Molecular qubits on surfaces

Introduction

Molecules provide suitable platforms to encode quantum information. In parallel to the well-studied qubits located at defect sites in solid-state materials, molecular qubits demonstrated comparable efficiency and substantially higher versatility. Molecules can be fabricated in large quantities, while offering high tunability of the spin environment and spontaneous aggregation in long-range ordered 2D or 3D lattices.

The rational design of molecular spin qubits have already achieved important milestones, such as the implementation of simple quantum computations and have raised expectations for the use of molecular spins qubits in future quantum computers. Chemistry becomes pivotal for the realization of high-performance molecular spin qubits. Molecules can be easily manipulated and transferred onto solids, whereby the extension of molecular design principles on surfaces represents one of the best suited strategies to tackle the major challenges towards the future implementation of molecular spins into solid state devices: addressability, coherence and scalability.

In this group, we study individual and assembled molecular spin qubits deposited on surfaces by combining several experimental techniques. With low-temperature STM, we determine how the molecules are oriented and assembled on the surface. In addition, we perform scanning tunneling spectroscopy to determine the electronic properties and how the surface adsorption influences the molecular orbitals. Using X-ray magnetic circular dichroism, we determine the spin properties and changes in the molecular structure occurring when the molecules are assembled in dense films. With ESR-STM, we study their quantum coherence and how the molecules interact with each other.

Our group has been developing a unique tool to explore the coherence properties of ensembles of molecular spin qubits on a surface. This tool, named Spin Resonance of Ensembles of Spins on Surfaces (Spin-RESS) is a surface-sensitive ESR spectrometer that allows the investigation of a single layer of surface-adsorbed molecules. It can operate in continuous wave and pulsed mode to characterize the magnetic anisotropy and the quantum coherence of the ensemble. The group recently developed the functionality of exciting the nuclear spin of molecules and detect their effect on the electron spins, a technique known as Electron Nuclear DOuble Resonance (ENDOR). Using this tool we want to explore molecular architectures with long coherence time and investigate their potential use as qubits on surfaces.

Longer-term Goals

• Demonstrate quantum-coherent control of molecular qubits on a surface
• Use molecular qubits on surfaces for quantum information
• Fast screening of quantum properties of potential molecular qubits

Near-term Goals

• Extend ESR-STM of molecules to more molecular qubits on surfaces
• Use Spin-RESS tool to measure quantum properties molecular qubits on surfaces
• Use synthetic chemistry lab to functionalize molecular qubits to match surfaces

Research results to date

QNS has adapted many of its main research tools to investigate the properties of molecules on surfaces. Some of the highlights include:

In 2021, QNS studied vanadyl-phtalocyanine (VO-Pc) molecules on several surfaces. From previous studies it was known that VO-Pc has a total spin of S=1/2 in powder and on some surfaces. This makes it a possible qubit. We used our Naomi STM system to image the VO-Pc molecules and compared them to another molecule with the same structure but without a spin, namely TiO-Pc. Our STM results were confirmed with X-ray spectroscopy measurements that we performed in 2021 at the Swiss Light Source. The results are being prepared for publication at which point updates will be included here.

In 2020, QNS demonstrated the first ESR STM of a single molecule. This work was done on our “Alice” system and utilized iron-phtalocyanine (Fe-Pc) molecules on MgO, see Figure 1. This work is extremely important since it extends the class of ESR-STM active spins from 3d transition metal atoms to the much broader class of magnetic molecules. Fe-Pc is usually a S=1 system and we were pleasantly surprised when we found clear evidence of a S=1/2 spin when deposited on a thin film of MgO. Our calculations showed that this happens due to a charge transfer with the metal substrate under the MgO. We found an isotropic magnetic behavior of the molecule in ESR STM and also found some intriguing magnetic interactions between molecules. This work is accepted for publication in Nature Chemistry (2021).

In 2020, QNS studied the properties of VO-Pc molecules adsorbed on Au(111). This is the surface that shows the herringbone reconstruction shown in Figure 2. We were quite surprised to see complex looking images and spectra until we realized that hydrogen molecules were coming in and out of the tunnel junction. While not directly related to the spin of VO-Pc, this work was nevertheless quite interesting and has attracted considerable attention due to hydrogen-molecule interactions. The work was published in Royal Society of Chemistry Advances (2021).

*Fig.1 Molecule ESR STM. STM image of two Fe-Pc molecules, one Fe and one Ti atom on MgO. Image from our “Alice” system, grid intersects: oxygen atoms of the MgO top layer.

*Fig.2 VO-Pc on gold. STM image overlaid with half a molecule. The 4-fold symmetry of the molecule is clearly visible. Image with our AFM-STM system at 8K.

Research Equipment

NAOMI LT-STM system

Commercial system
Delivery: September 2017
First Results: March 2019
Key results
• Molecular qubits on various surfaces
• Films of molecules on surfaces
• Rare earth atoms on various surfaces
Specs:
Temp: 8 K in continuous mode
Cryogen-free operation

Our LT-STM system dedicated to the investigation of molecules on surfaces is nicknamed “NAOMI” and is a commercial system, purchased from RHK-Technology. NAOMI is a closed cycle, cryogen-free PanScan STM system that operates in the range of 5×10-11 Torr. The 8 K chilled system runs uninterrupted and does not require liquid cryogens. It provides cost-free and non-stop low-temperature operations. The STM system is coupled with a preparation chamber with a base pressure of 3×10-10 Torr, equipped with a low electron energy diffraction (LEED) device. The NAOMI STM gives access to fast-paced, high-resolution investigation of a variety of systems and libraries of molecular spin qubit deposited on several substrates (conductors, thin insulating films supported on metals, semiconductors, and superconductors). Thanks to its fast operation it allows several sample preparation cycles per day and it is used for screening of qubit systems of interest. It also gives access to the characterization of chemical properties, stability, and reactivity of molecular qubits supported on a surface.

