Coherent controllers for optical-feedback cooling of quantum oscillators

We study the cooling performance of optical-feedback controllers for open optical and mechanical resonators in the Linear Quadratic Gaussian setting of stochastic control theory. We utilize analysis and numerical optimization of closed-loop models based on quantum stochastic differential equations to show that coherent control schemes, where we embed the resonator in an interferometer to achieve all-optical feedback, can outperform optimal measurement-based feedback control schemes in the quantum regime of low steady-state excitation number. These performance gains are attributed to the coherent controller's ability to simultaneously process both quadratures of an optical probe field without measurement or loss of fidelity, and may guide the design of coherent feedback schemes for more general problems of robust nonlinear and robust control.Comment: 15 pages, 20 figures. Submitted to Physical Review X. Follow-up paper to arXiv:1206.082

Robust Control Design for Laser Cavity Squeezing in Quantum Optical Systems

Quantum control theory is a rapidly evolving research field, which has developed over the last three decades. Quantum optics has practical importance in quantum technology and provides a promising means of implementing quantum information and computing device. In quantum control, it is difficult to acquire information about quantum states without destroying them since microscopic quantum systems have many unique characteristics such as entanglement and coherence which do not occur in classical mechanical system. Therefore, the Lyapunov-based control methodology is used to first construct an artificial closed-loop controller and then an open-loop law is derived by simulation of the artificial closed-loop system. This work considers the stabilization of laser cavity as the main integral part of quantum optical systems through squeezing the output beam of the cavity. As a comprehensive example of this type of system, quantum optomechanical sensors are investigated. To this end, a nonlinear model of quantum optomechanical sensors is first extended incorporating various noises. Then, linear quadratic Gaussian (LQG) control method is used to tackle the problem of mode-squeezing in optomechanical sensors. Coherent feedback quantum control is synthesized by incorporating both shot noise and back-action noise to attenuate the output noise well below the shot noise level (Two waves are said to be coherent if they have a constant relative phase). In the second phase of this work, due to entanglement of the system with critical uncertainties and technical limitations such as laser noise and detector imprecision, robust H [infinity] method is employed for the robust stabilization and robust performance of the system in practice. In H [infinity] methods, a control designer expresses the control problem as a mathematical optimization problem and then finds the controller that solves this. The effectiveness of the proposed control strategy in squeezing the cavity output beam is demonstrated by simulation

Broadband Measurement and Reduction of Quantum Radiation Pressure Noise in the Audio Band

One hundred years after Albert Einstein predicted the existence of gravitational waves in his general theory of relativity, the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first direct detection of gravitational waves. Since the first detection of gravitational waves from a binary black hole merger, LIGO has gone on to detect gravitational waves from multiple binary black hole mergers, and more recently from a binary neutron star merger in collaboration with telescopes around the world. The detection of gravitational waves has opened a new window to the universe and has launched the era of gravitational wave astronomy. With the first detection of gravitational waves now two years behind us, work has already begun on improving the sensitivity of Advanced LIGO and planning for future generations of gravitational wave interferometers. One of the main limiting noise sources for current and future gravitational wave detectors is quantum noise, which includes quantum radiation pressure noise that originates from the quantum nature of the photons that reflect off of the test masses. Chapter one provides an introduction to gravitational wave sources and detectors. It also describes the noise sources that limit the sensitivity of interferometeric gravitational wave detectors like Advanced LIGO and includes a detailed description of the origin of quantum noise and its effect in interferometers. Chapter two introduces the concept and properties of optical springs. Much of the experimental work presented in the rest of this thesis utilizes an optical spring. This thesis investigates quantum radiation pressure noise and techniques to reduce quantum noise in gravitational wave interferometers. The experimental research contained in this thesis uses an optomechanical Fabry-Perot cavity in which one of the cavity mirrors is a microresonator consisting of a micro-mirror suspended by a cantilever structure. Chapter three outlines the design and construction of the optomechanical cavity that is housed in a vacuum chamber and sits on a suspended optical breadboard to provide isolation from seismic motion. Chapter three also includes details on the design of the cantilever micro-mirror used in the optomechanical cavity. The experiments in this thesis can be divided into two main categories: the characterization of optical springs and the measurement of broadband quantum radiation pressure noise. Chapter four of this thesis focuses on the characterization of optical springs. I present results from an experiment that uses radiation pressure to control an optomechanical cavity and investigates the feedback control needed to keep the system stable. In chapter five, I present results from an experiment in which we create an optical spring using a beamsplitter rather than the canonical example of an optical spring in a detuned Fabry-Perot cavity. Chapter six of the thesis describes the experiment and results of a broadband measurement of quantum radiation pressure noise. I present a measurement of a noise spectrum in which the effects of quantum radiation pressure noise are observed between 2 kHz and 90 kHz, including a frequency band between 10 kHz and 30 kHz where the quantum radiation pressure noise is visible above all other noise sources. Chapter seven presents the results from two experiments in which we have successfully reduced the amount of quantum radiation pressure noise. The first experiment is done by detecting the light that is transmitted through the cavity by a photodetector. By detecting the light in transmission of the cavity rather than reflection, we are able to evade the presence of quantum radiation pressure noise in the measurement. The second experiment injects bright squeezed light into the optomechanical cavity in place of the coherent field used in the experiment in chapter six. The injection of squeezed light into the optomechanical cavity successfully reduces the amount of quantum radiation pressure noise. Finally, having made a measurement of quantum radiation pressure noise and two measurements in which the quantum radiation pressure noise is reduced, I outline a future experiment to measure the ponderomotive squeezing that is produced by the optomechanical cavity and the plans for making a measurement below the Standard Quantum Limit

