Garrett D. Cole

Demonstration of an ultracold micro-optomechanical oscillator in a cryogenic cavity

Preparing and manipulating quantum states of mechanical resonators is a highly interdisciplinary undertaking that now receives enormous interest for its far-reaching potential in fundamental and applied science. Up to now, only nanoscale mechanical devices achieved operation close to the quantum regime. We report a new micro-optomechanical resonator that is laser cooled to a level of 30 thermal quanta. This is equivalent to the best nanomechanical devices, however, with a mass more than four orders of magnitude larger (43 ng versus 1 pg) and at more than two orders of magnitude higher environment temperature (5 K versus 30 mK). Despite the large laser-added cooling factor of 4,000 and the cryogenic environment, our cooling performance is not limited by residual absorption effects. These results pave the way for the preparation of 100-m scale objects in the quantum regime. Possible applications range from quantum-limited optomechanical sensing devices to macroscopic tests of quantum physics.

Pulsed quantum optomechanics

Studying mechanical resonators via radiation pressure offers a rich avenue for the exploration of quantum mechanical behavior in a macroscopic regime. However, quantum state preparation and especially quantum state reconstruction of mechanical oscillators remains a significant challenge. Here we propose a scheme to realize quantum state tomography, squeezing, and state purification of a mechanical resonator using short optical pulses. The scheme presented allows observation of mechanical quantum features despite preparation from a thermal state and is shown to be experimentally feasible using optical microcavities. Our framework thus provides a promising means to explore the quantum nature of massive mechanical oscillators and can be applied to other systems such as trapped ions.

Observation of non-Markovian micromechanical Brownian motion.

All physical systems are to some extent open and interacting with their environment. This insight, basic as it may seem, gives rise to the necessity of protecting quantum systems from decoherence in quantum technologies and is at the heart of the emergence of classical properties in quantum physics. The precise decoherence mechanisms, however, are often unknown for a given system. In this work, we make use of an opto-mechanical resonator to obtain key information about spectral densities of its condensed-matter heat bath. In sharp contrast to what is commonly assumed in high-temperature quantum Brownian motion describing the dynamics of the mechanical degree of freedom, based on a statistical analysis of the emitted light, it is shown that this spectral density is highly non-Ohmic, reflected by non-Markovian dynamics, which we quantify. We conclude by elaborating on further applications of opto-mechanical systems in open system identification.

Cooling-by-measurement and mechanical state tomography via pulsed optomechanics

Observing a physical quantity without disturbing it is a key capability for the control of individual quantum systems. Such back-action-evading or quantum non-demolition measurements were first introduced in the 1970s for gravitational wave detection, and now such techniques are an indispensable tool throughout quantum science. Here we perform measurements of the position of a mechanical oscillator using pulses of light with a duration much shorter than a period of mechanical motion. Utilizing this back-action-evading interaction, we demonstrate state preparation and full state tomography of the mechanical motional state. We have reconstructed states with a position uncertainty reduced to 19u2009pm, limited by the quantum fluctuations of the optical pulse, and we have performed cooling-by-measurement to reduce the mechanical mode temperature from an initial 1,100 to 16u2009K. Future improvements to this technique will allow for quantum squeezing of mechanical motion, even from room temperature, and reconstruction of non-classical states exhibiting negative phase-space quasi-probability.

Femtosecond laser fabrication of high reflectivity micromirrors

High-quality freestanding micromirrors consisting of 40 dielectric layers on silicon have been fabricated by ultrashort-pulse laser ablation in combination with laser-assisted wet chemical etching. Backside material removal enables direct access to both faces of the dielectric coating. The amplitude reflectance of the micromirrors has been determined by Fabry–Perot interferometry; a finesse in excess of 8900±700, corresponding to a reflectivity exceeding 99.95%, has been found. The mechanical quality factor, Q, of the microresonators, measured at 20 K, is determined to be between 5000 and 6000.

Megahertz monocrystalline optomechanical resonators with minimal dissipation

We present detailed experimental and theoretical results for novel micro-optomechanical resonators, representing a significant improvement in the performance of such structures. These devices exhibit eigenfrequencies (fr) approaching 4 MHz, reflectivities exceeding 99.98% at 1064 nm, and mechanical quality factors (Q) of 0.8 × 105 (measured at 20 K and 2.5 × 10-7 millibar pressure); yielding a Q·fr product of 3.1 × 1011 Hz, while enabling a finesse of approximately 20,000 when used as an end mirror in an impedance-matched Fabry-Perot optical cavity. These results represent a breakthrough in the development of optomechanical devices applicable to the emerging field of quantum optomechanics.