*Fig.3 The "Naomi” LT-STM system (top). High resolution STM image of molecular spin qubit candidates (bottom-left). LEED pattern of an highly ordered 2D array of molecular spin qubit

SE-ESR Surface Ensemble ESR

FULLY home-built system
Start of assembly: December 2018
First Results: June 2019
Key results
• Continuous wave operation (June 2019)
• Pulsed electron spin resonance (March 2020)
• Sensitivity of 1 layer of spins (August 2020)
• Electron-electron Double Resonance (February 2021)
• Electron-Nuclear Double Resonance (March 2021)
• First original system investigated (May 2021)
Specs:
Temp: 5K with Helium flow
Room-temperature
Magnet: 3.2T high homogeneity magnet
Warm magnet bore
Cryogen-free magnet cooling
ESR: 8-12 GHz
ENDOR: 0 – 300 MHz
ELDOR : 0 – 200 MHz

This setup is a fully home-built surface-sensitive ESR spectrometer, whose purpose is the characterization of the quantum coherence of ensembles of surface spins, such as films of molecular spin qubits, two-dimensional materials, and magnetic atoms on surfaces. This setup is unique in the world for its specifications, and we will use it to explore and identify surface spins with long coherence time and potential use in quantum computing.

**Figure 4.** Shematics of the surface resonator (top).

In a conventional ESR spectrometer, the microwave power is focused in a small volume using a “resonator”. The working principle is the confinement of the microwave radiation using a metal cavity (pretty much as in a microwave oven), whose dimensions determine the wavelength that can be used for the experiment. In our systems, the spins are distributed on a surface plane, therefore when inserted in a conventional ESR spectrometer, most of the volume does not actually contain any spin. This makes the detection of the ESR signal inefficient.

We circumvented this problem by designing a surface-type resonator capable of maximizing the microwave power at the surface of our sample. This resonator is a coplanar waveguide antenna (Fig 4.), with copper electrodes grown on a sapphire single crystal substrate. After traveling through the transmission line, the microwave reaches the sample part, which is actually a few millimeter-long strips of epitaxially grown copper. In this part, the microwave gets confined within the dimensions of the resonator strip, which is crucial to enhance the detection of our surface spins.

In order to measure electron spin resonance, we apply an external magnetic field. The spins on the sample surface “precess” around the magnetic field direction with a characteristic frequency (Larmor frequency) that depends on the strength of the applied field. When the precession frequency and the microwave frequency are matched, the spins start absorbing some of the microwave’s power (spin resonance). We can detect this absorbed power by measuring the reflected microwave returning to the transmission line which allows us to determine the experimental condition to drive the spin resonance. To enhance our detection, we use several amplification stages. We also use a frequency mixing scheme (heterodyne) to detect the in-phase and out-of-phase response of our spin precession with respect to the incident microwave.

Spin-RESS is capable of operation in such a continuous wave, but also with pulsed ESR. We can control the shape and amplitude of our pulses with an arbitrary waveform generator. After sending the proper sequence of pulses, we measure the response of the spins by detecting their “echo”, a signal that is emitted when all spins precess with the same phase. We can use a variety of pulse sequences (Ramsey, Rabi, inversion recovery, etc…) to characterize the quantum coherence of surface electron spins and detect mutual coupling between electrons in electron double resonance (ELDOR) experiments. In addition, the spectrometer is equipped with a radiofrequency coil that can excite nuclear spins, whose quantum state can be detected using the coupling to nearby electrons (electron-nuclear double resonance ENDOR). These features allow us to not only determine the potential of surface spins as individual qubits, but also to exploit their mutual coupling to realize entangled systems in molecular architectures.

The key feature of Spin-RESS is the possibility to detach the sample part from the resonator (Figure 4) and move it in an ultra-high vacuum preparation chamber equipped with surface preparation tools (ion gun, thermal stage, molecule and metal evaporator). This operation is realized without breaking the vacuum, which allows us to prepare clean surfaces onto which we grow films of molecular spin qubit. Using clean surfaces is a very crucial point to achieve controlled coupling between the molecules and the substrate. It also allows us to study the same systems that are typically investigated with our STMs.

The combination of low-temperatures, pulsed ESR operation, ultra-high vacuum, and in situ sample preparation makes Spin-RESS unique in the world. The construction of this machine started in December 2018 with the delivery of the cryostat and cryogen-free magnet with room temperature bore, which allowed us to perform initial tests at room temperature and obtain the first ESR spectrum in June 2019. In 2020 we completed the pulsed ESR scheme and improved the sensitivity of the instrument down to a single molecule layer on the surface (Figure 5). In 2021 we added the ELDOR and ENDOR features, and finally completed and connected the preparation chamber. The machine is now ready to be used for exploring novel materials. The exploration started this year with a study on molecular 2D single-crystal flakes of metal-organic frameworks in collaboration with Prof. Chung Mingee of the University of Birmingham, UK.

*Fig.4 Shematics of the surface resonator (top). The surface of the resonator strip is the support of the spins that we investigate. This part can be detached from the waveguide (bottom) and moved to the preparation chamber for surface treatment and molecular film growth.