# Quantum entanglement demonstrated, quantum gate, computer could be next

PORTLAND, Ore. — The world's first proof of concept for quantum entanglement within a semiconductor was reported recently by Albert Chang, an adjunct professor at Purdue University who recently moved to Duke University.

Chang next plans to build the world's first quantum gate in an electronically controlled semiconductor device, enabling the creation of a key building block in quantum computation.

"We were able to obtain the first direct evidence for spin entanglement in a coupled double-quantum-dot system," said Chang. "This is why we are so excited about this new result."

Chang was a 12-year veteran of AT&T Bell Laboratories' Microstructure Physics Research Department before joining Purdue University.

While a professor at Purdue University in 2001, Chang demonstrated a serial quantum-dot system and predicted he would demonstrate quantum entanglement in two to five years — a promise he fulfilled with his recent demonstration.

Next, Chang predicts he will demonstrate the world's first electronically controlled semiconductor quantum gate within two years. This prediction is based on his new parallel configuration of two gallium arsenide quantum-dot transistors with a common source and drain.

"We have gone to a parallel geometry," said Chang. "They have a common source and drain and that has enabled us to tune the coupling — the interaction — between two quantum dots while maintaining the ability to detect the spin of the system."

This tuning enables Chang's design to selectively entangle the two parallel quantum states, thereby realizing a mechanism whereby future quantum gates can operate on quantum bits, or q-bits. Quantum computers operate on q-bits, rather than digital bits, which can only be "0" or "1," because q-bits can superimpose any number of parallel computations.

Q-bits can take on more than one value simultaneously because each q-bit uses an odd number of electrons — ideally a single one — and each one has an "up" or "down" spin. (The odd-number requirement ensures that the total quantum spin of the whole quantum dot is nonzero.)

"You need to have an odd number of electrons on each quantum dot, because that is what defines a q-bit," said Chang. "With an odd number of electrons you have a net spin on each quantum dot, which is the q-bit."

According to quantum computational theory, the exact state of the quantum dot cannot be determined because of the simultaneous superimposed logic states. However, the results of the computation can be read out after the computation is done, thereby enabling problem-solving procedures that would be intractable for any digital computer, such as uncrackable encryption codes.

While in their superimposed state, the dots perform computations by affecting each other at a distance, that is, by becoming entangled. Now that Chang can essentially turn entanglement on and off, his semiconductor architecture, which uses standard fabrication techniques, can potentially be used to build quantum gates. When used together, those gates could create a quantum computer.