J. Eisert, M. B. Plenio
We present a general necessary and sufficient criterion for the possibility of a state transformation from one mixed Gaussian state to another of a bi-partite continuous-variable system with two modes. The class of operations that will be considered is the set of local Gaussian completely positive trace-preserving maps.
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A powerful theoretical structure has emerged in recent years on the characterization and quantification of entanglement in continuous-variable systems. After reviewing this framework, we will illustrate it with an original set-up based on a type-II OPO with adjustable mode coupling. Experimental results allow a direct verification of many theoretical predictions and provide a sharp insight into the general properties of two-mode Gaussian states and entanglement resource manipulation.
We continue the study, from a semiclassical viewpoint, of Calvo and Picon's operators, as manipulating photon states in quantum communication. In a previous paper, we defined a one-dimensional model operator and studied it analytically before moving on to Calvo and Picon's full two-dimensional operators. In the present paper, we show how the one-dimensional operator may also be useful as an experimental model, since it allows manipulations of two-dimensional Laguerre-Gaussian modes; the intensity distributions (in physical space) of the Laguerre-Gaussian modes then approximately flow along the…
We investigate which non-Gaussian resources are needed, in addition to Gaussian operations and measurements, for implementation of arbitrary quantum gates on multimode quantum states of light. We show that an arbitrary set of states with finite expansion in Fock basis is sufficient for this task. As an illustration we present an explicit scheme for probabilistic implementation of the nonlinear sign gate using resource non-Gaussian states and Gaussian operations.
In this thesis, several new procedures for the generation and manipulation of entangled states in quantum optical systems are introduced. Each is evaluated in terms of realistic models of the imperfect apparatus of any real laboratory implementation. The first half of the thesis considers entanglement generation in Cavity QED, introducing several proposals for the creation of maximally entangled two-qubit states. The second half of the thesis focuses on the manipulation of entangled states of light pulses. In particular, an entanglement distillation procedure for Gaussian states is introduced.…
Free expansion of a Gaussian wavepacket using operator manipulations
本文将高斯波包视为频率匹配的谐振子基态，利用含时压缩算符及 Hadamard 引理等算符操作直接推导其自由膨胀，并服务于量子传感教学。
A scheme for controlling the entanglement of a two-qubit system by a local magnetic pulse is proposed. We show that the entanglement of the two-qubit system can be increased by sacrificing the coherence in ancillary degree of freedom, which is induced by a local manipulation.
Controlled generation and manipulation of photon states encoded in their spatial degrees of freedom is a crucial ingredient in many quantum information tasks exploiting higher-than-two dimensional encoding. Here, we prove the impossibility to arbitrarily modify $d$-level state superpositions (qu$d$its) for $d>2$, encoded in the transverse modes of light, with optical components associated to the group of symplectic transforms (Gaussian operations). Surprisingly, we also provide an explicit construction of how non-Gaussian operations acting on mode subspaces do enable to overcome the limit $d=2…
We study the entanglement capability of the evolution of a pair of qubits evolving under unitary dynamics, when the local dynamical parameters cannot be modified during the time-evolution. Unlike the fast local control regime, we find that local and non-local contributions to the dynamics are strictly interconnected. Moreover, it is possible to strongly increase the entanglement capability by suitably initializing the characteristic energies of the two parties.
We find a minimal set of necessary and sufficient conditions for the existence of a local procedure that converts a finite pure state into one of a set of possible final states. This result provides a powerful method for obtaining optimal local entanglement manipulation protocols for pure initial states. As an example, we determine analytically the optimal distillable entanglement for arbitrary finite pure states. We also construct an explicit protocol achieving this bound.
We provide a scheme for quantum computation in lattice systems via global but periodic manipulation, in which only effective periodic magnetic fields and global nearest neighbor interaction are required. All operations in our scheme are attainable in optical lattice or solid state systems. We also investigate universal quantum operations and quantum simulation in 2 dimensional lattice. We find global manipulations are superior in simulating some nontrivial many body Hamiltonians.
