Quantum measurement is inevitable to extract information from quantum systems. Exploring the power and limitation of quantum measurement has profound implications not only to foundational studies, but also to many practical applications. In most of these tasks, many identically prepared quantum systems are measured to acquire sufficient information. Although there is no entanglement or even no classical correlation among these identically systems, surprisingly, quantum collective measurements on them may extract more information than local measurements on individual systems, thereby leading to higher efficiency and precision in these tasks. Although the significance of collective measurements has been recognized for two decades, it is still very challenging to demonstrate their advantage in experiments. Here, we will introduce the first implementation of quantum collective measurement and it's applications on quantum orienting, reducing quantum backation of measurement, and so on.
Quantum measurement is inevitable to extract information from quantum systems. Exploring the power and limitation of quantum measurement has profound implications not only to foundational studies, but also to many practical applications. In most of these tasks, many identically prepared quantum systems are measured to acquire sufficient information. Although there is no entanglement or even no classical correlation among these identically systems, surprisingly, quantum collective measurements on them may extract more information than local measurements on individual systems, thereby leading to higher efficiency and precision in these tasks. Although the significance of collective measurements has been recognized for two decades, it is still very challenging to demonstrate their advantage in experiments. Here, we will introduce the first implementation of quantum collective measurement and it's applications on quantum orienting, reducing quantum backation of measurement, and so on.
We study the impact of entangling measurements on the efficiency of quantum measurement-based engines. We first demonstrate that for engines comprising many subsystems the efficiency can be enhanced through the implementation of entangling measurements. Then, we show that when the collective measurement yields the same local state for the subsystems as individual local measurements, the improvement is directly proportional to the amount of correlations among the subsystems. Finally, we present examples for two-level systems where this type of engine can operate at perfect efficiency yielding a finite amount of work in the limit of a large number of subsystems; we also discuss how to generalize this idea to larger systems.
A key aspect in quantum information is to understand the advantage offered by quantum systems over classical ones in communication tasks. In recent years, a fundamental approach to this problem has been developed, focusing on quantum correlations in prepare-and-measure scenarios. Inspired by the developments in Bell nonlocality and device-independent information processing, this line of research aims to characterize the possibilities and limits of quantum systems for communication, in particular to precisely capture the advantage they offer over classical systems. In addition to fundamental insights, these ideas also underpin the concept of semi-device-independent quantum information processing. Exploring trade-offs between security, performance and ease-of-implementation, this approach opens promising directions for novel quantum information processing technologies and devices. A number of protocols and proof-of-principle demonstrations have been reported in recent years, in particular for quantum randomness certification and key distribution. Here, we provide a comprehensive introduction to quantum prepare-and-measure correlations and semi-device independent applications.
Collective measurements on identically prepared quantum systems can extract more information than local measurements, thereby enhancing information-processing efficiency. Although this nonclassical phenomenon has been known for two decades, it has remained a challenging task to demonstrate the advantage of collective measurements in experiments. Here, we introduce a general recipe for performing deterministic collective measurements on two identically prepared qubits based on quantum walks. Using photonic quantum walks, we realize experimentally an optimized collective measurement with fidelity 0.9946 without post selection. As an application, we achieve the highest tomographic efficiency in qubit state tomography to date. Our work offers an effective recipe for beating the precision limit of local measurements in quantum state tomography and metrology. In addition, our study opens an avenue for harvesting the power of collective measurements in quantum information-processing and for exploring the intriguing physics behind this power.
