Quantum Light’s Wave-Particle Balance Now Fully Tunable

Scientists are increasingly focused on harnessing the fundamental properties of quantum mechanics to advance computational technologies. Kan Takase and colleagues from the Department of Applied Physics, School of Engineering, The University of Tokyo, report a significant step forward in controlling quantum light, working in collaboration with researchers at the Advanced ICT Research Institute, National Institute of Information and Communications Technology, NTT Device Technology Labs, NTT Inc, and alongside international collaborators Petr Marek, Radim Filip, Warit Asavanant and Akira Furusawa. Their research details the experimental demonstration of tunable generation of intermediate quantum states, lying between wave-like and particle-like behaviours, using a technique called generalized photon subtraction. This ability to continuously control the wave-particle duality, achieved by detecting multiple photons from squeezed light, offers a pathway to creating high-rate, optimised quantum states crucial for enhancing fault-tolerant quantum computation and ultimately addressing a key limitation in the development of optical quantum computers. Recent work demonstrates the tunable generation of these intermediate states using a technique called generalised photon subtraction (GPS), allowing for continuous control over the wave-particle duality of light. By detecting up to three photons from squeezed-light sources with a photon-number-resolving detector, researchers have successfully manipulated the balance between wave- and particle-like features in quantum states. This approach constructs a spectral family of states with high generation rates, optimised for potential applications in fault-tolerant quantum computation. By accessing the full spectrum of these intermediate states has remained a challenge, as previous methods, relying on standard photon subtraction, were largely confined to producing states exhibiting strong wave-like behaviour. Now, this GPS method surpasses those limitations by accessing the entire range of a key parameter, ‘s0’, which governs the balance between wave- and particle-like character, specifically, across the range 0 ≤ s0. Such control is not merely a conceptual advance. But a practical necessity for improving the performance of quantum bits, or qubits — the implications extend to the development of Gottesman-Kitaev-Preskill (GKP) qubits, a promising architecture for fault-tolerant quantum computers using light. GKP qubits require specific quantum states as resources. The ability to adapt these states with precision addresses a central bottleneck in optical quantum computing. Once generated, these intermediate states can be used in a breeding protocol. Where multiple states are combined and measured to create the desired GKP qubit. The minimum number of photons needed for detection and the overall success probability of GKP qubit generation are strongly influenced by the characteristics of the initial resource states. Therefore, a technique capable of generating intermediate states with lower values of s0 is a vital step towards practical quantum computation.

Scientists have experimentally demonstrated GPS-based generation of these intermediate states, combining squeezed-light sources with a superconducting-nanostrip photon detector. Through conditioning on the detection of up to three photons, they mapped out the full spectrum of wave-particle duality. Establishing a tunable state generator for future fault-tolerant quantum computers. This effort provides a important step toward overcoming resource generation limitations and opening a pathway to realising fault-tolerant quantum computation with light. Generation of entangled photon pairs via parametric down-conversion and multi-photon detection Initially, a continuous-wave laser operating at 1545.32nm was amplified and then frequency-doubled to produce pump light at 772.66nm. In turn, this pump light served as the driving source for generating two independent squeezed-vacuum modes. These modes were created by directing the pump light into periodically-poled lithium niobate (PPLN) waveguides, each measuring 45mm in length. Achieving parametric gains of 2.11 and 2.05 respectively. Then, these squeezed states underwent interference on a variable beam splitter, constructed from two polarization beam splitters and a half-wave plate, allowing for precise control over the beam splitting ratio via rotation of the half-wave plate. Beyond the beam splitter, the resulting entangled state was directed towards a photon-number-resolving detector, specifically an arrayed superconducting-nanostrip photon detector (SNSPD). Still, this detector possesses the capability to register up to three photons, forming the basis of the generalised photon subtraction (GPS) technique — by conditioning the quantum state on the detection of a specific number of photons. Researchers were able to project the state onto a non-Gaussian manifold, and the choice of detecting up to three photons, rather than fewer. Was intended to broaden the range of accessible intermediate states and enhance the generation rate of desired quantum