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Quantum Information and Computation

Fabrication of the structure for qubits using electrons on liquid helium

John M. Goodkind, Subrahmanyam Pilla

Published Dec 1, 2001• 7 citations
Publisher

Abstract

In the previous papers, the system of qubits using electrons on a liquid helium film was described. In this paper we describe the physical realization of the system that we have begun to fabricate. We will not in this brief discussion describe how we intend to operate the system. We will show that we are nano- and micro-fabricating a new type of electronic device that differs from other microelectronic devices in that the final step of the fabrication deposits a layer of helium rather than some other dielectric. The operation of the device will differ in that it manipulates single electrons and it must operate at low temperatures.

DOI
10.26421/qic1.s-11

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In the previous papers, the system of qubits using electrons on a liquid helium film was described. In this paper we describe the physical realization of the system that we have begun to fabricate. We will not in this brief discussion describe how we intend to operate the system. We will show that we are nano- and micro-fabricating a new type of electronic device that differs from other microelectronic devices in that the final step of the fabrication deposits a layer of helium rather than some other dielectric. The operation of the device will differ in that it manipulates single electrons and it must operate at low temperatures.
A quantum computer based on the manipulation and detection of coherent states of electrons localized above a helium film is described. Each quantum logic element (qubit) is made of a combination of the ground and first excited states of an electron trapped in the image potential well at the surface. Potentials applied to micro-electrodes located beneath each electron confine the lateral motion of the electron and are used to operate gates. Mechanisms for one- and two-qubit logic operations and a readout of the final state are discussed. The principle decay channel for the excited state is via the emission of one phonon into the bulk liquid. Dechoherence times are calculated to be of order of 100 μs and allow greater than 104 serial operations.
It has been proposed in literature that a quantum computer can be made utilizing the electronic states of electrons bound to the surface of liquid helium. One can model a single electron on the surface as a 1d hydrogenic atom, providing a set of quantum electronic states which are easily tunable, with an untuned energy gap of 0.488\\,meV (̃120 GHz) between the ground state and the first excited state, and it is these two energy levels that are proposed as the 0 and 1 state of a qubit. To that end, three microfabricated devices are needed: a low temperature electron source of low energy electrons, a detector capable of detecting single electrons, and a microstructure capable of trapping and Stark shifting the energy levels of individual electrons in proximity close enough to perform multiple qubit operations. This dissertation contains a description of the devices microfabricated for these purposes. An electron source based on porous silicon was fabricated, tested, and proven to provide low energy electrons. Other more conventional techniques based on a thoriated tungsten filament were also explored. For electron detection, we have fabricated a transition edge superconducting microbolometer. Tests have shown it is capable of detecting a few eV of energy. For the microstructure, we fabricated a series of columns 200 nm in diameter, 1.5[mu]m in height, separated by 500 nm. For later tests, a microelectrode exposed through a 10[mu]m diameter hole in a ground plane was used. Initial experiments describing bolometer designs and electron confinement are discussed, as well as proposed microfabrication redesign to continue this work. Numerical time series computations of both single (NOT, PHASE) and two qubit (SWAP, [square root]SWAP, CNOT) gates are also presented
Electrons trapped above cryogenic substrates such as liquid helium and solid neon form exceptionally clean physical systems, since they reside in vacuum and are largely isolated from environmental noise. A recent breakthrough has been the realization of charge qubits using electrons on solid neon surfaces, with coherence times reaching T2 = 2T1 ≈ 100 µs. In this article, we describe in detail the preparation of cryogenic substrates, electron deposition and the measurement of electron mobility. We further discuss the differences in surface morphology between helium and neon, their impact on mobility and qubit performance, and emphasize the necessity of systematic surface characterization as a means to optimize cryogenic substrate preparation and electron deposition.
Surface‐state electrons on liquid helium, have been proposed as condensed matter qubits. We have demonstrated that small numbers of electrons, including a single isolated electron, can be held in an electrostatic trap above the surface of superfluid helium and individually detected using a superconducting single‐electron transistor (SET). A new design for a quantum information processor, using an array of electron traps on liquid helium, has been fabricated on a Si wafer using a superconducting A1 SET.