Over the past two decades, both experimental and theoretical research works have suggested the possibility of observing nonclassical gravitational effects by detecting gravitationally induced quantum correlations. Most existing proposals focus on center-of-mass interactions in mesoscopic systems within the Newtonian regime. In this essay, we explore a different approach by examining the gravitational interaction of angular momentum eigenstates in the post-Newtonian limit. This offers a novel way to test general relativistic phenomena in a low-energy quantum mechanical setting. We also discuss the potential advantages and challenges of this approach compared to other existing schemes.

Photoisomerization, a photochemical process underlying many biological mechanisms, has been modeled recently within the quantum resource theory of thermodynamics. This approach has emerged as a promising tool for studying fundamental limitations to nanoscale processes independently of the microscopic details governing their dynamics. On the other hand, correlations between physical systems have been shown to play a crucial role in quantum thermodynamics by lowering the work cost of certain operations. Here we explore quantitatively how correlations between multiple photoswitches can enhance the efficiency of photoisomerization beyond that attainable for single molecules. Furthermore, our analysis provides insights into the interplay between quantum and classical correlations in these transformations.

Thermodynamical systems at the nanoscale, such as single molecules interacting with highly structured vibrational environments, typically undergo nonequilibrium physical processes that lack precise microscopic descriptions. Photoisomerization is such an example, which has emerged as a platform on which to study single-molecule ultrafast photochemical processes from a quantum resource theoretic perspective. However, the upper bounds on its efficiency have only been obtained under significant simplifications that make the mathematics of the resource-theoretical treatment manageable. Here we generalize previous models for the photoisomers, while retaining the full vibrational structure, and still obtain analytical bounds on the efficiency of photoisomerization. We quantify the impact of such a vibrational structure on the optimal photoisomerization quantum yield both when the vibrational coordinate has no dynamics of its own and when we take into account the vibrational dynamics. This work serves as an example of how to bridge the gap between the abstract language of quantum resource theories and the open system formulation of nanoscale processes.

The presence of quantum effects in photosynthetic excitation energy transfer has been intensely debated over the past decade. Nonlinear spectroscopy cannot unambiguously distinguish coherent electronic dynamics from underdamped vibrational motion, and rigorous numerical simulations of realistic microscopic models have been intractable. Experimental studies supported by approximate numerical treatments that severely coarse-grain the vibrational environment have claimed the absence of long-lived quantum effects. Here, we report the nonperturbative, accurate microscopic model simulations of the Fenna-Matthews-Olson photosynthetic complex and demonstrate the presence of long-lived excitonic coherences at 77 K and room temperature, which persist on picosecond timescales, similar to those of excitation energy transfer. Furthermore, we show that full microscopic simulations of nonlinear optical spectra are essential for identifying experimental evidence of quantum effects in photosynthesis, as approximate theoretical methods can misinterpret experimental data and potentially overlook quantum phenomena.

Nanothermometry within living cells is an important endeavor in physics, as the mechanisms of heat diffusion in such complex and dynamic environments remain poorly understood. In biology, nanothermometry may offer new insights into cellular biology and open new avenues for drug-discovery. Previous studies using various nanothermometers have reported temperature variations of up to several Kelvins during metabolic stimulation, but these findings have remained controversial as they appear to contradict the law of heat diffusion in the presence of heating rates that are consistent with physiological parameters. Here, nanodiamond nanothermometry are reported inside macrophages by measuring the optically detected magnetic resonance spectra of nitrogen-vacancy centers. The spectra are analyzed when cells are metabolically stimulated and after cell death. It is shown that, in the experimental setting, the apparent spin resonant spectral shifts can be misinterpreted as temperature changes but are actually caused by electrical field changes on the nanodiamond’s surface. These artifacts are addressed with optimized nanodiamonds and a more robust sensing protocol to measure temperature inside cells with precision down to 100 mK (52 mK outside cells). No significant temperature changes upon metabolic stimulation are found, a finding consistent with the implementation of the heat diffusion law and expected physiological heating rates.

Solid-state quantum systems with optical and spin degrees of freedom have found widespread application in emerging quantum technologies. Recently, molecular qubits came forward as precisely tunable entities that present a compelling alternative to well-established yet hard-to-tune point defects in solid-state systems. In this work, we disclose ground-state triplet carbenes as purely organic qubits comprising two unpaired electrons in close proximity that can be generated in a crystalline matrix with high spatial accuracy via in situ photoactivation. We further demonstrate how state-of-the-art multireference quantum chemical calculations provide insight into their fundamental spin characteristics. As a result, several key assets were realized in a single solid-state qubit material under cryogenic conditions: The exclusive use of light elements (C, H, N, O), photolithographic patterning, optical spin-selective transitions, and a large zero-field splitting in the GHz regime, which, taken together, lays the ground for optically detected magnetic resonance with remarkable fluorescence contrast of >40% and record-high spin coherence times of T2 = 157(4) μs at 5 K.