states. To achieve continuous control over the wave-particle duality necessitated careful calibration of the initial Gaussian entangled states. Even so, the parameter s0, governing the balance between wave-like and particle-like characteristics, was adjusted by manipulating the reflectivity of the variable beam splitter. At a fixed beam splitter setting, The team could then perform photon subtraction, generating a family of states with varying degrees of non-Gaussian character. A trialal setup was designed to allow for continuous tuning of s0, enabling the exploration of the entire spectrum of the method. To characterise the generated states, The team employed quadrature measurements, which provide information about the quantum state’s distribution in phase space, revealing the transition from wave-like (Schrödinger cat states) to particle-like (Fock states) behaviour as s0 was varied. Through analysing the quadrature distributions, the team confirmed the successful generation of the technique and validated the tunability of the GPS method. Such an approach differs from previous methods, which were largely confined to producing states with strong wave-like characteristics, as the current work provides access to the full range of s0 values. Tailoring quantum states across the full s0 parameter range for enhanced computation Measurements confirmed control over the parameter ‘s0’, which dictates the wave- and particle-like balance, across the complete range of 0 ≤ s0 ≤ 1. By employing generalised photon subtraction, researchers successfully generated a spectral family of quantum states with demonstrably high generation rates, optimised to meet the demands of fault-tolerant quantum computation. Accessing the full range of s0 values represents a substantial advancement in tailoring non-Gaussian quantum resources. A value of s0 approaching zero yields a state increasingly resembling a pure Fock state, indicative of strong particle-like behaviour, while larger values of s0 produce the technique pronounced wave-like features, displaying quantum interference fringes in phase space. The ability to continuously adjust s0 allows for the creation of hybrid states exhibiting intermediate characteristics. At the core of this effort lies the demonstration of tunable state generation using GPS. By detecting up to three photons, The project team achieved precise control over the quantum state’s properties. Since the parameter s0 governs the degree of non-Gaussianity, its full control is essential for optimising quantum information processing. The effort highlights the potential application of these states in breeding protocols for creating Gottesman, Kitaev, Preskill (GKP) qubits. Simulations suggest that this approach with s0 around 1 provide the most favourable performance as resource states for GKP qubit generation, requiring fewer detected photons and achieving higher success probabilities. This GPS method offers a pathway to efficiently generate the necessary resources for optical quantum computing. Beyond the experimental demonstration, The project establishes GPS as a flexible toolbox for tailoring non-Gaussian resources. Generalised photon subtraction unlocks tunable wave-particle duality for improved quantum qubit design The precise control of quantum states lying between wave and particle characteristics is now demonstrably within reach. A significant impediment to building practical optical quantum computers has been the difficulty in generating and manipulating these intermediate, non-classical states of light. Standard techniques, relying on photon subtraction, tended to favour strongly wave-like states. Limiting the potential for creating the more flexible quantum bits needed for fault tolerance. Recent work from Kan Takase and colleagues offers a solution, employing a technique called generalised photon subtraction to access the full spectrum of these elusive states. The implications extend beyond simply achieving a technical milestone, as continuously tuning the wave-particle duality, governed by a parameter ‘s0’ across its entire possible range, unlocks a pathway to better Gottesman-Kitaev-Preskill (GKP) qubits. Unlike prior schemes, this approach doesn’t force a compromise between wave-like coherence and particle-like discreteness. Instead offering a family of states optimised for quantum information processing. The project focuses on the generation and control of these states, rather than their integration into a fully functioning quantum computer. The ability to adapt quantum this approach such precision represents a substantial leap forward. Rather than being constrained by the limitations of previous methods, scientists now possess a more flexible toolbox for designing quantum resources. Beyond GKP qubits, this technique could prove valuable in other areas of quantum information science, potentially enabling new protocols for quantum communication and sensing. This advance is a welcome sign that the gap between theoretical possibility and practical realisation is steadily closing. 👉 More information 🗞 Tuning Wave-Particle Duality of Quantum Light by Generalized Photon Subtraction 🧠 ArXiv: https://arxiv.org/abs/2602.21629 Tags: