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Effective schemes are proposed for the generation of bipartite Bell states and theoretically scaling up to arbitrary multiparticle Greenberger–Horne–Zeilinger, cluster, and W states among quantum-dot (QD) spins. They are realized based on the interaction between an single incident photon and single QD spin. Errors from system nonuniformity and defective photon scattering are transformed into detectable photon losses. In the event of a failed experiment, the system allows for the reinjection of a single photon and the reinitiation of the quantum circuit, until a successful outcome is achieved. This loss-prediction and experiment-repeatability approach ensures that the schemes are implemented with uniform and robust fidelity. Compared with the previous methods, these schemes significantly simplify experimental procedures and analysis processes. Furthermore, upon successful execution of the experiment, the use of single-photon detector can faithfully differentiate between various types of entangled states, depending on the initial states of the QD spins. An analysis of the feasibility of the schemes under current experimental parameters suggests high efficiency even in the weak coupling region. [ABSTRACT FROM AUTHOR]

Quantum thermal machines can generate steady-state entanglement by harvesting spontaneous interactions with local environments. However, using minimal resources and control, the entanglement is typically weak. Here, we study entanglement generation in a two-qubit quantum thermal machine in the presence of a continuous feedback protocol. Each qubit is measured continuously and the outcomes are used for real-time feedback to control the local system-environment interactions. We show that there exists an ideal operation regime where the quality of entanglement is significantly improved, to the extent that it can violate standard Bell inequalities and uphold quantum teleportation. In agreement with (Khandelwal et al 2020 New J. Phys. 22 073039), we also find, for ideal operation, that the heat current across the system is proportional to the entanglement concurrence. Finally, we investigate the robustness of entanglement production when the machine operates away from the ideal conditions. [ABSTRACT FROM AUTHOR]

In this paper, we propose a protocol for generating N-qubit Greenberger–Horne–Zeilinger (GHZ) states in a superconducting system. The physical model contains N superconducting flux qubits and a cavity, where the N superconducting flux qubits are coupled to the cavity simultaneously. The preparation of GHZ states can be realized in one step by applying a pair of classical fields to each qubit. The protocol is promising for generating N-qubit GHZ states because the operation time is independent of the number of the qubits in the strong drive regime. Furthermore, we consider the effects of random noise and decoherence, and numerical simulations show that the protocol is insensitive to these disturbing factors. Therefore, the protocol may be useful in the generation of GHZ states. [ABSTRACT FROM AUTHOR]

Entanglement represents “the” key resource for several applications of quantum information processing, ranging from quantum communications to distributed quantum computing. Despite its fundamental importance, deterministic generation of maximally entangled qubits represents an on- going open problem. Here, we design a novel generation scheme exhibiting two attractive features, namely, i) deterministically generating different classes - particularly, GHZ-like, W-like and graph states - of genuinely multipartite entangled states, ii) not requiring any direct interaction between the qubits. Indeed, the only necessary condition is the possibility of coherently controlling - according to the indefinite causal order framework - the causal order among the unitaries acting on the qubits. Through the paper, we analyze and derive the conditions on the unitaries for deterministic generation, and we provide examples for unitaries practical implementation. We conclude the paper by discussing the scalability of the proposed scheme to higher dimensional genuine multipartite entanglement (GME) states and by introducing some possible applications of the proposal for quantum networks.

Entanglement between quantum network nodes is often produced using intermediary devices—such as heralding stations—as a resource. When scaling quantum networks to many nodes, requiring a dedicated intermediary device for every pair of nodes introduces high costs. Here, we propose a cost-effective architecture to connect many quantum network nodes via a central quantum network hub called an entanglement generation switch (EGS). The EGS allows multiple quantum nodes to be connected at a fixed resource cost, by sharing the resources needed to make entanglement. We propose an algorithm called the rate control protocol, which moderates the level of competition for access to the hub's resources between sets of users. We proceed to prove a convergence theorem for rates yielded by the algorithm. To derive the algorithm we work in the framework of network utility maximization and make use of the theory of Lagrange multipliers and Lagrangian duality. Our EGS architecture lays the groundwork for developing control architectures compatible with other types of quantum network hubs as well as system models of greater complexity.

