Stephan Thelen is an American-born, Swiss-based composer, guitarist, and mathematician known for his involvement in the fringes of classical, progressive rock, jazz, ambient, and various other forms of experimental music. Active since the early 1990s, Thelen has been involved in a wide range of projects including the experimental rock bands Radio Osaka and Sonar. Since releasing Broken Symmetry in 2002, he has recorded numerous solo projects and become increasingly involved in modern classical composition; he was commissioned by the Kronos Quartet to compose a piece, "Circular Lines," for their Fifty for the Future project. The piece later appeared, as recorded by Kronos Quartet, on Thelen's ambitious 2020 release World Dialogue.

A native of Santa Rosa, California, Thelen later relocated to Europe, where in 1990 he earned his PhD in mathematics at University of Zu¨rich. Putting down roots in the Swiss city, he worked as a mathematician while becoming enmeshed in its experimental music scene. His interest in the classical world led to deep-dive studies of Be´la Barto´k and Steve Reich, and he was greatly inspired by attending several Guitar Craft seminars by King Crimson's Robert Fripp. Thelen formed the Zu¨rich-based art rock band Radio Osaka in the early '90s, and the group remained active throughout in the 2000s. His first solo outing, 2002's Broken Symmetry, combined elements of groove-based math rock, electronic, and jazz. Other solo efforts including Epiphany (2005) and String Geometry (2008) followed, as did his ongoing work with Radio Osaka and increasing involvement with the jazz orchestra Root Down. He was even involved in a 2007 tribute album to Tim Buckley.

Thelen's creativity continued to diversify, and in the latter half of the 2000s, he expanded his focus to include modern classical and chamber music composition. In 2010, he became part of a new project called Sonar, a minimalist Swiss rock band known for exploring the harmonics of tritone-tuned guitars. Sonar made their debut with 2012's A Flaw of Nature and remained prolific over the coming years with releases on experimental labels like Ronin Rhythm and Cuneiform Records. Meanwhile, Thelen continued composing on his own, and through a mutual friend was put in touch with San Francisco's famed Kronos Quartet, who commissioned a piece from him as part of their ambitious music library project Fifty for the Future. The quartet first performed Thelen's propulsive "Circular Lines" in 2017 and later recorded it for inclusion on his 2020 album World Dialogue, which also featured performances by the Polish all-female Al Pari Quartet. During this time, Thelen dug deeper into his own guitar work and began an experimental effects-heavy project called Fractal Guitar. The first release, 2018's Fractal Guitar, was followed in early 2021 by Fractal Guitar 2. ~ Timothy Monger, Rovi