arXiv subjects

Meng-Jun Hu

Publications and source records attributed to Meng-Jun Hu.

28 records · Page 2Linked to original sources

arXiv2018

Experimental Verification of Anisotropic Invariance for Three-Qubit States

We experimentally test the recently predicted anisotropic invariance properties of pure three-qubit states, via generation and measurement of polarisation-path entangled three-qubit states. These properties do not require aligned reference frames and can be determined from measurements on any two of the qubits. They have several applications, such as a universal ordering of pairwise quantum correlations, strong monogamy relations for Bell inequalities and quantum steering, and a complementarity relation for Bell nonlocality versus 3-tangle, some of which we also test. The results indicate that anisotropic invariance, together with the three qubit Bloch vector lengths, can provide a robust and complete set of invariants for such states under local unitary transformations.

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arXiv2018

Observation of Nonlocality Sharing among Three Observers with One Entangled Pair via Optimal Weak Measurement

Bell nonlocality plays a fundamental role in quantum theory. Numerous tests of the Bell inequality have been reported since the ground-breaking discovery of the Bell theorem.Up to now, however, most discussions of the Bell scenario have focused on a single pair of entangled particles distributed to only two separated observers. Recently, it has been shown surprisingly that multiple observers can share the nonlocality present in a single particle from an entangled pair using the method of weak measurements [Phys. Rev. Lett. {\bf 114}, 250401 (2015)]. Here we report an observation of double CHSH-Bell inequality violations for a single pair of entangled photons with strength continuous-tunable optimal weak measurements in photonic system for the first time. Our results not only shed new light on the interplay between nonlocality and quantum measurements but may also be significant for important applications such as unbounded randomness certification and quantum steering.

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arXiv2018

Direct Measurement of the Two-dimensional Spatial Quantum Wavefunction via Strong Measurements

Wavefunction is the foundation of quantum theory, which is assumed to give a complete description of a quantum system. For a long time, wavefunction is introduced as an abstract element of the theory and there lacks effective ways to measure it directly. The situation, however, is somewhat changed when Lundeen et al. reported the direct measurement of the quantum wavefunction via weak measurements, which gives the wavefunction a clearly operational definition [Nature 474, 188 (2011)]. The weak measurement method requires sequential measurements of conjugate observables position and momentum with the position measurement is weak enough. Surprisingly, the recent research by Vallone and Dequal shows that performing sequential strong measurements realizes the same target, in which case no approximation has to be made compared to the case of weak measurements[Phys. Rev. Lett. 116, 040502 (2016)]. Here we experimentally report the direct measurement of the two-dimensional transverse wavefunction of photons via strong measurements for the first time, which implies that an accurate and clear operational definition can be given to wavefunction. We have measured the Gaussian and Laguerre-Gaussian of l = 1 spatial wavefunctions of photons with R-square are 0.97 and 0.93 respectively. As a potentially important application, we show that the direct measurement of two-dimensional wavefunction provides an alternative way to realize digital holography of three-dimensional objects. The results presented here will not only deepen our understanding of abstract wavefunction but also have significant applications in quantum information processing and quantum imaging.

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arXiv2018

Experimental Realization of Sequential Weak Measurements of Arbitrary Non-commuting Pauli Observables

Sequential weak measurements of non-commuting observables is not only fundamentally interesting in quantum measurement but also shown potential in various applications. The previous reported methods, however, can only realize limited sequential weak measurements experimentally. In this Letter, we propose the realization of sequential measurements of arbitrary observables and experimentally demonstrate for the first time the measurement of sequential weak values of three non-commuting Pauli observables by using genuine single photons. The results presented here will not only improve our understanding of quantum measurement, e.g. testing quantum contextuality, macroscopic realism, and uncertainty relation, but also have many applications such as realizing counterfactual computation, direct process tomography, direct measurement of the density matrix and unbounded randomness certification.

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arXiv2018

Quantum-Inspired Gravitational Wave Detector

A new gravitational-wave detector, which is devised based on quantum weak measurement amplification, is introduced and shown has the potential to significantly improve the strain sensitivity of gravitational-wave detection.