Coupled nonclassical systems for coherent backaction noise cancellation

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Silicon-Nitride Platform for Narrowband Entangled Photon Generation

CMOS-compatible photonic chips are highly desirable for real-world quantum optics devices due to their scalability, robustness, and integration with electronics. Despite impressive advances using Silicon nanostructures, challenges remain in reducing their linear and nonlinear losses and in creating narrowband photons necessary for interfacing with quantum memories. Here we demonstrate the potential of the silicon nitride (Si3N4) platform by realizing an ultracompact, bright, entangled photon-pair source with selectable photon bandwidths down to 30 MHz, which is unprecedented for an integrated source. Leveraging Si3N4's moderate thermal expansion, simple temperature control of the chip enables precise wavelength stabilization and tunability without active control. Single-mode photon pairs at 1550 nm are generated at rates exceeding 107 s-1 with mW's of pump power and are used to produce time-bin entanglement. Moreover, Si3N4 allows for operation from the visible to the mid-IR, which make it highly promising for a wide range of integrated quantum photonics applications.Comment: Please don't hesitate to email comments and suggestion

Enhanced nonlinear optomechanics in a coupled-mode photonic crystal device

The nonlinear component of the optomechanical interaction between light and mechanical vibration promises many exciting classical and quantum mechanical applications, but is generally weak. Here we demonstrate enhancement of nonlinear optomechanical measurement of mechanical motion by using pairs of coupled optical and mechanical modes in a photonic crystal device. In the same device we show linear optomechanical measurement with a strongly reduced input power and reveal how both enhancements are related. Our design exploits anisotropic mechanical elasticity to create strong coupling between mechanical modes while not changing optical properties. Additional thermo-optic tuning of the optical modes is performed with an auxiliary laser and a thermally-optimised device design. We envision broad use of this enhancement scheme in multimode phonon lasing, two-phonon heralding and eventually nonlinear quantum optomechanics.Comment: 20 pages, 9 figure

Subsystems for all-optical coherent quantum-noise cancellation

Quantum mechanics dictates that a measurement always disturb the measured system. In weak continuous measurements, the trade-off between measurement precision and back-action onto the system yields an optimal measurement sensitivity, which is known as the Standard Quantum Limit (SQL) in opto-mechanical measurements, such as gravitational-wave detection. It corresponds to finding the optimal optical power in a compromise between quantum shot noise and quantum radiation-pressure noise. Coherent quantum-noise cancellation (CQNC) aims at overcoming the SQL and reducing back-action noise via the introduction of an effective negative-mass oscillator. In an alloptical set-up, this oscillator is realised by a detuned optical resonator coupled to incoming light with a beam-splitter and a down-conversion interaction and needs to be matched to the measured system in resonance frequency, damping and coupling strengths. This thesis explores the nature of CQNC and a potential all-optical realisation in theory and experiment, with a particular emphasis on the beam-splitter and the down-conversion interaction. Two possible set-ups are compared theoretically and critical parameters determined. Available opto-mechanical devices were characterised and confirmed to be suitable for CQNC. The down-conversion coupling strength gDC is linked to experimentally obtainable parameters. More than 2.3 dB reduction in uncertainty of two-mode squeezed light were observed. The squeezing measurements yielded gDC = 2\pi\times200 kHz at 100mW pump power, which is well within the initially required range and is in agreement with results from two other measurement methods. Optical resonators coupled via a beam-splitter interaction are studied theoretically and experimentally. In this work, the beam-splitter interaction of strength gBS was realised by a wave plate. A simplified experiment design enabled stabilisation of the coupled resonators. Our experimental observations accurately confirmed our theoretical predictions. The observed mode splitting yielded gBS = 2\pi\times235 kHz, within the updated requirements. Losses and a limited measurement strength will be the limiting factors for CQNC. The updated set of parameters, backed by the conducted experiments, paves the way towards a reduction of radiation-pressure noise of up to 4.8 dB