Session M54: Quantum-Enhanced Sensing and Measurement

Stationary Inflection Point Based Hypersensitive Sensors with Enhanced Signal-to-Noise Ratio

Stationary inflection points (SIPs) are spectral singularities in the Bloch dispersion relations of periodic metamaterials. They emerge due to the formation of exceptional point degeneracies of the transfer matrix, indicating a coalescing of the Bloch modes of the system. At the SIP, both the group velocity and second derivative of the dispersion relation with respect to the wavenumber are equal to zero, allowing for slow light that is robust to losses and structural imperfections. One significant feature of SIP structures is the near-total conversion of an input signal into the slow mode, leading to the formation of the Frozen Mode Regime (FMR). We analyze the transport properties of nonlinear periodic structures exhibiting SIPs and their use in the realization of sublinear sensors, where the response of the reflectance is sublinear with respect to frequency detuning. We show that such sensors provide an enhanced signal-to-noise ratio, while also being hypersensitive to small perturbations. We demonstrate our proposal by using a complex photonic network that operates in the microwave regime.   

Non-contact friction in near-field optomechanical transducers

Nanophotonic devices offer excellent capabilities for probing neutral atoms, color centers, free electrons and nano-mechanical oscillators. In most applications, the near-field interaction, requires sub-wavelength gaps between the nanophotonic device and the system of interest. However, short-range interactions with the photonic structure can affect the delicate coherence properties of the quantum system. In this work, we study the interaction between a nano-mechanical resonator and a photonic crystal (PhC) cavity in a monolithic optomechanical system. A high-Q Si₃N₄ binary tree nanobeam resonator is suspended in parallel to the PhC with a few hundred nanometers gap. In the absence of the PhC, the fundamental mode of the nanobeam at 160 kHz has a quality factor of 70 million and a thermal-limited force sensitivity of 2 aN/sqrt(Hz) at room temperature. We observe a decrease in the Q of the fundamental mode with decreasing nanobeam-PhC gap––the Q decreases by more than an order of magnitude for gap of 200 nm. While the exact physical mechanism is still unclear, we present a systematic study of this phenomenon. The observed damping mechanism poses a challenge for realizing sensitive integrated optomechanical sensors.     

State-Insensitive trapping of alkaline-earth atoms in a nanofiber-based optical dipole trap

Neutral atoms that are optically trapped using the evanescent fields surrounding optical nanofibers are a promising platform for developing quantum technologies and exploring fundamental science, such as quantum networks and many-body physics of interacting photons. Building on the successful advancements with trapped alkali atoms, here we trap strontium-88 atoms, an alkaline-earth element, in a state-insensitive, nanofiber-based optical dipole trap using the evanescent fields of an optical nanofiber. Employing a two-color, double magic-wavelength trapping scheme, we realize state-insensitive trapping of the atoms for the kilohertz-wide ¹S₀−³P₁,|m|=1 intercombination transition, which we verify by performing high-resolution spectroscopy for an atom-surface distance of about 300 nm. Alkaline-earth atoms also exhibit nonmagnetic ground states and ultranarrow linewidth transitions making them ideal candidates for atomic clocks and precision metrology applications, especially with state-insensitive traps. Additionally, given the low collisional scattering length specific to strontium-88, this work also lays the foundation for developing versatile and robust matter-wave atomtronic circuits over nanophotonic waveguides.   

Application of optimal control to improve shaking protocols for quantum accelerometers

With the design and construction of fault-tolerant quantum computers still a number of years away, a more near-time application of quantum technology may lie in the field of quantum sensing. Here the interaction of quantum systems with their environment is used to extract information about the environment's properties – for instance the strength of a magnetic field or its acceleration with respect to a reference frame. Shaken lattice is an example to realize a quantum sensor to detect the latter: A cloud of cold atoms is trapped in a configurable optical lattice and manipulated by moving ("shaking") the lattice. The final momentum distribution of the atoms yields information about the presence of external accelerating forces on the atoms, where the sensitivity is dependent on the shaking control. With the goal to design high-precision sensors in mind, this inevitably motivates the application of quantum optimal control techniques to find shaking protocols that improve the system's sensitivity. In this talk we discuss the common approach derived from Mach-Zehnder interferometry and present two optimal control strategies to push the sensitivity.   

Coherent manipulation of nuclear spins in the strong driving regime

Spin-based quantum sensing and quantum computing rely on manipulating the spin state of the sample of interest or information qubits. Fast spin state manipulation is desirable to expedite experiments, to enhance sensitivity, and to enable application of elaborate pulse sequences with a limited coherence time. Strong driving using intense radio-frequency (RF) fields can thus facilitate fast manipulation and enable broadband excitation of spin species.

Here, we designed and implemented a spiral RF antenna capable of delivering intense fields to a sample. The planar antenna is tailored for quantum sensing experiments using the diamond's nitrogen vacancy (NV) center. We demonstrate the antenna by driving Rabi oscillations in $^1$H spins in an immersion oil sample on the diamond’s surface and measure Rabi frequencies higher than 500 kHz, corresponding to pi-pulses shorter than 1 $mu$s.

We discuss the implications of driving spins in a regime where the driving strength is comparable to the spin-state splitting, i.e., where the rotating wave is no longer a valid approximation. We present a simple recipe to optimize pulse fidelity in this regime, considering various driving field angles.   

