Modeling long-lived holes in silicon quantum dots

April 10, 2023

(Nanowerk News) The theoretical model developed by three RIKEN physicists to optimize semiconductor nanodevices will be useful for improving quantum hardware (Physical Review Letter, “Noise-induced dephasing in silicon spin-hole qubits”).

An electron trapped in a semiconductor device offers a promising building block for a future quantum computer. Electrons have a property called spin, which, when measured, is in one of two states—much like the binary information, or bits, used in conventional computing. But due to its quantum nature, spin can exist in a superposition of both states. These quantum bits, or qubits, lie at the heart of quantum information processing.

Electrons, or their positively charged pairs known as holes, can be isolated in tiny semiconducting blobs called quantum dots. Figure 1: Illustration of a qubit in a silicon quantum dot. Optimizing the size, shape and geometry of the quantum dot results in longer hole spins. (Image: Tony Melov)

But the spins of electrons and holes only maintain their quantum states for a limited time. Interference, or noise, from the spinning environment can change the spin state. “Once a quantum state is assigned to a qubit, it immediately begins to fade,” explains Peter Stano of the RIKEN Center for Emergent Matter Science (CEMS).

This inevitable decay, or dephasing, is a fundamental and profound difference to classical information, which can be made permanent. Understanding dephasing is essential for developing methods of mitigating it, and thus aiding the design of large-scale quantum computers.

Now, Stano, along with CEMS colleagues Ognjen Malkoc and Daniel Loss, have theoretically modeled a hole trapped in a silicon quantum dot. Using this model, they demonstrated that the length of time a hole spin maintains its quantum state depends on the size and shape of the quantum dot and the magnetic and electric fields applied to it.

The team identified robust configurations of quantum dots by going beyond established theoretical models.

“Our results show that by carefully designing the quantum dot and by placing the electric and magnetic fields in a certain way, we can find the sweet spot where the silicon spin-hole qubits are most resilient to electrical disturbances,” said Stano.

This highlights one of the main advantages of spin qubits—they are largely immune to electrical noise, which is the strongest type of noise present in any semiconductor device.

But dephasing is only one of the design considerations when optimizing quantum dots for quantum information processing. The speed and reliability of reading, writing, and operating quantum information is also important.

“All of these aspects will have the same sensitivity to the quantum dot design,” said Stano. “Our goal was to exploit the sensitivity also seen here and optimize the spin-qubit design.”

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