We develop a Gaussian state treatment that allows a transparent quantum description of the continuous, nondestructive imaging of and feedback on a Bose-Einstein condensate. We have previously demonstrated [Phys. Rev. Lett. \textbf{115}, 060401 (2015)] that the measurement back action of stroboscopic imaging leads to selective squeezing and entanglement of quantized density oscillations. Here, we investigate how the squeezing and entanglement are affected by the finite spatial resolution and geometry of the probe laser beam and of the detector and how they can be optimized.
We demonstrate that local transformations on a composite quantum system can be enhanced in the presence of certain entangled states. These extra states act much like catalysts in a chemical reaction: they allow otherwise impossible local transformations to be realised, without being consumed in any way. In particular, we show that this effect can considerably improve the efficiency of entanglement concentration procedures for finite states.
Highly degenerate chaotic radiation has a Gaussian density matrix and a large occupation number of modes $f $. If it is passed through a weakly transmitting barrier, its counting statistics is close to Poissonian. We show that a second identical barrier, in series with the first, drastically modifies the statistics. The variance of the photocount is increased above the mean by a factor $f$ times a numerical coefficient. The photocount distribution reaches a limiting form with a Gaussian body and highly asymmetric tails. These are general consequences of the combination of weak transmission and…
It is shown that a finite number of conditions are {\em not} sufficient to determine the locality of transformations between two probability distributions of pure states as well as the locality of transformations between two $d\times d$ mixed states with $d\geq 4$. As an example, an infinite, but minimal, set of necessary and sufficient conditions for the existence of a local procedure that converts one probability distribution of two pure pair of qubits into another one is found.
We derive an optimal bound for arbitrary entanglement manipulation based on the transmission of a pulse in coherent states over a lossy channel followed by local operations and unlimited classical communication (LOCC). This stands on a theorem to reduce LOCC via a local unital qubit channel to local filtering. We also present an optimal protocol based on beam splitters and a quantum nondemolition (QND) measurement on photons. Even if we replace the QND measurement with photon detectors, the protocol outperforms known entanglement generation schemes.
Flexible manipulation of bipartite and multipartite EPR steerings
本文提出一种利用空间结构泵浦和可调泵浦比例灵活操控二分及多分EPR导引的方案，可实现方向、类型及真正五方导引的调控，通用且实验可行。
We analyze approximate transformations of pure entangled quantum states by local operations and classical communication, finding explicit conversion strategies which optimize the fidelity of transformation. These results allow us to determine the most faithful teleportation strategy via an initially shared partially entangled pure state. They also show that procedures for entanglement manipulation such as entanglement catalysis [Jonathan and Plenio, Phys. Rev. Lett. 83, 3566 (1999)] are robust against perturbation of the states involved, and motivate the notion of non-local fidelity, which qua…
A strongly confined light field necessarily exhibits a local polarization that varies on a subwavelength scale. We demonstrate that a single optical mode of such kind can be used to selectively and simultaneously manipulate atomic ensembles that are less than a micron away from each other and equally coupled to the light field. The technique is implemented with an optical nanofiber that provides an evanescent field interface between a strongly guided optical mode and two diametric linear arrays of cesium atoms. Using this single optical mode, the two atomic ensembles can simultaneously be opti…
We present a modular design for integrated programmable multimode sources of arbitrary Gaussian states of light. The technique is based on current technologies, in particular recent demonstrations of on-chip photon manipulation and generation of highly squeezed vacuum states in semiconductors. While the design is generic and independent of the choice of integrated platform, we adopt recent experimental results on compound semiconductors as a demonstrative example. Such a device would be valuable as a source for many quantum protocols that range from imaging to communication and information pro…
Lattice Gauge Theories form a very successful framework for studying nonperturbative gauge field physics, in particular in Quantum Chromodynamics. Recently, their quantum simulation on atomic and solid-state platforms has been discussed, aiming at overcoming some of the difficulties still faced by the conventional approaches (such as the sign problem and real time evolution). While the actual implementations of a lattice gauge theory on a quantum simulator may differ in terms of the simulating system and its properties, they are all directed at studying similar physical phenomena, requiring th…
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