量子互文在中国科大的二十个春秋

量子互文性是量子力学最为奇妙和令人困惑的特征之一,它指的是量子力学中一个算符的测量结果取决于测量时实际被观测的所有算符的选择。中国科学技术大学在量子互文方向的研究具有悠久的历史。本文回顾了过去20年来中国科大在量子互文方向所取得的理论和实验进展。我们首先介绍著名的最简单的态无关量子互文性证明;而后展示几个基于光子系统的量子互文性实验测试;最后,我们讨论了量子互文性在广泛的量子信息科学中的作用及其在量子计算中的应用。

Stable single photon sources in the near C-band range above 400K

The intrinsic characteristics of single photons became critical issues since the early development of quantum mechanics. Nowadays, acting as flying qubits, single photons are shown to play important roles in the quantum key distribution and quantum networks. Many different single photon sources (SPSs) have been developed. Point defects in silicon carbide (SiC) have been shown to be promising SPS candidates in the telecom range. In this work, we demonstrate a stable SPS in an epitaxial 3CSiC with the wavelength in the near C-band range, which is very suitable for fiber communications. The observed SPSs show high single photon purity and stable fluorescence at even above 400 K. The lifetimes of the SPSs are found to be almost linearly decreased with the increase of temperature. Since the epitaxial 3C-SiC can be conveniently nanofabricated, these stable near Cband SPSs would find important applications in the integrated photonic devices.

Detection and quantification of entanglement with measurement-device-independent and universal entanglement witness

Entanglement is the key resource in quantum information processing,and an entanglement witness(EW)is designed to detect whether a quantum system has any entanglement.However,prior knowledge of the target states should be known first to design a suitable EW,which weakens this method.Nevertheless,a recent theory shows that it is possible to design a universal entanglement witness(UEW)to detect negative-partial-transpose(NPT)entanglement in unknown bipartite states with measurement-device-independent(MDI)characteristic.The outcome of a UEW can also be upgraded to be an entanglement measure.In this study,we experimentally design and realize an MDI UEW for two-qubit entangled states.All of the tested states are well-detected without any prior knowledge.We also show that it is able to quantify entanglement by comparing it with concurrence estimated through state tomography.The relation between them is also revealed.The entire experimental framework ensures that the UEW is MDI.

Experimental demonstration of tight duality relation in three-path interferometer

Bohr’s principle of complementarity has a long history and it is an important topic in quantum theory,among which the famous example is the duality relation.The relation between visibilityC and distinguishability D,C2+D2≤1,has long been recognized as the only representative of the duality relation.However,recent researches have shown that this inequality is not good enough because it is not tight for multipath interferometers.Meanwhile,a tight bound for the multipath interferometer has been put forward.Here we design and experimentally implement a three-path interferometer coupling with path indicator states.The wave property of photons is characterized by l1-norm coherence measure,and the particle property is based on distinguishability of the indicator states.The new duality relation of the three-path interferometer is demonstrated in our experiment,which bounds the union of a right triangle and a part of elliptical area inside the quadrant of a unit circle.Data analysis confirms that the new bound is tight for photons in three-path interferometers.

Linear optical approach to supersymmetric dynamics

The concept of supersymmetry developed in particle physics has been applied to various fields of modern physics.In quantum mechanics,the supersymmetric systems refer to the systems involving two supersymmetric partner Hamiltonians,whose energy levels are degeneracy except one of the systems has an extra ground state possibly,and the eigenstates of the partner systems can be mapped onto each other.Recently,an interferometric scheme has been proposed to show this relationship in ultracold atoms[Phys.Rev.A 96043624(2017)].Here this approach is generalized to linear optics for observing the supersymmetric dynamics with photons.The time evolution operator is simulated approximately via Suzuki–Trotter expansion with considering the realization of the kinetic and potential terms separately.The former is realized through the diffraction nature of light and the later is implemented using a phase plate.Additionally,we propose an interferometric approach which can be implemented perfectly using an amplitude alternator to realize the non-unitary operator.The numerical results show that our scheme is universal and can be realized with current technologies.

A von-Neumann-like photonic processor and its application in studying quantum signature of chaos

Photonic quantum computation plays an important role and offers unique advantages.Two decades after the milestone work of Knill-Laflamme-Milburn,various architectures of photonic processors have been proposed,and quantum advantage over classical computers has also been demonstrated.It is now the opportune time to apply this technology to real-world applications.However,at current technology level,this aim is restricted by either programmability in bulk optics or loss in integrated optics for the existing architectures of processors,for which the resource cost is also a problem.Here we present a von-Neumann-like architecture based on temporal-mode encoding and looped structure on table,which is capable of multimode-universal programmability,resource-efficiency,phasestability and software-scalability.In order to illustrate these merits,we execute two different programs with varying resource requirements on the same processor,to investigate quantum signature of chaos from two aspects:the signature behaviors exhibited in phase space(13 modes),and the Fermi golden rule which has not been experimentally studied in quantitative way before(26 modes).The maximal program contains an optical interferometer network with 1694 freely-adjustable phases.Considering current state-of-the-art,our architecture stands as the most promising candidate for real-world applications.

