An artist’s rendering of time multiplexing of control signals to a quantum computer. The control signals for single-qubit gates (short blocks) and two-qubit gates (long blocks) travel through common cables (tunnels) to switches, which distribute them among the qubits (spheres) based on switching signals (diamonds). By ordering the control signals in a clever way, akin to playing Tetris, traffic jams in the flow of control signals can largely be avoided and programs on the quantum computer can be executed almost as fast as if each qubit had its own cable for control signals.
Image Credit: Chalmers University of Technology/Boid
Scientific Frontline: Extended "At a Glance" Summary: Smart Cable Sharing in Quantum Computing
The Core Concept: Smart cable sharing (time-domain multiplexing) is a control architecture that allows multiple qubits to be operated sequentially via a single shared cable. This drastically reduces internal hardware requirements without significantly slowing down the system's computation time.
Key Distinction/Mechanism: In traditional quantum computing architectures, each qubit requires its own dedicated control cable (parallel control), which generates excess heat and takes up physical space. Smart cable sharing functions differently by utilizing time-domain multiplexing; it routes rapid, sequential control signals through shared cables down to microwave switches located directly next to the quantum processor to direct the signals to the correct target qubits.
Major Frameworks/Components:
- Superconducting Circuits: The foundational quantum hardware that must be cooled inside cryostats to near absolute zero (-273.15°C) to function properly.
- Time-Domain Multiplexing: The technique of sequencing control signals rapidly so that qubits do not require simultaneous, dedicated input.
- Microwave Switches: Rapid routing mechanisms installed directly next to the processor to distribute shared signals to individual qubits.
- Logarithmic Time Scaling: A critical mathematical finding from the research demonstrating that computational delay increases logarithmically—not linearly—as the number of qubits sharing a cable increases.
Branch of Science: Quantum Computing, Applied Quantum Physics, and Microtechnology.
Future Application: This multiplexing framework provides a clear path for scaling quantum hardware from hundreds of qubits to thousands. Such large-scale processing capabilities will be required to execute advanced algorithms in complex fields like molecular modeling for drug development, materials science, and global logistics optimization.
Why It Matters: The primary bottleneck to scaling superconducting quantum computers is the physical space and thermal radiation introduced by thousands of control cables. By implementing smart cable sharing, engineers can effectively bypass this hardware roadblock, representing a vital leap toward building fully realized, large-scale quantum computers capable of solving real-world societal challenges.
A major obstacle in the development of powerful quantum computers is the growing number of cables required to control a computer as the number of qubits increases. Researchers at Chalmers University of Technology in Sweden have now demonstrated that several qubits can share the same cable – without significantly increasing computation time. Their study is the most comprehensive of its kind and could become an important piece of the puzzle in developing quantum computers. These computers have the potential to revolutionize such areas as drug development and logistics.
The power of quantum computers lies in what are known as “qubits”. Unlike a conventional computer “bit”, which can have the value 1 or 0, a qubit can have the values 1 and 0 simultaneously – and all states in between, in any combination. This means a quantum computer with 20 qubits can simultaneously represent a combination of more than one million different states, resulting in enormous computational power.
“The global quantum technology race is in full swing, with tech giants currently in the lead with quantum computers based on more than 100 qubits. But to solve real-world societal challenges, quantum computers will need grow much further in size, with thousands or more well-functioning qubits,” says Anton Frisk Kockum, Associate Professor of Applied Quantum Physics at Chalmers University of Technology. At Chalmers, researchers have been developing Sweden’s largest quantum computer within the Wallenberg Centre for Quantum Technology.
Engineering challenges slow scaling
However, scaling up quantum computers comes with practical challenges. For many types of quantum computers to work – including those based on superconducting circuits – they must be cooled to temperatures close to absolute zero, that is -273.15°C. Cooling is achieved using helium in cryostats surrounding the quantum computer. To control quantum computations, signals are sent through cables from electronics outside the system to the cooled qubits inside it. But those cables emit heat that affects the temperature inside the cryostat, something which risks causing the qubits to lose their ability to continue the computation.
“Since each qubit currently requires its own cable, there’s a limit to how many qubits a system can contain before the temperature becomes too high, and the quantum computer stops working. There are also physical limitations, since the cables take up space in the cryostat,” says Ingrid Strandberg, a staff scientist in quantum technology at Chalmers.
Smart cable sharing challenges previous concerns
An alternative but relatively unexplored approach is to allow several qubits to share the same cable. Instead of controlling qubits in parallel with one cable for each, they are controlled sequentially in rapid succession using fewer cables. The process requires microwave switches to be installed next to the quantum processor to route each control signal to its target qubit – a procedure known as time-domain multiplexing.
However, the method involves a presumed trade-off. If qubits must “wait” their turn to receive signals, computations may take longer. To investigate how significant this delay really is, the researchers tested how different types of quantum processors are affected when the number of control cables is reduced. The results were surprisingly positive.
“We can see that for many common quantum algorithms; the number of cables can be drastically reduced without the computations becoming significantly slower or the runtime increasing very much. In some cases, such as the gates that connect two qubits, you can even share cables with no additional time cost at all, limited only by how the qubits are interconnected,” explains Marvin Richter, a PhD student in quantum technology at Chalmers and the study’s lead author.
Important step towards large-scale quantum computers
The Chalmers researchers’ computer simulations and mathematical analyses are the most comprehensive to have been conducted in this area. A particularly important conclusion from the results is that computation time increases logarithmically, not linearly, when individual qubits share cables.
“That's a slower increase than previously feared,” says Simone Gasparinetti, Associate Professor in Quantum Technology at Chalmers and co-author of the study. “Allowing multiple qubits to share cables could be an important step towards large-scale quantum computers. These results give us even stronger motivation to develop the necessary fast, low-dissipation microwave switches to implement this method.”
About the study: In the theoretical study, computer simulations were performed on quantum processors of different sizes, up to around 1,000 qubits. The main focus was on a processor with 121 qubits arranged in an 11×11 grid. In the study, the researchers varied the number of qubits per cable from one per cable and up to 121. In simulations of the largest systems, with up to 1,000 qubits, as many as eight qubits per cable were tested.
Funding: The study was funded through support from the Knut and Alice Wallenberg Foundation via the Wallenberg Centre for Quantum Technology (WACQT), the Swedish Foundation for Strategic Research, the EU Horizon Europe program, the OpenSuperQPlus100 project, the European Union and the European Research Council (ERC).
Published in journal: PRX Quantum
Title: Overhead in Quantum Circuits with Time-Multiplexed Qubit Control
Authors: Marvin Richter, Ingrid Strandberg, Simone Gasparinetti, and Anton Frisk Kockum
Source/Credit: Chalmers University of Technology | Lovisa Håkansson
Reference Number: qs041426_01
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