EMT/QIS: GaAs hole spins as qubits: Eliminating hyperfine interaction-induced decoherence Quantum Computing is an emerging area of science and engineering. Its broad goal is to harness the superposition of quantum states for computing purposes. The elementary building block of a quantum computer is the quantum bit (qubit). Quantum bits have been demonstrated in a wide variety of systems ranging from trapped ions, to electron spins in semiconductors, to superconductors. In order to be useful, a qubit must be properly initialized, measured, and coupled to other qubits. The quality of a qubit is characterized by a lifetime (T1) and a coherence time (T2). These time scales vary by several orders of magnitude from one system to another (e.g. trapped ion versus superconducting qubits). It is therefore customary to quote a quality factor, Q, which is the ratio of the coherence time to the typical gate operation time. We will build qubits based on hole spins in GaAs nanometer-size quantum dots. Hole spins are promising candidates since the hole wavefunctions have p-like orbitals. As a result, hyperfine interactions with the host crystal nuclei are expected to be negligible, leading to spin coherence times approaching the hole spin lifetime of 300 microseconds. Taking into account the typical gate operation time of ~150 ps, a hole spin qubit, if realized, could have a quality factor of nearly 2 million, well beyond the threshold for fault tolerance. The primary goal of this project is to accurately measure, and determine what limits, the quantum coherence times of hole spins in GaAs quantum dots.
Quantum computers may revolutionize the computing landscape by increasing the speed at which certain types of computation can be performed. Funds from this NSF award were used to develop quantum bits, the elementary building blocks of a quantum computer, using the spin degree of freedom of the electron. We developed new quantum devices, based on holes, and demonstrated reliable quantum point contacts. We also explored the use of silicon devices for quantum computing. Silicon has material properties that make it especially attractive for quantum computing. At the same time, it has been difficult to develop single electron silicon devices due to the complicated growth of Si/SiGe quantum well structures. We were able to demonstrate high mobility electron gases based on depletion mode and accumulation mode structures. We also demonstrated the fabrication of quantum dot devices that can be used to isolate a single electronic charge. Broader impacts: Several graduate and undergraduate students were supported by this award. They have been trained in highly technical electron beam lithography, low temperature measurements, and high speed electrical control. A senior graduate student graduated and is now a postdoctoral fellow at Harvard University. Three high school students were partially supported by this award. One of the students is now in electrical engineering at MIT, the other two students are now at Yale.
