Micro-manipulation of nanodiamonds containing NV centers for quantum applications

Highlights

• Micro-manipulations of NDs may extend magnetic sensing and imaging.

• Levitated NDs in vacuum could be used for ultra-sensitive mechanical spin hybrid systems and nano-optomechanics devices.

• Trapped or levitated NDs with NV centers relevant to fundamental quantum spin-photon studies and mechanical systems coupling.

• Technologies are under development with applications still challenging.

Abstract

Micro-manipulations of nanodiamonds (NDs) containing Nitrogen-Vacancy (NV) centers are here reviewed. Various methods such as optical tweezers, electro-kinetic trap, ions traps, optofluidics and plasmonics applied to the specific nanomaterial are reviewed, focusing on the advantages and achievements in controlling the NDs positioning for magnetic sensing. These approaches are relevant to extend magnetic sensing and imaging in different fluid environments. Levitated NDs in vacuum using optical tweezers or ions traps are also reviewed for applications in cavity optomechanics towards establishing ultra-sensitive mechanical spin hybrid systems or nano-optomechanics devices. The current demonstrations of trapped or levitated NDs containing NV centers are relevant to fundamental studies of quantum spin-photon and mechanical systems coupling but have not yet been applied directly to biological systems, as these fields of applications present several challenges.

Introduction

Manipulating micro/nanoscale particles, molecules, and atoms have been central in the last 20 years for the characterization and application of micro/nanoparticles. Several methods have been developed for trapping and manipulating microparticles, cells, bacteria, sub-cell organelles, bio-macromolecules, and nanoparticles, among these methods optical [1], magnetic [2] and electron tweezers [3], ions traps, dielectrophoresis [4], optofluidics [5] to name few. Most of these methods operate mainly based on electromagnetic interaction with trapping objects. Methods for trapping nanoparticles, more commonly based on optical tweezers microscopes for their micro-manipulation, are currently a tool also for probing processes happening inside living cells [6,7]. Several nanoprobes were used for in vivo tracking of processes. Similar modalities can be applied to control nanoparticles that can be used for quantum sensing [8] or to construct and test quantum technologies with hybrid systems [9], such as, for example, systems where optical, spin and mechanical modes can be coupled together to advance as example optomechanics measurement sensitivity. These systems are considered relevant for fundamental quantum mechanics studies and to enhance the sensitivity of conventional optical probes in the biological systems. In particular, nanodiamonds (NDs) containing nitrogen-vacancy (NV) centers have raised interest as multifunctional probes for their unique possibility of being fluorescent scanning probes, combining spin optical read-out [10,11] that makes them unique quantum sensors [8,12]. Being hosted in the inert diamond, their biocompatibility is also a relevant aspect as a functional biomarker in fluorescent confocal and wide-field microscopy for living cell imaging [[13], [14], [15]]. Quantum magnetic sensing with ND-NV scanning probes has been demonstrated and in general, the use of highly doped bulk diamond with NV spins has reached applications in magnetic sensing of biological systems [16].

As such recently trapping and micromanipulations of NDs containing NV centers have been the object of several studies both theoretical and experimental that in some cases has opened new avenues in quantum sensing, quantum metrology, and quantum mechanics fundamentals studies. However, several hurdles seem also to appear associated with the non-yet fully controlled material properties of the NDs compared to bulk diamond.

In this review, we will summarise the demonstrations of various methods to achieve micro-manipulation of NDs containing NV centers and their implications to fields of applications central to this nanomaterial such as quantum sensing, quantum metrology, and quantum optomechanics. The relevance of NDs in various fields of research is also briefly outlined below to understand how micro-manipulation methods could advance these research areas. NDs have emerged as new carbon nanomaterials and have attracted considerable attention in material and life science towards medical applications, due to their considerable biocompatibility, surface functionalization ability, and availability of energetically stable lattice defects, such as the nitrogen-vacancy (NV) centers. NV shows an elevated level of photostability. The NV center has two bright states of charge, NV⁰ and NV⁻, the fluorescence of both being characterized with a satisfactory contrast to the background cells and long lifetime above 30 ns for NV⁰ and up to 25 ns for NV⁻, low photo-bleaching, and readily detectable wavelengths, making it attractive in the histology field. Sensing is possible due to the NV-center charge state being influenced by environmental potentials, and room-temperature Optically Detected Magnetic Resonance (ODMR) is available with good magnetic sensitivity properties. Potential applications are thus attractive in diverse fields of medicine, in particular in cancer therapy, precise drug delivery, and fluorescent markers [17].

The formation and material properties of the NDs are currently variable but having as a common denominator the possibility to host the NV centers. Due to the many types of NDs from detonation [18], to high temperature-high-pressure [19], to chemical vapor deposition (CVD) [20] and in different sizes, the different material environment surrounding NV centers and the various surface properties of the same particles, each ND type has different properties with regards to fluorescence and qualify for different applications. These aspects have recently prompted a need for standardization to reach high quality in NDs fabrication and control [21].

