Current Research Directions:
Rare earth nanophotonic
Developing scalable quantum memories and other optical processors that operate near the telecommunication wavelengths is a significant milestone in the field of quantum information processing. High optical quality factors as well as strong light confinement can be achieved on chip by fabricating microscopic photonic resonators using proper materials. Silicon based materials are ideal candidates for realization of such scalable systems due to their compatibility with CMOS manufacturing and favorable optical properties near the telecomm band. Placing atoms in the vicinity of the optical mode of such miniaturized resonators provides strong light-atom interactions important for coherent optical information processing. In nano-cavities, the enhanced light atom coupling can reach the regime of strong cavity quantum electrodynamic, where a single photon can strongly interact with a single atom. Applications of such unique interaction ranges from photonic memories for scalable secure communication to logic gates for quantum photonic computation.
In HiQP Lab, we are interested in developing solid-state photonic systems that are scalable and also compatible with the current telecommunication technology. We develop nano-photonic devices integrated with rare-earth ions and study strong quantum interactions of optical information with such ions.
Hybrid quantum photonics
Multimode interaction of light with atomic spin integrated with micro/nano-photonic structures, exhibiting both optical and mechanical resonances, can provide a powerful platform for controlling quantum optical information on chip. Such multimode interactions in hybrid systems are investigated in our group and their application in developing quantum optical devices are being explored.
The interaction between light and an array of nano-mechanical oscillators can provide a multimode and fast sensing scheme to enhance measurement precision. These novel platforms has been the subject of many groundbreaking discoveries in recent years. For instance, radiation forces has been used to manipulate the motion of the mechanical oscillators with masses ranging over more than 15 orders of magnitude. At one end of the spectrum, nano-oscillators and deformable nanocavities have masses about 10-15 kg. At the other end, a table-top experiments have used a 1-gram mirror, while the Laser Interferometer Gravitational-wave Observatory (LIGO) has been reported laser cooling of a mirror with a mass of 2.7 kg. The promising prospects of developing nano-mechanical arrays for multimode quantum memories and sensors has been the derive to many applied research in recent years.
Quantum optomechanical interaction generally observed in systems with ultra-high quality factor resonances in both mechanical and optical modes, allowing coherent exchange of information between these degrees of freedom (such interaction schematically depicted in figure below). The exchange is mediated by radiation pressure, where the photons in the optical mode can be red- or blue-shifted by creating or destroying a phonon in the mechanical mode. In this way the motion of a mechanical oscillator can be coupled to an optical resonance that allows both control and read-out of the mechanical mode.
One of the research interests of our group is to employ experimental research to investigate the interaction of coherent laser fields with nano and micro mechanical objects that can be used for quantum information. Ultimately, careful engineering of strong interaction of light with atom-like nanomechanical oscillators paves the way for fabricating aritifical atoms with tunable and controllable interactions.
At the MIT (Vuletic group), we have investigated quantum nonlinear photon-photon interactions mediated by trapped atoms inside a high-Q optical resonator. Interaction of this kind allows us to observe peculiar quantum phenomena such as non-destructive detection of a single photon, large conditional phase shift induced by one photon, cavity-induced cooling of the atomic motion, atomic spin squeezing and many more fascinating effects. A schematic drawing of cavity QED experimental setup is shown below.
At the ANU (CQC2T - Lam group), we have carried out experiments to optically detect and cool nano-mechanical oscillators namely Ag2Ga nanowires by means of photothermal forces. We have demonstrated that by periodic feedback cooling of a nanowire its force sensitivity can be enhanced.
To reach the quantum regime with optomechanical systems, it is critical to minimize the thermalization and decoherence processes by reducing their coupling to environmental thermal reservoirs. Thus far, this has necessitated the use of cryogenic operating environments or complicated fabrication of nanostructures exhibiting phononic bandgap characteristics. One of the main sources of mechanical dissipation and low mechanical quality factor is the coupling to the reservoir via clamping and material supports. What if we could eliminate this clamping altogether? One way to eliminate clamping loss is to use an optical tweezer to trap an object. In these cases the scattering due to the optical trapping beam can lead to heating of the object and therefore creates a thermal mechanical state.
At the ANU (CQC2T- Lam group), we proposed and theoretically investigated possibility of scattering-free levitation of a macroscopic cavity mirror. The suspended mirror forms part of a low loss optical cavity. The proposed optical suspension will therefore be fully coherent. This can potentially lead to observation of an extremely high mechanical quality factor. The proposed levitation experiment is schematically depicted below.
As a new frontier, quantum information technology promises vastly powerful computing and perfectly secure communication, while introducing its own complications. Unlike conventional optical information systems, quantum optical data cannot be stored as a set of measurement results. A requirement of a quantum memory is that information should be stored without any measurement or disturbance. Another important criterion for a quantum memory is that the efficiency of the recall must exceed 50%. This is the crucial no-cloning limit for security of information guaranteeing that nobody can eavesdrop the transmitted information by secretly reading out the memory. Therefore, the laws of quantum physics, such as the no-cloning theorem, can be used to guarantee the absolute security of transmitted information. Quantum information, however, is extremely fragile and susceptible to loss, and similar to classical communication, purification (amplification) of quantum (classical) information is essential for long-distance communication. Although, deterministic noiseless amplification of quantum information is fundamentally impossible as a consequence of the no-cloning theorem, non-deterministic amplification is allowed by quantum mechanics. To implement long distance quantum communication, so-called quantum repeater devices are proposed that rely on both non-deterministic noiseless amplifier as well as high fidelity quantum memory devices to extend the communication distance. These devices also need to be compatible with telecom-wavelength so they can be integrated into current infra-structure. There has been a significant progress towards realization of such repeater systems, however, the outstanding question still remains whether a practical quantum repeater can be realized to provide secure communication over long distances?
At the ANU (CQC2T- Lam Group), we have developed an efficient quantum memory for optical information that works by creating a frequency gradient of atomic absorption that can be coherently reversed to recall stored information. This kind of reversible absorption is called gradient echo memory (GEM). Based on this technique, we have demonstrated a memory with arbitrary access to stored optical information where a string of pulses can be stored and recalled on-demand in arbitrary order allowing re-sequencing of the stored information.
At the University of Louisville (ERINC, Cohn group), we have carried out viscosity sub-surface force measurements of polymeric solutions by means of nanowire-tipped AFM cantilevers. We demonstrated that long metal nanoneedles grown on AFM probes could give very clear measurements of the surface tension, evaporation rate and viscosity of liquid crystals and polymeric solutions. To perform this measurement, we controllably grew Ag2Ga nanowires on AFM cantilevers by real-time observation of the formation process under STM microscope. Later, the startup company Nauganeedles was founded to develop and commercialized this nanowire production technique.