Quantum entanglement in spin qubits demonstrated for the first time

May 17, 2012

A double quantum dot. Each electron spin (SL or SR) defines one quantum bit (qubit) (credit: D. Loss and D. P. DiVincenzo)

Harvard scientists have demonstrated quantum entanglement between two spin qubits for the first time, using electrostatic interaction between electrons.

Spin qubits are tiny droplets of free electrons created within quantum dots, small devices fabricated from standard semiconductor materials.

Spin qubits have several advantages as elements in a quantum computer: they operate at room temperature (unlike other devices, which require extremely low temperatures and expensive support systems), they can be fabricated using standard photolithography techniques (for mass production), and the spin is easily controlled, using the electron spin (up-down) to act as the qubit.

However, environmental fluctuations can cause entanglement between the qubits to degrade because the electrostatic interaction is weak, said Amir Yacoby, Harvard University professor of physics and applied physics, who led the research.

His team overcame this problem by allowing the qubits to interact for a precise amount of time, then flipping them, which causes them to return to their initial state. This maintains the delicate relationship between the qubits while overcoming outside disturbances from the external environment.

But it takes time to create that entanglement, and during that time, various elements are trying to interact with the individual qubits, which cause them to lose their information, he said. “It took some creative thinking to design a system that would allow their entanglement to accumulate, but would limit their interaction with the rest of their environment.”

This method of quantum error correction has been used to correct disturbances in other systems — in superconducting qubits, for example — but this is the first time it has been demonstrated in spin qubits.

Ref.: M. D. Shulman et al., “Demonstration of Entanglement of Electrostatically Coupled Singlet-Triplet Qubits,” Science, 2012 [DOI: 10.1126/science.1217692]