Nanofiber-based high-Q microresonator for cryogenic applications

Abstract

We demonstrate a cryo-compatible, fully fiber-integrated, alignment-free optical microresonator. The compatibility with low temperatures expands its possible applications to the wide field of solid-state quantum optics, where a cryogenic environment is often a requirement. At a temperature of 4.6 K we obtain a quality factor of (9.9 ± 0.7) × 10 6 . In conjunction with the small mode volume provided by the nanofiber, this cavity can be either used in the coherent dynamics or the fast cavity regime, where it can provide a Purcell factor of up to 15. Our resonator is therefore suitable for significantly enhancing the coupling between light and a large variety of different quantum emitters and due to its proven performance over a wide temperature range, also lends itself for the implementation of quantum hybrid systems. IntroductionAchieving efficient coupling between quantum emitters and light is an important goal of research in quantum communication and computation, sensor applications and fundamental studies. Such an efficient interaction can for example be realized by means of an optical cavity or by reducing the mode area of the light field to the size of the emitter's interaction cross-section. Optical nanofibers offer such a strong transverse confinement of the light field.This makes them a versatile tool for interfacing different types of emitters such as atoms [1-3], single molecules [4], quantum dots [5][6][7] and color centers in diamond [8,9]. For many proof-of-principle experiments, cold atoms were the emitters of choice as they represent a well-controlled and isolated system. However, the experimental overhead for a cold-atom set-up is significant and solid-state emitters are much more suitable for practical and scalable platforms for quantum networks or nanosensors [10,11].Yet, the optical transitions of solid state emitters also couple to the phononic degrees of freedom of the system, leading to dephasing and inelastic scattering. In order to avoid this problem, the phonons of the system have to be frozen out and the branching ratio of emission into the coherent zero-phonon line has to be maximized. Further, cryogenic temperatures may be required to be able to spectrally address individual solid state emitters in the case, where many are present in the same host system [4]. This calls for a cryo-compatible optical microresonator with high quality factor, Q, that selectively accelerates the desired optical transition via the Purcell effect [12][13][14][15].Here, we show that a fully fiber-based optical microresonator that consists of a tapered optical fiber with two integrated fiber Bragg gratings (FBGs) [16,17], as demonstrated in [18], can be employed at cryogenic temperatures. In particular, we confirm that a high Q factor prevails after contact gas-cooling from room temperature to liquid helium temperature. Consequently, the resonator is still compatible with reaching the strong coupling regime. Furthermore, for usage at cryogenic temperatures, the alignment-free character of our resonator represen...

Thermal tuning of a fiber-integrated Fabry-Pérot cavity

“…The temperature control together with the finite width of the stop bands of the individual fiber Bragg gratings that function as the mirrors of this Fabry-Perot type cavity has further allowed us to reversibly and controllably omit and re-establish the cavity effect. This can be especially important when coupling solid-state quantum emitters to such cavities as can be readily done by placing an emitter on a tapered fiber section between the gratings [13], thereby creating a high-Q alignment-free cavity. This tunability then allows for convenient reference measurements with and without the cavity, something which is otherwise often not possible with permanently deposited solid-state quantum emitters.…”

“…to two spectral lines at a time. The highest finesse at a particular wavelength then indicates the wavelength of best overlap of the two stop bands at a particular temperature of the second grating [13].The finesse is plotted as a function of grating temperature in Fig. 2.…”

“…By using two gratings to form a Fabry-Perot cavity they have also shown the potential to function as narrowband filters [10,11]. Their suitability for forming an alignment-free Fabry-Perot type cavity [12] has also opened up their application in the field of quantum optics, where the fiber connecting two such fiber Bragg gratings can be readily tapered to guide the light between the gratings as an evanescent wave [13][14][15][16]. This allows for efficient coupling of quantum emitters to the guided light field of such nanofiber-based cavities.…”

Spontaneous emission and energy shifts of a Rydberg rubidium atom close to an optical nanofiber

“…Here, we envision experiments performed at a few degrees Kelvin in a cryogenic environment, as recently described in Ref. [38]. In such conditions, the effect of black-body radiation is negligible.…”

Feedback-cooling the fundamental torsional mechanical mode of a tapered optical fiber to 30 mK

“…In the future, cooling to lower temperatures could be achieved by placing the TOF in a colder environment, e.g., in a commonly available 4K cryostat [41], where k B T /h is only 83 GHz. In this case, our Qf product would be sufficiently large for ground-state cooling, provided that the mechanical properties are not altered and that the nanofiber thermalizes at this temperature.…”