NC State University

Quantum Computing/Information Processing

The primary goal is to identify the most suitable physical system/implementation for quantum computing/information processing and analyze/design the optimum structure. Our proposed design is based on a linear array of quantum dots that are defined by metal electrodes on silicon with voltage applied so that a single electron is trapped at each quantum dot at low temperature. Placed in an external magnetic field, the Zeeman spin states of these electrons constitute the qubits of the quantum computer. The proximity of the quantum dots assures exchange coupling between electron spins (qubits) on neighboring dots. To explore the feasibility of this structure, we are pursuing the following issues: (1) development and use of 3D models to explore design of optimum/robust QD (i.e., qubit) structures as well as the spin-SET detector, (2) description for the generation of the dynamic electric and magnetic fields for address operation, and (3) investigation of mechanisms for the loss of spin coherence.

ECE Nano Image - Cross section of two QDs

Our embodiment of a nanoscale quantum information system (QIS), illustrated in Figure 1, relies on trapping a single electron within each quantum dot (QD) of a linear array of quantum dots and has the following attributes:

  • Device specifications are: quantum dot contact diameter of 20-40 nm, separation between quantum dots of ~50-90 nm. Each QIS may have ~ 103 QDs;
  • This linear array of electron spins in a semiconductor chip, placed in a microwave cavity in an external magnetic field, constitutes the qubits of a quantum computer;
  • Close proximity of the quantum dots enables nearest neighbor exchange coupling, whose magnitude is dynamically controlled by electrical potential applied to the elements of an overlying interdigitated linear electrode array (see Fig. 1):
  • A linear chain of exchange coupled quantum dots defines each QIS that could then be duplicated many times within a larger array of quantum dots so that a large number of identical quantum computer replicas are formed;
  • Unique addressing of each electron spin (qubit) is accomplished simultaneously in every member of the replicas by momentary magnetic bias fields produced by selective address currents in an interconnect array patterned on the chip (see Fig. 1);
  • The QIS is operated by a programmed sequence of microwave pulses, each executing a unitary transformation, delivered synchronously with address pulses to the chip in a microwave cavity (in an external magnetic field) resonant at the Zeeman frequency of the addressed qubits;
  • At the conclusion of algorithm execution, either an electron spin-echo measurement detects the final state, in the case of a large number of identical QISs, or a single electron transistor (SET) spin-detector (spin-SET) measures the spin orientation of the end element of the QIS, in which case only a single linear-array QIS is required. The spin-echo measurement requires a large number of QISs solely to enhance signal to noise, and is not an ensemble measurement as for NMR quantum computers operated in solution at room temperature. The quantum computer initial state is a pure state since our QIS is operated at ~ 1K with Zeeman splitting ~ 100 GHz. Spin-echo detection, although more difficult, has the advantage of being phase sensitive and qubit addressable, while the SET spin-detector device is not, therefore requiring special consideration in the sequence execution of the algorithm.
  • Both the single qubit and coupled-qubit compute rate are programmable and the T2 parameter appears to easily meet Preskill's [1] condition for error correction. This type of design with acceptable coherence appears to be readily scalable since it relies only on established integrated circuit (IC) fabrication techniques. All other non-spin-based quantum computer possibilities may suffer from a lack of possible commercial manufacturing as well as inadequate coherence. Additionally, this design also appears to be extendable to include quantum memory where information is stored in the spin of an array of nuclei coupled at the periphery or throughout the QIS array. Information is transferred to the nuclear spin via the hyperfine coupling of the electron spin in the neighboring quantum dot. The long T2 of the nuclear spin (as long as several hours) enables storage and retrieval of quantum information.
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