Cryogenic Quantum Computing future: from mK to Mega-Qubits
Future Cryogenic Quantum Computing: A Comprehensive Scaling Solution Overview
APL Quantum reviews cutting-edge technologies needed to overcome hardware constraints and create classical interfaces to manage and control the next generation of large-scale cryogenic quantum computers.
Quantum processors could revolutionise computing like semiconductors did. Researchers and industry leaders face significant challenges in scaling up systems beyond the noisy intermediate-scale quantum (NISQ) era, where quantum computers are limited to hundreds of qubits controlled by large, noisy, room-temperature electronics. The development of classical technology to interface with quantum systems is important.
The article discusses cryogenic quantum systems, a leading quantum computer architecture. Dilution refrigerators are used to achieve millikelvin (mK) temperatures for these systems.
Limitations of Conventional Systems
Scaling quantum computers is essential for IBM's 100,000-qubit system. To scale, quantum decoherence, qubit quality, robust error correction, and hardware scalability must be addressed.
Standard designs place readout and control electronics several meters from mK-temperature qubits at ambient temperature. Due to its unmanageable size and power consumption, the standard microwave technique, which expands control and readout lines proportionally to qubits, is unsustainable.
Technology in the cryostat is limited by cooling power. At 20 mK, a high-powered cooling system can only deliver 30 μW of cooling power, limiting the size and power dissipation of internal classical interfaces.
Next Generation Cryogenic Quantum Computing Interfaces
The review covers several intriguing heat load, latency, and connectivity technologies:
CMOS Cryotech
This technology moves control and readout circuitry to cryogenic temperatures, usually 4 K or mK, to revolutionise quantum computing.
Moving electronics reduce signal delay, enabling quantum feedback and mistake correction.
FDSOI and bulk CMOS are the main technologies. Advanced FDSOI reduces cryogenic power consumption and leakage currents.
Problems: Google, Intel, and IBM have tried to combine cryo-CMOS control systems, but control lines must grow linearly with qubits. Qubit operation may be disturbed if heat dissipation surpasses cryostat cooling. Two to thirty mW is estimated for each physical qubit.
Single-Flux Quantum Logic
SFQ technology processes classical data using digital, superconducting Josephson junctions and magnetic flux quantisation.
Control: A series of coherent SFQ pulses activates qubits, with pulse spacing corresponding to qubit period. High anticipated fidelity (>99.99%). Scalability: SFQ controllers at the mK level eliminate most wires between the controller and qubit chip. Recent research shows that digital demultiplexing can save space and heat.
Due to its power efficiency, SFQ logic may be better for large-scale computing than CMOS. Power consumption per physical qubit is expected to be less than 1 nW. Using Cryo CMOS for Quantum Computing Scales Spin Qubits
Readout/Optical Control
By using optical fibres to convey messages, electro-optical systems reduce cryostat heat dissipation from coaxial cable.
Electro-optical modulators (EOMs) encode electrical signals onto optics. At cryogenic temperatures, photodiodes detect electrical signals.
Full qubit reading with radio-over-fiber down to mK temperatures eliminates the need for active or passive cryogenic microwave equipment.
Efficiency: Optic fibres replace coaxial driving and readout wires in optical communications. The main cause of energy loss is optical-RF conversion. Different conversion directions result in power dissipation ranging from <10 nW (Optical-RF) to <10μW (RF-Optical) per qubit.
Wireless readout/control
Wireless connectivity replaces cable connections in the QC system in this new research.
Wireless communication may reduce cryostat sizes by keeping big, complex circuits at ambient temperature. Signals travel 33% faster through refrigerator coils than coaxial connections, reducing latency.
Wireless communication supports hundreds of channels by bypassing refrigerator size qubit upscaling. Because they only have passive features like antennae and matching circuits, mK transceivers consume less power.
Efficiency: Wireless technology requires less than 1 nW per qubit, making it very efficient. THz cryo-CMOS backscatter transceivers reduce thermal heat in cryogenic environments through contactless connection.
Wireless connections reduce driving and readout wires, however flux lines may be needed for qubit tuning DC signals. Researchers must also address channel interference, uncertain signal propagation, and mK transceivers.
Future Perspective
These advanced control systems must be built soon for quantum computing to keep up with qubit production and quantum software advances. Creating a classical interface for globally applicable quantum computers is crucial. This scenario is similar to choosing the best roads, railroads, or fibre optics to connect a rapidly growing city (the quantum processor) to the administrative and power hubs (the classical control systems) without causing traffic jams or overheating the central districts.