Abstract
Several schemes have been proposed to extend Quantum Key Distribution protocols aiming at improving their security or at providing new physical substrates for qubit implementation. We present a toolbox to jointly create, manipulate and measure qubits stored in polarization and transverse-modes degrees of freedom of single photons. The toolbox includes local operations on single qubits, controlled operations between the two qubits and projective measurements over a wide variety of non-local bases in the four dimensional space of states. We describe how to implement the toolbox to perform an extended version of the BB84 protocol for this Hilbert space (ideally transmitting two key bits per photon). We present the experimental implementation of the measurement scheme both in the regimes of intense light beams and with single photons. Thus, we show the feasibility of implementing the protocol providing an interesting example of a new method for quantum information processing using the polarization and transverse modes of light as qubits.
Highlights
Quantum Key Distribution (QKD) protocols exploit the quantum non-cloning theorem [1] and the indistinguishability of quantum states belonging to unbiased bases [2] to accomplish secure distribution of cryptographic keys
We describe how to implement the toolbox to perform an extended version of the BB84 protocol for this Hilbert space
We present the experimental implementation of the measurement scheme both in the regimes of intense light beams and with single photons
Summary
Quantum Key Distribution (QKD) protocols exploit the quantum non-cloning theorem [1] and the indistinguishability of quantum states belonging to unbiased bases [2] to accomplish secure distribution of cryptographic keys. In this paper we present a scheme to transmit two key bits on a single photon encoding two qubits on two different photonic degrees of freedom. For this we use the polarization and the transverse-modes (TM) degrees of freedom. To implement any QKD scheme it is necessary to prepare and measure any quantum state of a set of mutually unbiased bases (MUBs). Such states are the primary resource required to encode and transmit a bit of key.
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