QuTech researchers, a partnership between the Delft University of Technology and TNO, have achieved six silicon-based spin qubits created in a fully interoperable array for the first time in history. Notably, the qubits reduce error rate thanks to a new chip design, an automated calibration procedure, and qubit initialization and readout methods. These developments will help create a silicon-based, scalable quantum computer with intrinsic Qubits constructed from silicon.
Silicon-based Qubits
Qubits can be produced from various materials. Still, no one is certain which material will be the most effective for creating a large-scale quantum computer. Only small silicon quantum devices with excellent qubit operations have been demonstrated thus far.
QuTech researchers, led by Prof. Lieven Vandersypen, have created and run a six-qubit processor with an emphasis on:
- Careful Hamiltonian engineering.
- A high level of abstraction for programming the quantum circuits.
- Efficient background calibration.
All of which are required to achieve high fidelities. This advancement will allow for the testing of more important quantum protocols and will be a significant step toward large-scale quantum computers.
Electron spin qubits in semiconductor quantum dots hold promise for practical large-scale quantum processing because of their scaling potential due to their tiny size, long-lived coherence, and compatibility with modern semiconductor fabrication processes. However, spin qubits are now out of scale when compared to superconducting, trapped ions, and photonic platforms, all of which have proven control of dozens of qubits.
QuTech scientists studied a system of six spin qubits in a linear quantum dot array. They tested its performance using techniques like multi-layer gate patterns for independent control of the two-qubit exchange interaction and micromagnet gradients for electric-dipole spin resonance (EDSR) and selective qubit addressing.
The multi-layer gate arrangement allows for precise control of the charge occupation of each quantum dot and the tunnel couplings between quantum dots. These characteristics are individually controlled by virtual gates, which are linear combinations of gate voltages.
Embodying Qubits in Silicon
A silicon chip, an essential element in every computer chip, is used to create the quantum dot array. A qubit is defined by a quantum mechanical feature called spin, and the orientation of the qubit determines whether it is in the 0 or 1 logical state, binary.
The team manipulated and measured the spin of individual electrons and made them interact with one another using precisely calibrated microwave radiation, magnetic fields, and electric potentials. They focused on short measurement cycles combined with high-fidelity readout when creating the qubit measurement system, which speeds up testing all other parts of the experiment.
QuTech researchers utilized Pauli Spin Blockade (PSB) to probe the parity of the two spins (rather than discriminating between singlet and triplet states) to measure the outer qubit pairs, taking advantage of the fact that the T0 triplet relaxes to the singlet well before the end of the 10 μs readout window.
“The quantum computing challenge today consists of two parts,””Developing qubits that are of good enough quality, and developing an architecture that allows one to build large systems of qubits. Our work fits into both categories. And since the overall goal of building a quantum computer is an enormous effort, I think it is fair to say we have made a contribution in the right direction.”
Mr. Stephan Philips.
The spin of the electron is a sensitive quality. The error rate rises due to tiny variations in the electromagnetic environment’s spin direction. The QuTech team developed new techniques for preparing, managing and interpreting the spin states of electrons based on their prior knowledge in constructing quantum dots. With this new qubit configuration, they could instantly entangle systems of two or three electrons and build logic gates.
Intel the Chip Giant And Silicon Qubits
Intel has been working on quantum computing with QuTech in the Netherlands. In 2017, they presented a novel superconducting chip to Intel’s research partner in the Netherlands, QuTech, employing improved material science and manufacturing processes. They also developed the Horseridge II, a cryogenic control chip, which according to Intel, has improved capabilities and better degrees of integration, enabling elegant control of the quantum system.
Their team in Oregon and Arizona have developed a method to produce 17-qubit circuits with an architecture that improves reliability at higher temperatures and reduces RF interference between each qubit.
Intel is concentrating on developing spin qubits or quantum dot technology since spin qubits have a significant advantage over superconducting qubits due to orders of magnitude less die area per qubit.
Intel and QuTech published the results of their work on silicon qubits in April of this year. Intel’s D1 production facility in Hillsboro, Oregon, manufactured the first silicon qubits at scale. They can now fit over 10,000 quantum dot arrays onto a single 300 mm wafer with over 95% yield. Using spin qubits also allows them to manufacture quantum dots in the same high-volume facility where they manufacture microprocessors.
Alternative Technologies to Silicon Qubits
Rigetti manufactures superconducting quantum processors for their hardware. Each superconducting qubit on the device comprises a non-linear Josephson inductance connected with an ultra-low-loss capacitor to form a resonant structure in the 3-6GHz range.
Aspen-M’s latest processor has two independent quantum processors, each with 40 qubits totalling 80 qubits. As a result, according to the company, Aspen-M delivers 250% quicker quantum processing speeds than previous-generation hardware and a 50% reduction in readout mistakes.
IonQ combines physical performance, perfect qubit replication, optical networkability, and highly optimized algorithms to produce a scalable trapped ion quantum computer that will support various applications across various industries. The qubits of IonQ are ionized ytterbium atoms, a silvery rare-earth metal. Every ytterbium atom in the cosmos is identical to every other ytterbium atom.
Superconducting qubits have been used to create quantum arrays with over 50 qubits. But the promise of a more straightforward transition from research to business makes silicon engineering infrastructure globally accessible. Before this work by the QuTech team, only arrays of up to three qubits could be constructed in silicon without compromising quality. Silicon presents significant technical obstacles.