Diamond-based quantum sensors have enabled high-resolution NMR spectroscopy at the microscale in scenarios where fast molecular motion averages out dipolar interactions among target nuclei. However, in samples with low-diffusion, ubiquitous dipolar couplings challenge the extraction of relevant spectroscopic information. In this work we present a protocol that enables the scanning of nuclear spins in dipolarly-coupled samples at high magnetic fields with a sensor based on nitrogen vacancy (NV) ensembles. Our protocol is based on the synchronized delivery of radio frequency (RF) and microwave (MW) radiation to eliminate couplings among nuclei in the scanned sample and to efficiently extract target energy-shifts from the sample’s magnetization dynamics. In addition, the method is designed to operate at high magnetic fields leading to a larger sample thermal polarization, thus to an increased NMR signal. The precision of our method is ultimately limited by the coherence time of the sample, allowing for accurate identification of relevant energy shifts in solid-state systems.

Quantum collision models are pivotal for simulating open quantum systems, yet lack comprehensive error certification. We analytically derive Markovian and non-Markovian collision models from chain mapping techniques, identifying a critical error source, thus elevating collision models to numerically exact methods and enhancing their reliability across quantum simulations.

Two-dimensional electronic spectroscopy (2DES) is a powerful tool for exploring quantum effects in energy transport within photosynthetic systems and investigating novel material properties. However, simulating the dynamics of these experiments poses significant challenges for classical computers due to the large system sizes, long timescales, and numerous experiment repetitions involved. This paper introduces the probe qubit protocol (PQP)-for quantum simulation of 2DES on quantum devices-addressing these challenges. The PQP offers several enhancements over standard methods, notably reducing computational resources, by requiring only a single-qubit measurement per circuit run and achieving Heisenberg scaling in detection frequency resolution, without the need to apply expensive controlled evolution operators in the quantum circuit. The implementation of the PQP protocol requires only one additional ancilla qubit, the probe qubit, with one-to-all connectivity and two-qubit interactions between each system and probe qubits. We evaluate the computational resources necessary for this protocol in detail, demonstrating its function as a dynamic frequency-filtering method through numerical simulations. We find that simulations of the PQP on classical and quantum computers enable a reduction on the number of measurements, i.e., simulation runtime, and memory savings of several orders of magnitude relative to standard quantum simulation protocols of 2DES. The paper discusses the applicability of the PQP on near-term quantum devices and highlights potential applications where this spectroscopy simulation protocol could provide significant speedups over standard approaches such as the quantum simulation of 2DES applied to the Fenna-Matthews-Olson (FMO) complex in green sulfur bacteria.

Nuclear hyperpolarization is a known method to enhance the signal in nuclear magnetic resonance (NMR) by orders of magnitude. The present work addresses the ¹³C hyperpolarization in diamond micro- and nanoparticles, using the optically pumped nitrogen-vacancy center (NV) to polarize ¹³C spins at room temperature. Consequences of the small particle size are mitigated by using a combination of surface treatment improving the ¹³C relaxation (T₁) time, as well as that of NV, and applying a technique for NV illumination based on a microphotonic structure. Adjustments to the dynamical nuclear polarization sequence (PulsePol) are performed, as well as slow sample rotation, to improve the NV-¹³C polarization transfer rate. The hyperpolarized ¹³C NMR signal is observed in particles of 2-micrometer and 100-nanometer median sizes, with enhancements over the thermal signal (at 0.29-tesla magnetic field) of 1500 and 940, respectively. The present demonstration of room-temperature hyperpolarization anticipates the development of agents based on nanoparticles for sensitive magnetic resonance applications.

We present an open-source tensor network Python library for quantum many-body simulations. At its core is an abelian-symmetric tensor, implemented as a sparse block structure managed by a logical layer on top of a dense multi-dimensional array backend. This serves as the basis for higher-level tensor network algorithms, operating on matrix product states and projected entangled pair states. An appropriate backend, such as PyTorch, gives direct access to automatic differentiation (AD) for cost-function gradient calculations and execution on GPU and other supported accelerators. We show the library performance in simulations with infinite projected entangled-pair states, such as finding the ground states with AD and simulating thermal states of the Hubbard model via imaginary time evolution. For these challenging examples, we identify and quantify sources of the numerical advantage exploited by the symmetric-tensor implementation.

We propose a state preparation protocol based on sequential measurements of a central spin coupled with a spin ensemble, and investigate the usefulness of the generated multi-spin states for quantum enhanced metrology. Our protocol is shown to generate highly entangled spin states, devoid of the necessity for non-linear spin interactions. The metrological sensitivity of the resulting state surpasses the standard quantum limit, reaching the Heisenberg limit under symmetric coupling strength conditions. We also explore asymmetric coupling strengths, identifying specific preparation windows in time for optimal sensitivity. Our findings introduce a novel method for generating large-scale, non-classical, entangled states, enabling quantum-enhanced metrology within current experimental capabilities.