In the race to build scalable quantum computers, one critical issue has remained unsolved: wiring. As the qubit count increases in superconducting quantum processors, the demand for individual control lines per qubit becomes an insurmountable obstacle—introducing excessive wiring complexity that results in greater errors, higher costs, and packaging and thermal challenges within cryogenic systems.
This inefficient design could require millions of wires in future architectures (see Fig. 1). A new superconducting quantum architecture engineered with a novel control mechanism has demonstrated potential for a dramatic reduction in the wiring demands required to operate large arrays of qubits.
This breakthrough design developed by Planckian replaces the traditional model of individual qubit control with a global control approach to enable multiple qubits to be manipulated via shared control lines. By decoupling control complexity from the number of qubits, this efficient and cost-effective solution keeps wiring at manageable levels as the system grows, which reduces thermal demands and operating costs.
Counterintuitive control
Sending the same control pulse to many qubits at once using global control makes it challenging to get only one qubit to respond while the others remain untouched. How is it possible to perform a logic operation on one qubit if all qubits are being driven with the same signal?
Planckian’s solution counterintuitively uses a type of qubit interaction typically held to a minimum. ZZ coupling (see Fig. 2) is a type of interaction between qubits in which the energy levels and resonance frequency of a qubit shift slightly, depending on the state of its neighboring qubit (0 or 1). While these state-dependent qubit interactions are necessary to control and excite a qubit as well as to create entanglement and conduct gate operations, this approach can also lead to crosstalk and errors.
As a result, most superconducting platforms minimize these interactions, instead opting for tunable couplers for dynamic control. But these alternatives often increase circuit complexity and create additional noise sources.
Rather than avoiding ZZ coupling, Planckian’s platform exploits it by using the state of a neighboring qubit to block a target qubit from being excited or manipulated. Applying a specific sequence of pulses across qubits driven by the same control lines can use ZZ coupling to create a “blockade effect” that prevents the simultaneous excitation of two nearby qubits.
When two qubits are coupled via ZZ interaction, it’s possible to design pulses so only one of them can be excited at a time. If qubit A is already in the excited state, then qubit B becomes off-resonant with the global pulse. It can’t absorb energy and stays in place.
This global control approach uses this blockade effect for selective control so that operations are performed only where and when needed. This isolates individual qubits within a globally addressed network and enables them to perform logical operations.
Operations behind the blockade
The system’s unique arrangement of superconducting qubits—with fixed always-on ZZ interactions—configures qubits in such a way that only one location at a time has the right local neighborhood configuration to respond to the global pulse.
When a sequence of pulses is sent across the processor, only the target qubit flips or rotates, while others remain untouched due to the blockade. With the blockade effect on the front lines to protect against unwanted excitations, the approach forms the basis for a globally controlled quantum processing unit (QPU) where logical operations are performed by broadcasting carefully timed microwave pulses along shared lines.
This design is natively universal and can implement all necessary gate operations for quantum computation. In its latest iteration, where qubits are connected within a closed-loop geometry, the system supports single-qubit gates and a direct one-shot implementation of the three-qubit Toffoli gate, a fundamental component for many quantum algorithms and error correction protocols.
Tackling the scalability crisis
Major quantum players have made headlines with superconducting quantum processors surpassing hundreds of qubits, but these platforms typically require two coaxial cables per qubit to deliver and read out control signals. This means a million-qubit processor could demand as many as 2 million individual wires—and it creates unscalable constraints on cryostat design, thermal management, and signal integrity.
Even at smaller scales, a cryostat for a 150-qubit processor with coaxial wiring costs approximately $5M, with as much as 80% of the cost allocated for wiring.
Planckian’s global control approach significantly reduces wiring density to minimize crosstalk, noise, and operational costs while simplifying packaging and cryogenic integration.
As the broader quantum industry marches toward the million-qubit era, this approach demonstrates that innovation in control architectures—not just qubit counts—is crucial to unlocking the power of practical, scalable quantum computers.
Future experiments
While global control schemes have long intrigued quantum theorists, practical implementation was hindered by issues like fidelity, overhead, and selectivity. The new architecture, however, serves as a promising rebuttal—proving it’s possible to build a scalable QPU with high fidelities and universal quantum logic.
The next step is further experimental validation, which will include gathering relevant data on key performance metrics to inform its future scale-up, as well as derisking the technology.
With fewer wires, lower costs, and less noise, this elegant and practical design is unlocking a more efficient path to scalable quantum computation. As a result, this architecture is uniquely positioned to bridge the gap between today's experimental prototypes and tomorrow’s fault-tolerant machines.
Marco Polini
Marco Polini is the chief scientific officer of Planckian and a full professor of condensed matter physics at the University of Pisa (Italy).