We discuss the generation and the long-time persistence of entanglement in open two-qubit systems whose reduced dissipative dynamics is not a priori engineered but is instead subjected to filtering and Markovian feedback. In particular, we analytically study (1) whether the latter operations may enhance the environment capability of generating entanglement at short times and (2) whether the generated entanglement survives in the long-time regime. We show that, in the case of particularly symmetric Gorini–Kossakowski–Sudarshan–Lindblad it is possible to fully control the convex set of stationary states of the two-qubit reduced dynamics, therefore the asymptotic behaviour of any initial two-qubit state. We then study the impact of a suitable class of feed-back operations on the considered dynamics. [ABSTRACT FROM AUTHOR]

The effect of environmental coherence on bipartite and tripartite quantum entanglements using quantum collision models is studied. In the collision model, an open quantum system of two qubits indirectly interacts with the reservoir via auxiliary qubits. The reservoir is modeled as a collection of initially uncorrelated qubits prepared in an identically nonthermal state. It is shown that the coherence of the nonthermal reservoir contributes to the generation and protection of entanglement. It is found that adding coherence to the initial state of the reservoir qubit can significantly enhance the steady‐state entanglement of the system, and the presence of coherence in the reservoir allows steady‐state entanglement to be obtained in a higher temperature region. [ABSTRACT FROM AUTHOR]

We propose two schemes that generate four-photon cluster states based on quantum dot microcavity coupling systems. In the first scheme, the four-photon cluster state is generated from four single-photon states assisted by three quantum dot nondemolition detectors. In the second scheme, a photon polarization controlled-Z (C-Z) gate is constructed firstly by exploiting a quantum dot microcavity coupling system, and then the four-photon cluster state is generated from two EPR pairs, which come from currently available spontaneous parametric down-conversion technique. Both nondemolition detectors and a photon polarization C-Z gate are realized by quantum dot microcavity coupling systems, which can be realized with near unity success probability and nearly perfect fidelity as the side leakage rate is small and the coupling strength is strong. [ABSTRACT FROM AUTHOR]

We propose a protocol based on a tunneling plus particle-detection process aimed at generating tripartite entanglement in a system of 3 indistinguishable fermions in a triple-well potential, initially prepared in a state exhibiting only exchange correlations. Particular attention is paid to the generation of fermionic ghz - and w -type states, which are analogous to the usual ghz - and w -type states defined in composites of distinguishable qubits. The protocol succeeds in generating fermionic w -type states, and the ensuing state becomes effectively equivalent to a 3-distinguishable-qubit w -type state shared among three localized parties. The protocol, however, is unable to generate ghz -type states, a result that highlights the fundamental inequivalence between these two types of states, and throws light into the characterization of processes that guarantee the emergence of specific kinds of multipartite entanglement in systems of identical parties. Our findings suggest new paths for the exploration, generation and exploitation of multipartite entanglement in composites of indistinguishable particles, as a useful resource for quantum information processing. • Entanglement generation in indistinguishable-particle systems. • Multipartite entanglement in fermion systems. • W and GHZ states in fermion systems. [ABSTRACT FROM AUTHOR]

Quantum state transfer (QST) and entangled state generation (ESG) are important building blocks for modern quantum information processing. To achieve these tasks, convention wisdom is to consult the quantum adiabatic evolution, which is time-consuming, and thus is of low fidelity. Here, using the shortcut to adiabaticity technique, we propose a general method to realize high-fidelity fast QST and ESG in a cavity-coupled many qubits system via its dark pathways, which can be further designed for high-fidelity quantum tasks with different optimization purpose. Specifically, with a proper dark pathway, QST and ESG between any two qubits can be achieved without decoupling the others, which simplifies experimental demonstrations. Meanwhile, ESG among all qubits can also be realized in a single step. In addition, our scheme can be implemented in many quantum systems, and we illustrate its implementation on superconducting quantum circuits. Therefore, we propose a powerful strategy for selective quantum manipulation, which is promising in cavity coupled quantum systems and could find many convenient applications in quantum information processing. [ABSTRACT FROM AUTHOR]

