First spin-entangled electrons on a chip

July 2, 2015

False color scanning electron microscope image of the device The two green spots are the quantum dots located in the gap between the two (red) electrodes.(credit: RIKEN)

A team from the RIKEN Center for Emergent Matter Science, along with collaborators from the University of Tokyo and University of Osaka, have successfully produced pairs of spin-entangled electrons and demonstrated, for the first time, that these electrons remain entangled even when they are separated from one another on a chip.

This research could allow information contained in quantum bits (qubits) to be shared between many elements on a chip, a key requirement to scale up the power of a quantum computer.

“We set out to demonstrate that spin-entangled electrons could be reliably produced,” said Russell Deacon. “So far, researchers have been successful in creating entangled photons, since photons are extremely stable and do not interact. Electrons, by contrast, are profoundly affected by their environment. We chose to try to show that electrons can be entangled through their spin, a property that is relatively stable.”

Entangled electrons in separate quantum dots

To perform the feat, Deacon and his collaborators began the painstaking work of creating a tiny device, just a few hundred nanometers in size. The idea was to take a Cooper pair (a pair of electrons that allows electricity to flow freely in superconductors) and get them, while tunneling — a quantum phenomenon — across a junction between two superconductor leads, to pass through two separate “quantum dots” (small crystals that have quantum properties).

“If we could detect a superconducting current,” Deacon said, “this would mean that the electrons, which can be used as qubits, remain entangled even when they have been separated between the quantum dots. We confirm this separation by measuring a superconducting current that develops when they split and are recombined in the second lead.”

The quantum dots, each around 100 nanometers in size, were grown at random positions on a semiconductor chip. This chip was examined using an atomic force microscope … [to select] around 20 devices. Of those just two worked.”

By measuring the superconducting current, the team was able to show clearly that the spin of the electrons remained entangled as they passed through the separate quantum dots. “Since we have demonstrated that the electrons remain entangled even when separated,” says Deacon, “this means that we could now use a similar, albeit more complex, device to prepare entangled electron pairs to teleport qubit states across a chip.”

According to Seigo Tarucha, leader of the laboratory that conducted the work, “This discovery is very exciting, as it could lead eventually to the development of applications such as quantum networks and quantum teleportation. Though it is technically difficult to handle, electron spin is a very promising property for these applications, as it is relatively free from the environment and lasts comparatively long. It could be combined with photons, by using the spin-entangled electrons to create photons that themselves would be entangled. This could allow us to create large networks to share quantum information in a widely distributed way.”

The open-access paper was published in Nature Communications.


Abstract of Cooper pair splitting in parallel quantum dot Josephson junctions

Devices to generate on-demand non-local spin entangled electron pairs have potential application as solid-state analogues of the entangled photon sources used in quantum optics. Recently, Andreev entanglers that use two quantum dots as filters to adiabatically split and separate the quasi-particles of Cooper pairs have shown efficient splitting through measurements of the transport charge but the spin entanglement has not been directly confirmed. Here we report measurements on parallel quantum dot Josephson junction devices allowing a Josephson current to flow due to the adiabatic splitting and recombination of the Cooper pair between the dots. The evidence for this non-local transport is confirmed through study of the non-dissipative supercurrent while tuning independently the dots with local electrical gates. As the Josephson current arises only from processes that maintain the coherence, we can confirm that a current flows from the spatially separated entangled pair.