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Quantum Information Processing

Quantum computation with electrons trapped on liquid Helium by using the centimeter-wave manipulating techniques

Yufen Li, Suirong He, Miao Zhang, Lian-Fu Wei

Published Aug 5, 2024• 2 citations
Publisher

Abstract

Abstract Surface-state electrons floating on liquid Helium have been served as one of the great potential experimental platforms to implement quantum computation, wherein the qubits are usually encoded by either the lowest two levels of the vertical vibrations (i.e., Hydrogen-like atoms) or the electronic spins. Given the relevant operations require additional techniques, such as the corresponding millimeter-wave or magnetic field manipulations, here we investigate how to implement the scalable quantum computation with a trapped electron array by alternatively using the usual centimeter-wave manipulating techniques. This is because the eigenfrequency of the present qubit, encoded by the two lowest levels of the lateral vibration of the trapped electron, is limited in the centimeter-wave band. We show that, by biasing the electrodes properly and driving the coplanar waveguide transmission line resonator, the electrons can be individually trapped in a series of anharmonic potentials on liquid Helium. Therefore, the well-developed circuit quantum electrodynamics technique for the implementation of superconducting quantum computation can be conveniently utilized here in the present quantum computing platform (proposed firstly in Phys Rev Lett 105:040503, 2010, to implement the fundamental logic gates, typically such as the single-qubit rotations of the individually addressable trapped electrons, the switchable two-qubit manipulations between the electrons trapped in the distant traps, and also the high-fidelity readouts of the target qubits. The feasibility of the proposal is also discussed by numerical simulations.

DOI
10.1007/s11128-024-04498-4

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Abstract Surface-state electrons floating on liquid Helium have been served as one of the great potential experimental platforms to implement quantum computation, wherein the qubits are usually encoded by either the lowest two levels of the vertical vibrations (i.e., Hydrogen-like atoms) or the electronic spins. Given the relevant operations require additional techniques, such as the corresponding millimeter-wave or magnetic field manipulations, here we investigate how to implement the scalable quantum computation with a trapped electron array by alternatively using the usual centimeter-wave manipulating techniques. This is because the eigenfrequency of the present qubit, encoded by the two lowest levels of the lateral vibration of the trapped electron, is limited in the centimeter-wave band. We show that, by biasing the electrodes properly and driving the coplanar waveguide transmission line resonator, the electrons can be individually trapped in a series of anharmonic potentials on liquid Helium. Therefore, the well-developed circuit quantum electrodynamics technique for the implementation of superconducting quantum computation can be conveniently utilized here in the present quantum computing platform (proposed firstly in Phys Rev Lett 105:040503, 2010, to implement the fundamental logic gates, typically such as the single-qubit rotations of the individually addressable trapped electrons, the switchable two-qubit manipulations between the electrons trapped in the distant traps, and also the high-fidelity readouts of the target qubits. The feasibility of the proposal is also discussed by numerical simulations.
The system of electrons on liquid helium is an interesting candidate to implement quantum computation, due to the long coherence times of the qubits encoded by the electronic spins. In order to implement the quantum logic operations between the spins, we propose here a configuration, similarly to the cooled ions in a trap, to couple the distant electrons via manipulating their center of mass (CM) vibrations. First, we show that the electrons could be confined in a common harmonic oscillator potential by using an electrostatic field. Then, with a single current pulse (applied on the micro-electrode below the liquid helium) the distant electronic spins can be coupled simultaneously to the CM mode. Finally, by adiabatically eliminating the CM mode, effective interaction between the distant spins is induced for implementing the desired quantum computing.
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.
Electrons trapped on the surface of cryogenic substrates (liquid helium, solid neon, or hydrogen) are an emerging platform for quantum information processing made attractive by the inherent purity of the electron environment, the scalability of trapping devices, and the predicted long lifetime of electron spin states. Here we demonstrate the spatial control and detection of single electrons above the surface of liquid helium at temperatures above 1 K. A superconducting coplanar waveguide resonator is used to read out the charge state of an electron trap defined by gate electrodes beneath the helium surface. Dispersive frequency shifts are observed as the trap is loaded with electrons, from several tens down to single electrons. These frequency shifts are in good agreement with our theoretical model that treats each electron as a classical oscillator coupled to the cavity field. This sensitive charge readout scheme can aid efforts to develop large-scale quantum processors that require the high cooling powers available in cryostats operating above 1 K.
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
The quantized lateral motional states and the spin states of electrons trapped on the surface of superfluid helium have been proposed as basic building blocks of a scalable quantum computer.Circuit quantum electrodynamics allows strong dipole coupling between electrons and a high-Q superconducting microwave resonator, enabling such sensitive detection and manipulation of electron degrees of freedom.Here, we present the first realization of a hybrid circuit in which a large number of electrons are trapped on the surface of superfluid helium inside a coplanar waveguide resonator.The high finesse of the resonator allows us to observe large dispersive shifts that are many times the linewidth and make fast and sensitive measurements on the collective vibrational modes of the electron ensemble, as well as the superfluid helium film underneath.Furthermore, a large ensemble coupling is observed in the dispersive regime during experiment, and it shows excellent agreement with our numeric model.The coupling strength of the ensemble to the cavity is found to be ≈1 MHz per electron, indicating the feasibility of achieving single electron strong coupling.