By creating entanglement between quantum bits on distant chips, Prof. Ronald Hanson and his team have laid a basis for quantum information processing.
Nature describes the work by the Kavli-team as “towards the realization of a solid-state quantum network” and released their article as an advanced online paper. A “quantum network”, in their view, is a grid of multi-qubit registers linked by photonic channels.
Drawing on Hanson’s latest work, the outlines of the long elusive quantum computer are beginning to emerge. The long-lived qubits are diamond-based and housed in cryostats. Their electron-spins can be set by microwave pulses and read out with fibre-coupled lasers. Laser pulses can even couple distant qubits in an entangled state, a unique quantum resource for information storage, processing and communication.
When two particles are entangled, their quantum states are coupled in a way physics cannot explain. Measure the state of one of the particles, and the state of the other is determined immediately. Entanglement of electron spins on different chips is a novelty.
Prof. Hanson (Quantum Transport Group at Applied Sciences) has previously demonstrated the stability on nitrogen-vacancy (NV) defect centres in diamond as well as the possibility of shielding these qubits with microwaves, the ability to read them out and the induction of entanglement of qubits on a single diamond chip. This week’s news is the macroscopic distance of three meters between entangled qubits.
The lab setup entails two cryostats on opposite sides of the table, green and red lasers, a veritable wood of lenses, splitters and mirrors to control the photon flows and microwave equipment, which serves to control the qubits’ spins.
An entanglement attempt consists of bringing both qubits in a superposition of spin-up and spin-down states. A short 2-nanosecond laser pulse entangles each qubit with a photon. The photons from the qubits are combined onto a beam splitter and detected in two output ports. After the half-transparant mirror (or beam splitter) it’s impossible to say from which qubit the photon came. Quantum mechanics then states it came from both, making them entangled.
This is not always the case. Indeed, it’s a pretty rare event with a probability as low as 1 in 10 million. With an attempt rate of 20 kHz, one entanglement happens every ten minutes and it took a week to register 739 distant entanglements.
Even so, it’s the first time that entanglement between distant chip-based qubits has been produced and Hanson isn’t worried about the current success rate. What he sees is the future of quantum computing:
Hanson writes: ‘When combined with future advances in nanofabricated integrated optics and electronics, the use of electrons and photons as quantum links and nuclear spins for quantum processing and memory offers a compelling route forward towards the realization of solid-state quantum networks.’
And down that route he steadily goes, step by step.
H. Bernien, B. Hensen, R. Hanson et. al., ‘Heralded entanglement between solid-state qubits separated by 3 meters’, Nature AOP