Jian-Shun Tang

Measuring non-Markovianity of processes with controllable system-environment interaction

Non-Markovian processes have recently become a central topic in the study of open quantum systems. We realize experimentally non-Markovian decoherence processes of single photons by combining time delay and evolution in a polarization-maintaining optical fiber. The experiment allows the identification of the process with strongest memory effects as well as the determination of a recently proposed measure for the degree of quantum non-Markovianity based on the exchange of information between the open system and its environment. Our results show that an experimental quantification of memory in quantum processes is indeed feasible which could be useful in the development of quantum memory and communication devices.

Revisiting Bohr's principle of complementarity with a quantum device

Bohrs principle of complementarity (BPC) is the cornerstone of quantum mechanics. According to this principle, the total wavelike and particlelike information of a particle is limited by the Englert-Greenberger (EG) duality relation. Here, by introducing a quantum detecting device into the experiment, we find that the limit of the EG duality relation is exceeded because of the interference between the wave and particle properties of the photon. A generalized EG duality relation is further developed. Our work provides a generalization of BPC and gives new insights into quantum mechanics.

Storage of multiple single-photon pulses emitted from a quantum dot in a solid-state quantum memory

Quantum repeaters are critical components for distributing entanglement over long distances in presence of unavoidable optical losses during transmission. Stimulated by the Duan–Lukin–Cirac–Zoller protocol, many improved quantum repeater protocols based on quantum memories have been proposed, which commonly focus on the entanglement-distribution rate. Among these protocols, the elimination of multiple photons (or multiple photon-pairs) and the use of multimode quantum memory are demonstrated to have the ability to greatly improve the entanglement-distribution rate. Here, we demonstrate the storage of deterministic single photons emitted from a quantum dot in a polarization-maintaining solid-state quantum memory; in addition, multi-temporal-mode memory with 1, 20 and 100 narrow single-photon pulses is also demonstrated. Multi-photons are eliminated, and only one photon at most is contained in each pulse. Moreover, the solid-state properties of both sub-systems make this configuration more stable and easier to be scalable. Our work will be helpful in the construction of efficient quantum repeaters based on all-solid-state devices.

Experimental detection of polarization-frequency quantum correlations in a photonic quantum channel by local operations

The measurement of correlations between different degrees of freedom is an important, but, in general, extremely difficult task in many applications of quantum mechanics. Here, we report an all-optical experimental detection and quantification of quantum correlations between the polarization and the frequency degrees of freedom of single photons by means of local operations acting only on the polarization degree of freedom. These operations only require experimental control over an easily accessible two-dimensional subsystem, despite handling strongly mixed quantum states comprised of a continuum of orthogonal frequency states. Our experiment thus represents a photonic realization of a scheme for the local detection of quantum correlations in a truly infinite-dimensional continuous-variable system, which excludes an efficient finite-dimensional truncation.

Robust bidirectional links for photonic quantum networks

Jin-Shi Xu, ∗ Man-Hong Yung, 3, ∗ Xiao-Ye Xu, Jian-Shun Tang, Chuan-Feng Li, † and Guang-Can Guo Key Laboratory of Quantum Information, University of Science and Technology of China, CAS, Hefei, 230026, People’s Republic of China Department of Chemistry and Chemical Biology, Harvard University, Cambridge MA, 02138, USA Center for Quantum Information, Institute for Interdisciplinary Information Sciences, Tsinghua University, Beijing, 10084, People’s Republic of China (Dated: May 8, 2013)Researchers experimentally realize a promising method for creating robust bidirectional quantum communication links. Optical fibers are widely used as one of the main tools for transmitting not only classical but also quantum information. We propose and report an experimental realization of a promising method for creating robust bidirectional quantum communication links through paired optical polarization-maintaining fibers. Many limitations of existing protocols can be avoided with the proposed method. In particular, the path and polarization degrees of freedom are combined to deterministically create a photonic decoherence-free subspace without the need for any ancillary photon. This method is input state–independent, robust against dephasing noise, postselection-free, and applicable bidirectionally. To rigorously quantify the amount of quantum information transferred, the optical fibers are analyzed with the tools developed in quantum communication theory. These results not only suggest a practical means for protecting quantum information sent through optical quantum networks but also potentially provide a new physical platform for enriching the structure of the quantum communication theory.

