Centre for Quantum Technologies

The narrowness of the optical transitions of rare-earth-ion dopants makes them highly sensitive probes of their environment. We measured the optical transitions Er$^{3+}$ dopants to determine the previously unknown magnetic ordering of Gd$_{2}$SiO$_{5}$ -- a promising host for quantum applications of rare-earth dopants. By measuring the transitions' magnetic-field dependence we determined an antiferromagnetic ordering with spins oriented along or slightly canted from the crystal's $a^*$ axis. The optical transitions are narrower than the coupling to gadolinium spins revealing information about the coupling strengths. We further optically measured a Néel temperature of $1.86\pm0.01_\mathrm{stat.}\pm0.07_\mathrm{syst.}$ K, and assembled a phase diagram in applied field and temperature showcasing a triple point where two gadolinium sites order semi-independently from each other. At high applied field the erbium dopants show long optical coherence times up to 0.4 ms at 3 T; at low fields these are probably limited by three low-frequency magnon modes below 10 GHz, observed directly. This study can be used to benchmark a method of magnetic structure determination.

Optical control offers a non-contact, high-precision and ultrafast route to manipulating quantum material properties1-5. Fractional Chern ferromagnetic states in moiré superlattices are apromisingplatform by whichto pursue topological quantum computing6-10, but an effective optical control protocol has remained elusive. Here we demonstrate robust optical switching of integer and fractional Chern ferromagnets in twisted molybdenum ditelluride (MoTe2) bilayers using continuous-wave circularly polarized light. Highly efficient optical manipulation of spin orientations in the topological ferromagnet regime is realized at zero field using a pump light power as low as 28 nW µm-2. Using this optically induced transition, we also demonstrate magnetic bistate cycling and spatially resolved writing of ferromagnetic domain walls. This work establishes a reliable and efficient optical control scheme for moiré Chern ferromagnets, paving the way for dissipationless spintronics and quantized Chern junction devices.

On-chip integrable circularly polarized light (CPL) detectors hold significant potential for applications in optical communications, bioimaging, quantum information, and intelligent sensing, enabling chip-scale chiral light detection and information processing. However, current developments in materials and structural designs struggle to achieve wide-spectral detection capability while maintaining high circular polarization discrimination. This limitation significantly constrains the practical applicability of such detectors in complex environments and object detection scenarios. Herein, we propose a CPL detector based on a twist-stacked black phosphorus (b-p) system. By utilizing twisted b-p as an artificially constructed unit with chiral response and designing a symmetry-stacked sandwich structure in space, we achieve bipolar circular polarization detection and imaging across a wide wavelength range (from visible light to mid-infrared), with peak responsivity reaching 0.1 A/W. This design demonstrates both theoretically and experimentally the feasibility of utilizing a twist-stacked b-p system for wide-spectral CPL detection. This advancement offers valuable insights for addressing the bottlenecks in chip-integrated CPL detection technology.

We introduce quantum Kolmogorov-Arnold networks (QKAN), a quantum algorithmic framework inspired by the recently proposed Kolmogorov-Arnold Networks (KAN). QKAN inherits the compositional structure of KAN and is based on block-encodings, constructed recursively from a single layer using quantum singular value transformation. We demonstrate the algorithmic utility of QKAN in two applications. First, we introduce and analyze QKAN as a quantum learning model, treating the eigenvalues of block-encoded matrices as neurons and applying parametrized activation functions on the edges of the network. We show that QKAN is a wide-and-shallow neural architecture, where shallow depth is compensated by exponentially wide layers whenever efficient block-encodings of inputs are available. We further discuss how to parametrize and train QKAN using parametrized quantum circuits and quantum linear algebra subroutines. Second, we demonstrate that QKAN can serve as a multivariate quantum state-preparation protocol for functions with shallow compositional structure. We demonstrate this by efficiently preparing a multivariate Gaussian quantum state using a two-layer QKAN. Looking forward, we anticipate that QKAN's compositional and modular design will enable new applications in quantum machine learning and quantum state preparation.

Superfluorescence (SF) is a cooperative quantum emission process characterized by intense, ultrafast bursts of light, with promising applications in quantum photonics. In solid-state systems, SF typically requires cryogenic conditions, with quasi-2D hybrid metal halide perovskites being a notable exception where room-temperature SF has been reported. However, the mechanisms enabling high-temperature SF in these materials remain poorly understood, and the distinction between SF and amplified spontaneous emission (ASE) is often overlooked. Here, SF in hybrid quasi-2D perovskite phenylbutylammonium cesium lead bromide (PBA:CsPbBr3) is reported with the lowest threshold fluence recorded to date in perovskites, observed across 78-180 K. The phase behavior of emission under varying temperature and excitation fluence is mapped, identifying transitions between SF, ASE, and spontaneous emission regimes. A simple ab-initio model has been developed to predict the emission density, correlation, and trace distance for understanding the cooperative phenomena and the unified theory of SF and ASE. The analysis reveals the underlying dynamics that differentiate cooperative from non-cooperative emission, offering new insight into light-matter interactions in perovskites. These findings deepen the understanding of SF in solid-state systems and inform the design of quantum optical materials operable at elevated temperatures.

Abstract The two-state vector formalism of quantum mechanics is a time-symmetrized approach to standard quantum theory. In our work, we aim to establish rigorous foundations for the future investigation within this formalism. We introduce the concept of a story—a compatible pair consisting of a two-state vector and an ideal measurement. Using this concept, we examine the structure of the space comprising all two-state vectors. We analyze the problem of distinguishability and confirm that some pairs of two-state vectors or their statistical mixtures cannot be physically distinguished. In particular, we discuss an example of a two-state vector that is indistinguishable from a statistical mixture of separable two-state vectors and provide an example of a two-state vector that can be distinguished from every such mixture. This leads us to formulate the definition of a strictly non-separable two-state vector as a genuine manifestation of entanglement between the past and the future.

Efficient data transfer between memory and photonic components is crucial for a wide range of applications. However, this necessity brings forth energy-efficient data movement challenges associated with the memory wall, underscoring the demand for a fast and low-energy electro-optic photonic memory solution. Here, we demonstrate a class of energy-efficient electro-optic devices, namely Pockels photonic memory, that combines low-field switchable ferroelectrics with lithium niobate’s Pockel’s effect. Among such devices, this article will describe in detail the integrated embodiment of a ferroelectric field-effect transistor with lithium niobate on insulator micro ring resonator. We achieve switchable and non-volatile multiple optical memory states (6 states per transistor) with ultra-low energy cost (femto Joule/state), while achieving robust 10 year data retention and read-write endurance exceeding 107 cycles. Furthermore, we demonstrate the possibility of linear memory state stacking. The Pockels photonic memory enables the scaling of reconfigurable photonic systems into the femto Joule/state energy efficiencies.

The study of undecidability in problems arising from physics has experienced a renewed interest, mainly in connection with quantum information problems. The goal of this review is to survey this recent development. After a historical introduction, we first explain the necessary results about undecidability in mathematics and computer science. Then we briefly review the first results about undecidability in physics which emerged mostly in the 80s and early 90s. Finally, we focus on the most recent contributions, which we divide in two main categories: many body systems and quantum information problems.