Entanglement quantification via weak measurements assisted by deep learning

Entanglement has been recognized as being crucial when implementing various quantum information tasks.Nevertheless, quantifying entanglement for an unknown quantum state requires nonphysical operations or post-processing measurement data. For example, evaluation methods via quantum state tomography require vast amounts of measurement data and likely estimation.

Remarkable extension of Noether's theorem in PT-symmetric systems

Noether’s theorem[1],relating the symmetry of a physical system to its constant of motion,is a fundamental physical law. It has been shown that the expectation value of a time-independent operator is a constant of motion if it commutes with the Hermitian Hamiltonian. However,applying Noether’s theorem to open systems and obtaining the corresponding conserved quantities are still tricky.

Experimental demonstration of separating the wave-particle duality of a single photon with the quantum Cheshire cat

As a fundamental characteristic of physical entities,wave-particle duality describes whether a microscopic entity exhibits wave or particle attributes depending on the specific experimental setup.This assumption is premised on the notion that physical properties are inseparable from the objective carrier.However,after the concept of the quantum Cheshire cats was proposed,which makes the separation of physical attributes from the entity possible,the premise no longer holds.Furthermore,an experimental demonstration of the separation of the wave and particle attributes inspired by this scenario remains scarce.In this work,we experimentally separated the wave and particle attributes of a single photon by exploiting the quantum Cheshire cat concept for the first time.By applying a weak disturbance to the evolution of the system,we achieve an effect similar to the quantum Cheshire cat and demonstrated the separation of the wave and particle attributes via the extraction of weak values.Our work provides a new perspective for the in-depth understanding of wave-particle duality and promotes the application of weak measurements in fundamentals of quantum mechanics.

Measuring a dynamical topological order parameter in quantum walks

Quantum processes of inherent dynamical nature,such as quantum walks,defy a description in terms of an equilibrium statistical physics ensemble.Until now,identifying the general principles behind the underlying unitary quantum dynamics has remained a key challenge.Here,we show and experimentally observe that split-step quantum walks admit a characterization in terms of a dynamical topological order parameter(DTOP).This integer-quantized DTOP measures,at a given time,the winding of the geometric phase accumulated by the wavefunction during a quantum walk.We observe distinct dynamical regimes in our experimentally realized quantum walks,and each regime can be attributed to a qualitatively different temporal behavior of the DTOP.Upon identifying an equivalent manybody problem,we reveal an intriguing connection between the nonanalytic changes of the DTOP in quantum walks and the occurrence of dynamical quantum phase transitions.

Optical demonstration of quantum fault-tolerant threshold

A major challenge in practical quantum computation is the ineludible errors caused by the interaction of quantum systems with their environment. Fault-tolerant schemes, in which logical qubits are encoded by several physical qubits, enable to the output of a higher probability of correct logical qubits under the presence of errors. However, strict requirements to encode qubits and operators render the implementation of a full fault-tolerant computation challenging even for the achievable noisy intermediate-scale quantum technology. Especially the threshold for fault-tolerant computation still lacks experimental verification. Here, based on an all-optical setup, we experimentally demonstrate the existence of the threshold for the fault-tolerant protocol. Four physical qubits are represented as the spatial modes of two entangled photons, which are used to encode two logical qubits. The experimental results clearly show that when the error rate is below the threshold, the probability of correct output in the circuit, formed with fault-tolerant gates, is higher than that in the corresponding non-encoded circuit. In contrast, when the error rate is above the threshold, no advantage is observed in the fault-tolerant implementation. The developed high-accuracy optical system may provide a reliable platform to investigate error propagation in more complex circuits with fault-tolerant gates.

Experimental verification of group non-membership in optical circuits

The class quantum Merlin–Arthur(QMA),as the quantum analog of nondeterministic polynomial time,contains the decision problems whose YES instance can be verified efficiently with a quantum computer.The problem of deciding the group non-membership(GNM)of a group element is conjectured to be a member of QMA.Previous works on the verification of GNM,which still lacks experimental demonstration,required a quantum circuit with O(n~5)group oracle calls.Here,we provide an efficient way to verify GNM problems,in which each quantum circuit only contains O(1)group of oracle calls,and the number of qubits in each circuit is reduced by half.Based on this protocol,we then experimentally demonstrate the new verification process with a four-element group in an all-optical circuit.The new protocol is validated experimentally by observing a significant completeness-soundness gap between the probabilities of accepting elements in and outside the subgroup.This work efficiently simplifies the verification of GNM and is helpful in constructing more quantum protocols based on the near-term quantum devices.