A theoretical scheme for the stable entanglement generation of two remote mechanical modes is introduced. Two identical optomechanical systems, each constituting a single‐mode optical field as well as a qubit and a moveable mirror driven by an external pump field, are considered. To make the model more realistic, atomic, photonic, and phononic dissipations are considered. The time‐dependent state of each subsystem is obtained, analytically. Then, by an appropriate Bell‐state measurement (BSM) on the state of the whole system (first approach), the entanglement of two mechanical modes is created. As a second parallel approach, appropriate atomic measurement is applied after the BSM. According to these numerical simulations via concurrence, the noteworthy features are as follows. There exists optimum value of photon–phonon coupling strength for obtaining the steady‐state entanglement; by choosing appropriate values of dissipation, photon–phonon coupling, and pump amplitude, a significant degree and stable entanglement between the two mechanical modes are accessible. The present scheme opens new practical perspectives in constructing stable entanglement between two remote mechanical modes and can be helpful to realize quantum memories for quantum information processing and establishing long‐distance quantum communication networks. [ABSTRACT FROM AUTHOR]

A full-fledged quantum network relies on the formation of entangled links between remote location with the help of quantum repeaters. The famous Duan-Lukin-Cirac-Zoller quantum repeater protocol is based on long distance single-photon interference (SPI), which not only requires high phase stability but also cannot generate maximally entangled state. Here, we propose a quantum repeater protocol using the idea of post-matching, which retains the same efficiency as the SPI protocol, reduces the phase-stability requirement and can generate maximally entangled state in principle. We also outline an implementation of our scheme based on the Kerr nonlinear resonator. Numerical simulations show that our protocol has its superiority by comparing with existing protocols under a generic noise model and show the feasibility of building a large-scale quantum communication network with our scheme. We believe our work represents a crucial step towards the construction of a fully-connected quantum network., (© 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)

In this paper, we propose a simple method to simultaneously generate the entanglement between macroscopic objects (mechanical oscillators) and entanglement between microscopic objects (photons) based on a coupled cavity-optomechanical system. We show that when the cavity-optomechanical system works in the single-photon strong-coupling regime and in the deep quantum regime, the radiation pressure of a photon which is created by the atom-cavity interaction, can directly induce these joint entangled states. Thus, no measurement on this system is required. Numerical simulation shows that the fidelity of the prepared joint entangled states can reach 0.954. [ABSTRACT FROM AUTHOR]

In this paper, we show that multi-phonon NOON states can be generated in a two-dimensional anisotropic trapped-ion system. In the proposal, two laser pulses are applied to an ion along different directions in the ion trap plane to exchange the information between the external and internal states of the ion. Different from the previous frameworks, the proposal is outside the Lamb–Dicke regime. The distinct advantage of the proposed scheme is that the entanglement generation is deterministic and no measurement on the system is required. Numerical simulations show that the fidelity of the prepared entangled states is strongly affected by Lamb–Dicke parameters. [ABSTRACT FROM AUTHOR]

Recently, strong coupling dominating dissipative and decoherence effects between the atom and a quantum cavity has been investigated experimentally. In this paper, we study the entanglement dynamics of two two-level atoms coupling to a quantum cavity where each mirror is formed by a chain of atoms trapped near a one-dimensional waveguide. We consider the effects of both Markovian dynamics and non-Markovian dynamics on the entanglement evolution of the two central atoms. Our results show that the steady entanglement can be generated between the two central atoms whether the system is under the Markov regime or non-Markov regime when the number of atoms of the atomic mirrors is finite. When the number of the atomic mirrors is large enough, the entanglement of two central atoms takes oscillating behavior without a stable value. The effect of detuning and dissipation on entanglement dynamics is also discussed and the results imply that the detuning plays an important role in improving the entanglement amplitude. [ABSTRACT FROM AUTHOR]

In this paper, we introduce a new measure of correlation for bipartite quantum states. This measure depends on a parameter $\alpha $ , and is defined in terms of vector-valued $\textit {L}_{\textit {p}}$ -norms. The measure is within a constant of the exponential of $\alpha $ -Rényi mutual information, and reduces to the trace norm (total variation distance) for $\alpha =1$. We will prove some decoupling type theorems in terms of this measure of correlation, and present some applications in privacy amplification as well as in bounding the random coding exponents. In particular, we establish a bound on the secrecy exponent of the wiretap channel (under the total variation metric) in terms of the $\alpha $ -Rényi mutual information according to Csiszár’s proposal. [ABSTRACT FROM AUTHOR]

