Experimental study of a Back Action Evading device for continuos measurements on a macroscopic harmonic oscillator at the quantum limit level

The Back Action Evading technique is a particular kind of quantum non demolition measurement, first proposed by Caves et al. in 1980 [3]. We present an experimental study to implement the Back Action Evading measurement scheme in monitoring the amplitude of an harmonic oscillator excited by a classical force. Results showing the agreement of our theoretical model with the experimental behaviour of our apparatus in the classical regime are presented. We discuss also the optimization of the performance of our set-up, which should allow to monitor our oscillator in quantum regime even below the standard quantum limit level. Received: 22 March 1996     

冷原子系综内单集体激发态的相干操纵

原子系综内部分原子发生相干态转移后所处量子态被称为集体激发态.如果激发数目在单原子量级则被称为单激发态.在量子存储过程中,单光子以单激发态的形式在原子系综内进行存储.因此,研究单激发态的制备、演化、转化、干涉等过程是量子存储及其应用研究的关键.本文总结了近年来作者所在研究团队针对冷原子系综体系在此研究方向取得的若干成果.主要包括采用动量模式调控、三维光晶格等手段抑制单激发态的退相干,采用环形腔增强原子至光子的转化效率,发展基于拉曼光的单激发态相干转移技术,利用单量子态不同模式间干涉制备光与原子纠缠,利用里德伯阻塞机制提升纠缠制备效率等.此外,简要回顾了基于多个单激发态的量子中继及量子网络实验.     

Error-detected single-photon quantum routing using a quantum dot and a double-sided microcavity system

Based on a hybrid system consisting of a quantum dot coupled with a double-sided micropillar cavity, we investigate the implementation of an error-detected photonic quantum routing controlled by the other photon. The computational errors from unexpected experimental imperfections are heralded by single photon detections, resulting in a unit fidelity for the present scheme, so that this scheme is intrinsically robust. We discuss the performance of the scheme with currently achievable experimental parameters. Our results show that the present scheme is efficient. Furthermore, our scheme could provide a promising building block for quantum networks and distributed quantum information processing in the future.     

Causal Intuition and Delayed-Choice Experiments

The conventional explanation of delayed-choice experiments appears to violate our causal intuition at the quantum level. I reanalyze these experiments using time-reversed and time-symmetric formulations of quantum mechanics. The time-reversed formulation does not give the same experimental predictions. The time-symmetric formulation gives the same experimental predictions but actually violates our causal intuition at the quantum level. I explore the reasons why our causal intuition may be wrong at the quantum level, suggest how conventional causation might be recovered in the classical limit, propose a quantum analog to the classical block universe viewpoint, and speculate on implications of the time-symmetric formulation for cosmological boundary conditions.     

Intersubband optical transitions in one dimensional quantum wire structures

One dimensional (1D) quantum wire structures are emerging as the new generation of semiconductor nanostructures offering exciting physical properties which have significant potential for novel device applications. These structures have been the subject of intensive investigation recently including extensive theoretical and experimental studies of their interband optical properties. In this work we present the results of our study of the intersubband optical transitions in 1D semiconductor quantum wires. The crescent shaped quantum wire structures used for this research were grown on non-planar GaAs substrates. The intersubband transition energy spectra, the selection rules, and the two dimensional envelope wavefunctions were theoretically investigated by using our new LENS (local envelope states) expansion. We present recent experimental results on modulation doped V-groove quantum wires, including PL, PLE, TEM, CL, and infrared polarization resolved spectroscopy. We have observed a very unusual absorption lineshape at the far-infrared wavelengths that we assigned to phonon assisted Fano resonance in a modulation doped quantum wire structure.     

Experimental demonstration of probabilistic quantum cloning

The method of quantum cloning is divided into two main categories: approximate and probabilistic quantum cloning. The former method is used to approximate an unknown quantum state deterministically, and the latter can be used to faithfully copy the state probabilistically. Thus far, many approximate cloning machines have been experimentally demonstrated, but probabilistic cloning remains an experimental challenge, as it requires more complicated networks and a higher level of precision control. In this work, we design an efficient quantum network with a limited amount of resources and perform the first experimental demonstration of probabilistic quantum cloning in a NMR quantum computer. In our experiment, the optimal cloning efficiency proposed by Duan and Guo [Phys. Rev. Lett. 80, 4999 (1998)] is achieved.     

