Session N1: Quantum Optics II

A study in the optimal fidelity of continuous-variable teleportation.

We introduce three tunable parameters to optimize the fidelity of quantum teleportation with continuous-variable in nonideal scheme. Using the characteristic function formalism, we present the condition that the teleportation fidelity is independent of the amplitude of input coherent states for any entangled resource. Then we investigate the effects of tunable parameters on the fidelity with or without the presence of environment and imperfect measurements, by analytically deriving the expression of fidelity for three different input coherent state distributions. It is shown that, for the linear distribution, the optimization with three tunable parameters is the best one with respect to single- and two-parameter optimization. Our results reveal the usefulness of tunable parameters for improving the fidelity of teleportation and the ability against the decoherence.   

A local hidden variable model of the GHZ puzzle

Greenberger-Horne-Zeilinger (GHZ) states have been used to study quantum nonlocality and provide an all-or-nothing, no-go theorem for local hidden variable models. Recent experiments using coincident-detected entangled photons prepared in a three-particle GHZ state have shown significant violations of the Mermin inequality under strict locality conditions, a result believed to be inconsistent with local realism. As an argument against this conclusion, a local hidden variable model is presented which shows similar violations of the Mermin inequality, a result made possible by postselecting on coincident-detection events, as was done in the experiment.   

An ideal experiment to determine the `past of a particle' in the nested Mach-Zehnder Interferometer

An ideal experiment is designed to determine the past of a particle in the nested Mach-Zehnder interferometer (MZI) by using standard quantum mechanics with quantum non-demolition measurements. We find that when the photon reaches the detector, it only follows one arm of the outer interferometer and leaves no trace in the inner MZI; while when it goes through the inner MZI, it cannot reach the detector. Our result obtained from the standard quantum mechanics is contradict to the statement based on two state vector formulism, “the photon did not enter the (inner) interferometer, the photon never left the interferometer, but it was there”.   

Quantum Teleportation without Classical Channel

We show that quantum teleportation can be achieved without the need of classical channel. Our protocol does not require prearranged entangled photon pairs and Bell measurements. By utilizing quantum Zeno effect and couterfactuality, we can entangle and disentangle a photon and an atom by non-local interaction. It is shown that quantum information is completely transferred from an atom to photon due to controllable disentanglement processes. There is no need to cross check teleportation results via classical channels. Our protocol is a complement to the conventional quantum teleportation.   

Classical Emulation of a 2-Qubit Quantum Computer Using Analog Voltage signals

The Hilbert space mathematical structure of a gate-based quantum computer may be reproduced by mapping the computational basis states to corresponding functions in the space of complex exponentials and identifying an $L^2$-inner product between any two such functions. The span of these complex basis exponentials is then identified with the finite dimensional Hilbert space of the quantum computer. By using classical analog electronic components, such as four-quadrant multipliers and operational amplifiers, voltage signals representing arbitrary four-dimensional quantum states, along with the equivalent gate and measurement operations of a quantum computer have been physically realized through the corresponding circuitry. The fidelity of the emulation is measured using both a direct evaluation of the signal as well as through an emulation of quantum state tomography to infer the quantum state. For both state synthesis and gate operations, our quantum emulation device is capable of achieving over $99\%$ fidelity.   

Quantum measurement reversal: Influence of finite detector efficiency

Amplitude-damping decoherence can be suppressed by applying a partial measurement and a reversal operation before and after the damping process respectively. In this scheme, environment is monitored to assist in discarding collapsed states. However, ideal detector efficiency is assumed in recent studies. Here we study the protection of single-qubit states and two-qubit entangled states under finite detector efficiency. A demonstration experiment setup based on linear optical system is described. Fidelity and concurrence are studied to evaluate the effect of the protection. We show that this scheme still works when the efficiency is large enough, however, its effect is weakened. Otherwise, it causes worse decoherence than that of states damping alone without any protection. A criterion of detector efficiency for deciding whether to apply this scheme is then given.   

Hybrid Quantum-Classical Approaches to NP-complete Problems

The observations by Feynman and the landmark discoveries of Deustch, Shor, Grover and others has opened the door toward huge advancements in computational power possible by shifting toward the paradigm of quantum computing. Problems in the computational complexity class \textbf{NP-complete} pose a unique challenge as there are no known algorithms to efficiently solve these problems in either the classical or quantum framework. It is known however, that many of these \textbf{NP-complete} problems admit a formulation as an unstructured search problem and can benefit from the quadratic speedup of Grover's algorithm. In many problems of practical interest this can amount to a monumental time savings and open the door to new classes of applications. Current quantum computing systems are highly constrained in the number of qubits they can support. In this presentation we look at a particular instance of a \textbf{NP-complete} problem known as 3SAT and investigate the speedups possible by switching to a hybrid approach using a small number of qubits.   

Entanglement of two movable mirrors with a single photon superposition state

In this talk, we will discuss a simple scheme to generate deterministic entanglement between two movable end mirrors in a Fabry–Perot cavity using a single photon superposition state. We will introduce the background of our study and the basic theory of cavity optomechanics. Analytical and numerical results are obtained to show that strong entanglement can be obtained either in the single-photon strong coupling regime deterministically or in the single-photon weak coupling regime conditionally.   

Robust quantum state recovery from amplitude damping within a mixed states framework

Due to the interaction with the environment, a quantum state is subjected to decoherence which becomes one of the biggest problems for practical quantum computation. Amplitude damping is one of the most important decoherence processes. We show that general two-qubit mixed states undergoing an amplitude damping can be almost completely restored using a reversal procedure. This reversal procedure, through CNOT and Hadamard gates, could also protect the entanglement of two-qubit mixed states, when it undergoes general amplitude damping. Moreover, in the presence of uncertainty in the underlying system, we propose a robust recovering method with optimal characteristics of the problem.