Quantum correlations,local interactions and error correction

Abstract

We consider the effects of local interactions upon quantum mechanically entangled systems. In particular we demonstrate that non-local correlations cannot increase through local operations on any of the subsystems, but that through the use of quantum error correction methods, correlations can be maintained. We provide two mathematical proofs that local general measurements cannot increase correlations, and also derive general conditions for quantum error correcting codes. Using these we show that local quantum error correction can preserve non-local features of entangled quantum systems. We also demonstrate these results by use of specific examples employing correlated optical cavities interacting locally with resonant atoms. By way of counter example, we also describe a mechanism by which correlations can be increased, which demonstrates the need for non-local interactions.     

Cost of Energy Quantum Correlation in a Spin Model

Identification of quantum phase transitions has been a long standing issue in quantum systems. In this work, we study the renormalization of Wigner–Yanase skew information in XY spin chain and compare its relation with cost of energy quantum correlation near the critical point. This study is presented by implementing the quantum renormalization group (RG) technique. We apply the (RG) method to examine existing phase transition in XY spin chain by using the cost of energy quantum correlations and Wigner–Yanase skew information. We demonstrate that cost of energy quantum can provide crucial information about quantum phase transitions as well as quantum correlations.

     

Quantum properties and dynamics of X states

X states are a broad class of two-qubit density matrices that generalize many states of interest in the literature. In this work, we give a comprehensive account of various quantum properties of these states, such as entanglement, negativity, quantum discord and other related quantities. Moreover, we discuss the transformations that preserve their structure both in terms of continuous time evolution and discrete quantum processes.     

Quantum games

Abstract

In these lecture notes we investigate the implications of the identification of strategies with quantum operations in game theory beyond the results presented Eisert et al. (1999, Phys. Rev. Lett., 83, 3077). After introducing a general framework, we study quantum games with a classical analogue in order to flesh out the peculiarities of game theoretical settings in the quantum domain. Special emphasis is given to a detailed investigation of different sets of quantum strategies.     

Investigation on the performances of multi-quantum barriers in a single quantum well solar cell

Multi-quantum barriers (MQBs) has been introduced into a single quantum well (SQW) solar cell to prevent the photo-generated electrons in the quantum well from leaking into the p-side, and is therefore intended for a new method to reduce the dark current and thereby improving the overall spectral response of the quantum well solar cell. Here we report on the performances of the MQB by dark current and photoluminescence measurement. It is found that the leakage possibility of the photo-generated electrons in the SQW into p-side and thus the dark current is suppressed for solar cell with MQB compared with that without MQB.     

Having It Both Ways: Distinguishable Yet Phase-Coherent Mixtures of Bose-Einstein Condensates

We have begun a series of experiments on mixed bosonic quantum fluids. Our system is mixed Bose-Einstein condensates in dilute Rb-87. By simultaneously trapping the atoms in two different hyperfine states, we are able to study the dynamics of component separation and of the relative quantum phase of two interpenetrating condensates. Population can be converted from one state to the other at a rate that is sensitive to the relative quantum phase.     

Wavelet transform in the context of quantum mechanics and new orthogonal property of mother wavelets in parameter space

Based on the quantum mechanical version of wavelet transform (WT) W _ψ f(μ,s)?=??ψ|U(μ,s)|?f?? [Fan, H.Y.; Lu, H.L. Opt. Lett. 2006, 31, 407] we further prove Parseval theorem and inversion formula for WT in the context of quantum mechanics. The concise proof not only manifestly shows the merit of Dirac's representation theory, but also leads to a new orthogonal property of mother wavelets in parameter space.     

Non-Fermi Behavior Near Ferromagnetic Quantum Phase Transition

We propose a model for the explanation of the non-Fermi behavior near the ferromagnetic quantum phase transition. The scaling equations have been used to calculate the specific heat and we showed that the quantum effects are responsible for the T ln T term from the specific heat. The results are in agreement with recent experimental data obtained in the Ni _x Pd_{1 – x} system. The relation with the other approaches was also discussed.     

Electrical Manipulation of Spin Injection into a Single InAs Quantum Dots

We demonstrate spin injection from a n-Zn_0.96Mn_0.04Se layer into individual InAs quantum dots (SQDs) in a p–i–n diode structure using cw polarization resolved magneto-micro photoluminescence spectroscopy. Interestingly, we find that the spin injection efficiency strongly varies from dot to dot. We obtain a single quantum dot circular polarization degree ranging from 2% to almost 50% (at B=4 T) at zero biasing and within the spectral range studied here, we found 2 maxima of the degree of the circular polarization at SQD energies separated by ∼33 meV. Importantly, we demonstrate that the spin injection efficiency can be manipulated by external forward biasing (U _ext).     

Quantum non-demolition measurements using cold atoms in an optical cavity

Abstract

In this paper we present a theoretical analysis of a recent quantum non-demolition experiment in optics using cold atoms in a magneto-optical trap as a nonlinear medium. A signal beam and a meter beam from two independent lasers are coupled within a A-type three-level scheme in the D1 line of ⁸⁷Rb atoms. The experimental results for the relevant quantum correlations of the fields represent up to now the best achievement for a single back-action evading measurement. Moreover, they are found to be in remarkably good agreement with the theoretical predictions from a fully quantum model for three-level atoms in a doubly resonant cavity.     

Quantum coherence and phase transitions in granular superconductors with dissipation. I. Ordered arrays

We investigate the effect of dissipative currents on a superconducting phase transition in two- and three-dimensional granular arrays. We find the corresponding phase diagrams and show that, depending on dissipation, both long-range and local quantum fluctuations may be effective in destroying the global phase coherence. For large values of charging-to-Josephson coupling energy ratio and atT=0, we find the universal critical resistanceR _c only for three dimensional arrays while for two dimensional onesR _c ^–1 diverges logarithmically with increasing of the grain charging energy. For large values of a Josephson coupling energy, quantum phase slip effects play the main role and lead to the universal critical resistance in both cases. We also investigate the effect of dissipation on the critical temperature of theXY model phase transition and show that strong dissipation is extremely effective in suppressing quantum effects.     

Perspectives of an Experiment for Detecting Macroscopic Quantum Coherence

In the second half of 1993 we presented to INFN a proposal of an experiment for detecting macroscopic quantum coherence with a system of SQUIDs. It was based essentially on ideas presented first by Leggett and collaborators and developed in many articles in the 1980s. The experimental work started in the 1994 just after the approval of the INFN. As an introduction, the experimental method and the setup we choose in order to perform a set of measurements on a system of SQUIDs is described. Then the measuring procedures and their purposes are explained and discussed.As a future perspective, a possible test on the existence of the quantum gravity is discussed. According to a theoretical proposal by John Ellis and collaborators, the quantum gravitational friction could induce the transition from QM behavior of microscopic states to the classical behavior of macroscopic states, i.e., states containing an Avogadro number (M _pl/M _e ) of elementary particles, as may be obtained in a SQUID.     

Fabrication of Au/Ti TESs for Optical Photon Counting

High efficiency and negligible dark-count rates of transition-edge sensor (TES) microcalorimeters as single photon detector at telecommunications and optical visible wavelengths make them powerful tools for quantum information and quantum computation. In this work we report details on the fabrication of Au/Ti for photon counting and analyse the effects on the critical temperature and the transition steepness of the structuring process and wiring material. Au/Ti films deposited by electron-beam at substrate temperature lower than 435 K show sharp transition and reproducible T _c . Moreover, we observe that TES with Al wiring are more stable and have better characteristics of TES with Nb wiring. Using 20 micron×20 micron Ti/Au TES single photon detection has been obtained in the UV-visible range.      

Full-Blind Delegating Private Quantum Computation

The delegating private quantum computation (DQC) protocol with the universal quantum gate set {X,Z,H,P,R,CNOT} was firstly proposed by Broadbent et al. [Broadbent (2015)], and then Tan et al. [Tan and Zhou (2017)] tried to put forward a half-blind DQC protocol (HDQC) with another universal set {H,P,CNOT,T}. However, the decryption circuit of Toffoli gate (i.e. T) is a little redundant, and Tan et al.’s protocol [Tan and Zhou (2017)] exists the information leak. In addition, both of these two protocols just focus on the blindness of data (i.e. the client’s input and output), but do not consider the blindness of computation (i.e. the delegated quantum operation). For solving these problems, we propose a full-blind DQC protocol (FDQC) with quantum gate set {H,P,CNOT,T}, where the desirable delegated quantum operation, one of {H,P,CNOT,T}, is replaced by a fixed sequence (H,P,CZ,CNOT,T) to make the computation blind, and the decryption circuit of Toffoli gate is also optimized. Analysis shows that our protocol can not only correctly perform any delegated quantum computation, but also holds the characteristics of data blindness and computation blindness.     

