Multi-photon entangled states of light such as N00N (N is the photon number) and squeezed states are routinely used in quantum metrological experiments. Spatially structuring these quantum optical states improves the sensitivity and enhances resolution by using the high dimensional spatial degree of freedom. However, progress in quantum structured light mainly focused on either increasing the number of photons, or increasing the mode numbers, separately [1]. The reason is that generating high (mode number>> 10) dimensional structured light states is usually a lossy process that degrades multi-photon states of light.
Multi-photon entangled states of light such as N00N (N is the photon number) and squeezed states are routinely used in quantum metrological experiments. Spatially structuring these quantum optical states improves the sensitivity and enhances resolution by using the high dimensional spatial degree of freedom. However, progress in quantum structured light mainly focused on either increasing the number of photons, or increasing the mode numbers, separately [1]. The reason is that generating high (mode number>> 10) dimensional structured light states is usually a lossy process that degrades multi-photon states of light.
ABSTRACT Structured light enhances quantum information processing by combining quantum states of light and shaped wavefronts. However, until now, the advantages of high photon‐number states and of structured light have only been demonstrated separately. Here, we report a rotation sensor that combines ‐dimensional spatial modulation with four‐photon quantum states, yielding significantly improved angular resolution that scales as their product, . Our work shows that it is possible to use both multi‐photon states of light together with high spatial mode numbers, opening avenues in quantum sensing, communications, and quantum computing.
Quantum-enhanced sensors that exploit N -photon entanglement surpass the shot-noise limit by N and achieve N -fold resolution gains [1, 2]. Structured light can further boost sensitivity [3]. However, shaping entanglement beyond two photons remains difficult because of inefficient optical setups and the low generation probability of multi-photon states. Consequently, demonstrations with high-order structured light have so far been restricted to N ≤ 2 [1], and larger- N interferometers lacked large spatial structure, limiting improvement to N -fold resolution. Our work unifies these capabilities.
Quantum states of light, such as squeezed states or entangled states, can be used to make measurements (metrology), produce images, and sense objects with a precision that far exceeds what is possible classically, and also exceeds what was once thought to be possible quantum mechanically. The primary idea is to exploit quantum effects to beat the shot-noise limit in metrology and the Rayleigh diffraction limit in imaging and sensing. Quantum optical metrology has received a boost in recent years with an influx of ideas from the rapidly evolving field of optical quantum information processing. Both areas of research exploit the creation and manipulation of quantum-entangled states of light. We will review some of the recent theoretical and experimental advances in this exciting new field of quantum optical metrology, focusing on examples that exploit a particular two-mode entangled photon state – the High-N00N state.
Structured light has become topical of late, where controlling light in all its degrees of freedom has offered novel states of light long predicted, enhanced functionality in applications, and a modern toolbox for probing fundamental science. Structuring light as single photons and entangled states allows the spatial modes of light to be used to encode a large alphabet, accessing high dimensional Hilbert spaces for fundamental tests of quantum mechanics and improved quantum information processing tasks. In this tutorial, we outline the basic concepts of high dimensional quantum states expressed in a basis of spatial modes (structured light) and explain how to create, control, and detect such quantum states in the laboratory with a focus on transverse spatial modes such as the orbital angular momentum and pixel (position) modes. Finally, we highlight some example applications of such quantum structured light, from communications to imaging.
Photonic N00N-states, i.e. states of light where N photons are in an extremal superposition between two orthogonal states 21(∣N,0⟩+∣0,N⟩) , have an increased phase-sensitivity in comparison to their classical counterparts. This phase sensitivity has been harnessed in multiple proof-of-principle experiments with one implementation even showing this advantage unconditionally [1] . Using the increased phase sensitivity, that quantum states of light offer, in conjunction with the increased angular sensitivity of states with large orbital angular momentum (OAM) [2] , has been the target of theoretical studies [3] as well experiments utilizing squeezed states [4] and entangled photon pairs [5] . Until recently however [6] , a reliable way of generating two-photon OAM N00N-states, in a single beam-line and for arbitrary OAM-values, had not been demonstrated.
The field of high-dimensional quantum photonics involves the use of multimode photonic degrees-of-freedom such as the spatial, temporal, or spectral structure of light to encode multi-level quantum states. Recent years have seen rapid progress in the development of methods to generate, manipulate, and distribute such quantum states of light and their use in a range of quantum technology applications that offer practical advantages over conventional qubit-based approaches. High-dimensional quantum states of light encoded in photonic time-bins, frequency-bins, transverse-spatial modes, waveguide paths, and temporal modes have enabled noise-robust fundamental tests of quantum mechanics, error-resilient and high-capacity quantum communication protocols, andas well as efficient approaches for quantum information processing, to name just a few examples. However, research in this field has progressed fairly independently, with little exchange across different photonic degrees-of-freedom or between experiment and theory and no comprehensive comparison between degrees-of-freedom. This roadmap aims to bridge this gap by surveying progress in each area and identifying shared challenges and opportunities that cut across two or more photonic degrees-of-freedoms. We review early work and state-of-the-art experimental techniques under development for high-dimensional quantum states encoded in single and entangled photons, as well as theoretical tools for their measurement and certification. We outline the main outstanding challenges for theory and each experimental degree-of-freedom, identifying promising future directions of research that may enable these to be overcome. We end by discussing interconnections and shared challenges centered around their distribution, measurement, and manipulation, with a view towards their integration into next-generation quantum technology platforms and applications.