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arXiv2017

Gravitational Wave Detection via Weak Measurements Amplification

A universal amplification scheme of ultra-small phase based on weak measurements is given and a weak measurements amplification based laser interferometer gravitational-wave observatory (WMA-LIGO) is suggested. The WMA-LIGO has potential to amplify the ultra-small phase signal to at least $10^{3}$ order of magnitude such that the sensitivity and bandwidth of gravitational-wave detector can be further improved. Our results not only shed a new light on the quantum measurement but also open a new way to the gravitational-wave detection.

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arXiv2017

Cascaded Weak Measurements Amplification of Ultra-small Phases

The weak measurements based amplification of ultra-small phase was proposed in our previous work. Due to the technical imperfections, the ability of amplification is usually limited in practice. Here we introduce the concept of cascaded weak measurements amplification so that the ability of weak measurements amplification can be further improved. A formula of determining the cascaded number of weak measurements amplification is derived. The method presented here could be found useful in high precision measurement e.g., gravitational waves detection.

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arXiv2016

Experimental creation of superposition of unknown photonic quantum states

As one of the most intriguing intrinsic properties of quantum world, quantum superposition provokes great interests in its own generation. Oszmaniec [Phys. Rev. Lett. 116, 110403 (2016)] have proven that though a universal quantum machine that creates superposition of arbitrary two unknown states is physically impossible, a probabilistic protocol exists in the case of two input states have nonzero overlaps with the referential state. Here we report a heralded quantum machine realizing superposition of arbitrary two unknown photonic qubits as long as they have nonzero overlaps with the horizontal polarization state $|H\rangle$. A total of 11 different qubit pairs are chosen to test this protocol by comparing the reconstructed output state with theoretical expected superposition of input states. We obtain the average fidelity as high as 0.99, which shows the excellent reliability of our realization. This realization not only deepens our understanding of quantum superposition but also has significant applications in quantum information and quantum computation, e.g., generating non-classical states in the context of quantum optics and realizing information compression by coherent superposition of results of independent runs of subroutines in a quantum computation.

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arXiv2016

Unified view of quantum amplification based on quantum transformation

A general framework of quantum state amplification using the language of quantum state transformation is given systematically for the first time. The concept of amplification of quantum states is defined specifically and the amplification of a set of quantum states is formulated generally as the transformation of quantum states. Three different kinds of important quantum amplifications, i.e., deterministic noisy quantum amplification, probabilistic noiseless quantum amplification, and deterministic noiseless quantum amplification are identified and discussed. For deterministic quan- tum amplification, the linearity of amplification is proven to be incompatible with the noiseless amplification while it is not true for probabilistic quantum amplification. However, deterministic noiseless quantum amplification is shown physically attainable if the linearity of amplification is given up. The connection between the gain of amplification and the successful probability is discussed for probabilistic quantum amplification. Assuming that successful probability is the same for all quantum states to be amplified, we obtain a generally valid relation between the gain of amplication and the successful probability. Particular interest is given to phase-preserving quantum amplication of Gaussian states which has been shown of theoretical interest and of practical importance in quantum information and quantum communication recently. Our results of quantum state amplification not only enrich the research of quantum amplification but also can be helpful for further practical applications.

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arXiv2016

Are observables necessarily Hermitian?

Observables are believed that they must be Hermitian in quantum theory. Based on the obviously physical fact that only eigenstates of observable and its corresponding probabilities, i.e., spectrum distribution of observable are actually observed, we argue that observables need not necessarily to be Hermitian. More generally, observables should be reformulated as normal operators including Hermitian operators as a subclass. This reformulation is consistent with the quantum theory currently used and does not change any physical results. The Clauser-Horne-Shimony-Holt (CHSH) inequality is taken as an example to show that our opinion does not conflict with conventional quantum theory and gives the same physical results. Reformulation of observables as normal operators not only coincides with the physical facts but also will deepen our understanding of measurement in quantum theory.

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