Quantum mechanics as quantum information,mostly

Abstract

In this paper, I try to cause some good-natured trouble. The issue is, when will we ever stop burdening the taxpayer with conferences devoted to the quantum foundations? The suspicion is expressed that no end will be in sight until a means is found to reduce quantum theory to two or three statements of crisp physical (rather than abstract, axiomatic) significance. In this regard, no tool appears better calibrated for a direct assault than quantum information theory. Far from a strained application of the latest fad to a time-honoured problem, this method holds promise precisely because a large part—but not all—of the structure of quantum theory has always concerned information. It is just that the physics community needs reminding.     

Beta Spectrometry with Magnetic Calorimeters

Absolute activity measurements of alpha, beta and gamma emitting radioactive sources are important in numerous fields such as therapeutic radiology and the characterization of nuclear waste. Conventional ionization and liquid scintillation detectors, which are commonly used for these applications, have an energy dependent quantum efficiency and severe limitations in energy resolution. As a novel alternative we have developed a detector based on a metallic magnetic calorimeter (MMC) with a gold absorber that covers the full solid angle of 4π around the radioactive source. Deposition of energy in the absorber causes a temperature rise and results in a change of magnetization of a parametric Au:Er sensor, which can be measured by a low-noise high-bandwidth dc-SQUID. The detector has equal sensitivity for beta and gamma radiation. In this paper we describe a detector which has a deviation from linear behavior for energies up to 700 keV of smaller than 0.5% and an overall quantum efficiency for beta particles in this energy range close to unity. We show the data of our experiments measuring the decay of ³⁶Cl and compare the results to the theoretically expected spectrum for this second order forbidden non-unique β-decay. We discuss the observed contributions to noise, the quantum efficiency and the achieved energy resolution.     

Patterning nanoroads and quantum dots on fluorinated graphene

Using ab initio methods we have investigated the fluorination of graphene and find that different stoichiometric phases can be formed without a nucleation barrier, with the complete “2D-Teflon” CF phase being thermodynamically most stable. The fluorinated graphene is an insulator and turns out to be a perfect matrix-host for patterning nanoroads and quantum dots of pristine graphene. The electronic and magnetic properties of the nanoroads can be tuned by varying the edge orientation and width. The energy gaps between the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO) of quantum dots are size-dependent and show a confinement typical of Dirac fermions. Furthermore, we study the effect of different basic coverage of F on graphene (with stoichiometries CF and C₄F) on the band gaps, and show the suitability of these materials to host quantum dots of graphene with unique electronic properties.     

Ground state cooling,quantum state engineering and study of decoherence of ions in Paul traps

Abstract

We investigate single ions of ⁴⁰Ca⁺ in Paul traps for quantum information processing. Superpositions of the S_½ electronic ground state and the metastable D_5/2; state are used to implement a qubit. Laser light on the S_½ ? D_5/2 transition is used for the manipulation of the ion's quantum state. We apply sideband cooling to the ion and reach the ground state of vibration with up to 99.9% probability. Starting from this Fock state (n = 0), we demonstrate coherent quantum state manipulation. A large number of Rabi oscillations and a ms-coherence time is observed. Motional heating is measured to be as low as one vibrational quantum in 190ms. We also report on ground state cooling of two ions.     

Collapse of a two-mode squeezed state after registering n -single mode photons

By enquiring what happens to the remaining mode when one makes one-mode n-photon counting for the two-mode squeezed state, we show that the two-mode squeezed state will collapse to a counting operator in the remaining mode with a new smaller quantum efficiency of the detector. We also derive its P-representation.     

The information role of entanglement and interference operators in Shor quantum algorithm gate dynamics

Abstract

Shor algorithm dynamics of quantum computation states are analysed from the classical and the quantum information theory points of view. The Shannon entropy is interpreted as the degree of information accessibility through measurement, while the von Neumann entropy is employed to measure the quantum information of entanglement. The intelligence of a state with respect to a subset of qubits is defined. The intelligence of a state is maximal if the gap between the Shannon and the von Neumann entropy for the chosen result qubits is minimal. We prove that the quantum Fourier transform creates maximally intelligent states with respect to the first n qubits for Shor's problem, since it annihilates the gap between the classical and quantum entropies for the first n qubits of every output state.