Optical Cycling Research Articles

Reduced spin measurement back-action for a phase sensitivity ten times beyond the standard quantum limit

Fundamental quantum noise limits the precision of quantum-based detectors, for example limiting the ultimate precision of atomic clocks, which have applications in communication, navigation and tests of fundamental physics. Collective measurements of many quantum spins can project the ensemble into an entangled, spin-squeezed state with improved quantum-limited measurement resolution. However, measurement back-action has limited previous implementations of collective measurements to only modest observed enhancements in precision. Here, we experimentally demonstrate a collective measurement with reduced measurement back-action to produce and directly observe, with no background subtraction, a spin-squeezed state with phase resolution improved by a factor of 10.5(1.5) in variance, or 10.2(6) dB, compared to the initially unentangled ensemble of N = 4.8 × 105 87Rb atoms. The measurement uses a cavity-enhanced probe of an optical cycling transition, mitigating back-action associated with state-changing transitions induced by the probe. This work establishes collective measurements as a powerful technique for generating useful entanglement for precision measurements. The phase of a collection of spins is measured with a sensitivity ten times beyond the limit set by the quantum noise of an unentangled ensemble of 87Rb atoms. A cavity-enhanced probe of an optical cycling transition is employed to mitigate back-action associated with state-changing transitions induced by the probe.

Near‐Infrared Spectrally Selective Plasmonic Electrochromic Thin Films

A plasmonic electrochromic effect in which electrochemical doping reversibly modulates near-infrared surface plasmon absorption of aluminium-doped zinc oxide and tin-doped indium oxide nanocrystals is reported. Optical performance, switching kinetics, and cycling durability point to high-performance NIR selective plasmonic electrochromic coatings based on earth-abundant materials.

Laser cooling of a diatomic molecule

It has been roughly three decades since laser cooling techniques produced ultracold atoms, leading to rapid advances in a wide array of fields. Laser cooling has not yet been extended to molecules because of their complex internal structure. However, this complexity makes molecules potentially useful for a wide range of applications. For example, heteronuclear molecules possess permanent electric dipole moments that lead to long-range, tunable, anisotropic dipole-dipole interactions. The combination of the dipole-dipole interaction and the precise control over molecular degrees of freedom possible at ultracold temperatures makes ultracold molecules attractive candidates for use in quantum simulations of condensed-matter systems and in quantum computation. Also, ultracold molecules could provide unique opportunities for studying chemical dynamics and for tests of fundamental symmetries. Here we experimentally demonstrate laser cooling of the polar molecule strontium monofluoride (SrF). Using an optical cycling scheme requiring only three lasers, we have observed both Sisyphus and Doppler cooling forces that reduce the transverse temperature of a SrF molecular beam substantially, to a few millikelvin or less. At present, the only technique for producing ultracold molecules is to bind together ultracold alkali atoms through Feshbach resonance or photoassociation. However, proposed applications for ultracold molecules require a variety of molecular energy-level structures (for example unpaired electronic spin, Omega doublets and so on). Our method provides an alternative route to ultracold molecules. In particular, it bridges the gap between ultracold (submillikelvin) temperatures and the ∼1-K temperatures attainable with directly cooled molecules (for example with cryogenic buffer-gas cooling or decelerated supersonic beams). Ultimately, our technique should allow the production of large samples of molecules at ultracold temperatures for species that are chemically distinct from bialkalis.

Direct absorption imaging of ultracold polar molecules

We demonstrate a scheme for direct absorption imaging of an ultracold ground-state polar molecular gas near quantum degeneracy. A challenge in imaging molecules is the lack of closed optical cycling transitions. Our technique relies on photon shot-noise limited absorption imaging on a strong bound-bound molecular transition. We present a systematic characterization of this imaging technique. Using this technique combined with time-of-flight (TOF) expansion, we demonstrate the capability to determine momentum and spatial distributions for the molecular gas. We anticipate that this imaging technique will be a powerful tool for studying molecular quantum gases.

Optical cycling paves the way for laser-cooled molecules

Molecules have more degrees of freedom than atoms do, so guiding them into the same few quantum states is a challenge.

Radiative Force from Optical Cycling on a Diatomic Molecule

We demonstrate a scheme for optical cycling in the polar, diatomic molecule strontium monofluoride (SrF) using the X2Sigma+ --> A2Pi(1/2) electronic transition. SrF's highly diagonal Franck-Condon factors suppress vibrational branching. We eliminate rotational branching by employing a quasicycling N = 1 --> N' = 0 type transition in conjunction with magnetic field remixing of dark Zeeman sublevels. We observe cycling fluorescence and deflection through radiative force of an SrF molecular beam using this scheme. With straightforward improvements our scheme promises to allow more than 10(5) photon scatters, possibly enabling the direct laser cooling of SrF.