Continuous variable quantum key distribution

Quantum key distribution enables unconditionally secure key distribution between two legitimate users.The information-theoretic security is guaranteed by the fundamental laws of quantum physics.Initially,the quantum key distribution protocol was proposed based on the qubits.Later on,it was found that quantum continuous variables can also be exploited for this target.The continuous variable quantum key distribution can build upon standard telecommunication technology and exhibits a higher secret key rate per pulse at a relatively short distance due to the possibility of encoding more than 1 bit per pulse.In this article,we review the current status of the continuous variable quantum key distribution research,including its basic principle,experimental implementations,security and future directions;the experimental progress in this field made by our group is also presented.     

多光子纠缠实验新进展--五光子纠缠技术及终端开放的隐形传态实验

在实验上发展了多光子纠缠技术,利用已有的四光子纠缠技术结合新发展的单光子源技术,在世界上第一次实现了五光子纠缠．并在此基础上实现了新型的量子隐形传态——终端开放的隐形传态．实验结果在量子力学基础问题的检验、信息论、密码学和量子计算等重要应用方向上,都具有显著的意义和价值．实验方法将大大促进未来网络化量子通信、线性光学量子计算、量子力学基础检验等重要科学问题的研究。     

Experimental realization of quantum controlled teleportation of arbitrary two-qubit state via a five-qubit entangled state

Quantum controlled teleportation is the transmission of the quantum state under the supervision of a third party. This paper presents the theoretical and experimental results of an arbitrary two-qubit quantum controlled teleportation scheme, in which the sender Alice only needs to perform two Bell state measurements and the receiver Bob can perform an appropriate unitary operation to reconstruct the arbitrary two-qubit states under the control of the supervisor Charlie. The operation process of the scheme is verified on the IBM quantum experience platform, and the accuracy of the transmitted quantum state is further checked by performing quantum state tomography. Meanwhile, a good fidelity is obtained by using the theoretical density matrix and the experimental density matrix. A sequence of photonic states is introduced to analyze the possible intercept-replace-resend, intercept-measure-resend, and entanglement-measure-resend attacks on this scheme. The results proved that our scheme is highly secure.     

Optimization of multiple quantum well electroabsorption modulators

A novel and simple approach for optimization of a multiple quantum well electroabsorption waveguide modulator is presented. In our approach, all four of the modulator characteristic parameters (on-off ratio, insertion loss, bandwidth, and driving voltage) are included. Design examples for GaAs/AlGaAs multiple quantum well modulators are presented. The accuracy of our model is confirmed by comparison between calculated and published experimental data.     

Continuous variable quantum communication with bright entangled optical beams

In this paper, we briefly introduce the basic concepts and protocols of continuous variable quantum communication, and then summarize the experimental researches accomplished by our group in this field. The main features of quantum communication systems used in our experiments are: (1) The bright entangled optical beams with the anticorrelated amplitude quadratures and the correlated phase quadratures that serve as the entanglement resources and (2) The Bell-state direct detection systems are utilized in the measurements of quantum entanglement and transmitted signals instead of the usually balanced homodyne detectors.     

One-shot quantum measurement using a hysteretic dc SQUID

We propose a single shot quantum measurement to determine the state of a Josephson charge quantum bit (qubit). The qubit is a Cooper pair box and the measuring device is a two junction superconducting quantum interference device (dc SQUID). This coupled system exhibits a close analogy with a Rydberg atom in a high Q cavity, except that in the present device we benefit from the additional feature of escape from the supercurrent state by macroscopic quantum tunneling, which provides the final readout. We test the feasibility of our idea against realistic experimental circuit parameters and by analyzing the phase fluctuations of the qubit.     

A quantum identification scheme based on polarization modulation

A quantum identification scheme including registration and identification phases is proposed. The users‘ passwords are transmitted by qubit string and recorded as a set of quantum operators. The security of the proposed scheme is guaranteed by the no-cloning theorem. Based on photon polarization modulation, an experimental approach is also designed to implement our proposed scheme.     

Controlled Quantum State Transfer with Separate Measurements in Cavity QED

An experimental scheme on controlled quantum state transfer is proposed in cavity quantum electrodynamics (QED) without joint Bell-state measurement (BSM). A quantum state can be transferred perfectly with the help of the cooperation of the third side by constructing a three-atom GHZ entangled state as the controlled channel. This scheme is insensitive to the cavity decay and the thermal field. The probability of the success in our scheme is 1.0.     

