Quantum electronics and optics at the interface of solid neon and superfluid helium

Abstract

We predict a new quantum electronic structure at the interface between two condensed phases of noble-gas elements: solid neon and superfluid helium. An excess electron injected onto this interface self-confines its wavefunction into a nanometric dome structure whose size varies with pressure. A collection of such electrons can form a classical Wigner crystal visualizable by midinfrared photons. The ultralong spin-coherence time allows the electrons in this system to serve as perfect quantum bits. They can be deterministically arranged on-chip at a spacing of several microns. Their spin states can be controlled and readout by single-electron devices. This unique system offers an appealing new architecture for scalable quantum information processing.Noble-gas elements helium (He) and neon (Ne) are the two most stable chemical elements in the universe. When an excess electron (e − ) approaches helium or neon (He/Ne) atomic cores, it experiences a strong repulsive force due to the Pauli exclusion from the occupied orbital electrons. To enter a condensed liquid or solid He/Ne at low temperature, the excess electron must overcome a potential barrier of the order of 1 eV [1][2][3][4][5]. Inside bulk He/Ne, the ground-state electron pushes away all the surrounding atoms and opens up a nanometric cavity, known as an electron bubble [6,7]. On a He/Ne-to-vacuum surface, the ground-state electron is weakly attracted by its image charge and forms a dimple structure [8]. The dimple radius is over 500 nm on liquid He surface [9] and much larger on solid Ne surface [10]. A collection of electrons on these surfaces can form a classical two-dimensional electron gas (2DEG) [9,10].In the past decades, there has been considerable interest of using the long-coherence orbital and spin states of the electrons on liquid He surface to engineer quantum bits (qubits) [11][12][13][14][15][16]. In particular, since 4 He (excluding 3 He) has zero atomic spin and constitutes an ultraclean superfluid below 1 K [17], electrons in this environment possess a spin-coherence time over 100 s [13,16]. Likewise, 20 Ne and 22 Ne (excluding 21 Ne) also have zero atomic spin, and can form ultrapure solids below 23 K [5,18]. They can potentially serve as a new solidstate storage matrix for qubits. However, by far it is still challenging to precisely position every electron, control and readout its spin states on either of these surfaces. The large dimple radius can result in overlapped electron wavefunctions in nanoscale devices [16,19].In this paper, we reveal a new quantum electronic structure at the interface between solid Ne and superfluid He. We show that each electron is self-confined into a dome structure, whose flat side attaches to the solid Ne and curved side dips into the superfluid He. Employing a bosonic density functional theory (DFT) which can accurately reproduce essential properties of superfluid He and electron bubble [20][21][22], we calculate the ground state, excited states, and optical transitions. The dome diameter decreases with incr...

Single electrons on solid neon as a solid-state qubit platform

“…It has been theoretically calculated that the in-plane motional coherence of an electron on solid Ne surface can be several milliseconds 61 . Ultimately, utilizing the spin states through engineered spin-orbital coupling 16,20,52 can yield ultralong qubit coherence in excess of 1 s 52,[54][55][56] .…”

“…Without optimization, the measured T 1 = 15 µs and T 2 200 ns have already reached the state of the art for a charge qubit 51 , highlighting the promise of this new material environment. With projected development employing spin-charge conversion 16,20,52 , we anticipate the nearly perfect spinless environment formed by solid neon 53 to support electron spin qubits with estimated coherence time over 1 s 52,[54][55][56] . Beyond quantum computing, this novel solid-state single-electron qubit platform creates an appealing hybrid quantum framework that can connect various qubit platforms, thereby paving new pathways in quantum information science and technology.…”

“…A repulsive barrier, U ≈ 0.7 eV, occurs due to the Pauli exclusion between the excess electron and atomic shell electrons. In addition, an attractive polarization potential, V (z) = −( − 1)/[( + 1)e 2 /4z], (z > b), with a dielectric constant = 1.244 and short-range cutoff b ≈ 2.3 Å, occurs due to the induced image charge inside solid Ne 56,63,64 . With this potential, the electron's z-motion has a ground-state energy E z0 = −15.8 meV and an eigen-wavefunction peaked at about 1 nm distance from the surface (Fig.…”

“…While the electron-on-solid-Ne (eNe) system can be considered conceptually as an extension to the historically more studied electronon-liquid-He (eHe) system, it renders much stronger surface rigidity that suppresses decoherence through surface excitations 52,68,69 . Compared with eHe that was proposed as a qubit platform over two decades ago 52,[68][69][70][71][72][73] , eNe embodies a potentially even more fascinating, solid-state qubit platform 56,61,74 .…”

Electron spin coherence on a solid neon surface

“…When an excess electron approaches a condensed liquid He or solid Ne from vacuum, it can form surface states under two effects: (a) a repulsive barrier on the order of 1 eV due to the Pauli exclusion between the excess electron and atomic shell electrons; (b) an attractive potential from the image charge inside the liquid or solid due to the polarization by the electron [21][22][23] . In the past two decades, there have been considerable effort in utilizing the motional (charge) or spin states of a single electron on a liquid (superfluid) He surface [24][25][26][27][28][29][30][31][32][33] to form a qubit. However, due to the practical challenges in suppressing liquid-surface vibration and performing single-spin readout, the electron qubits on liquid He surface have not been achieved so far.…”