The measurement of discrete quantum systems is a vital ingredient for quantum information processors. These systems, being fragile in nature, are characterized by a perturbation or collapse of their states whenever they are measured due to measurement-induced decoherence. We propose a quantum measurement model that allows for the extraction of information from any superposed quantum system without inducing a total collapse. The methodology used involves the coupling of a quantum probe through suitable interactions that create a partial entanglement between probe and quantum system. This entanglement in turn transfers information about the quantum system to the probe. Therefore, by making measurements on the probe rather than the superposed quantum system itself, we avert a total collapse to the quantum system. Our measurement model is applied to quantum computers for the measurement of their superposed states. The use of the measurement scheme for solving computational problems on the quantum computer is presented. In addition, the measurement scheme is invoked as an eavesdropping scheme on quantum cryptography systems; we show the degree to which their security is comprised. Lastly, the system performance of multi-user quantum cryptography networks is simulated, culminating in the experimental demonstration of a six-user quantum cryptographic network system. ^
The measurement of discrete quantum systems is a vital ingredient for quantum information processors. These systems, being fragile in nature, are characterized by a perturbation or collapse of their states whenever they are measured due to measurement-induced decoherence. We propose a quantum measurement model that allows for the extraction of information from any superposed quantum system without inducing a total collapse. The methodology used involves the coupling of a quantum probe through suitable interactions that create a partial entanglement between probe and quantum system. This entanglement in turn transfers information about the quantum system to the probe. Therefore, by making measurements on the probe rather than the superposed quantum system itself, we avert a total collapse to the quantum system. Our measurement model is applied to quantum computers for the measurement of their superposed states. The use of the measurement scheme for solving computational problems on the quantum computer is presented. In addition, the measurement scheme is invoked as an eavesdropping scheme on quantum cryptography systems; we show the degree to which their security is comprised. Lastly, the system performance of multi-user quantum cryptography networks is simulated, culminating in the experimental demonstration of a six-user quantum cryptographic network system. ^
The measurement of discrete quantum systems is a vital ingredient for quantum information processors. These systems, being fragile in nature, are characterized by a perturbation or collapse of their states whenever they are measured due to measurement-induced decoherence. We propose a quantum measurement model that allows for the extraction of information from any superposed quantum system without inducing a total collapse. The methodology used involves the coupling of a quantum probe through suitable interactions that create a partial entanglement between probe and quantum system. This entanglement in turn transfers information about the quantum system to the probe. Therefore, by making measurements on the probe rather than the superposed quantum system itself, we avert a total collapse to the quantum system.
Abstract In the context of the century-long debate on quantum measurement problem, the current work proposes a model that describes the process of collapse of state by quantum interaction, which resolves the controversies of the framework of quantum mechanics and describes the entire domain of quantum-to-classical world including the weak measurement and partial collapse. ‘Measurement’, being the process of physically interacting with a system in order to extracting information from it, is theorized in the current model by synthesizing the quantum interaction between system and measuring apparatus with the information entropy of such process. The model assumes Schrödinger equation to be the only guiding equation for all physical systems including the measuring apparatus, and does not presuppose ‘superposition principle’, rather derives it theoretically from the formulation. The superposed state is shown to be independent of the choice of measurement operator (observable) or basis states (pointers) of the measuring apparatus. Most interestingly, the current model explains the non-observance of ‘superposition principle’ by classical systems as the classical limit of such quantum description of measurement. Along with solving the quantum measurement problem, the work also explains weak measurement and partial collapse, which can be further extended to investigate such several emerging critical phenomena.
Different quantum information schemes, such as eavesdropping in quantum cryptography, dictate the necessity of extracting information about pair conjugate observables from a single copy of a quantum system. Mathematically, quantum measurements are usually described by an uncertainty relation. The difference between the simultaneous measurement uncertainty relation form those known from the textbooks on quantum mechanics is the additional uncertainty associated with the measurement procedure itself in contrast to the state preparation uncertainties described by the Schroedinger-Robertson type uncertainty relations. We present here an overview of our approach based on the state estimation theory and maximum likelihood strategy. We make a theoretical analysis and an experimental verification of minimum-uncertainty product of the two-states quantum system simultaneous measurement based on partially entangled photon pairs.
Abstract In the context of the century-long debate on quantum measurement problem, the current work proposes a model that describes the process of collapse of state by quantum interaction, which resolves the controversies of the framework of quantum mechanics and describes the entire domain of quantum-to-classical world including the weak measurement and partial collapse. ‘Measurement’, being the process of physically interacting with a system in order to extracting information from it, is theorized in the current model by synthesizing the quantum interaction between system and measuring apparatus with the information entropy of such process. The model assumes Schrödinger equation to be the only guiding equation for all physical systems including the measuring apparatus, and does not presuppose ‘superposition principle’, rather derives it theoretically from the formulation. The superposed state is shown to be independent of the choice of measurement operator (observable) or basis states (pointers) of the measuring apparatus. Most interestingly, the current model explains the non-observance of ‘superposition principle’ by classical systems as the classical limit of such quantum description of measurement. Along with solving the quantum measurement problem, the work also explains weak measurement and partial collapse, which can be further extended to investigate such several emerging critical phenomena.
Decoherence, often caused by unavoidable coupling with the environment, leads to degradation of quantum coherence. For a multipartite quantum system, decoherence leads to degradation of entanglement and, in certain cases, entanglement sudden death. Tackling decoherence, thus, is a critical issue faced in quantum information, as entanglement is a vital resource for many quantum information applications including quantum computing, quantum cryptography, quantum teleportation, and quantum metrology. Here, we propose and demonstrate a novel scheme to protect entanglement from decoherence. Our entanglement protection scheme makes use of the quantum measurement itself for actively battling against decoherence and it can effectively circumvent even entanglement sudden death (ESD).
In connection with the recent experiments on quantum measurement, such as the “which-way”detection using the Bragg scattering of cold atoms and quantum interference using a small molecular C 60 quantum system, we introduce the quantum measurement problem and its relevant fundamental concepts and ideas, such as quantum coherence, quantum decoherence and quantum entanglement. In order to understand the loss of quantum coherence for a quantum system due to its interaction with another system behaving classically or macroscopically, we discuss some quantum dynamical models to describe the quantum decoherence and quantum dissipation induced by couplings to the external environment or inner motions. In the classical or macroscopic limit, it is shown that the wave function of the total system formed by the quantum system plus another system interacting with it is factorized as an entangled state with correlation between the measured quantum states and the configurations of the external large system or the inner degrees of freedom. In association with a novel viewpoint about quantum measurement, which has been directly verified by recent experiments (e.g.,S. Durr et al., Nature,1998,33:359), it is shown that quantum entanglement is indeed responsible for the quantum decoherence, and thus it can be regarded as a “clean” quantum measurement when the large system behaves as a classical object. These arguments give us a possible solution to the “Schrdinger cat paradox” and to whether the coherent superposition of macroscopic states exists. These discussions are also applied to the quantum decoherence problems in quantum information including quantum computing and quantum communication.