Heisenberg-scaling measurement of the single-photon Kerr non-linearity using mixed states

Improving the precision of measurements is a significant scientific challenge. Previous works suggest that in a photon-coupling scenario the quantum fisher information shows a quantum-enhanced scaling of N2, which in theory allows a better-than-classical scaling in practical measurements. In this work, utilizing mixed states with a large uncertainty and a post-selection of an additional pure system, we present a scheme to extract this amount of quantum fisher information and experimentally attain a practical Heisenberg scaling. We performed a measurement of a single-photon’s Kerr non-linearity with a Heisenberg scaling, where an ultra-small Kerr phase of ≃6 × 10−8 rad was observed with a precision of ≃3.6 × 10−10 rad. From the use of mixed states, the upper bound of quantum fisher information is improved to 2N2. Moreover, by using an imaginary weak-value the scheme is robust to noise originating from the self-phase modulation.Quantum metrology usually relies on entanglement or squeezing for pursuing Heisenberg-limited precision. In this work, instead, the authors demonstrate Heisenberg-scaling measurement of a single photon Kerr’s nonlinearity using less-demanding mixed states.

Measurement-induced quantum coherence recovery

We show that measurement can recover the quantum coherence of a qubit in a pure dephasing environment. The experimental demonstration of this in an optical system by comparing the visibilities (and fidelities) of the final states with and without measurement is provided here. This method can be extended to other two-level quantum systems and entangled states in a dephasing evolution environment. It may also be used to implement other types of quantum information processing.

Degenerate cavity supporting more than 31 Laguerre–Gaussian modes

Photons propagating in Laguerre-Gaussian modes have characteristic orbital angular momenta, which are fundamental optical degrees of freedom. The orbital angular momentum of light has potential application in high-capacity optical communication and even in quantum information processing. In this work, we experimentally construct a ring cavity with four lenses and four mirrors that is completely degenerate for Laguerre-Gaussian modes. By measuring the transmitted peaks and patterns of different modes, the ring cavity is shown to support more than 31 Laguerre-Gaussian modes. The constructed degenerate cavity opens a new way for using the unlimited resource of available angular momentum states simultaneously.

Measurement of the inhomogeneous broadening of a bi-exciton state in a quantum dot using Franson-type nonlocal interference

The inhomogeneous broadening of the bi-exciton state in quantum dots, i.e., the inhomogeneous broadening of the upper level of the cascade process, is not only a fundamental problem in quantum dots, but also closely related with the coherent control of this complex system and the quality of the entangled photon pairs, especially the time-bin entangled photon pairs. This inhomogeneous broadening is inherently a two-photon correlated phenomenon. In this work, we construct a genuine Franson-type nonlocal interference process to measure the inhomogeneous broadening of the bi-exciton state. The results show that the inhomogeneous broadening of the bi-exciton state is considerably smaller than that of the exciton state, that is why the entangled photon pairs can be generated by the cascade process in the quantum dot.

Experimental implementation of a degenerate optical resonator supporting more than 46 Laguerre-Gaussian modes

Great efforts have been made to investigate photons orbital angular momentum (OAM) due to its comprehensive applications ranging from micro-manipulation to biosciences. Recently, it has been proposed that the unlimited OAM number can be used as synthetic degrees of freedom and can be used for quantum simulation. Here, we demonstrate a vital step in manipulating such kind of unlimited degree of freedom simultaneously. We construct an optical resonator with four spherical mirrors, which is predicted to support lights in different Laguerre-Gaussian modes with well-defined OAMs. The transmitted peaks of more than 46 Laguerre-Gaussian modes are observed to be overlapped within the bandwidth of the resonator. The transmitted beam profiles are further obtained by locking the resonator. Our experimental results establish the critical techniques to manipulate multiple-OAM degrees of freedom, which are useful for quantum simulation.