WEST LAFAYETTE, Ind. Quantum-dot techniques have produced the first examples of quantum computing in a semiconductor at Purdue University. Using electron-beam lithography to deposit small metal islands over a gallium arsenide (GaAs) heterostucture interface, scientists created isolated regions that trap only a few electrons. More important, two of the dots were placed close enough for the team to observe quantum-spin interactions, a discovery that might lead to semiconductor-based quantum computers.
"The special thing about what we have been able to accomplish is to put two quantum dots together and observe an effect that is related both to the spin physics of the system and the interaction, or coupling, between the dots. That has never been done before," said lead researcher Albert Chang, a Purdue professor and a 12-year veteran of AT&T Bell Laboratories' Microstructure Physics Research Department. "This is field-opening work for implementing qubits [quantum bits] for quantum computation in a semiconductor-based system."
Chang was assisted by Michael Melloch, a professor of electrical and computer engineering at Purdue, and doctoral candidate Heejun Jeong.
In theory, tiny quantum dots can be used to create computers that fit hundreds of millions of qubits onto chips. While past research has shown that devices using electrical-field interactions basically the same physical mechanism that underlies standard FETs can be constructed, the new work's next step is to reveal actual "quantum entanglement," a fundamentally different principle, the researchers said.
The quantum properties of electrons would allow information to be encoded in qubits, leading to a more powerful form of computation. Qubit codes keep data in a "superposition" of states until a final read operation fixes their value. In this way, a qubit can simultaneously represent the values of both 1 and 0, while doing calculations that would ordinarily require separate steps for each. For instance, adding two qubits is equivalent to adding 1 + 1, 1 + 0, 0 + 1 and 0 + 0 in a single step. The result is then "read out" by making an observation on the final state of the system.
Rather than a charge, qubits represent the "spin" value of the electrons residing inside their quantum dots "up" or "down" to represent 1 and 0. Traditional computers ignore electron spin and merely assign a 1 when there are enough electrons to drive a signal's voltage above 2.4 V (in a 5-V system) or 0, when the voltage drops below 2.4 V. Qubits, however, can operate with a single electron, allowing it to represent a 1 when the spin is up and a 0 when it is down.
The spin of an electron is not fixed; quantum mechanics specifies only a varying probability of any given electron's spin being up or down, 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") and needs to be resolved only at its end, when the final result is read.
What the Purdue researchers have done is demonstrate that traditional GaAs fabrication equipment can be used to fashion quantum dots each representing a single qubit in domains as small as 50 nm in diameter (a nanometer is about 5 to 10 atoms wide). They did not, however, fabricate a complete single-electron transistor (SET) with its gate, source and drain connected to metal leads. Instead the group used the metal layer to fabricate electrodes that drove electrons out of the surrounding area, leaving behind quantum-dot domains. The resulting quantum dot itself was centered inside that domain, and measured between 50 and 120 nm, depending on the voltages applied to the electrodes.
"We have the smallest dots, and that's really crucial," said Chang. "The lithographic diameter of the dots is 180 nm, but after you energize the electrodes, the actual quantum dot which sits inside the semiconductor material underneath the surface of the chip could be 100 to 120 nm, or in extreme cases as small as 50 nm." Size matters, Chang said, because "the smaller they are, the closer to room temperature we can operate them at."
Without gate, source and drain leads, the researchers had to infer that their quantum dots were working by observing a related macroscopic phenomenon called the Kondo effect, which explains the enhanced resistivity, at low temperatures, of bulk materials with impurities.
For quantum dots operating as SETs, the Kondo effect suppresses conductance at low temperatures for a ground state with an even number of electrons. If the ground state has an odd number of electrons, then conductance through the dot can reach zero resistance due to the formation of a "puddle" of electrons that "screens" the spin of the dot. "By observing this kind of screening characteristic, we know that we have a spin effect happening inside the quantum dot," Chang said.
Chang said the Purdue work is "the first time that anyone has been able to use the Kondo effect to identify the spin state of each of the dots." With an even number of electrons on a dot, the spins cancel out in pairs, eliminating the spin effect. "In order to have it perform as a qubit you just need an odd number of electrons it doesn't matter if it's one, three or 31," Chang said. "But in order to know that you have a spin effect, you need to observe it through some spin-dependent phenomenon like the Kondo effect."
By using the Kondo effect, the researchers were able to verify that their quantum dots were manifesting the necessary spin state. Next, the team sought to verify that their two quantum dots were close enough together to interact in ways that could be useful for quantum calculations. Interaction between the dots was confirmed by measuring the differential conductance of the transistor as the bias voltage between the source and drain was varied.
In passing current through the quantum dot, the team observed that two peaks existed in the differential conductance of the SET as voltage was varied between the source and drain. An isolated SET would have only a single, zero-bias peak in the differential conductance. However, the researchers observed the splitting of this peak into two, thereby verifying that the quantum dots were interacting with each other.
"That kind of interaction will give you the means for doing the elementary quantum-computational process," said Chang.
The quantum dots were fabricated by spin-coating a GaAs substrate with a polymer, which was then exposed to an electron beam along the lines that would act as its electrodes. A solvent was used to remove the polymer, and a metal layer was deposited over the whole chip. Then, both the metal and the polymer were flaked off, leaving behind metal electrodes atop the GaAs substrate. The electrodes were then energized to drive electrons out of the substrate everywhere except where two quantum dots resided, leaving a pair of two-dimensional puddles containing about 21 to 41 electrons, isolated from the rest of the substrate.
"The quantum dots, which are just little puddles of electrons, sit beneath the top layer of the semiconductor," Chang said.
For the future, the researchers plan to put their chip to work by demonstrating that it can manifest quantum entanglement. If they can show entanglement, and if their measurements show that the dots are maintaining a coherence period long enough to do a quantum calculation, then they plan to build a real quantum computer, albeit one that performs a single operation. "We will try to implement the quantum exclusive operation," said Chang, who estimated that it will take several years to complete these next research steps.
"We are already making devices to look for entanglement, but we are looking at a two- to five-year timeline to make further significant progress," he said.
An audio recording of reporter R. Colin Johnson's full interview with Albert Chang can be found online at AmpCast.com/RColinJohnson.