Photonic Clays: A New Family of Functional 1D Photonic Crystals

Clays have shown potential as intelligent optical sensing platforms when integrated into a one-dimensional photonic crystal (PC) environment. The clay component imparts intrinsic functionality to the multilayer system by combining the signature ion exchange with the tunable structural color of photonic crystals, giving rise to environmentally sensitive photonic clay architectures. We have fabricated different Laponite-based 1D PCs and clay defect PCs by simple bottom-up self-assembly methodologies and elaborate their working principles and chemically encoded optical response. Accessibility of the multilayer system to analytes is studied on the background of the barrier properties of clays and diffusion control by the mesoporous oxide layers. The time dependence of analyte uptake and the extent and driving force for analyte release are pointed out and discussed in the context of different interactions between the clay layers and analytes. We demonstrate the possibility of optical cycling associated with repeated analyte uptake and removal processes, rendering photonic clays recyclable and low cost sensing platforms with simple optical read-out.

Influence of coloring voltage on the optical performance and cycling stability of a polyaniline–indium hexacyanoferrate electrochromic system

An electrochromic system based on the multielectrochromic polyaniline (PANI) and pseudo-transparent indium hexacyanoferrate (InHCF) thin-film electrodes was studied in this work. In combination with a hybrid H +/K +-conducting solid polymer electrolyte—KCl-doped poly(2-acrylamido-2-methylpropanesulfonic acid) (K-PAMPS), a precoloring-free PANI–InHCF electrochromic device (ECD) with an active area of 3×3 cm 2 was fabricated and exhibited yellowish-green-blue multicolor electrochromism. From in situ spectroelectrochemical experiments, we found that the performance of a PANI/K-PAMPS/InHCF ECD was significantly affected by the operating voltages, especially by the coloring voltage. Both the bleached and yellowish state of the ECD could be attained reversibly by applying a voltage ranging from +1.5 to +1.7 V (InHCF vs. PANI). Different coloring voltages resulted in different optical properties and cycling stabilities, however. For instance, the device biased at −1.6 V (InHCF vs. PANI) showed a deep blue color, but the optical activity decayed quickly (less than 50 cycles) when the device was switched between +1.6 and −1.6 V. Nevertheless, the device could be reversibly operated between +1.6 and 0 V for several hundred cycles, although a narrower electrochromic extent (yellowish-to-green) was observed correspondingly. The optimization of the coloring voltage is therefore of paramount importance to the PANI/K-PAMPS/InHCF ECD.

Multiphoton femtosecond phase-coherent two-dimensional electronic spectroscopy

In this paper the authors compare 400 nm one-photon and 800 nm two-photon two-dimensional Fourier transform electronic spectra of the organic laser dye Coumarin 102 in methanol using collinear optical pulse sequences and phase cycling. Results from the two different experiments show differences in the photon echo peak positions and shapes, reflecting differences in the two-photon and one-photon selection rules.

Diamonds for scalable quantum information systems

Enormous potential lies in the weird quantum world for a new generation of information systems that address important and unsolved problems in secure communications, highperformance computing, data storage, and simulation. Surprisingly, single crystal diamond (SCD), long known for its allure as a gemstone, has just the right properties for fabricating critical components that will serve as building blocks of this new quantum technology. A key requirement for useful quantum information devices is the ability to create shared quantum entanglement among large numbers of quantum bits (qubits) independent of their physical locations. This is best done using optical photons, especially for long-distance secure quantum communication applications. However, because photons do not make good quantum memories, a system with long-lived qubits, such as spins and a highfidelity quantum interface to photons would be highly desirable. Although trapped ions are potentially capable of fulfilling this need, there is no substitute for solid-state implementation for ease of scaling and integration. Our approach is to use nitrogen-vacancy (NV) color centers in diamond. These consist of a nitrogen atom paired with a vacancy in the diamond lattice. They provide strong optical transitions that scatter photons efficiently enough to allow individual NVs to be easily visualized with a confocal microscope.1 Unlike most optical emitters, the NV has a ground state with three electron spin sublevels. Proximal nuclear spins provide additional degrees of freedom,2 resulting in numerous choices for qubit encoding. Spin coherence times for such qubits are unusually long, even at room temperature, rivaled only by ion qubits trapped in high vacuum. It is estimated that up to a million single qubit operations can be performed within the NV spin coherence lifetime. Through a fortunate interaction with the singlet excited states, the NV can be spin-polarized to a high degree by illumination with broadband light, such as an LED flashlight, even at room temperature. Equally fortuitous is the fact that one of the electron spin sublevels undergoes an optical cycling transition that is pure enough, at liquid helium temperatures, to allow single-shot spin readout.1 Even at room temperature, this cycling transition is adequate for such a readout if half of the emitted photons can be detected. Encouragingly, our recent theoretical estimates3 and pilot experiments4 with optical plasmon nanowires suggest this might soon be possible. At room temperature, we have demonstrated each of the key elements needed to build few-qubit quantum processors with enough functionality for quantum repeater nodes.5 Specifically, we showed that arbitrary electron spin coherences can be transferred to a nuclear spin where they can be stored for up to a fraction of second and recovered with high fidelity. Furthermore, the stored nuclear spin is sufficiently robust for optical re-initialization cycles of the electron spin, as required for a quantum repeater memory. We also observed multi-qubit interactions between the NV electron spin and nearby nuclear6 and electron7spins up to nanometers away. We are currently in the process of creating long-distance entanglement between pairs of NVs using optical measurements. So far, we have demonstrated narrowband, electrically tunable,8 optical transitions stable enough to allow spontaneous photons, emitted minutes apart, to produce optical interference fringes.9 We have also demonstrated optical Raman spin-flip transitions on one branch of the excited state while simultaneously maintaining an optical cycling transition on the other.10 In summary, we can report surprising progress in realizing scalable quantum information systems with NV color centers in diamond (see Table 1). In the future, we plan to develop large-scale photonic circuits that optically couple numerous NV qubits to construct advanced quantum processors, and highperformance spintronic devices.11