NV centers magnetometry [22], a central application, is based on the color center energy levels properties and its spin 1 quantum state. Upon continuous optical excitation using a confocal microscope, the electrons associated with the color centers are polarized on the ground state level m_s = 0, after occasional transitioning to a non-radiative metastable state. By adding a microwave excitation at the resonance frequency of the ground state transition from m_s = 0 to the degenerate m_s = ±1, the electrons population is forced to the higher energy spin level, interrupting the optical transition from excited to the ground state, and thus providing reduced photoluminescence (PL) when the microwave excitation is on. By applying a magnetic field, the m_s = ±1 degenerate level split and a distinct photoluminescence reduction are observed corresponding to the resonance frequencies of the +1 and −1 spin level transition to the m_s = 0. This is called optical detected magnetic resonance (ODMR) [23], and it can be related to the modulus and direction of the applied magnetic field and used as a magnetometer. More complex magnetometry schemes [[24], [25], [26]] together with improved control of the diamond material permit to improve the sensitivity, which was 50 pT/√Hz in a bulk diamond ensemble of NVs [27] with a record of 15 pT/√Hz realized in [28], and recently using two ferrite cones in a bowtie configuration a 1 pT/√Hz sensitivity has been achieved [29]. The magnetic sensitivity is however 9.1 nT/√Hz in bulk diamonds single NV [30]. Regarding NDs, one of the best magnetic field sensitivity reported is 290nT/√Hz obtained by chemical vapor deposition (CVD) growth of high purity Nanocrystals with single NV, owing to a coherence time of 200 μs [31]. It is noted that the record coherence time exceeding 700 μs was obtained in the controlled fabrication of cylindrical diamonds particles with diameter and height ranging from 100 to 700 and 500 nm to 2 μm, respectively using high-purity, single-crystal diamond membranes with shallow-doped NV centers, holding the promises of ultrahigh sensitivity micro- and nanoscale sensors [32]. Details of the PL emission and spin properties of NV in diamond and NDs can be found in several review papers [33,34] and some of the applications in another review as an example [16,22,[35], [36], [37]].

One of the most outstanding applications of NDs with NV center is as Nanoprobes in scanning-based techniques such as scanning probe magnetometry, which relies on attaching NDs to AFM tips or diamond cantilever to AFM tips [11,12]. The methods for devising the ND probes to achieve the highest magnetic field sensitivity are sometimes not-scalable [38] and they are not always applicable to a fluidic environment.

As such, other methods to micro-manipulate precisely NDs with NV centers in various environments to test their sensitivity have been proposed and studied. Several examples are presented where the NDs mechanical micro-manipulation is achieved using optical tweezers (OTs), anti-Brownian electrokinetic trap (ABEL), optical levitation, ion-trap, and optomechanical traps. The main objective is to achieve Nano and micromanipulation control of the NDs sensing probe in a variety of fluids environments, targeting applications that need accurate positioning of the NV center down to the Nanoscale within existing systems as, for example, the controlled labeling of a single biological cell. The first group of traps has been identified to have the potentials to improve living cells magnetometer applications, where the NV has been widely employed [13,15,28].

Other traps applied to NV center are meant to produce hybrid systems for quantum mechanical fundamental studies, owing to the fundamental properties of the NV as a long coherence time spin qubit that can be entangled with nuclear spins and remotely entangled using flying qubits as a single-photon source in a quantum network [[39], [40], [41], [42]]. Ultimately the quest is to use high vacuum levitated NV in NDs spin degree of freedoms together with their mechanical center of mass control for strong coupling of mechanical and spin modes of the system, experiments also ongoing in bulk diamond [43,44].

Here, we will focus to review the demonstrations of trapping and micro-manipulating of NDs functionalized with NV centers to study its sensing modalities in various fluid and vacuum environments. We review these methods also in the context of fundamental studies of quantum mechanics, where levitated NDs are needed. We will first briefly describe the operation principles of the traps applied to NDs in Section 2. In particular, we review first the attempts of 3D micro-manipulations of NV NDs using optical tweezers (Section 3), Anti-Brownian Electro-kinetic trap (ABEL trap) (Section 4), and other trapping methods such as optofluidics and near field plasmonics methods (Section 5). The first three groups of approaches are more applicable to non-invasive tracking of biological processes in a living organism, while near field plasmonic approaches are more applicable for controllable nanofabrication of quantum photonics devices that use NV in NDs.

Then we will review trapping methods to achieve levitated NDs in vacuum or low atmospheric pressure for applications in the domain of quantum optomechanics (6 Levitated NDs: a route towards quantum optomechanics, 7 NDs trapped and levitated in ions trap), introducing and underlining the role of NV in quantum optomechanics in Section 6. In Section 6 we focus on the use of OTs for achieving levitated NDs, while in Section 7 ions trap demonstrations of trapped and levitated NDs are described. Levitated NDs in vacuum using optical tweezers or ions traps and within a cavity optomechanics [45,46] are discussed given their applications in ultra-sensitive mechanical spin hybrid systems as, for example, for force detection. In Section 8 examples of levitated optomechanics such as Nano-optomechanics envisioned devices with optically levitated NDs are reviewed. Here nano-optomechanical systems based on ‘levitated nanoparticles’ are applied in low vacuum or vacuum conditions, envisioning applications to fundamental studies of NDs with NV centers sensory properties, when they are not interacting with other fluids or air, to create new spin–optomechanical hybrid devices for testing of quantum wave-function collapse models [[47], [48], [49]] and quantum gravity [50,51]. An extremely sensitive torque detector can be built by making use of a levitated ND [52], which can also act as a room-temperature mass spectrometer [53]. In Section 9 levitated NDs experiments and theory for fundamental tests of quantum mechanics are reviewed, while in Section 10 levitated NDs application in quantum sensing and metrology are finally presented. We conclude summarising the current achievements and challenges to implement micro-positioning and control of NDs, while we envision possible other approaches or demonstrations which could further advance this field of research.