Quantum twisted double-slits experiments:confirming wavefunctions'physical reality

Are quantum states real? This most fundamental question in quantum mechanics has not yet been satisfactorily resolved, although its realistic interpretation seems to have been rejected by various delayedchoice experiments. Here, to address this long-standing issue, we present a quantum twisted double-slit experiment. By exploiting the subluminal feature of twisted photons, the real nature of a photon during its time in flight is revealed for the first time. We found that photons' arrival times were inconsistent with the states obtained in measurements but agreed with the states during propagation. Our results demonstrate that wavefunctions describe the realistic existence and evolution of quantum entities rather than a pure mathematical abstraction providing a probability list of measurement outcomes. This finding clarifies the long-held misunderstanding of the role of wavefunctions and their collapse in the evolution of quantum entities.

Orbital angular momentum photonic quantum interface

Light-carrying orbital angular momentum(OAM)has great potential in enhancing the information channel capacity in both classical and quantum optical communications.Long distance optical communication requires the wavelengths of light are situated in the low-loss communication windows,but most quantum memories currently being developed for use in a quantum repeater work at different wavelengths,so a quantum interface to bridge the wavelength gap is necessary.So far,such an interface for OAM-carried light has not been realized yet.Here,we report the first experimental realization of a quantum interface for a heralded single photon carrying OAM using a nonlinear crystal in an optical cavity.The spatial structures of input and output photons exhibit strong similarity.More importantly,single-photon coherence is preserved during up-conversion as demonstrated.

Experimental demonstration of switching entangled photons based on the Rydberg blockade effect

The long-range interaction between Rydberg-excited atoms endows a medium with large optical nonlinearity.Here,we demonstrate an optical switch to operate on a single photon from an entangled photon pair under a Rydberg electromagnetically induced transparency configuration.With the presence of the Rydberg blockade effect,we switch on a gate field to make the atomic medium nontransparent thereby absorbing the single photon emitted from another atomic ensemble via the spontaneous fourwave mixing process.In contrast to the case without a gate field,more than 50%of the photons sent to the switch are blocked,and finally achieve an effective single-photon switch.There are on average 1-2 gate photons per effective blockade sphere in one gate pulse.This switching effect on a single entangled photon depends on the principal quantum number and the photon number of the gate field.Our experimental progress is significant in the quantum information process especially in controlling the interaction between Rydberg atoms and entangled photon pairs.

High-dimensional entanglement between distant atomic-ensemble memories

Entangled quantum states in high-dimensional space show many advantages compared with entangled states in two-dimensional space.The former enable quantum communication with higher channel capacity,enable more efficient quantum-information processing and are more feasible for closing the detection loophole in Bell test experiments.Establishing high-dimensional entangled memories is essential for long-distance communication,but its experimental realization is lacking.We experimentally established high-dimensional entanglement in orbital angular momentum space between two atomic ensembles separated by 1 m.We reconstructed the density matrix for a three-dimensional entanglement and obtained an entanglement fidelity of(83.9±2.9)%.More importantly,we confirmed the successful preparation of a state entangled in more than three-dimensional space(up to seven-dimensional)using entanglement witnesses.Achieving high-dimensional entanglement represents a significant step toward a high-capacity quantum network.

Quantum interface for high-dimensional quantum states encoded in an orbital angular momentum space

Quantum information science involves the study of information pro-cessing tasks that can be accomplished using quantum mechanical sys-tems.The basic unit of quantum information is a quantum bit(qubit),which exists in a two-dimensional space spanned for example by the or-thogonal polarization states of a photon.In this case,each photon carries only one qubit.However,if the photon is encoded in a high-dimensional space.

旋转坐标空间中的双色Einstein-Podolsky-Rosen纠缠实验

在量子信息领域,一对粒子在位置和动量上的纠缠称为位置-动量Einstein-Podolsky-Rosen (EPR)纠缠.它是一种重要的纠缠资源,在量子计算和量子通信中有着重要的研究意义. EPR纠缠已经在不同的物理体系中被研究,但这些体系都处在固定的坐标空间中.这里纠缠的双光子是利用热原子系综的自发四波混频过程产生的,作者在旋转的位置-动量空间中观察EPR纠缠光子的鬼成像和鬼干涉.通过不同的成像物体,测量到的位置-动量关联展现出有趣的动力学特征,包括关联的逐渐消退和与坐标轴无关的EPR纠缠特性.在旋转坐标空间中操纵EPR纠缠的这些结果展现出它在量子通信和远程量子图像处理中的潜在应用前景.

Harmonics-assisted optical phase amplifier

The change in the relative phase between two light fields serves as a basic principle for the measurement of the physical quantity that guides this change.It would therefore be highly advantageous if the relative phase could be amplified to enhance the measurement resolution.One well-known method for phase amplification involves the use of the multi-photon number and path-entangled state known as the NOON state;however,a high-number NOON state is very diffcult to prepare and is highly sensitive to optical losses.Here we propose and experimentally demonstrate in principle a phase amplifer scheme with the assistance of a harmonic generation process.The relative phase difference between two polarization modes in a polarized interferometer is amplified coherently four times with cascaded second-harmonic generation processes.We demonstrate that these amplification processes can be recycled and therefore have the potential to realize much higher numbers of multiple amplification steps.The phase amplification method presented here shows considerable advantages over the method based on NOON states and willbe highly promising for use in precision optical measurements.

Angular-spectrum-dependent interference

Optical interference is not only a fundamental phenomenon that has enabled new theories of light to be derived but it has also been used in interferometry for the measurement of small displacements,refractive index changes,and surface irregularities.In a two-beam interferometer,variations in the interference fringes are used as a diagnostic for anything that causes the optical path difference(OPD)to change;therefore,for a specified OPD,greater variation in the fringes indicates better measurement sensitivity.Here,we introduce and experimentally validate an interesting optical interference phenomenon that uses photons with a structured frequency-angular spectrum,which are generated from a spontaneous parametric down-conversion process in a nonlinear crystal.This interference phenomenon is manifested as interference fringes that vary much more rapidly with increasing OPD than the corresponding fringes for equal-inclination interference;the phenomenon is parameterised using an equivalent wavelength,which under our experimental conditions is 29.38 nm or about 1/27 of the real wavelength.This phenomenon not only enriches the knowledge with regard to optical interference but also offers promise for applications in interferometry.