Quantum states and measurements are two central physical objects in the theory of quantum mechanics. The preparation, control and measurement of quantum states lies at the heart of modern quantum information processing protocols ranging from secure communication to quantum computation and precision sensing. And yet, the reliable operation of these advanced protocols ultimately rests on a far more essential prerequisite - our ability to calibrate, certify and benchmark the quantum states we prepare and the quantum measurements we perform. Quantum information theory provides a natural framework for these tasks, meaningfully connecting the resources we expend for characterisation with the accuracy and precision we can expect in return. While decades of advances in quantum information have refined our understanding of quantum characterisation and sharpened our practical ability to calibrate with enhanced precision, important questions about the fundamental limits of quantum characterisation remain unanswered. Although incorporating entanglement at the measurement stage is known to enhance the estimation of unknown quantum states, the extent of this improvement remains poorly understood. This leaves open a fundamental question on the utility of complex entangling measurement strategies, impeding progress towards bridging the significant gap between practically achievable precision and the ultimate theoretical quantum limit. Complementary to quantum state estimation, which is relatively well-established in theory and practice, the characterisation of quantum measurements lacks a rigorous information-theoretic foundation, leading to a dearth of principled benchmarks and provably optimal strategies despite growing experimental interest. This thesis develops and applies information-theoretic tools to answer questions in quantum characterisation, with an eye to enabling practical improvements beyond current techniques in the field. First, we introduce a computable measure of entanglement enhancement for quantum state estimation and study its variation across general multi-parameter estimation problems for higher-dimensional quantum states. We show that the entanglement enhancement is maximised for full qudit state tomography but grows only linearly with system dimension even when allowing an asymptotically large number of entangling operations. Our proposed tight upper bound to entanglement enhancement therefore constitutes a strong limitation on the utility of such entangling measurements. Next, we formulate the dual problem of quantum detector estimation, developing a comprehensive framework for maximally precise calibration centred around a newly introduced quantity - the detector quantum Fisher information. This information-theoretic measure stipulates the ultimate precision attainable for measurement calibration and identifies the optimal input states that achieve this precision. Further, our framework enables the first provably optimal detector estimation experiment and reveals the scope for entanglement-based enhancements, thereby placing measurement characterisation on equal footing with state characterisation. Considered together, these results suggest two contrasting conclusions about quantum characterisation. On the one hand, they show that for quantum state estimation, increasingly powerful entangling measurements offer only diminishing returns at the ultimate precision limit. On the other hand, they highlight practical quantum advantages available for detector estimation by providing a framework that can improve calibration performance and guide future experimental design. More broadly, these results complete and advance the triad of state, process, and detector characterisation, positioning quantum information measures as not merely abstract bounds, but practical guides for deciding when complex quantum resources are genuinely worth deploying and where near-term quantum advantages are most likely to be realised.
Quantum mechanics governs the behavior of particles, and its inherent probabilistic nature presents a significant challenge to precise measurement. Traditional measurement techniques often involve the direct observation of a system's state, leading to potential loss of information. This research explores a novel approach to quantum measurement – the non-local quantification of statistical properties without requiring a direct physical observation of the system's state. We propose a method based on entanglement and quantum correlations to extract information about the system's distribution, offering a potential paradigm shift in measurement techniques and potentially unlocking new capabilities in quantum information processing. The core mechanism centers around creating a measurement protocol that leverages entanglement to establish a correlation between measurements, thereby allowing for the extraction of statistical information without the inherent disturbance of direct observation. This work addresses the limitations of current measurement methods by introducing a fundamentally different approach, emphasizing preservation of information through entanglement and quantum correlations.
Abstract The recently established resource theory of quantum coherence allows for a quantitative understanding of the superposition principle, with applications reaching from quantum computing to quantum biology. While different quantifiers of coherence have been proposed in the literature, their efficient estimation in today’s experiments remains a challenge. Here, we introduce a collective measurement scheme for estimating the amount of coherence in quantum states, which requires entangled measurements on two copies of the state. As we show by numerical simulations, our scheme outperforms other estimation methods based on tomography or adaptive measurements, leading to a higher precision in a large parameter range for estimating established coherence quantifiers of qubit and qutrit states. We show that our method is accessible with today’s technology by implementing it experimentally with photons, finding a good agreement between experiment and theory.