Quantum simulation of quantum many-body systems with ultracold two-electron atoms in an optical lattice

Ultracold atoms in an optical lattice provide a unique approach to study quantum many-body systems, previously only possible by using condensed-matter experimental systems. This new approach, often called quantum simulation, becomes possible because of the high controllability of the system parameters and the inherent cleanness without lattice defects and impurities. In this article, we review recent developments in this rapidly growing field of ultracold atoms in an optical lattice, with special focus on quantum simulations using our newly created quantum many-body system of two-electron atoms of ytterbium. In addition, we also mention other interesting possibilities offered by this novel experimental platform, such as applications to precision measurements for studying fundamental physics and a Rydberg atom quantum computation.     

Exploiting quantum parallelism to simulate quantum random many-body systems

We present an algorithm that exploits quantum parallelism to simulate randomness in a quantum system. In our scheme, all possible realizations of the random parameters are encoded quantum mechanically in a superposition state of an auxiliary system. We show how our algorithm allows for the efficient simulation of dynamics of quantum random spin chains with known numerical methods. We propose an experimental realization based on atoms in optical lattices in which disorder could be simulated in parallel and in a controlled way through the interaction with another atomic species.     

Dissipation-driven phase transition in two-dimensional Josephson arrays

We analyze the interplay of dissipative and quantum effects in the proximity of a quantum phase transition. The prototypical system is a resistively shunted two-dimensional Josephson junction array, studied by means of an advanced Fourier path-integral Monte Carlo algorithm. The reentrant superconducting-to-normal phase transition driven by quantum fluctuations, recently discovered in the limit of infinite shunt resistance, persists for moderate dissipation strength but disappears in the limit of small resistance. For large quantum coupling our numerical results show that, beyond a critical dissipation strength, the superconducting phase is always stabilized at sufficiently low temperature. Our phase diagram explains recent experimental findings.     

Quantum Experimental Data in Psychology and Economics

We prove a theorem which shows that a collection of experimental data of probabilistic weights related to decisions with respect to situations and their disjunction cannot be modeled within a classical probabilistic weight structure in case the experimental data contain the effect referred to as the ‘disjunction effect’ in psychology. We identify different experimental situations in psychology, more specifically in concept theory and in decision theory, and in economics (namely situations where Savage’s Sure-Thing Principle is violated) where the disjunction effect appears and we point out the common nature of the effect. We analyze how our theorem constitutes a no-go theorem for classical probabilistic weight structures for common experimental data when the disjunction effect is affecting the values of these data. We put forward a simple geometric criterion that reveals the non classicality of the considered probabilistic weights and we illustrate our geometrical criterion by means of experimentally measured membership weights of items with respect to pairs of concepts and their disjunctions. The violation of the classical probabilistic weight structure is very analogous to the violation of the well-known Bell inequalities studied in quantum mechanics. The no-go theorem we prove in the present article with respect to the collection of experimental data we consider has a status analogous to the well known no-go theorems for hidden variable theories in quantum mechanics with respect to experimental data obtained in quantum laboratories. Our analysis puts forward a strong argument in favor of the validity of using the quantum formalism for modeling the considered psychological experimental data as considered in this paper.     

微型光学偶极阱中单原子相干操控Rabi振荡的实现及其应用

文章综述了Rabi振荡的基本原理以及微型光学偶极阱中单原子相干操控Rabi振荡的研究进展,同时介绍了其在单光子源、量子寄存器、量子计算等方面的应用,并简要介绍了作者所在小组在微型光学偶极阱中单原子操控方面的实验进展.     

Experimental two-photon,three-dimensional entanglement for quantum communication

Orbital angular momentum entangled photons emitted by a down-conversion source are in higher dimensional entangled states. Here we report the experimental confirmation by demonstrating a violation of a generalized Clauser-Horne-Shimony-Holt-type Bell inequality in three dimensions by more than 18 standard deviations. Higher dimensional entangled states allow the realization of new types of quantum communication protocols. They also provide a more secure quantum cryptography scheme. Therefore our experimental results are likely to have applications in future quantum communication technology.