Experimental verification of generalized eigenstate thermalization hypothesis in an integrable system

Identifying the general mechanics behind the equilibration of a complex isolated quantum system towards a state described by only a few parameters has been the focus of attention in non-equilibrium thermodynamics.And several experimentally unproven conjectures are proposed for the statistical description of quantum(non-)integrable models.The plausible eigenstate thermalization hypothesis(ETH),which suggests that each energy eigenstate itself is thermal,plays a crucial role in understanding the quantum thermalization in non-integrable systems;it is commonly believed that it does not exist in integrable systems.Nevertheless,integrable systems can still relax to the generalized Gibbs ensemble.From a microscopic perspective,understanding the origin of this generalized thermalization that occurs in an isolated integrable system is a fundamental open question lacking experimental investigations.Herein,we experimentally investigated the spin subsystem relaxation in an isolated spin-orbit coupling quantum system.By applying the quantum state engineering technique,we initialized the system with various distribution widths in the mutual eigenbasis of the conserved quantities.Then,we compared the steady state of the spin subsystem reached in a long-time coherent dynamics to the prediction of a generalized version of ETH and the underlying mechanism of the generalized thermalization is experimentally verified for the first time.Our results facilitate understanding the origin of quantum statistical mechanics.

Improving the precision of optical metrology by detecting fewer photons with biased weak measurement

In optical metrological protocols to measure physical quantities,it is,in principle,always beneficial to in crease photon number n to improve measurement precision.However,practical constraints prevent the arbitrary increase of n due to the imperfections of a practical detector,especially when the detector response is dominated by the saturation effect.In this work,we show that a modified weak measurement protocol,namely,biased weak measurement significantly improves the precision of optical metrology in the presence of saturation effect.This method detects an ultra-small fraction of photons while main tains a considerable amount of metrological information.The biased pre-coupling leads to an additi onal reduction of photons in the post-selection and gene rates an extinction point in the spectrum distribution,which is extremely sensitive to the estimated parameter and difficult to be saturated.Therefore,the Fisher information can be persistently enhanced by increasing the photon number.In our magnetic-sensing experiment,biased weak measurement achieves precision approximately one order of magnitude better than those of previously used methods.The proposed method can be applied in various optical measurement schemes to remarkably mitigate the detector saturation effect with low-cost apparatuses.

Experimental verification of a coherence factorization law for quantum states

As a quantum resource,quantum coherence plays an important role in modern physics.Many coherence measures and their relations with entanglement have been proposed,and the dynamics of entanglement has been experimentally studied.However,the knowledge of general results for coherence dynamics in open systems is limited.Here we propose a coherence factorization law that describes the evolution of coherence passing through any noisy channels characterized by genuinely incoherent operations.We use photons to implement the quantum operations and experimentally verify the law for qubits and qutrits.Our work is a step toward understanding of the evolution of coherence when the system interacts with the environment,and will boost the study of more general laws of coherence.

Optical charge state manipulation of divacancy spins in silicon carbide under resonant excitation

Spin defects in silicon carbide(SiC)have attracted much attentions in various quantum technologies.In this work,we study the optical manipulation of charge state and coherent control of multifold divacancy spins ensemble in SiC under resonant excitation.The results reveal that the resonantly excited divacancy ensemble counts have dozens of enhancements by repumping a higher-energy laser.Moreover,it has a similar optimal repump laser wavelength of around 670 nm for multiple divacancies.On the basis of this,the optically detected magnetic resonance(ODMR)experiment shows that repump lasers with different wavelengths do not affect the ODMR contrast and line width.In addition,the repump lasers also do not change the divacancy spins'coherence times.The experiments pave the way for using the optimal repump excitation method for SiC-based quantum information processing and quantum sensing.

Experimental observation of an anomalous weak value without post-selection

Weak measurement has been shown to play important roles in the investigation of both fundamental and practical problems.Anomalous weak values are generally believed to be observed only when post-selection is performed,i.e.,only a particular subset of the data is considered.Here,we experimentally demonstrate that an anomalous weak value can be obtained without discarding any data by performing a sequential weak measurement on a single-qubit system.By controlling the blazing density of the hologram on a spatial light modulator,the measurement strength can be conveniently controlled.Such an anomalous phenomenon disappears when the measurement strength of the first observable becomes strong.Moreover,we find that the anomalous weak value cannot be observed without post-selection when the sequential measurement is performed on each of the components of a two-qubit system,which confirms that the observed anomalous weak value is based on sequential weak measurement of two noncommutative operators.

Silicon carbide based quantum networking

1.Spin defects in silicon carbide Solid-state qubits are critical for scalable quantum information pro-cessing.One of the most studied optically addressable solid-state spin systems is the negatively charged nitrogen-vacancy(NV)center in di-amond,which has been widely used in quantum communication and quantum sensing.However,the visible wavelength emission of NV cen-ters in diamond causes a large loss in fiber transmission and has shallow penetration depth in biological tissues.Moreover,there is still a lack of mature growing and nanofabrication methods for diamond.