Quantum entanglement is an important resource for quantum information processing tasks. However, realistic multipartite entangled state production is very difficult. In this paper, we propose an efficient single-step scheme for generating many body Greenberger–Horne–Zeilinger (GHZ) states on superconducting circuits by using a superconducting transmission-line resonator (TLR) interact with N superconducting transmon qubits. The distinct merit of our proposal is that it does not require the qubit-resonator coupling strengths to be the same, which is usually impractical experimentally, and thus is one of the main reasons for entanglement generation infidelity in previous single-step schemes. The removing of the uniform interaction requirement is achieved by modulating the qubits splitting frequencies with ac microwave fields, which results in tunable individual qubit-resonator coupling strength, and thus effective uniform qubit–qubit interaction Hamiltonian can be obtained. Since microwave control is conventional nowadays, our proposal can be directly tested experimentally, which makes previous multipartite entangled states generation schemes more efficient. [ABSTRACT FROM AUTHOR]

In contrast to classical systems, actual implementation of non-Hermitian Hamiltonian dynamics for quantum systems is a challenge because the processes of energy gain and dissipation are based on the underlying Hermitian system–environment dynamics, which are trace preserving. Recently, a scheme for engineering non-Hermitian Hamiltonians as a result of repetitive measurements on an ancillary qubit has been proposed. The induced conditional dynamics of the main system is described by the effective non-Hermitian Hamiltonian arising from the procedure. In this paper, we demonstrate the effectiveness of such a protocol by applying it to physically relevant multi-spin models, showing that the effective non-Hermitian Hamiltonian drives the system to a maximally entangled stationary state. In addition, we report a new recipe to construct a physical scenario where the quantum dynamics of a physical system represented by a given non-Hermitian Hamiltonian model may be simulated. The physical implications and the broad scope potential applications of such a scheme are highlighted.

In this paper, we investigate the entanglement generation of n-qubit states in a model consisting of n independent qubits, each coupled to a harmonic oscillator which is in turn coupled to a bath of N additional harmonic oscillators with nearest-neighbor coupling. With analysis, we can find that the steady multipartite entanglement with different values can be generated after a long-time evolution for different sizes of the quantum system. Under weak coupling between the system and the harmonic oscillator, multipartite entanglement can monotonically increase from zero to a stable value. Under strong coupling, multipartite entanglement generation shows a speed-up increase accompanied by some oscillations as non-Markovian behavior. Our results imply that the strong coupling between the harmonic oscillator and the N additional harmonic oscillators, and the large size of the additional oscillators will enhance non-Markovian dynamics and make it take a very long time for the entanglement to reach a stable value. Meanwhile, the couplings between the additional harmonic oscillators and the decay rate of additional harmonic oscillators have almost no effect on the multipartite entanglement generation. Finally, the entanglement generation of the additional harmonic oscillators is also discussed.

Motivated by some recent results of the optimal control (OC) theory, we study anisotropic XXZ Heisenberg spin-1/2 chains with control fields acting on a single spin, with the aim of exploring how maximally entangled state can be prepared. To achieve the goal, we use a numerical optimization algorithm (e.g., the Krotov algorithm, which was shown to be capable of reaching the quantum speed limit) to search an optimal set of control parameters, and then obtain OC pulses corresponding to the target fidelity. We find that the minimum time for implementing our target state depending on the anisotropy parameter Δ of the model. Finally, we analyze the robustness of the obtained results for the optimal fidelities and the effectiveness of the Krotov method under some realistic conditions. [ABSTRACT FROM AUTHOR]

Quantum teleportation circumvents the uncertainty principle using dual channels: a quantum one consisting of previously-shared entanglement, and a classical one, together allowing the disembodied transport of an unknown quantum state over distance. It has recently been shown that a classical bit can be counterfactually communicated between two parties in empty space, Alice and Bob. Here, by using our dual version of the chained quantum Zeno effect to achieve a counterfactual CNOT gate, we propose a protocol for transporting an unknown qubit counterfactually, that is without any physical particles travelling between Alice and Bob—no classical channel and no previously-shared entanglement.

