GaAs device could play quantum logic role
Academics at Cambridge University have produced the first GaAs-based quantum cellular automata (QCA) cell, which could be used to store quantum information, in the form of a quantum bit, or qubit.
“There is a whole computing architecture predicated on getting such a QCA cell to work and this is a demonstration that it can be achieved in a GaAs heterostructure,” said Charles Smith, in whose group the work was performed.
Today s transistor-based bits are somewhat different to the qubit. Like a regular bit, a qubit can take an “on” or “off” state, but it can also exist in a form which is a combination, or superposition, of both.
These three possible states can be used to encode information which, when stored in multiple qubits, are quantum-mechanically interrelated in a manner that has no analog in conventional computing.
This entanglement theoretically offers exponential performance benefits compared with existing computers, and consequently there has been a proliferation of proposed candidate qubits. Some examples include polarization based photon systems and spin or charge-based electron systems.
Described in the July 16 edition of Applied Physics Letters, the QCA cell comprises layers of GaAs and AlGaAs, deposited by MBE, on top of which quantum dots are made using electron-beam patterned Ti/Au gates.
On application of negative voltages small regions under the gates are depleted of electrons. These regions then act as quantum dots.
Four such quantum dots are arranged in a square configuration, forcing electrons to occupy opposite corners of the square due to Coulomb repulsion.
Simpler and more robust
The electrons may thus occupy either of the two diagonals, or inhabit the superposition of these two states simultaneously. These possibilities allow the QCA to behave as a qubit, rather than simply as a bit.
Quantum dot pairs have previously been demonstrated as an example of qubits that use electronic charge to store information, but in the QCA cell the interaction of the four dots suppresses information loss from unwanted external interactions more effectively.
QCA cells have been demonstrated in other materials, including phosphorus-doped silicon, but GaAs simplifies the system down to a single electron per dot.
“This removes the problem of complex electron-electron interactions within each dot,” commented Smith.
“Being able to get to the last electron allows interactions to be studied involving spin.”
The Applied Physics Letters paper suggests this QCA design could be scaled up to more complex logic operations, offering computation based on spin or charge.
A quantum computer needs only 10000 interacting qubits to perform computation far exceeding that of a conventional computer, according to Smith.
Each dot in his group's device is 30 nm in diameter, meaning that a quantum computer based on GaAs QCAs could fit in an area just 10 microns square - but the current need to operate such a device at only 4 K will inevitably pose a significant limitation.
“A quantum computer would be a high value product where the cost of the component would not be in the material,” said Smith, “but would be in the cryogenics and electronics required to run it.”
