Enable keyboard navigation for single posts in WordPress. Source: http://techglimpse.com/create-keyboard-navigation-for-your-website/

enable-keyboard-navigation.php

<?php

//* Euqueue keyboard navigation script
add_action( 'wp_enqueue_scripts', 'customprefix_add_keyboard_navigation' );
function customprefix_add_keyboard_navigation() {
    if ( is_single() ) {
        wp_enqueue_script( 'keyboard-navigation', get_bloginfo( 'stylesheet_directory' ) . '/js/keyboard-navigation.js', array( 'jquery' ), '1.0.0' );
    }
}

keyboard-navigation.js

jQuery(document).ready(function() {
    jQuery(document).keydown(function(e) {
        var url = false;
        if (e.which == 37) { // Left arrow key code
            url = jQuery('.pagination-previous a').attr('href');
        } else if (e.which == 39) { // Right arrow key code
            url = jQuery('.pagination-next a').attr('href');
        }
        if (url) {
            window.location = url;
        }
    });
});