Recently, entanglement plays an important role in quantum information science. Here we propose an efficient and applicable method which transforms arbitrary three-qubit unknown state to a maximally entangled Greenberger-Horne-Zeilinger state, and the proposed method could be further generalized to multi-qubit case. The proposed setup exploits only linear optical elements and quantum non-demolition detectors using cross-Kerr media. As the quantum non-demolition detection could reveal us the output state of the photons without destroying them. This property may make our proposed setup flexible and can be widely used in current quantum information science and technology. [ABSTRACT FROM AUTHOR]

We present an alternate derivation of the geometric phase gate based on the ac Stark shift for two qubits coupled to a shared cavity mode. Our derivation differs from the standard one in that the qubit-light coupling occurs purely as an ac Stark shift, rather than a state-dependent force. We derive expressions for the geometric phases induced on the qubit states arising from the displacement of coherent light. Simple analytic expressions are obtained for sinusodial driving of the cavity field. [ABSTRACT FROM AUTHOR]

In this paper, we show how Einstein-Podolsky-Rosen-like entanglement between a pair of spatially separated propagating continuous-mode Gaussian fields can be generated via a coherent feedback loop that connects two spatially distant nondegenerate optical parametric amplifiers (NOPAs) over two transmission channels. In particular, the scheme generates entanglement in a distributed manner using spatially distributed resources. It is shown that similar to a single NOPA, the coherent feedback scheme has parameters that determine the achievable frequency-dependent two-mode squeezing and entanglement bandwidth between the pair of continuous-mode fields. It is also shown that in ideal scenarios, the feedback connection is able to yield an increase in the quality of the entanglement while consuming less power, compared to conventional distribution of entanglement using a single NOPA and a two-cascaded NOPA system. Furthermore, in contrast to the two conventional systems, under the same pump power, the coherent feedback system provides more entanglement in the presence of transmission losses, which indicates that the feedback scheme increases tolerance to transmission losses. [ABSTRACT FROM AUTHOR]

The quantum vacuum is unstable under the influence of an external electric field and decays into pairs of charged particles, a process which is known as the Schwinger pair production. We propose and demonstrate that this electric field can generate entanglement. Using the Schwinger pair production for constant and pulsed electric fields, we study entanglement for scalar particles with zero spins and Dirac fermions. One can observe the variation of the entanglement produced for bosonic and fermionic modes with respect to different parameters. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the dynamics of entanglement of three-qubit states of a system dissipating into a common environment. By using the tripartite negativity as entanglement measure, our results imply that the three-qubit entanglement can be generated among the three qubits which have no interaction with each other, but interact with the common environment independently. From our analysis, we find that the three-qubit entanglement increases from zero to a stable value which varies with the size of the system with the increasing of the scaled time. Additionally, the extension of the entanglement generation to an arbitrary size of a subsystem is made and some discussion is given. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the input–output process with nitrogen-vacancy (NV) centers fixed on the surface of microtoroidal resonators (MTRs) with quantized whispering-gallery modes (WGMs). We demonstrate a photon–NV hybrid controlled phase flip (CPF) gate in decoherence-free subspace (DFS) with the input–output process. Efficient schemes of entanglement generation and quantum state transfer (QST) in DFS are proposed. Our schemes removed the requirements of high-Q and strongly coupling for the physical systems, and DFS encoding makes our schemes immune to collective dephasing. The experimental feasibility and challenge are discussed by considering the current experimental technology. [Copyright &y& Elsevier]

We propose theoretical schemes to generate n-electron GHZ state, n-electron cluster state, and four-electron | χ〉 state by using quantum-dot spins in double-sided optical microcavities. With single-photon detection and local unitary operations, these entangled states could be deterministically generated. The fidelities of our schemes are also calculated, which illustrate that our schemes have high performance under appropriate parameter conditions. [ABSTRACT FROM AUTHOR]

Abstract: In this Letter, we investigate the control of finite dimensional ideal quantum systems in which the quantum states are represented by the density operators. A new Lyapunov function based on the Hilbert–Schmidt distance and mechanical quantity of the quantum system is given. We present a theoretical convergence result using LaSalle invariance principle. Applying the proposed Lyapunov method, the generation of the maximally entangled quantum states of two qubits is obtained. [Copyright &y& Elsevier]

A system of two interacting qubits off-resonantly coupled to a common non-Markovian reservoir at zero temperature is analyzed. Comparing with the results in Markovian case, we find that much higher values of entanglement can be obtained for an initially factorized state of the two-qubit system. The maximal value of the entanglement increases as the strength of dipole-dipole interaction and detuning grow. Moreover, the entanglement induced by non-Markovian reservoir is more robust against the asymmetrical couplings between the two qubits and the reservoir. [ABSTRACT FROM AUTHOR]