Raman spectroscopic studies of gasochromic a-WO 3 thin films

The gasochromic mechanism in amorphous tungsten oxide films has been investigated using Raman scattering measurements and optical cycling tests. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm −1 due to vibrations of the OW 6+O and W 6+O bonds, respectively. When the Pd/a-WO 3 films are gasochromically colored by exposure to diluted hydrogen gas, extra Raman peaks due to OW 5+O and W 5+O bonds appear at 330 and 450 cm −1, respectively. Optical cycling tests confirm that the Pd/a-WO 3 films can not be bleached without oxygen. The Raman spectra of a-WO 3 films bleached with isotopic heavy oxygen ( 18O) show that the gasochromic coloration in a-WO 3 films is directly related to the double injection of hydrogen ions and electrons.

Electrochromic properties of spin-coated nickel oxide films

Nickel 2-ethylhexanoate was spin-coated from organic solution, followed by pyrolysis in air at 380°C. The scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis showed that the spin-coated films consisted of fine particles of NiO crystallites, 10–20 nm in diameter. The optical response and cycling ability of the films were measured by applying a rectangular potential pulse between 0.0 and 0.5 V vs. SCE in a 1 M KOH electrolyte. The influence of the processing temperature on the electrochromic properties of the films was studied. Although the films pyrolyzed in air at 380°C showed the response time of 1–2 s for coloration/bleaching, the degradation was observed within a few thousands to 10 4 cycles. The cycling ability could be much improved by post heat-treatment at 500°C for 20 min in air without sacrificing the response speed.

Effect of supporting electrolyte on the polymerization of electrochemically deposited poly(o-anisidine) films

Poly(o-anisidine) films deposited galvanostatically exhibit the formation of different oxidation states of the polymer with varying pH of the solution. The influence of supporting electrolyte on the extent of formation of the conducting phase at pH 2.1 has been investigated with the help of optical, thermal and potential cycling techniques. The optical spectra of the films reveal the proportionate increase in the fraction of the conducting emeraldine salt phase with the amount of supporting electrolyte (NaCl) added to the solution. Similarly, clarity in the decomposition steps of the thermal patterns and redox peaks in the cyclic voltammograms are found to increase as a function of NaCl concentration.

An Optical Study of Hydrogen Insertion in the Anodic Oxide of Tungsten

Ellipsometry is used to study electrochromic processes in anodic oxide films on tungsten electrodes. The oxide films are grown by galvanostatic anodization in , and electrochromic cycles are applied at 5V intervals beginning at 10V. The anodic oxide is transparent and has in its initial (bleached) state. After a cathodic cycle designed to give a hydrogen to oxygen ratio (assuming 100% efficiency) the film is optically absorbing (colored) and has an extinction coefficient . For anodizing potentials below 50V the optical cycling data exhibit little hysteresis and fit a model in which the hydrogen concentration builds up uniformly in the film. We conclude, therefore, that hydrogen moves freely through tungsten oxide even at a low concentration gradient. At higher anodizing potentials the optical cycling data exhibit progressively more hysteresis and fit neither a uniform buildup nor a stratified layer model of the conversion process. We attribute this behavior to inhomogeneity introduced by the transport of hydrogen along the pores which act as breakdown sites at higher potential. There is evidence that a substantial fraction of the hydrogen inserted during a coloring cycle is still present in the film when the field is reestablished at the end of the bleaching cycle. Since this is true for both short and extended cycles, the field reestablishment cannot be a consequence of a conductor‐insulator transition at a specific bulk hydrogen concentration in the film.

An Ellipsometric Study of Electrodeposition and Electrochemical Conversion of Lead Dioxide Films

An automated ellipsometer is used to study the galvanostatic electrodeposition of lead dioxide films onto a platinum substrate from an electrolyte containing lead acetate. The films exhibit regular optical properties over the thickness range from 0 to 600 nm. At a wavelength of 632.8 nm, the refractive index and extinction coefficient are 2.03 and 0.045, respectively. If the deposition process is assumed to require the transfer of two electrons per molecule deposited, the density of the deposited film is calculated to be 7.13 g/cm3. When the lead dioxide films are subjected to alternate anodic and cathodic galvanostatic cycles in neutral borate electrolyte, the optical data exhibit a complex but reproducible structure with inflections at points where the potential‐time curve is featureless. A limited quantitative analysis of the optical cycling data can be carried out by requiring consistency between results at widely different film thicknesses. It is possible that the complex optical cycling behavior is due to the presence of two different types of sites for hydrogen within the film.