Room temperature continuous wave mid-infrared VCSEL operating at 3.35um

Tunable vertical cavity surface emitting lasers (VCSELs) offer a potentially low cost tunable optical source in the 3-5 μm range that will enable commercial spectroscopic sensing of numerous environmentally and industrially important gases including methane, ethane, nitrous oxide, and carbon monoxide. Thus far, achieving room temperature continuous wave (RTCW) VCSEL operation at wavelengths beyond 3 μm has remained an elusive goal. In this paper, we introduce a new device structure that has enabled RTCW VCSEL operation near the methane absorption lines at 3.35 μm. This device structure employs two GaAs/AlGaAs mirrors wafer-bonded to an optically pumped active region comprising compressively strained type-I InGaAsSb quantum wells grown on a GaSb substrate. This substrate is removed in processing, as is one of the GaAs mirror substrates. The VCSEL structure is optically pumped at room temperature with a CW 1550 nm laser through the GaAs substrate, while the emitted 3.3 μm light is captured out of the top of the device. Power and spectrum shape measured as a function of pump power exhibit clear threshold behavior and robust singlemode spectra.

Low-Noise Substrate-Transferred Crystalline Coatings for Precision Interferometry

We outline advances in ultralow Brownian noise substrate-transferred crystalline coatings with key results being the demonstration of optical performance rivaling that of IBS multilayers including sub-ppm absorption and scatter losses down to 3 ppm.

Crystalline coatings for near-IR ring laser gyroscopes

Substrate-transferred crystalline coatings represent an entirely new concept in high-performance optical coatings. This technology was originally developed as a solution to the long-standing thermal noise limitation found in ultrastable optical interferometers, impacting cavity-stabilized laser systems for precision spectroscopy and optical atomic clocks, as well as interferometric gravitational wave (GW) detectors [1]. The ultimate stability of these systems is currently dictated by coating Brownian noise, driven by the excess mechanical losses of the materials that comprise the highly reflective elements of the cavity end mirrors. Compared with state-of-the art ion-beam sputtered dielectric reflectors, crystalline coatings, comprising substrate-transferred GaAs/AlGaAs heterostructures, exhibit competitive reflectivity together with a significantly enhanced mechanical quality, resulting in a thermally-limited noise floor consistent with a tenfold reduction in mechanical damping at room temperature [2]. Building upon this initial demonstration, we have recently developed high-performance crystalline supermirrors with parts-per-million levels of optical losses, including both absorption and scatter, at wavelengths spanning 1000 to nearly 4000 nm, with experimentally verified absorption coefficients below 0.1 cm-1 in the near infrared [3]. These advancements have opened up additional application areas including the focus of this work. Here we demonstrate the first implementation of crystalline supermirrors in an active laser system, expanding the core application area of these low-thermal noise cavity end mirrors to inertial sensing systems and specifically next-generation high-sensitivity ring-laser gyroscopes [4,5].

Quantum optical control of micro-mechanical resonators

Optomechanical interactions in high-finesse cavities offer a new promising route for the ongoing experimental efforts to achieve and to control the quantum regime of massive mechanical systems using the available toolbox of quantum optics. For example, they allow to cool mechanical degrees of freedom of movable mirrors via radiation-pressure backaction, in principle even into their quantum ground state. Ground-state cooling will eventually require to realize the scheme in a cryogenic environment. We have taken this next step and realized stable operation of a high-finesse cavity inside a 4He cryostat [1]. This allowed us to show radiation-pressure laser cooling of a micro-mechanical kHz resonator from a base temperature of 5 K to approximately 1.3 mK, which corresponds to a thermal occupation factor of ≪n≫ = 32 ± 4 [2]. Heating effects, e.g. due to absorption of photons in the micromirror, could not be observed. The cooling performance is only limited by the thermal coupling to the environment, which can be further reduced by improving the mechanical quality factor of the mechanical resonator and by further reducing the environmental temperature. We will discuss the relevance of these results for the preparation and control of mechanical quantum states.