We present a general, analytic recipe to compute the entanglement that is generated between arbitrary, discrete modes of bosonic quantum fields by Bogoliubov transformations. Our setup allows the complete characterization of the quantum correlations in all Gaussian field states. Additionally, it holds for all Bogoliubov transformations. These are commonly applied in quantum optics for the description of squeezing operations, relate the mode decompositions of observers in different regions of curved spacetimes, and describe observers moving along non-stationary trajectories. We focus on a quantum optical example in a cavity quantum electrodynamics setting: an uncharged scalar field within a cavity provides a model for an optical resonator, in which entanglement is created by non-uniform acceleration. We show that the amount of generated entanglement can be magnified by initial single-mode squeezing, for which we provide an explicit formula. Applications to quantum fields in curved spacetimes, such as an expanding universe, are discussed. [ABSTRACT FROM AUTHOR]

We investigate the generation of tripartite field states inside the high-Q cavities using the cavity QED. The main goal is to successfully generate the entanglement in tripartite systems by passing two-level atoms through three identical high-Q cavities. Our scheme gives the successful generation of entangled tripartite W and GHZ states for pre-determined interaction times of atoms with the cavity fields. The dynamics of initial entangled states is studied as the system evolves in the dissipative environments. [ABSTRACT FROM AUTHOR]

The degree of entanglement is determined for an arbitrary state of a broad class of $\mathcal{PT}$-symmetric bipartite composite systems. Subsequently we quantify the rate with which entangled states are generated and show that this rate can be characterized by a small set of parameters. These relations allow one in principle to improve the ability of these systems to entangle states. It is also noticed that many relations resemble corresponding ones in conventional quantum mechanics. [ABSTRACT FROM AUTHOR]

The evolution of entanglement in a 3-spin chain with nearest-neighbor Heisenberg-XY interactions for different initial states is investigated here. In an NMR experimental implementation, we generate multipartite entangled states starting from initial separable pseudo-pure states by simulating nearest-neighbor XY interactions in a 3-spin linear chain of nuclear spin qubits. For simulating XY interactions, we follow algebraic method of Zhang et al. [ Phys. Rev. A 72 (2005) 012331]. Bell state between end qubits has been generated by using only the unitary evolution of the XY Hamiltonian. For generating W-state and GHZ-state a single qubit rotation is applied on second and all the three qubits, respectively after the unitary evolution of the XY Hamiltonian. [ABSTRACT FROM AUTHOR]

In [Piani et al., PRL106 (2011) 220403], an activation protocol was introduced which maps the general non-classical (multipartite) correlations between given systems into bipartite entanglement between the systems and local ancillae by means of a potentially highly entangling interaction. Here, we study how this activation protocol can be used to entangle the starting systems themselves via entanglement swapping through a measurement on the ancillae. Furthermore, we bound the relative entropy of quantumness (a naturally arising measure of non-classicality in the scheme of Piani et al. above) for a special class of separable states, the so-called classical-quantum states. In particular, we fully characterize the classical-quantum two-qubit states that are maximally non-classical. [ABSTRACT FROM AUTHOR]

We propose two schemes for entanglement generation and quantum state transfer via a double-quantum-dot system. Our schemes only need spin-parity measurements and single-qubit measurements combined with additional qubits. Discussions about the feasibility of the current scheme show that they would be realized within the current experimental technology. [ABSTRACT FROM AUTHOR]

Abstract: We present a scheme for generating entanglement between two magnetic impurities in a solid-state system via electron scattering. The scheme applies to impurities of arbitrary quantum spin number. We show that resonance conditions yield generation of a maximally entangled state of the impurities'' spins, regardless of the value of the electron–impurity coupling constant and the impurity spin quantum number. The mechanism behind the scheme is explained in terms of resonance-induced selection rules. [Copyright &y& Elsevier]

We show that the primary cause of errors in a broad class of optical quantum-logic gates are due to the higher-order photon terms in parametric downconversion sources. A model describing real-life imperfections in these entangling gates is presented and tested in an experiment where we entangle dependent photons from the same downconversion source using a controlled-z gate, and measure the state tomographically. We find good agreement between the modelled and measured results. Our investigations demonstrate that, although small, these noise terms are amplified by the intrinsic non-determinism of the gates. It is worth considering alternative schemes based on weak nonlinearities to see if they are more resilient to this degradation. [ABSTRACT FROM AUTHOR]