A Bell-type test of energy-time entangled qutrits

We have performed a Bell-type test for energy-time entangled qutrits. A method of inferring the Bell violation in terms of an associated interference visibility is derived. Using this scheme we obtained a Bell value of $2.784 \pm 0.023$, representing a violation of $34 \sigma$ above the limit for local variables. The scheme has been developed for use at telecom wavelengths and using proven long distance quantum communication architecture to optimize the utility of this high dimensional entanglement resource.Comment: replaced lost acknowledement

Stationary state entanglement and total correlation of two qubits or qutrits

We investigate the mutual information and entanglement of stationary state of two locally driven qubits under the influence of collective dephasing. It is shown that both the mutual information and the entanglement of two qubits in the stationary state exhibit damped oscillation with the scaled action time $\gamma{T}$ of the local external driving field. It means that we can control both the entanglement and total correlation of the stationary state of two qubits by adjusting the action time of the driving field. We also consider the influence of collective dephasing on entanglement of two qutrits and obtain the sufficient condition that the stationary state is entangled.Comment: 5 pages, 2 figures, RevTex

Waveguide-based OPO source of entangled photon pairs

In this paper we present a compact source of narrow-band energy-time entangled photon pairs in the telecom regime based on a Ti-indiffused Periodically Poled Lithium Niobate (PPLN) waveguide resonator, i.e. a waveguide with end-face dielectric multi-layer mirrors. This is a monolithic doubly resonant Optical Parametric Oscillator (OPO) far below threshold, which generates photon pairs by Spontaneous Parametric Down Conversion (SPDC) at around 1560nm with a 117MHz (0.91 pm)- bandwidth. A coherence time of 2.7 ns is estimated by a time correlation measurement and a high quality of the entangled states is confirmed by a Bell-type experiment. Since highly coherent energy-time entangled photon pairs in the telecom regime are suitable for long distance transmission and manipulation, this source is well suited to the requirements of quantum communication.Comment: 13 page

Mixed State Entanglement: Manipulating Polarisation-Entangled Photons

There has been much discussion recently regarding entanglement transformations in terms of local filtering operations and whether the optimal entanglement for an arbitrary two-qubit state could be realised. We introduce an experimentally realisable scheme for manipulating the entanglement of an arbitrary state of two polarisation entangled qubits. This scheme is then used to provide some perspective to the mathematical concepts inherent in this field with respect to a laboratory environment. Specifically, we look at how to extract enhanced entanglement from systems with a fixed rank and in the case where the rank of the density operator for the state can be reduced, show how the state can be made arbitrarily close to a maximally entangled pure state. In this context we also discuss bounds on entanglement in mixed states.Comment: 12 pages, 10 figure

Ancilla-assisted quantum process tomography

Complete and precise characterization of a quantum dynamical process can be achieved via the method of quantum process tomography. Using a source of correlated photons, we have implemented several methods investigating a wide range of processes, e.g., unitary, decohering, and polarizing. One of these methods, ancilla-assisted process tomography (AAPT), makes use of an additional ``ancilla system,'' and we have theoretically determined the conditions when AAPT is possible. All prior schemes for AAPT make use of entangled states. Our results show that, surprisingly, entanglement is not required for AAPT, and we present process tomography data obtained using an input state that has no entanglement. However, the use of entanglement yields superior results.Comment: To appear in Physical Review Letter

Security bound of two-bases quantum key-distribution protocols using qudits

We investigate the security bounds of quantum cryptographic protocols using $d$-level systems. In particular, we focus on schemes that use two mutually unbiased bases, thus extending the BB84 quantum key distribution scheme to higher dimensions. Under the assumption of general coherent attacks, we derive an analytic expression for the ultimate upper security bound of such quantum cryptography schemes. This bound is well below the predictions of optimal cloning machines. The possibility of extraction of a secret key beyond entanglement distillation is discussed. In the case of qutrits we argue that any eavesdropping strategy is equivalent to a symmetric one. For higher dimensions such an equivalence is generally no longer valid.Comment: 12 pages, 2 figures, to appear in Phys. Rev.

Decoherence-based exploration of d-dimensional one-way quantum computation

We study the effects of amplitude and phase damping decoherence in d-dimensional one-way quantum computation (QC). Our investigation shows how information transfer and entangling gate simulations are affected for d>=2. To understand motivations for extending the one-way model to higher dimensions, we describe how d-dimensional qudit cluster states deteriorate under environmental noise. In order to protect quantum information from the environment we consider the encoding of logical qubits into physical qudits and compare entangled pairs of linear qubit-cluster states with single qudit clusters of equal length and total dimension. Our study shows a significant reduction in the performance of one-way QC for d>2 in the presence of Markovian type decoherence models.Comment: 8 pages, 11 figures, RevTeX

Nonlinear interaction between two heralded single photons

Harnessing nonlinearities strong enough to allow two single photons to interact with one another is not only a fascinating challenge but is central to numerous advanced applications in quantum information science. Currently, all known approaches are extremely challenging although a few have led to experimental realisations with attenuated classical laser light. This has included cross-phase modulation with weak classical light in atomic ensembles and optical fibres, converting incident laser light into a non-classical stream of photon or Rydberg blockades as well as all-optical switches with attenuated classical light in various atomic systems. Here we report the observation of a nonlinear parametric interaction between two true single photons. Single photons are initially generated by heralding one photon from each of two independent spontaneous parametric downconversion sources. The two heralded single photons are subsequently combined in a nonlinear waveguide where they are converted into a single photon with a higher energy. Our approach highlights the potential for quantum nonlinear optics with integrated devices, and as the photons are at telecom wavelengths, it is well adapted to applications in quantum communication.Comment: 4 pages, 4 figure

Qudit Quantum State Tomography

Recently quantum tomography has been proposed as a fundamental tool for prototyping a few qubit quantum device. It allows the complete reconstruction of the state produced from a given input into the device. From this reconstructed density matrix, relevant quantum information quantities such as the degree of entanglement and entropy can be calculated. Generally orthogonal measurements have been discussed for this tomographic reconstruction. In this paper, we extend the tomographic reconstruction technique to two new regimes. First we show how non-orthogonal measurement allow the reconstruction of the state of the system provided the measurements span the Hilbert space. We then detail how quantum state tomography can be performed for multi qudits with a specific example illustrating how to achieve this in one and two qutrit systems.Comment: 6 pages, 4 figures, submitted to PR

GHz QKD at telecom wavelengths using up-conversion detectors

We have developed a hybrid single photon detection scheme for telecom wavelengths based on nonlinear sum-frequency generation and silicon single-photon avalanche diodes (SPADs). The SPAD devices employed have been designed to have very narrow temporal response, i.e. low jitter, which we can exploit for increasing the allowable bit rate for quantum key distribution. The wavelength conversion is obtained using periodically poled Lithium niobate waveguides (W/Gs). The inherently high efficiency of these W/Gs allows us to use a continuous wave laser to seed the nonlinear conversion so as to have a continuous detection scheme. We also present a 1.27GHz qubit repetition rate, one-way phase encoding, quantum key distribution experiment operating at telecom wavelengths that takes advantage of this detection scheme. The proof of principle experiment shows a system capable of MHz raw count rates with a QBER less than 2% and estimated secure key rates greater than 100 kbit/s over 25 km.Comment: 12 pages, 7 figure