Fujitsu has built a working diamond-spin quantum computer prototype using tin-vacancy centres and photonic circuits to tackle one of quantum computing’s nastier scaling problems, which do not involve potentially dead or alive cats.
Writing in its bog, Fujitsu claims it is the world’s first working prototype combining tin-vacancy, or SnV, centres with photonic integrated circuits. Unlike superconducting quantum computers, which typically operate at about -273.13°C, Fujitsu’s machine runs at a relatively toasty -271.6°C, which should please any cat unsure whether it is dead, alive or merely freezing.
The prototype stems from research begun in 2020 between Fujitsu, Delft University of Technology and QuTech. Fujitsu wants to use the technology for modular quantum computers, linking separate quantum modules optically instead of squeezing an increasingly ridiculous number of qubits into one box.
Diamond-spin systems use defects in a diamond crystal, known as colour centres, as qubits. Fujitsu uses tin-vacancy centres, where a tin atom sits between two adjacent vacancies in the diamond lattice, rather than the better-known nitrogen-vacancy approach.
Fujitsu says SnV centres are less vulnerable to external noise and emit light roughly 10 times brighter than conventional NV centres. That makes optical links between quantum modules more practical, assuming the photons cooperate and don’t behave differently when someone looks at them.
The prototype integrates nanometre-sized diamond crystals containing SnV centres with alumina optical waveguides. Fujitsu had to bond tin-ion-implanted diamond substrates onto alumina and silicon dioxide, then thin the diamond from several hundred micrometres to several hundred nanometres before it could be used in the chips.
Controlling the system requires light, microwaves and radio-frequency signals. Fujitsu developed conversion technology that translates quantum circuits into the physical control sequences the hardware needs.
The machine can be accessed through the Fujitsu Hybrid Quantum Computing Platform, sparing users from worrying about the lasers, microwaves and frozen diamonds underneath. Previous Fujitsu and Delft work demonstrated two-qubit gate operations using electron and nuclear spins in nitrogen-vacancy centres with an error probability of less than 0.1 per cent.
Diamond’s attraction is its ability to preserve quantum states with high fidelity, potentially allowing logical qubits to be built from fewer physical qubits than some rival approaches. Optical links can generate entanglement between electron-spin qubits on separate chips or cryostats, leaving Schrödinger’s cat with even more boxes to be uncertain about.
Fujitsu corporate executive officer, corporate vice president and chief technology officer Vivek Mahajan said: “The diamond-spin approach we have applied in this prototype not only offers exceptional scalability in its own right, but also has the potential to be integrated with superconducting quantum computers to further extend their capabilities, enabling more complex and large-scale computations.”
Fujitsu plans a multi-module diamond-spin quantum computer prototype by 2027 and wants 250 logical qubits by fiscal 2030, rising to 1,000 logical qubits by fiscal 2035. By then, someone may finally have checked on the cat.