Stress and field sensing behavior of the NV center in a nanodiamond: First principles density functional theory and experimental approach

The nitrogen-vacancy (NV) center in a nanodiamond (ND) crystal is a promising material for quantum information processing, sensing, and computing applications. It is one of the best candidate materials for quantum sensing and metrology applications at elevated temperatures and pressures. A number of studies have shown that a negatively charged NV center in an ND crystal could be used for magnetic field sensing with sensitivity up to an order of a few nano Tesla per square Hertz. In this work, we present computational and analytical modeling of electronic structures of an NV center under the application of stress. Ground state band structures are analyzed to capture the effect of stress on the zero-phonon lines and spin sublevels. In addition, experimental studies for the optically detected magnetic resonance (ODMR) are presented for field sensing applications. Results for transverse spin relaxation measurements for the NV center are also discussed. The presentation concludes by providing a model for free spins detection of the rare earth ions.   

Towards faster and quantum-noise limited maser materials; developments in miniaturising and searching for new organic maser gain media

There has been rapid development for masers at room temperature, the microwave analogue of lasers, where their ability to amplify the weakest electrical signals and superb frequency stability give them the potential to revolutionise medical sensors and frequency standards for GPS communication. The talk will be about a promising new material for room temperature masers; diazapentacene doped para-terphenyl (DAP). DAP holds the record as the maser with the fastest startup time, can be pumped by longer wavelengths of red light, and is more chemically stable than the first room temperature maser made from pentacene. We will also report on recent successes on the miniaturisation of organic masers, including pentacene, through optimized dielectric cavities and pumping sources. We have produced a fully operating device with the highest masing peak powers yet achieved, all while forgoing bulky magnetic fields and being able to demonstrate strong coupling for cavity quantum electrodynamics experiments at room temperature. The extensive optical and paramagnetic characterisations that will be presented will provide fertile ground for studying and finding new maser materials for quantum sensors, spintronics, and the exploration of tailored materials in optoelectronics.   

Sub-ppm Nanomechanical Absorption Spectroscopy of Silicon Nitride

Material absorption is a key limitation in nanophotonic systems; however, its characterization is often obscured by diffraction and scattering loss. Here we show that nanomechanical frequency spectroscopy can be used to characterize the absorption of a dielectric thin film at the parts-per-million level, and use it to characterize the absorption of stoichiometric silicon nitride (Si3N4), a ubiquitous low-loss optomechanical material. Specifically, we track the frequency shift of a high-Q Si3N4 trampoline resonator in response to photothermal heating by a ~10 mW laser beam, and infer the absorption of the thin film from a model including thermal stress relaxation and both radiative and conductive heat transfer. A key insight is the presence of two thermalization timescales, a short (~100 msec) black-body thermalization of the thin film resonator, and a long (~100 sec) thermalization of the silicon substrate due to conductive heating. We infer the extinction coefficient of Si3N4 to be between 0.1 and 1 ppm in the 633 - 950 nm range, lower than previous upper bounds set by waveguide resonators and membrane-in-the-middle cavity optomechanics. Our approach is applicable to a broad variety of photonic materials and may offer new insights into their potential.   

Polynomial Generated Bosonic Code for Environment-Assisted Quantum Transduction

Recent findings suggest that engineering the environment of a bosonic channel using a non-Gaussian state can enhance its quantum capacity. Such an environment-assisted scheme is extremely valuable for quantum transduction schemes where limited interaction is available. It also holds significance for quantum communication with lossy bosonic modes. This work introduces a novel family of bosonic codes generated by polynomials, capable of transmitting quantum information with high fidelity under an arbitrary weak inter-mode coupling constant. Through detailed simulations and analysis, we demonstrate that our solution offers several advantages and is both feasible and practical for near-future realization.   

Using Feynman Diagrams to Analytically Compute Higher Order Quantum Corrections to Atom Interferometer Phase Shifts

In atom interferometry, the differential phase accumulated between two arms due to spatially varying gravitational fields is often analyzed under a semi-classical approximation that disregards the finite spatial extent of an atom's wavefunction. Deviations from this approximation have not yet been measured definitively, but as atom interferometers become more sensitive, higher order quantum corrections could potentially be observed. These measurements would offer insight into the connection between quantum mechanics and gravity and reveal a novel source of systematic error. We introduce a Feynman diagram based approach to analytically computing the phase shift in an atom interferometer which incorporates these higher-order quantum corrections. This approach can also be used to calculate phase shifts caused by spatially varying magnetic fields and optical potentials.   

Harnessing Quantumness of States using Discrete Wigner Functions and Protecting it using Weak Measurement from Non-Markovian Quantum Noise

The negativity of the discrete Wigner functions (DWFs) is a measure of non-classicality and is often used to quantify the degree of quantum coherence in a system. Studying Wigner's negativity and its evolution under different noisy non-Markovian quantum channels provides insight into the stability and robustness of quantum states. The variation of DWF negativity of qubit, qutrit, and two-qubit systems under the action of (non)-Markovian random telegraph noise and amplitude damping noise is investigated. Different negative quantum states that can be used as a resource for quantum computation and quantum teleportation are constructed. Quantum computation and teleportation success is estimated for these states under (non)-Markovian evolutions. Weak measurement (WM) and quantum measurement reversal (QMR) protect against quantum states' collapse and are used to preserve and enhance quantum correlations and universal quantum teleportation protocol.

We study the quantum correlations, maximal fidelity, and fidelity deviation of the two-qubit negative quantum states developed using DWFs with (without) WM and QMR. We evolve the states via the above-mentioned non-Markovian channels to consider the effect of the noisy environment. To benchmark the performance of negative quantum states, we compare our results with the two-qubit maximally entangled Bell state. Interestingly, we observe that some negative quantum states perform better with WM and QMR than the Bell state for different cases under evolution via noisy quantum channels.