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Molecular Quantum Opto-Spintronics

Periodic Reporting for period 4 - MoQuOS (Molecular Quantum Opto-Spintronics)

Período documentado: 2022-01-01 hasta 2022-06-30

In molecular quantum spintronics, the molecules’ electron spin degree of freedom plays a key role in storing, transmitting, and processing information. Different molecular devices such as molecular spin-transistors, spin-valves, and spin-filters enable the read-out and manipulation of the spin states of magnetic molecules. Such devices paved the way to milestone results such as the implementation of a Grover algorithm in the four states of a molecule nuclear spin. Concerning quantum computation and fast quantum sensing, though, the electron and nuclear spin single-shot read-out times have been limited to the milliseconds or even seconds. To further advance the field of molecular quantum spintronics and to tackle the bottleneck of slow single-shot read-out times, advanced optical methods have been exploited to manipulate and read out the different spin states of magnetic molecules:

A. Quantum emitter based magnetometry
The nitrogen vacancy (NV) center is a versatile and nicely controllable solid-state quantum system. We exploited its atomic-like characteristics to detect and measure extremely weak fields originating from molecular spins.
Their single emitter quality and distinguished electronic and nuclear spin properties make them a remarkable tool for many quantum technological applications both at room and cryogenic temperatures.

B. Molecular fluorescence spectroscopy
We performed ensemble and single molecule fluorescence spectroscopy to pick a designed molecular compound suited for the integration into opto-spintronic devices.
We aim to use fluorescent probes which are directly attached to the qubit molecule. The immediate proximity and chemical bonding enable an interaction between the fluorescent ligands and its central magnetic core. Hereby, the qubit spin state can be mapped on the optically detected signal of the fluorescent ligand.

C. Light interaction with molecular devices
We adressed molecular spin-transistors optically to manipulate and read out the qubit spin state. The microfabricated gold junctions of the spin-transistors allowed us to exploit an enhanced light-matter interaction.
To receive a spin-transistor, we placed a single-molecule magnet in a nanometer sized gap between two gold junctions. The ligands of the molecule are tunnel coupled to the junctions, enabeling to address the molecule electronically. Additional local electrodes further allowed the application of fast gate pulses and microwave electric fields, which influence to the spin system via the Stark effect.
During the period of examination, the PI moved his place of employment from Institut Néel (Grenoble, FR) to the Karlsruhe Institute of Technology (Karlsruhe, DE). Together with his new team at KIT he set up a completely new optical laboratory for the purpose of the ERC Advanced Grant objectives. This includes the whole process starting with the acquisition of totally new lab instruments, especially in relation to optics (optical components and tables, lasers, spectrometer, photo detectors, and more), followed by the consecutive installation and set up of these, up to the implementation of a new software infrastructure for the complete experimental control.

The group set up a home-built scanning confocal microscope at room temperatures for the purpose of quantum emitter investigation and application. This includes a fast and reliable optics for fluorescence excitation (pulsed and continuous wave), a 3D piezo setup for full spatial control, and a micro-wave and magnetic field setup for the application of quantum gates. At first, the nitrogen-vacancy center (NV) in diamond was the quantum emitter of interest. As this field of research was completely new for the group of the PI, the focus was first set on the general investigation of this color center and the replication of fundamental results. With the gained experience and knowledge, the group is now able to perform basic magnetometry measurements both with ensemble NV and single NV.
Furthermore, the group set up two new home-built dilution cryostats for optical experiments at milli-Kelvin temperatures. In more detail, this involves a complete scanning confocal microscope where the samples of interest can be investigated at temperatures down to 30 mK. Special care has been taken by minimizing the vibrational background of the cryostat and its gas handling system. After the successful execution of proof-of-principle experiments (all-optical), the setup is now upgraded. The new design includes a superconducting vector magnet configuration, and micro-wave and DC feedlines for mK, enabling the full quantum control of our systems of interest.

The group focused also on molecular quantum emitter. After the installation of a new broadband tunable laser system and the integration of this in both confocal microscopes, molecules such as DBATT and rare earth compounds have been intensively under investigation at room temperatures and cryogenic temperatures. New optical characterization and readout techniques have been carried out with spintronic devices. In addition, a new chemical vapor deposition system for the synthesis of carbon nanotubes needed for the molecular spin-valves was set up and successfully used to build devices. To increase the possibility and reliability to produce complex carbon nanotube circuit in spin-valve devices a nanotube stapling technique inside a scanning electron microscope has been set up and used.
The technical developments of the two low-temperature setups of MoQuOS were very successful and led to the foundation of the start-up Qinu GmbH (https://qinu.de/). Qinu aims at the development, production, worldwide distribution, and consulting of solutions in the field of cryogenics and quantum technologies, with special focus on optical measurements like those developed in MoQuOS. It was founded in early 2021 in Karlsruhe and has already a wide range of international cooperation partners. With its systems Qinu focuses primarily on the fast-developing market of quantum technologies and supports worldwide customers in industry and research in the implementation of their quantum innovations. Qinu accumulates the research experience of MoQuOS, including specialized electrical cabling for controlling and reading out the experiments, optimized electronic circuits like amplifiers and filters, electromagnets for cryogenic temperatures, optomechanical and optoelectronic elements for optic-based experiments, and piezoelectric components for the mechanical control of the applications. Qinu provides the know-how for the final installation of the individual experiments and technologies into its own systems, as well as for the optimal operation of the cryogenics. Besides milli-Kelvin temperatures, this includes access for optical excitation and readout, microwave access for quantum gate applications, superconducting magnetic system as well as the possibility to scan the samples of interest optically.
Scheme of the project Molecular Quantum Opto-Spintronics (MoQuOS)