is on in state 1, or off, in state 0. Physical changes to these states allow computers to encode information. Logic gates are made up of a specific arrangement of transistors. A bunch of transistors can make up an integrated circuit which can store chunks of data. These circuits are all interconnected on the surface of a chip.

Qubits work differently from bits, and quantum gates work differently than classical gates. Unlike classical bits, which can have a value of 1 or 0, under the right conditions, qubits can stay in the wave-like, quantum superposition state, which represents a combination of all possible configurations—0, 1, or both at the same time. Firing microwave photons at qubit-specific frequencies allows researchers to control their behavior, which can be to hold, change, or read out units of quantum information.

Unfortunately, qubits are quite fragile: They are heat-sensitive, unstable, and error-prone. When qubits talk to each other or to the wiring in their environment, they can lose their quantum properties, making calculations less accurate. When describing how long they can stay in their superposition states, experts refer to their “coherence time.” The coherence time and how long it takes to do a gate set the limit on how big of a quantum calculation you can do with a set of qubits.

Classical computers have cores, which are groupings of transistors that can run multiple tasks in parallel. You can envision it as having multiple checkout registers open at a supermarket instead of having everyone line up for one. CPUs that offer multiple cores, or multi-threading, can split up a big task into smaller pieces that can be fed to the different cores for processing.

Now, IBM wants to apply this concept to quantum computing as well, through a technique called circuit knitting. This “effectively takes large quantum circuits, finds ways to break them down into smaller, more digestible quantum circuits, which can be almost parallelly run across a number of processors,” Chow explains. “With this classical parallelization, it increases the types of problems and capabilities that we’re able to address.” Parallelization could also be useful for decreasing error rates.

This design offshoot is separate from the development of Osprey or Condor, which are on track to hit 433 and 1,121 qubits, respectively, in the next few years. “But we also want to have some modularity built-in that will allow us to scale even further. At some level, just the amount of the number of qubits that we’re going to be able to pack into a single chip will start to become limited,” says Chow. “We’re testing some of those boundaries with Osprey and with Condor currently.”

With Heron, the idea is for engineers to test ways to establish quantum links across multiple quantum chips. “We’re exploring what we call these modularly couplers that will allow us to effectively have multiple chips that are connected together,” Chow says.

To scale even more, IBM is also working on long-range couplers that can connect up clusters of quantum processors through a meter-long cryogenic cable (superconducting qubits need to be kept very cold). “We’re calling this the inter-quantum communication link,” says Chow, and it can extend quantum coherent connections within the shared cryogenic environment.

Combining parallelization, chip-to-chip connection, as well as long-range coupling is what could enable them to achieve their 2025 goal of a 4,158-qubit system: The Kookaburra.