Haitan Xu

Ultra-sensitive chip-based photonic temperature sensor using ring resonator structures

Resistance thermometry provides a time-tested method for taking temperature measurements. However, fundamental limits to resistance-based approaches has produced considerable interest in developing photonic temperature sensors to leverage advances in frequency metrology and to achieve greater mechanical and environmental stability. Here we show that silicon-based optical ring resonator devices can resolve temperature differences of 1 mK using the traditional wavelength scanning methodology. An even lower noise floor of 80 μK for measuring temperature difference is achieved in the side-of-fringe, constant power mode measurement.

Tunable Spin-Qubit Coupling Mediated by a Multielectron Quantum Dot.

We present an approach for entangling electron spin qubits localized on spatially separated impurity atoms or quantum dots via a multielectron, two-level quantum dot. The effective exchange interaction mediated by the dot can be understood as the simplest manifestation of Ruderman-Kittel-Kasuya-Yosida exchange, and can be manipulated through gate voltage control of level splittings and tunneling amplitudes within the system. This provides both a high degree of tunability and a means for realizing high-fidelity two-qubit gates between spatially separated spins, yielding an experimentally accessible method of coupling donor electron spins in silicon via a hybrid impurity-dot system.

Observation of optomechanical buckling transitions

Correlated phases of matter provide long-term stability for systems as diverse as solids, magnets and potential exotic quantum materials. Mechanical systems, such as buckling transition spring switches, can have engineered, stable configurations whose dependence on a control variable is reminiscent of non-equilibrium phase transitions. In hybrid optomechanical systems, light and matter are strongly coupled, allowing engineering of rapid changes in the force landscape, storing and processing information, and ultimately probing and controlling behaviour at the quantum level. Here we report the observation of first- and second-order buckling transitions between stable mechanical states in an optomechanical system, in which full control of the nature of the transition is obtained by means of the laser power and detuning. The underlying multiwell confining potential we create is highly tunable, with a sub-nanometre distance between potential wells. Our results enable new applications in photonics and information technology, and may enable explorations of quantum phase transitions and macroscopic quantum tunnelling in mechanical systems.

Optomechanics with high-contrast gratings

High-contrast gratings fabricated in free-standing membranes of silicon nitride are a remarkable new platform for optomechanics, as they combine high reflectivity, low mass, and a high mechanical quality factor in a single device. In an effort to further improve on our earlier designs, we are now fabricating high-contrast gratings from stoichiometric silicon nitride. The new gratings have a diameter of 80 μm, a thickness of 250 μm, and are patterned in square membranes from 100 μm to 500 μm on a side. We find reflectivities R < 0.994 for these devices, and fundamental mechanical resonance frequencies above 1.5 MHz. In addition, we have incorporated HCGs fabricated from low-stress silicon nitride into a “membrane-in-the-middle” setup, and observe that the cavity transmission spectrum is distorted from a constant free spectral range of 3 GHz to one characterized by anticrossings separated by 72 ± 2 MHz.

Thermodynamic limits for optomechanical systems with conservative potentials

The mechanical force from light -- radiation pressure -- provides an intrinsic nonlinear interaction. Consequently, optomechanical systems near their steady state, such as the canonical optical spring, can display non-analytic behavior as a function of external parameters. This non-analyticity, a key feature of thermodynamic phase transitions, suggests that there could be an effective thermodynamic description of optomechanical systems. Here we explicitly define the thermodynamic limit for optomechanical systems and derive a set of sufficient constraints on the system parameters as the mechanical system grows large. As an example, we show how these constraints can be satisfied in a system with

Nonlinear Optics via Hybrid Quantum Systems

\mathbb{Z}_2

From Membrane-in-the-Middle to Mirror-in-the-Middle with a High-Reflectivity Sub-Wavelength Grating

symmetry and derive a free energy, allowing us to characterize this as an equilibrium phase transition.

Optical temperature sensor and use of same

Realization of a quantum optical nonlinearity at the single-photon level remains an outstanding challenge. Here we consider optomechanical and microwave-domain approaches for such nonlinearities, and develop applications in quantum sensing and simulation.