Zhihao Bian

Realization of Single-Qubit Positive-Operator-Valued Measurement via a One-Dimensional Photonic Quantum Walk.

We perform generalized measurements of a qubit by realizing the qubit as a coin in a photonic quantum walk and subjecting the walker to projective measurements. Our experimental technique can be used to realize, photonically, any rank-1 single-qubit positive-operator-valued measure via constructing an appropriate interferometric quantum-walk network and then projectively measuring the walkers position at the final step.

Experimental investigation of the stronger uncertainty relations for all incompatible observables

The Heisenberg-Robertson uncertainty relation quantitatively expresses the impossibility of jointly sharp preparation of incompatible observables. However, it does not capture the concept of incompatible observables because it can be trivial even for two incompatible observables. We experimentally demonstrate that the new stronger uncertainty relations proposed by Maccone and Pati [Phys. Rev. Lett. 113, 260401 (2014)] relating to the sum of variances are valid in a state-dependent manner and that the lower bound is guaranteed to be nontrivial when two observables are incompatible on the state of the system being measured. The behavior we find agrees with the predictions of quantum theory and obeys the new uncertainty relations even for the special states which trivialize the Heisenberg-Robertson relation. We realize a direct measurement model and perform an experimental investigation of the strengthened relations.

Detecting topological invariants in nonunitary discrete-time quantum walks

We report the experimental detection of bulk topological invariants in nonunitary discrete-time quantum walks with single photons. The nonunitarity of the quantum dynamics is enforced by periodically performing partial measurements on the polarization of the walker photon, which effectively introduces loss to the dynamics. The topological invariant of the nonunitary quantum walk is manifested in the quantized average displacement of the walker, which is probed by monitoring the photon loss. We confirm the topological properties of the system by observing localized edge states at the boundary of regions with different topological invariants. We further demonstrate the robustness of both the topological properties and the measurement scheme of the topological invariants against disorder.

Perfect state transfer and efficient quantum routing: a discrete-time quantum walk approach

architecture than state transfer. We show how arbitrary networks can be designed for routing multi-qubit quantum states between arbitrary sites with a time scaling that is linear to the distance to be covered. In this paper, we present a scheme on perfectly transferring unknown state and routing quantum information in architecture of QWs on regular network. We transfer the coin states between arbitrary sites by manipulating the coin flipping. Our protocol benefits from the full control of the walker+coin system. Compared to the previous protocols on quantum state transfer, transferring the coin state is more feasible and easier to extend to multiqubit entanglement transfer releasing the requirement of periodicity of QWs. The walker carries the coin prepared in a certain state, which needs to be transferred, and walks from the original site to the target site with timedependent coin flippings for each step. Thus the coin state is perfectly transferred between two sites. Furthermore, we extend the method to distribute multi-qubit entanglement between arbitrary sites resulting in a possible implementation of efficient quantum routing. This article is organized as follows. In Sec. II, we give a brief introduction to a discrete-time QW on line, and illustrate two kinds of biased coin flipping operators, which play a critical role in our scheme. The content of the scheme for realizing quantum routing based on controllable perfect state transfer via discrete-time QW is showed in Sec. III. We extend the scheme from one dimensional quantum routing to two dimensional case, routing entangled qubits to two arbitrary positions. We also show the scheme can be extended to high dimensional case.

Localized state in a two-dimensional quantum walk on a disordered lattice

We realize a pair of simultaneous ten-step one-dimensional quantum walks with two walkers sharing coins, which we prove is analogous to the ten-step two-dimensional quantum walk with a single walker holding a four-dimensional coin. Our experiment demonstrates a ten-step quantum walk over an 11x11 two-dimensional lattice with a line defect, thereby realizing a localized walker state.

Experimental test of uncertainty relations for general unitary operators

Uncertainty relations are the hallmarks of quantum physics and have been widely investigated since its original formulation. To understand and quantitatively capture the essence of preparation uncertainty in quantum interference, the uncertainty relations for unitary operators need to be investigated. Here, we report the first experimental investigation of the uncertainty relations for general unitary operators. In particular, we experimentally demonstrate the uncertainty relation for general unitary operators proved by Bagchi and Pati [ Phys. Rev. A94, 042104 (2016)], which places a non-trivial lower bound on the sum of uncertainties and removes the triviality problem faced by the product of the uncertainties. The experimental findings agree with the predictions of quantum theory and respect the new uncertainty relation.

Linear optical demonstration of quantum speed-up with a single qudit.

Quantum algorithm acts as an important role in quantum computation science, not only for providing a great vision for solving classically unsolvable problems, but also due to the fact that it gives a potential way of understanding quantum physics. We experimentally realize a quantum speed-up algorithm with a single qudit via linear optics and prove that even a single qudit is enough for designing an oracle-based algorithm which can solve a certain problem twice faster than any classical algorithm. The algorithm can be generalized to higher-dimensional systems with the same two-to-one speed-up ratio.

Experimental Detection of Information Deficit in a Photonic Contextuality Scenario

Contextuality is an essential characteristic of quantum theory, and supplies the power for many quantum information processes. Previous tests of contextuality focus mainly on the probability distribution of measurement results. However, a test of contextuality can be formulated in terms of entropic inequalities whose violations imply information deficit in the studied system. This information deficit has not been observed on a single local system. Here we report the first experimental detection of information deficit in an entropic test of quantum contextuality based on photonic setup. The corresponding inequality is violated with more than 13 standard deviations.

Enhanced violations of Leggett-Garg inequalities in an experimental three-level system

Leggett-Garg inequalities are tests of macroscopic realism that can be violated by quantum mechanics. In this letter, we realise photonic Leggett-Garg tests on a three-level system and implement measurements that admit three distinct measurement outcomes, rather than the usual two. In this way we obtain violations of three- and four-time Leggett-Garg inequalities that are significantly in excess of those obtainable in standard Leggett-Garg tests. We also report violations the quantum-witness equality up to the maximum permitted for a three-outcome measurement. Our results highlight differences between spatial and temporal correlations in quantum mechanics.

A one-dimensional quantum walk with multiple-rotation on the coin.

We introduce and analyze a one-dimensional quantum walk with two time-independent rotations on the coin. We study the influence on the property of quantum walk due to the second rotation on the coin. Based on the asymptotic solution in the long time limit, a ballistic behaviour of this walk is observed. This quantum walk retains the quadratic growth of the variance if the combined operator of the coin rotations is unitary. That confirms no localization exhibits in this walk. This result can be extended to the walk with multiple time-independent rotations on the coin.