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Quantum computing

Quantum computing

Physicists break distance record for electron spin-state transmission in spin qubits

10 Oct 2019 Shi En Kim 
Electron spin state transfer via swapping in a linear spin qubit array. Credit: John Nichol

For quantum computers to be feasible, quantum error correction is necessary to protect the information in qubit arrays even if individual qubits become corrupted. Its implementation requires that multiple qubits can interact with one another.

Researchers at the University of Rochester and Purdue University, US, have demonstrated the ability to manipulate the interactions between electron spin qubits in the form of spin swapping between electron pairs. They were able to transmit electron spin states with single-electron precision in a linear array of spin qubits – a significant step towards realizing fault-tolerant quantum information processing.

A spin on quantum state transmission

Led by John Nichol at Rochester, the researchers successfully demonstrated coherent spin-state transfer along an array of four electrons confined in a quadruple quantum dot in a GaAs/AlGaAs heterostructure. When they applied a voltage pulse to a gate between two quantum dots, the electrons in the dots exchanged their spin states via Heisenberg exchange coupling, which occurs when the wavefunctions of neighbouring electrons overlap. By applying a series of voltage pulses to specific gates, the researchers were able to shuttle the spin states of the electrons back and forth. They were also able to transmit the spins of entangled electrons using the same technique.

“Spin swapping in electron pairs has been known for some time, but we were the first to experimentally apply it to long-distance quantum information transfer,” said Nichol. His team demonstrated the transmission of spins in four electrons, breaking the previous record of two.

These demonstrations are significant in two ways. Firstly, the process of spin transfer by Heisenberg exchange coupling is scalable to larger spin qubit arrays. Secondly, the researchers were able to transmit spins without having to move a single electron. This is akin to a row of rugby players standing in a row and swapping their jerseys without moving. However, unlike jerseys, an electron cannot be stripped of its spin. After all, spin is a fundamental property of the electron. Yet Nichol et al. were able to shuffle spin states as if they were physical objects to be moved around arbitrarily – the spins had become independent of the parent electrons.

A promising outlook

There is no theoretical limit on how far spin transmission can reach. Nichol and his group are planning to go to greater lengths – literally – and incorporate more electrons in the same GaAs/AlGaAs system.

Additionally, they aim to recreate these observations in silicon spin qubits. Unlike GaAs/AlGaAs, silicon can be isotopically purified to remove nuclear spins, which can serve as a source of magnetic noise. Nichol predicts that the transmission of electron spins in silicon qubits can be conducted with much greater fidelity, propelling humankind towards a quantum future.

“Having the capability to transmit quantum states paves the way for more bizarre tricks such as quantum teleportation – the instantaneous transfer of quantum information from one physical location to another,” said Nichol. “Our coherent spin state transfer should in principle allow us to do that, which is pretty exciting.”

Full details of this research are reported in Nature.

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