Single Electrons on Solid Neon as Charge Qubits

Short answer: Researchers have reported charge qubits made from single electrons trapped on a solid neon surface in vacuum. A 2022 Nature paper reported a T1 of 15 microseconds, and a 2023 Nature Physics paper reported relaxation and coherence times both on the order of 0.1 ms. A later theoretical paper says calculated limits are at least an order of magnitude above the observed values.

What is an electron-on-neon qubit?

According to the 2022 Nature abstract, the platform uses isolated single electrons trapped on an ultraclean solid neon surface in vacuum. The authors integrate an electron trap in a circuit quantum electrodynamics architecture and achieve strong coupling between the motional states of a single electron and a single microwave photon in an on-chip superconducting resonator. Quantum information lives in the electron's motional states and not in its spin.

What were the first results?

In 2022 the authors implemented qubit gate operations and dispersive readout, and measured an energy relaxation time T1 of 15 microseconds and a phase coherence time T2 over 200 ns. They conclude that the qubit already performs near the state of the art for a charge qubit.

The 2023 Nature Physics abstract begins from a known problem: electron charge qubits built on conventional semiconductors and superconductors suffer severe charge noise that limits coherence to the order of one microsecond. The authors report electron charge qubits that exceed this limit, with measured relaxation and coherence times both on the order of 0.1 ms. They describe this as surpassing all existing charge qubits and rivalling state-of-the-art superconducting transmon qubits, which is their own comparison. They also demonstrate simultaneous strong coupling of two qubits to a common resonator, which they call a first step towards two-qubit entangling gates.

What limits coherence?

A later theoretical paper in an IOP journal studies the decoherence mechanisms of an electron's lateral motional states on solid neon. Its abstract says the roughly 0.1 ms coherence time is believed to result from the intrinsic purity of solid neon. At the typical experimental temperature of about 10 mK, the authors identify two main mechanisms, phonon-induced displacement of the neon surface and phonon-induced modulation of the neon permittivity. They calculate that as qubit frequency rises from 1 GHz to 10 GHz, coherence time falls from about 366 s to 7 ms for one mechanism and from about 864 s to 24 ms for the other. At about 6.4 GHz, the calculated coherence times are at least one order longer than the observed ones, which the authors read as plenty of room for experimental improvement.

How to read these numbers

These are laboratory experiments on small numbers of qubits, plus a theory paper whose figures are model calculations and not measurements. The 2022 and 2023 values come from different devices and are not a single series. This post relies on abstracts and visible summary text and has not checked full texts. It makes no claim about products or deployed systems.

FAQ

What encodes the quantum information?

The motional states of a single electron trapped above a solid neon surface, strongly coupled to microwave photons in a superconducting resonator, according to the 2022 and 2023 abstracts.

Why do charge qubits usually struggle?

The 2023 abstract says charge noise in conventional semiconductors and superconductors limits coherence to the order of one microsecond.

Is a two-qubit gate demonstrated?

The 2023 authors demonstrate simultaneous strong coupling of two qubits with a common resonator and call it a first step towards entangling gates. They do not report a gate in the abstract.

Why might coherence improve?

The theoretical paper calculates coherence limits at least one order longer than observed at about 6.4 GHz, suggesting room for improvement.