The uncertainty principle, which bounds the uncertainties involved in obtaining precise outcomes for two complementary variables defining a quantum particle, is a crucial aspect in quantum mechanics. Recently, the uncertainty principle in terms of entropy has been extended to the case involving quantum entanglement 1 . With previously obtained quantum information for the particle of interest, the outcomes of both non-commuting observables can be predicted precisely, which greatly generalizes the uncertainty relation. Here, we experimentally investigated the entanglement-assisted entropic uncertainty principle for an entirely optical set-up. The uncertainty is shown to be near zero in the presence of quasi-maximal entanglement. The new uncertainty relation is further used to witness entanglement. The verified entropic uncertainty relation provides an intriguing perspective in that it implies the uncertainty principle is not only observabledependent but is also observer-dependent 2 .In quantum mechanics, the outcomes of an observable can be predicted precisely by preparing eigenvectors corresponding to the state of the measured system. However, the ability to predict the precise outcomes of two conjugate observables for a particle is restricted by the uncertainty principle. Originally observed by Heisenberg 3 , the uncertainty principle is best known as the Heisenberg-Robertson commutationwhere R ( S) represents the standard deviation of the corresponding variable R (S). It can be seen that the bound on the right-hand side is state-dependent and can vanish even when R and S are non-commuting. To avoid this defect, the uncertainty relation has been re-derived in terms of an information-theoretic model 5 in which the uncertainty relating to the outcomes of the observable is characterized by the Shannon entropy instead of the standard deviation. The entropic uncertainty relation for any two general observables was first given by Deutsch 6 . Soon afterwards, an improved version was proposed by Kraus 7 and then proved by Maassen and Uiffink 8 . The improved relation reads as follows:where H is the Shannon entropy, c = max i,j | a i |b j | 2 and represents the overlap between observables R and S, and |a i (|b j ) represents the eigenvectors of R (S).Although we cannot obtain the precise outcomes of both the two conjugate variables, even when the density matrix of the prepared state is known, the situation would be different if 1 Key Laboratory of Quantum Information, University of Science and Technology of China, CAS, Hefei, 230026, China, 2 Center for Quantum Technologies, National University of Singapore, 2 Science Drive 3, 117542, Singapore. † These authors contributed equally to this work. *e-mail: cfli@ustc.edu.cn.we invoked the effect of quantum entanglement. The possibility of violating the Heisenberg-Robertson uncertainty relation was identified early by Einstein, Podolsky, and Rosen in their famous paper, and was originally used to challenge the correctness of quantum mechanics (EPR paradox) 9 . Popper also p...

It is well known that many operations in quantum information processing depend largely on a special kind of quantum correlation, that is, entanglement. However, there are also quantum tasks that display the quantum advantage without entanglement. Distinguishing classical and quantum correlations in quantum systems is therefore of both fundamental and practical importance. In consideration of the unavoidable interaction between correlated systems and the environment, understanding the dynamics of correlations would stimulate great interest. In this study, we investigate the dynamics of different kinds of bipartite correlations in an alloptical experimental setup. The sudden change in behaviour in the decay rates of correlations and their immunity against certain decoherences are shown. Moreover, quantum correlation is observed to be larger than classical correlation, which disproves the early conjecture that classical correlation is always greater than quantum correlation. Our observations may be important for quantum information processing.

No abstract

Solid-state color centers with manipulable spin qubits and telecom-ranged fluorescence are ideal platforms for quantum communications and distributed quantum computations. In this work, we coherently control the nitrogen-vacancy (NV) center spins in silicon carbide at room temperature, in which the telecomwavelength emission is detected. Through carefully optimizing the implanted conditions, we improve the concentration of NV centers for about 4 times. Based on this, the coherent control of NV center spins is achieved at room temperature and the coherence time T2 * can be reached around 1 μs. Furthermore, the investigation of fluorescence properties of single NV centers shows that they are room temperature photostable single photon sources at telecom range. Taking the advantages of the technological mature materials, the experiment demonstrates that the NV centers in silicon carbide are promising systems for large-scale integrated quantum photonics and long-distance quantum networks.

