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Bulletin of the American Physical Society

Exploiting silicon chip technology for control of electrons on superfluid helium.

F. R. Bradbury, Maika Takita, Kevin Eng, Thomas M. Gurrieri, Kathy J. Wilkel, S. A. Lyon

Published Mar 1, 2010• 0 citations
OpenAlex

Abstract

Electrons on the surface of superfluid helium have extremely high mobilities and long predicted spin coherence times, making them ideal mobile qubits. Previous work has shown that electrons localized in helium filled channels can be reliably transported between multiple underlying gates. Silicon chips have been designed, fabricated, and post processed by reactive ion etching to leverage the large scale integration capabilities of silicon technology. These chips, which serve as substrates for the electrons on helium research, utilize silicon CMOS for on-chip signal amplification and multiplexing and the uppermost metal layers for defining the helium channels and applying electrical potentials for moving the electrons. We will discuss experimental results for on-chip circuitry and clocked electron transport along etched channels.

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Electrons on the surface of superfluid helium have extremely high mobilities and long predicted spin coherence times, making them ideal mobile qubits. Previous work has shown that electrons localized in helium filled channels can be reliably transported between multiple underlying gates. Silicon chips have been designed, fabricated, and post processed by reactive ion etching to leverage the large scale integration capabilities of silicon technology. These chips, which serve as substrates for the electrons on helium research, utilize silicon CMOS for on-chip signal amplification and multiplexing and the uppermost metal layers for defining the helium channels and applying electrical potentials for moving the electrons. We will discuss experimental results for on-chip circuitry and clocked electron transport along etched channels.
Electrons bound to the surface of liquid helium are an emerging quantum computing platform, offering the potential for highly mobile spin qubits that can be manipulated using CMOS-fabricated devices. Here, as a step toward realizing this technology, we demonstrate selective two-dimensional shuttling of electrons across a helium film condensed on the surface of a CMOS control chip. The electrons are moved in packets containing, on average, several tens down to single electrons. We perform CCD-style electron shuttling in any of 128 transport microchannels, each of which links electron storage zones and sensing zones in the 2D plane. Shuttling sequences can be repeated at least 109^99 times with no detectable electron loss. The device serves as a prototype quantum information processing platform that is readily scalable to control large monolithically integrated arrays of single electron spins.
Unprecedented transport efficiency is demonstrated for electrons on the surface of micron-scale superfluid helium-filled channels by co-opting silicon processing technology to construct the equivalent of a charge-coupled device. Strong fringing fields lead to undetectably rare transfer failures after over a billion cycles in two dimensions. This extremely efficient transport is measured in 120 channels simultaneously with packets of up to 20 electrons, and down to singly occupied pixels. These results point the way towards the large scale transport of either computational qubits or electron spin qubits used for communications in a hybrid qubit system.
Electrons floating on the surface of superfluid helium have been suggested as promising mobile spin quantum bits (qubits). Transferring electrons extremely efficiently in a narrow channel structure with underlying gates has been demonstrated, showing no transfer error while clocking 10 9 pixels in a 3-phase charge coupled device (CCD). While on average, one electron per channel was clocked, it is desirable to reliably obtain a single electron per channel. We have designed an electron turnstile consisting of a narrow (0.8μm) channel and narrow underlying gates (0.5μm) operating across seventy-eight parallel channels. Initially, we find that more than one electron can be held above the small gates. Underlying gates in the turnstile region allow us to repeatedly split these electron packets. Results show a plateau in the electron signal as a function of the applied gate voltages, indicating quantization of the number of electrons per pixel, simultaneously across the seventy-eight parallel channels.
Unprecedented transport efficiency is demonstrated for electrons on the surface of micron-scale superfluid helium-filled channels by co-opting silicon processing technology to construct the equivalent of a charge-coupled device.Strong fringing fields lead to undetectably rare transfer failures after over a billion cycles in two dimensions.This extremely efficient transport is measured in 120 channels simultaneously with packets of up to 20 electrons, and down to singly occupied pixels.These results point the way towards the large scale transport of either computational qubits or electron spin qubits used for communications in a hybrid qubit system.
Electrons floating on the surface of liquid helium are possible spin qubits for quantum information processing. Varying electric potentials are not expected to modify spin states, which allows their transport on helium using a charge-coupled device (CCD)-like array of underlying gates. This approach depends upon efficient intergate transfer of individual electrons. Measurements are presented here of the charge transfer efficiency of few electrons clocked back and forth above a short microscopic CCD-like structure. A charge transfer efficiency of 0.999 999 92 is obtained for a clocking frequency of 800kHz.
Quantum computing promises new ways of solving some mathematical problems by making use of the state superposition properties of quantum bits (qubits) to compute in parallel all the results of a mathematical expression, which, in some cases, provides a computational advantage over classical computing. Various physical systems have been suggested as possible qubits. All of them have met significant challenges in their implementation into a quantum computer, in part because the usefulness of quantum computing is tied to its scalability. Electrons floating above superfluid helium are potential qubits because the exceptional lack of interaction with their surroundings should translate into extended quantum coherence times. Varying electric potentials are not expected to modify spin states, which allows their transport on helium using a charge-coupled device (CCD)-like array of underlying gates. That transport could move single qubits between memory, interaction and read-out gates that would constitute a quantum computer. The scalability of this scheme depends critically on efficient inter-gate electron transfer. This dissertation starts by presenting Shor's prime factorization algorithm as a motivation for quantum computing, justifying the use of electron spins on liquid helium as qubits and explaining the inner-workings of this type of quantum computer. But the essence of this work is experimental. Our setup had to be built from scratch. We then measured the inter-gate charge transfer efficiency (CTE) on thick (∼ 0.9 mm) helium and on helium in shallow (∼ 3 µm deep) channels. On thick helium, at low frequencies, we measured a CTE of 0.9990 (at a density of 4 electrons/µm2) that was governed by the diffusion of electrons through our millimeter-size CCD gates. In channels (12 × 10 µm gates), the CTE was found to be 0.99999992 ±6 × 10-8 for about one electron per gate and probably limited by shallow fabrication-related potential traps. This ability to reliably clock few charges for long distances is an important step towards a proof of the scalability of electron spins on helium as a quantum computing scheme.