Gunnar Björk

Quantum Key Distribution Using Multilevel Encoding

We extend the original Bennett and Brassard for quantum key distribution protocol by using M mutually complementary bases and N orthogonalvectors in each base.

Quantum key distribution using multilevel encoding: security analysis

We propose an extension of quantum key distribution based on encoding the key into quNits, i.e. quantum states in an N-dimensional Hilbert space. We estimate both the mutual information between the legitimate parties and the eavesdropper, and the error rate, as a function of the dimension of the Hilbert space. We derive the information gained by an eavesdropper using optimal incoherent attacks and an upper bound on the legitimate party error rate that ensures unconditional security when the eavesdropper uses finite coherent eavesdropping attacks. We also consider realistic systems where we assume that the detector dark count probability is not negligible.

Improved light extraction from emitters in high refractive index materials using solid immersion lenses

Solid immersion lenses (SILs) are optically transparent, truncated spheres, brought in contact with a sample to be imaged. The combination of a conventional optical microscope and a SIL results in a highly effective numerical aperture of the imaging system that can improve the resolution. In addition, when imaging high refractive index samples, such as semiconductors, the light collection efficiency can be increased drastically. We investigate the collection efficiency as a function of the SILs geometry and refractive index, using an analytical expression for the light dispersion through an arbitrarily truncated sphere. The theoretical results are compared to experimental measurements obtained on single quantum dots and are found to be in good agreement.

Comprehensive experimental test of quantum erasure

Abstract:In an interferometer, path information and interference visibility are incompatible quantities. Complete determination of the path will exclude any possibility of interference, rendering zero visibility. However, it is, under certain conditions, possible to trade the path information for improved (conditioned) visibility. This procedure is called quantum erasure. We have performed such experiments with polarization-entangled photon pairs. Using a partial polarizer, we could vary the degree of entanglement between the object and the probe. We could also vary the interferometer splitting ratio and thereby vary the a priori path predictability. This allowed us to test quantum erasure under a number of different experimental conditions. All experiments were in good agreement with theory.

Applications of entangled-state interference

Interference phenomena lead to a wealth of applications in many areas of physics. Entangled quantum states allow one to surpass the classical measurement sensitivity or resolution in polarimetry, interferometry, and imaging. In this paper we shall review, in some depth, polarization properties of quantized two-mode electromagnetic fields and show how interference and quantum entanglement lead to new phenomena. We shall also briefly discuss subwavelength quantum lithography.

Description of entanglement in terms of quantum phase

We explore the role played by the phase in an accurate description of the entanglement of bipartite systems. We first present an appropriate polar decomposition that leads to a truly Hermitian operator for the phase of a single qubit. We also examine the positive operator-valued measures that can describe the qubit phase properties. When dealing with two qubits, the relative phase seems to be a natural variable to understand entanglement. In this spirit, we propose a measure of entanglement based on this variable.