Advanced complementary metal–oxide–semiconductor (CMOS) technology is an attractive platform for delivering such interfaces. Scaled-up quantum computers will require control interfaces capable of the manipulation and readout of large numbers of qubits, which usually operate at millikelvin temperatures. High-fidelity control and readout of a superconducting qubit is performed with a low-noise optical fibre link that delivers microwave signals directly to the millikelvin quantum computing environment. Leveraging the low thermal conductivity and large intrinsic bandwidth of optical fibre enables the efficient and massively multiplexed delivery of coherent microwave control pulses, providing a path towards a million-qubit universal quantum computer. By demonstrating high-fidelity control and readout of a superconducting qubit, we show that this photonic link can meet the stringent requirements of superconducting quantum information processing7. Here we introduce a photonic link using an optical fibre to guide modulated laser light from room temperature to a cryogenic photodetector6, capable of delivering shot-noise-limited microwave signals directly at millikelvin temperatures. The complexity and heat load associated with the multiple coaxial lines per qubit limits the maximum possible size of a processor to a few thousand qubits5. In superconducting quantum processors4, each qubit is individually addressed with microwave signal lines that connect room-temperature electronics to the cryogenic environment of the quantum circuit. Our results demonstrate universal gate fidelity beyond the fault-tolerance threshold and may enable scalable silicon quantum computers.ĭelivering on the revolutionary promise of a universal quantum computer will require processors with millions of quantum bits (qubits)1–3. We realize Deutsch–Jozsa and Grover search algorithms with high success rates using our universal gate set. We identify the qubit rotation speed and coupling strength where we robustly achieve high-fidelity gates. Here we demonstrate a two-qubit gate fidelity of 99.5 per cent, along with single-qubit gate fidelities of 99.8 per cent, in silicon spin qubits by fast electrical control using a micromagnet-induced gradient field and a tunable two-qubit coupling.
![quantum error correction threshold for surface code quantum error correction threshold for surface code](http://www.inference.org.uk/qecc/QSUMMARYUBNM4D.png)
Electron spin qubits in silicon7–15 are particularly promising for a large-scale quantum computer owing to their nanofabrication capability, but the two-qubit gate fidelity has been limited to 98 per cent owing to the slow operation¹⁶. Among the many qubit platforms, only superconducting circuits⁴, trapped ions⁵ and nitrogen-vacancy centres in diamond⁶ have delivered this requirement.
![quantum error correction threshold for surface code quantum error correction threshold for surface code](https://i1.rgstatic.net/publication/45909775_Surface_code_quantum_error_correction_incorporating_accurate_error_propagation/links/0fcfd5111ae6e0560d000000/largepreview.png)
One of the most promising error correction codes is the surface code², which requires universal gate fidelities exceeding an error correction threshold of 99 per cent³.
![quantum error correction threshold for surface code quantum error correction threshold for surface code](https://images.squarespace-cdn.com/content/v1/5d52f7bd9d7b3e0001819015/1597229790415-JMQ4KVONUIMLREF2EQX8/ke17ZwdGBToddI8pDm48kDFmBK5yOg9djtqEo39X98JZw-zPPgdn4jUwVcJE1ZvWEtT5uBSRWt4vQZAgTJucoTqqXjS3CfNDSuuf31e0tVHABVMZwSBH_fazBg6zr-3J7KS69AlX-gubHxi9CZbX820nsU3dfn6w--du8-EjPUE/2020-08-12+11_56_13-Window.png)
The JoFET amplifier completes the suite of semiconductor-based options for quantum control, information processing, and readout.įault-tolerant quantum computers that can solve hard problems rely on quantum error correction¹. In contrast to metallic superconducting amplifiers, our device is compatible with magnetic fields. Accordingly, we demonstrate a total added noise that approaches the fundamental limits placed by quantum mechanics. The gain is sufficient for integration into a measurement chain with conventional semiconductor amplifiers. The JoFET amplifier has 20 dB of gain with a 4 MHz instantaneous bandwidth, and a resonant frequency that is tunable over 2 GHz via the field effect. Here, we demonstrate a quantum-limited amplifier using a Josephson field-effect transistor (JoFET).
![quantum error correction threshold for surface code quantum error correction threshold for surface code](https://image.slideserve.com/540852/quantum-errors-l.jpg)
A semiconductor-based solution for the crucial task of quantum-limited amplification is, however, conspicuously absent. More broadly, the inherent scalability of semiconductors has motivated a great deal of research on quantum applications, including the scalable generation of quantum-control signals, and the processing of quantum information at fault-tolerant thresholds. Whereas later parts of the chain are dominated by semiconductor-based devices, the quantum-limited step can currently only be performed using metallic superconductors. Quantum-limited amplifiers are the first link in the quantum signal processing chain, allowing minute signals to be measured by noisy, classical electronics.