Atomic-like charge qubit in a carbon nanotube enabling electric and magnetic field nano-sensing

I. Khivrich,S. Ilani

doi:10.1038/s41467-020-16001-5

Abstract

Quantum sensing techniques have been successful in pushing the sensitivity limits in numerous fields, and hold promise for scanning probes that study nano-scale devices and materials. However, forming a nano-scale qubit that is simple and robust enough to be placed on a scanning tip, and sensitive enough to detect various physical observables, is still a great challenge. Here, we demonstrate, in a carbon nanotube, an implementation of a charge qubit that achieves these requirements. Our qubit’s basis states are formed from the natural electronic wavefunctions in a single quantum dot. Different magnetic moments and charge distributions of these wavefunctions make it sensitive to magnetic and electric fields, while difference in their electrical transport allows a simple transport-based readout mechanism. We demonstrate electric field sensitivity better than that of a single electron transistor, and DC magnetic field sensitivity comparable to that of NV centers. Due to its simplicity, this qubit can be fabricated using conventional techniques. These features make this atomic-like qubit a powerful tool, enabling a variety of imaging experiments.

Highlights

Quantum sensing techniques have been successful in pushing the sensitivity limits in numerous fields, and hold promise for scanning probes that study nano-scale devices and materials
A large variety of scanning probe sensors have been developed, optimized to measure specific physical quantities: Magnetic fields are primarily imaged via scanning SQUIDs1, Hall probes[2] and NV centers[3], while electric fields are primarily probed with Kelvin probes[4], scanning tunneling potentiometry[5], and scanning single electron transistors (SET)[6]
Single quantum dot devices in carbon nanotubes have been successfully used as ultra-sensitive scanning SETs, allowing to image oxide interfaces[20], Wigner crystals[21], as well as the mapping of ballistic[22] and hydrodynamic[23] electron flows, demonstrating the compatibility of these devices with scanning probe applications

Summary

Results

The basis of our qubit is given by two electronic wavefunctions in a single quantum dot, formed in a suspended carbon nanotube. 4-fold degeneracy at B|| = 0, and Coulomb repulsion changes the simple non-interacting wavefunctions into Wigner crystals with finer real-space structures[21], yet, since the valley remains a good quantum number, the simple picture in which tuning B|| leads to crossing between levels with different magnetic moments and different spatial structures remains valid for the discussion below. To make an atomic-like qubit that is sensitive to local electric and magnetic fields, we choose a crossing between a high-lying K state and a low-lying K′ state with opposite spin directions, which we will denote as Kn and Km0 (n ≫ m). The Kn and Km0 states have opposite orbital momenta (Fig. 1b, center), endowing the qubit transition a large magnetic moment (~20 μB, μB is the Bohr magneton). The relevant triple point for our experiment separates the state jNi, having N holes, from the two qubit states, jBi 1⁄4 jNi þ Kn, and

Kn x

HEMT b

ΔVG VG

Additional information