Session C08: Sensing, Metrology, and Communications with Light

Optimized measurements for multiple state discrimination at the single-photon level

Measurements for nonorthogonal states of light, such as coherent states, are central for quantum and optical communication. The nonorthogonality of coherent states with small amplitude allows for practical implementations of quantum key distribution for secure communication. However, this nonorthogonality also prevents measurements from perfectly distinguishing among different coherent states, which fundamentally limits the amount of transmitted information in optical communication, particularly at low powers. Here, I describe our work on optimized measurements of nonorthogonal coherent states of light based on photon counting, optical displacement operations, and feedback. These elements provide a way to optimize measurements with enhanced sensitivities and reach sensitivity levels surpassing the limit of conventional Gaussian measurements, referred to as the quantum noise limit (QNL). We experimentally investigate various optimized strategies for discrimination of multiple nonorthogonal states with different sensitivities. Our experimental demonstration achieves discrimination below the QNL at the single-photon level in the presence of noise and loss. These optimized measurements can be used for enhancing information transfer in communication compared to the ideal Heterodyne limit in lossy channels and may be useful for quantum communication.   

Quantum Enhanced Plasmonic Sensing

One of the long-standing goals of quantum optics has been the use of quantum states of light to enhance the sensitivity of devices. Plasmonic sensors, which are widely used in biological and chemical sensing applications and serve as a robust diagnostic tool, offer a unique opportunity to bring such an enhancement to real-life devices. In this talk I will describe our work on the interface between quantum states of light, known as twin beams, and plasmonic sensors that consist of an array of subwavelength nanoholes. In particular, I will present recent experiments that show that continuous-variable entanglement survives the transduction, or transfer from photons to plasmons and back to photons, through a plasmonic structure and that the reduced noise properties of the twin beams can enhance the sensitivity of plasmonic sensors for refractive index measurements. We have shown a quantum enhancement of 56\% with respect to the shot noise limit for plasmonic sensors operating at the current state-of-the-art.   

Coherent storage and processing of broadband light via the Autler-Townes effect in cold Rb atoms

Techniques for coherently controlling light with matter open up the possibility for storing and manipulating optical signals, including those at the quantum level. These ideas were revolutionized by the realization of electromagnetically induced transparency (EIT), which relies upon quantum interference and leads to the well-known phenomenon of slow-light. In our laboratory, we have found that optical signals can be coherently controlled outside of the regime of EIT and quantum interference, which is described by the Autler-Townes effect and does not require slow light. We have developed a new Autler-Townes splitting (ATS) protocol that facilitates dynamically controlled coherent storage and manipulation of optical signals, and which can be implemented in almost any physical three-level system due to its robustness to decoherence. This technique, which relies on the absorption of the signal over a wide spectral region, is inherently broadband, well-suited to quantum memory applications, and reduces many of the technical constraints imposed by other memory techniques. We experimentally demonstrate the proof-of-principle of this technique for several applications in a laser-cooled sample of $^{87}$Rb atoms: the storage of short (30 ns) optical signals; the temporal/spectral compression and stretching of optical pulses; coherent temporal and spectral beamsplitting operations; and wavelength conversion. Finally, we show that weak optical pulses with less than one average photon per pulse can be stored and retrieved with this method, demonstrating the low-noise operation of our approach for applications in quantum information processing.   

Boson-Sampling-Inspired Quantum Metrology

Aaronson and Arkhipov recently used computational complexity theory to argue that classical computers very likely cannot efficiently simulate linear, multimode, quantum-optical interferometers with arbitrary Fock-state inputs in what is known as Boson Sampling. Here we present an elementary argument that utilizes only techniques from quantum optics. We explicitly construct the Hilbert space for such an interferometer and show that its dimension, in the number-path degrees of freedom, scales exponentially with all the physical resources. Since number-path entanglement, such as in N00N states, is a resource for quantum metrology, we show theoretically and experimentally how such interferometers can beat the shotnoise limit using only single photons, single photon detectors, and linear optics.