Quantum control in the solid-state

High precision quantum sensing, secure communication, and quantum information processing all rely on a perfect understanding and control of a quantum system. Using an ensemble quantum system as a coherent controllable system offers enhanced sensitivity and fidelity, however understanding its collective effects and stabilizing the surrounding disturbances is still an active challenge [1].

One particular ensemble quantum system that we are investigating is collections of Nitrogen Vacancy (NV) centres in diamond. It is a naturally occurring, photostable and bio compatible quantum system that allows polarization and readout of the internal spin states by optical means at ambient conditions [2]. As a single photon source, the NV centre can be coherently controlled and manipulated using Microwaves and has experimentally demonstrated as a stable qubit at room temperature. As an ensemble of NV emitters in nanodiamond, we have observed intrinsic cooperativity such as superradiance [3]. The observation of intrinsic superradiance shows the increased complexities of using ensemble emitters whilst also showing the possible benefits after gaining more systematic control over it. Ultimately we want to achieve deterministic control over an NV ensemble to use it as a single system offering enhanced sensitivity and readout signal.

[1] Choi, Soonwon, Norman Y. Yao, and Mikhail D. Lukin. “Quantum metrology based on strongly correlated matter.” arXiv preprint arXiv:1801.00042 (2017).

[2] Doherty, Marcus W., et al. “The nitrogen-vacancy colour centre in diamond.” Physics Reports 528.1 (2013): 1-45.

[3] Bradac, Carlo, et al. “Room-temperature spontaneous superradiance from single diamond nanocrystals.” Nature communications 8.1 (2017): 1205.

Quantum polaritonics and low-temperature cavity quantum electrodynamics

Photons in free space are exceptional carrier of information and are ideal candidates for quantum communication. However, they barely interact at low energies, and this limits our ability to exploit them for quantum applications. When photons are strongly coupled to matter excitations (excitons), half-light half-matter quasi-particles are formed, named polaritons. Two independent photons can now “see” each other through the interaction of their corresponding excitonic parts. In these conditions, light behaves as a gas of interacting photons, and exciting phenomena, like Bose-Einstein condensation and superfluidity of polaritons can be observed. Yet, polariton interactions are weak, and photon nonlinearities are observed only when a large number of polaritons is populated. Therefore, experiments remains described within the semiclassical limit.

At the low-temperature cavity QED lab, we aim to enter the quantum regime by increasing nonlinearites up to a level where the presence of a polariton blocks the excitation of a second one, a phenomenon known as polariton blockade [1]. Our group has recently made a significant step forward towards the blockade regime [2], by using of a home-built semi-integrated fibre cavity [3]. Such a system enables strong photonic confinement and in-situ tuning of the exciton interaction, and it is ideal for quantum polaritonics. Our research focuses mainly on two aspects: on one side we work on photonic engineering to achieve stronger photonic confinement; on the other side, we use novel materials, such as two-dimensional materials, to achieve stronger exciton-exciton interactions.

[1] Verger et al., Phys. Rev. B, 73, 193306 (2006)

[2] Matutano et al., Nat. Mater., 18, 213-218 (2019)

[3] Besga et al., Phys. Rev. Applied, 3, 014008 (2015)

Optical trapping and levitation

In recent years, fluorescent nanodiamonds have become increasingly popular as biomarkers and imaging agents, and many potential applications have been suggested or are already being pursued [1, 2]. Nanodiamonds are highly biocompatible and their surface chemistry is highly controllable. They can host a large number of colour centres which exhibits bright and stable fluorescence. Protected by the diamond matrix, these colour centres usually preserve their characteristic quantum optical properties even at room temperature. Optical tweezers usually rely on the interaction of light and the polarizability of the nanoparticle. We here want to exploit electronic resonances of optical centres embedded in solid-state matrix to enhance the optical forces [1]. We explore this effect in nanocrystals with a high concentration of active centers. It has been shown [2] that if the atoms are close enough to each other, they can act cooperatively, enhancing further the optical forces. We are currently investigating the resonant scattering optical force (radiation pressure) on dense SiV and NV nanodiamonds in a microfluidic chip setup.

We are also pursuing the study of the dipole force (tweezer part) on bright nanodiamonds and rare-earth ions doped nanocrystals. Applying the powerful toolbox of atomic physics, we will be able to regime new regime of control for quantum optomechanics experiments. These studies might help us understand collective effects in the solid-state and help us harvest a relatively new effect for quantum technologies of mesoscopic systems. These investigations are carried out in vacuum (levitation) and wet environment (microfluidic chip).

Optical levitation in vacuum offers a unique platform for investigating and manipulating particles where the only interactions occurring are between the light field and the particle itself. In addition, the oscillatory motion in the optical trap offers additional modalities for sensing such as for measuring external vibrations. One of the main limitation of optically levitated systems in vaccum is heating due to the trapping field. Rare-earth ions doped nanocrystals are uniquely suited for that purpose. Indeed using laser refrigeration, we can extract heat from the crystal through anti-Stokes fluorescence [3]. Together with the aforementioned ‘atomic’ enhancement we expect to develop the ‘perfect’ quantum system: isolated, cold and highly coherent. Such a mesoscopic quantum system will allow us to investigate the interplay between quantum physics and gravity and probe the frontier between the classical and quantum world.

[1] M. L. Juan & al., Nat. Phys. 13, 241–245 (2017)

[2] B. Prasanna Venkatesh & al., Phys. Rev. Lett. 120, 033602 (2018)

[3] A. T. M. Anishur Rahman & P. F. Barker, Nature Phot. 11, 634–638 (2017).

Fabrication and characterization of new quantum materials

We are currently investigating the growth and fabrication of high-quality bulk and nanodiamonds with high concentration of Si, Ge and N vacancy color centres. The nanocrystals are fabricated using state of the art equipment at the Australian National Fabrication Facility (ANFF) and CSIRO Lindfield. Precisely, nanodiamonds are deposited by plasma enhanced chemical vapour deposition techniques on surface engineered substrates. Indeed, by tuning the composition and surface nanostructure of the substrate, different atomic impurities can be introduced in the diamond lattice. These are color centres, the building blocks for quantum photonic applications.