The simulation of low-temperature properties of many-body systems remains one of the major challenges in theoretical [1][2][3] and experimental [4-6] quantum information science. We present, and demonstrate experimentally, a universal cooling method which is applicable to any physical system that can be simulated by a quantum computer [7][8][9]. This method allows us to distill and eliminate hot components of quantum states, i.e., a quantum Maxwell's demon [10]. The experimental implementation is realized with a quantum-optical network, and the results are in full agreement with theoretical predictions (with fidelity higher than 0.978). These results open a new path for simulating low-temperature properties of physical and chemical systems that are intractable with classical methods.From quantum field theories [1] to high-Tc superconductivity [2] and chemical reactions [3], quantum simulation [7][8][9] provides a unique opportunity for both theorists and experimentalists to explore a domain of science that goes beyond the applicability of any known classical computing method. Modern low-temperature physics has advanced primarily due to the development of efficient cooling methods [11,12]. The same is also true for quantum information science. Physical cooling can drive quantum states towards the physical ground states, which are pure states. However, in the context of quantum simulation, pure states are not necessarily "cooler" than mixed states. For example, pure states with an equal superposition of all eigenstates correspond to infinite-temperature states [13,14]. In order to achieve cooling for quantum simulation, in general, one not only needs to employ physical cooling to avoid decoherence [15], but also be able to prepare states that have low entropy in the basis of the eigenstates of the Hamiltonian of the system being simulated.An attractive approach for achieving cooling for spins (or qubits) is the heat-bath algorithmic cooling (HBAC) method [16]. The main idea of HBAC is to reduce the entropy of spin systems by unevenly distributing more entropy to one of the spins that can release the excess entropy to a heat bath through thermalization. The feasibility of HBAC has been demonstrated experimentally with NMR technology [4]. However, HBAC is not a uni- FIG. 1: (Color online). Overview of the cooling method. (a)Logic diagram of the feedback cooling system. The cooling module produces two outcomes, correlated with heating and cooling. The measurement result can be mapped into the position x of a 1D random walker; when the walker goes beyond its starting position x = 0 to the negative position x = −1, the particle is either rejected or recycled. (b) The quantum circuit diagram of the cooling module. It includes a controlled evolution for time t and an energy bias parameter γ for optimizing the performance.versal method for cooling quantum many-body systems; it is primarily employed for preparing polarized spins as initial states, or ancilla qubits, for quantum computation. Furthermore, the temperature of ...

Defects in silicon carbide have been explored as promising spin systems in quantum technologies. However, for practical quantum metrology and quantum communication, it is critical to achieve the on-demand shallow spin-defect generation. In this work, we present the generation and characterization of shallow silicon vacancies in silicon carbide by using different implanted ions and annealing conditions. The conversion efficiency of silicon vacancy of helium ions is shown to be higher than that by carbon and hydrogen ions in a wide implanted fluence range. Furthermore, after optimizing annealing conditions, the conversion efficiency can be increased more than 2 times. Due to the high density of the generated ensemble defects, the sensitivity to sense a static magnetic field can be research as high as 11.9 / z B TH   , which is about 15 times higher than previous results. By carefully optimizing implanted conditions, we further show that a single silicon vacancy array can be generated with about 80 % conversion efficiency, which reaches the highest conversion yield in solid state systems. The results pave the way for using on-demand generated shallow silicon vacancy for quantum information processing and quantum photonics. Keyword:Silicon carbide, silicon vacancy, implantation, magnetic sensing, single photon sources In recent years, color centers in silicon carbide (SiC) have been demonstrated as promising physical platforms for quantum science 1-11 . SiC is a well-known semiconductor material which has wide applications in high-power and high-temperature electronic devices. Moreover, SiC has technological advantages due to the welldeveloped device fabrication protocols and inch-scale growth. Besides some bright single photon emitters 3-7 , SiC also has two types of defect spins, including the silicon vacancy and divacancy defects 1,2,[8][9][10][11] . Similar with nitrogen-vacancy (NV) centers in diamond 12 , these spins can be polarized by optics and manipulated by microwaves at room temperature (RT). Moreover, their photoluminescence (PL) spectrum are in the

Entanglement and wave function description are two of the core concepts that make quantum mechanics such a unique theory. A method to directly measure the wave function, using Weak Values, was demonstrated by Lundeen et al., Nature 474, 188(2011). However it is not applicable to a scenario of two disjoint systems, where nonlocal entanglement can be a crucial element, since that requires obtaining Weak Values of nonlocal observables. Here, for the first time, we propose a method to directly measure a nonlocal wave function of a bipartite system, using Modular Values. The method is experimentally implemented for a photon pair in a hyper-entangled state, i.e. entangled both in polarization and momentum degrees of freedom.

Quantum coherence is the most distinguished feature of quantum mechanics. It lies at the heart of the quantum-information technologies as the fundamental resource and is also related to other quantum resources, including entanglement. It plays a critical role in various fields, even in biology. Nevertheless, the rigorous and systematic resource-theoretic framework of coherence has just been developed recently, and several coherence measures are proposed. Experimentally, the usual method to measure coherence is to perform state tomography and use mathematical expressions. Here, we alternatively develop a method to measure coherence directly using its most essential behavior-the interference fringes. The ancilla states are mixed into the target state with various ratios, and the minimal ratio that makes the interference fringes of the "mixed state" vanish is taken as the quantity of coherence. We also use the witness observable to witness coherence, and the optimal witness constitutes another direct method to measure coherence. For comparison, we perform tomography and calculate l_{1} norm of coherence, which coincides with the results of the other two methods in our situation. Our methods are explicit and robust, providing a nice alternative to the tomographic technique.