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Register a new userQuantum structured light: Non-classical spin texture of twisted single-photon pulses
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Classical structured light with controlled polarization and orbital angular momentum (OAM) of electromagnetic waves has varied applications in optical trapping, bio-sensing, optical communications and quantum simulations. The classical electromagnetic theory of such structured light beams and pulses have advanced significantly over the last two decades. However, a framework for the quantum density of spin and OAM for single-photons remains elusive. Here, we develop a theoretical framework and put forth the concept of quantum structured light for space-time wavepackets at the single-photon level. Our work marks a paradigm shift beyond scalar-field theory as well as the paraxial approximation and can be utilized to study the quantum properties of the spin and OAM of all classes of twisted quantum light pulses. We capture the uncertainty in full three-dimensional (3D) projections of vector spin demonstrating their quantum behavior beyond the conventional concept of classical polarization. Even in laser beams with high OAM along the propagation direction, we predict the existence of large OAM quantum fluctuations in the transverse plane which can be verified experimentally. We show that the spin density generates modulated helical texture beyond the paraxial limit and exhibits distinct statistics for Fock-state vs. coherent-state twisted pulses. We introduce the quantum correlator of photon spin density to characterize the nonlocal spin noise providing a rigorous parallel with fermionic spin noise operators. Our work paves the way for quantum spin-OAM physics in twisted single photon pulses and also opens explorations for new phases of light with long-range spin order.
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Much of the richness in nature arises due to the connection between classical and quantum mechanics. In advanced science, the tools of quantum mechanics was not only applied in microscopic description but also found its efficacy in classical phenomena, broadening the fundamental scientific frontier. A pioneering inspiration is substituting Fock state with structured spatial modes to reconstruct a novel Hilbert space. Based on this idea, here we propose the classical analogy of squeezed coherent state for the first time, deriving classical wave-packets by applying squeezed and displacement operators on free space structured modes. Such a generalized structured light not only creates new degrees of freedom into structured light, including tunable squeezed degree and displacement degree but also exhibits direct correlation between quadrature operator space and real space. Versatile generalized classical squeezed states could be experimentally generated by a simple large-aperture off-axis-pumped solid-state laser. On account of its tunablity, we initially put forward a blueprint using classical structured light, an analogy of squeezed states to realize super-resolution imaging, providing an alternative way to beat diffraction limit as well as opening an original page for subsequent applications of high-dimensional structured light, such as high-sensitive measurement and ultra-precise optical manipulation.
Using background-free detection of spin-state-dependent resonance fluorescence from a single-electron charged quantum dot with an efficiency of 0:1%, we realize a single spin-photon interface where the detection of a scattered photon with 300 picosecond time resolution projects the quantum dot spin to a definite spin eigenstate with fidelity exceeding 99%. The bunching of resonantly scattered photons reveals information about electron spin dynamics. High-fidelity fast spin-state initialization heralded by a single photon enables the realization of quantum information processing tasks such as non-deterministic distant spin entanglement. Given that we could suppress the measurement back-action to well below the natural spin-flip rate, realization of a quantum non-demolition measurement of a single spin could be achieved by increasing the fluorescence collection efficiency by a factor exceeding 20 using a photonic nanostructure.
The study of light propagation has been a cornerstone of progress in physics and technology. Recently, advances in control and shaping of light have created significant interest in the propagation of complex structures of light -- particularly under realistic terrestrial conditions. While theoretical understanding of this research question has significantly grown over the last two decades, outdoor-experiments with complex light structures are rare, and comparisons with theory have been nearly lacking. Such situations show a significant gap between theoretical models of atmospheric light behaviour and current experimental effort. Here, in an attempt to reduce this gap, we describe an interesting result of atmospheric models which are feasible for empirical observation. We analyze in detail light propagation in different spatial bases and present results of the theory that the influence of atmospheric turbulence is basis-dependent. Concretely, light propagating as eigenstate in one complete basis is stronger influenced by atmosphere than light propagating in a different, complete basis. We obtain these results by exploiting a family of the continuously adjustable, complete basis of spatial modes -- the Ince-Gauss modes. Our concrete numerical results will hopefully inspire experimental efforts and bring the theoretical and empirical study of complex light patterns in realistic scenarios closer together.
Spatial modes of light can potentially carry a vast amount of information, making them promising candidates for both classical and quantum communication. However, the distribution of such modes over large distances remains difficult. Intermodal coupling complicates their use with common fibers, while free-space transmission is thought to be strongly influenced by atmospheric turbulence. Here we show the transmission of orbital angular momentum modes of light over a distance of 143 kilometers between two Canary Islands, which is 50 times greater than the maximum distance achieved previously. As a demonstration of the transmission quality, we use superpositions of these modes to encode a short message. At the receiver, an artificial neural network is used for distinguishing between the different twisted light superpositions. The algorithm is able to identify different mode superpositions with an accuracy of more than 80% up to the third mode order, and decode the transmitted message with an error rate of 8.33%. Using our data, we estimate that the distribution of orbital angular momentum entanglement over more than 100 kilometers of free space is feasible. Moreover, the quality of our free-space link can be further improved by the use of state-of-the-art adaptive optics systems.
Quantum random number generation (QRNG) harnesses the intrinsic randomness of quantum mechanical phenomena. Demonstrations of such processes have, however, been limited to probabilistic sources, for instance, spontaneous parametric down-conversion or faint lasers, which cannot be triggered deterministically. Here, we demonstrate QRNG with a quantum emitter in hexagonal boron nitride; an emerging solid-state quantum source that can generate single photons on demand and operates at room temperature. We achieve true random number generation through the measurement of single photons exiting one of four integrated photonic waveguides, and subsequently, verify the randomness of the sequences in accordance with the National Institute of Standards and Technology benchmark suite. Our results open a new avenue to the fabrication of on-chip deterministic random number generators and other solid-state-based quantum-optical devices.
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