Portland, Ore. The world's first proof of concept for quantum entanglement in 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, a 12-year veteran of AT&T Bell Laboratories' Microstructure Physics Research Department before joining Purdue, performed the work with Jeng-Chung Chen, who received his doctorate at Purdue and is now a postdoctoral associate at the University of Tokyo under the supervision of professor Susumu Komiyama. Also contributing was Michael Melloch, a professor in Purdue's School of Electrical and Computer Engineering, and Purdue doctoral candidate Heejun Jeong.
In 2001, while a professor at Purdue University, Chang demonstrated a serial quantum-dot system (see www.eetimes.com/story/OEG20010924S0101) and predicted he would demonstrate quantum entanglement in two to five years a promise he fulfilled with his recent demonstration.
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 lets Chang's design selectively entangle two parallel quantum states, realizing a mechanism in which future quantum gates can operate. 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 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."
Under quantum computational theory, a quantum dot's exact state cannot be determined because of the simultaneous superimposed logic states. But the results can be read out after the computation, enabling problem-solving procedures that would be intractable for any digital computer, like uncrackable encryption codes.
Quantum computer next?
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.
The quantum properties of electrons allow information to be encoded in q-bits that maintain a "superposition" of logic states, until a final read operation fixes their value. In this way, a q-bit could simultaneously represent the values of both 1 and 0, while doing calculations that would ordinarily require separate steps for each. The result is selected from among the simultaneous calculations by making an observation of the final state of the system.
Instead of an electrical charge, q-bits represent the spin state of the electrons residing inside a quantum dot. A value of 1 or 0 is assigned to a single electron (or an odd number of electrons), depending on whether the spin at readout is up or down. Traditional computers ignore electron spin and merely assign a 1 when there are enough electrons to drive a signal's voltage above 2.4 volts (in a 5-V system) or 0, when the voltage drops below 2.4 V.
Since quantum mechanics teaches that electrons are not just particles, but also have the properties of waves, then both up and down spins can be simultaneously represented on the electron waveform, thereby allowing a single electron to simultaneously represent both 1 and 0 to varying degrees. With proper initial conditions, this nebulous state can be maintained throughout a calculation (called the coherence period), which only needs to be resolved at its end, when the final result is read out.