Quantum Guitar: A Hybrid Instrument

• Quantum Guitar is an instrument that maps guitar string states to qubits, enabling live quantum state manipulation and innovative audio synthesis.
• It integrates a dual-channel system combining classical electric signals with programmable quantum synth outputs to enable real-time crossfade transitions.
• The design leverages MIDI-controlled operations for continuous quantum rotations and projective measurements, enriching performance improvisation.

A Quantum Guitar is an instrument whose architecture and performance practice are grounded in quantum information science, employing qubits to represent and manipulate the playable states of guitar strings. Rather than operating solely as a classical electrical or acoustic guitar, the Quantum Guitar integrates quantum synthesis and control mechanisms—affording the performer access to hybrid quantum-classical sound regimes, live qubit manipulation (including measurement-induced collapse), and programmable quantum–inspired modulations via external controllers. This paradigm enables new modes of sound production, improvisation, and kinematic interaction, as demonstrated in public performances and music technology research.

1. Qubit Association with Guitar Strings

The fundamental innovation of the Quantum Guitar lies in the mapping of each string’s vibratory mode (or "playable state") onto a qubit. In this design, a string's state is captured not merely as a single frequency or classical waveform, but as a quantum state in a two-dimensional Hilbert space: $|\psi\rangle = \alpha|0\rangle + \beta|1\rangle$ with complex amplitudes satisfying $|\alpha|^2 + |\beta|^2 = 1$ (Coecke, 3 Sep 2025). The physical act of playing (plucking, bending, etc.) is extended to interact with, and even sculpt, the quantum state parameters $(\alpha, \beta)$. The model is engineered to allow real-time, interactive control of quantum operations through external interfaces (such as MIDI foot pedals), which mediate the application of unitary gates to the per-string qubits: $U_y(\theta) = \exp(-i\theta \sigma_y/2)$ where $\sigma_y$ is the Pauli Y operator and $\theta$ is a parameter controlled by pedal position. This endows the instrument with the ability to realize smooth quantum state rotations and, when required, to projectively measure the quantum state, manifesting a stochastic outcome mapped to a distinct sonic event.

2. System Architecture: Quantum-Classical Duality and Control

The Quantum Guitar is realized as an electric guitar augmented with a MIDI-controlled quantum synthesizer, specifically the Moth's Actias quantum synth (Coecke, 3 Sep 2025). The instrument's architecture supports the production of two concurrent sound layers:

• The classical channel encompasses the conventional electrical (or acoustic) signal path of the guitar.
• The quantum channel consists of the Actias synth’s sonification of the current qubit state(s), translating the abstract quantum register into discrete timbres, textures, or sample triggers.

A distinctive characteristic is the provision of dual, independently operable volume pedals, each regulating the amplitude of the quantum and classical channels, respectively. By adjusting these in real time, the performer effects a continuous interpolation (crossfade) between quantum and classical sound regimes, or vice versa. This "quantum-classical transition" is analogous to the gradual collapse or resurgence of quantum coherence from the perspective of the overall musical texture.

3. Qubit Manipulation and Live Measurement

Qubit manipulation is central to the expressive vocabulary of the Quantum Guitar, enabled through integration with MIDI expression and footswitch controllers. The performer can:

• Apply continuous or stepwise quantum rotations (e.g., $U_y(\theta)$) to a given string’s qubit, dynamically altering its state on the Bloch sphere.
• Trigger a projective measurement event (typically via a foot switch), collapsing the qubit to either $|0\rangle$ or $|1\rangle$, with resulting sound mapped to distinct sonic outcomes, e.g., a pad for $|0\rangle$, a lead tone for $|1\rangle$.
• Realize protocols that involve nontrivial multiqubit states. For instance, by playing the role of one qubit of an entangled Bell pair (e.g., $$|\Phi^+\rangle = (|00\rangle + |11\rangle)/\sqrt{2}$$), the Quantum Guitar’s measurement events may be mapped onto, or synchronized with, sound or control events in a complementary instrument (e.g., a grand piano), realizing a sonified Bell-pair paradigm in live chamber music settings.

4. Quantum Synthesis: Mapping Qubit States to Audio Output

The Actias quantum synth transforms the state of each qubit into sonic material via programmed mappings:

• For single qubits, basis measurement outcomes modulate the choice of timbre: different sound patches are assigned to $|0\rangle$, $|1\rangle$, or superposed/intermediate states.
• In cases involving two or more entangled qubits (Bell states), the measurement of one string can trigger correlated sound events on another instrument or channel, thus producing musical textures that directly reflect quantum correlations.
• This mapping is programmable, allowing for expanded sonic vocabularies—including cross-synthesis, granular processing, or other DSP-based modulation approaches linked directly to quantum outcomes.

5. Performance Practice, Genres, and Demonstrated Versatility

The Quantum Guitar has been featured in public performances across diverse musical contexts, including industrial music with the band Black Tish and experimental performances with piano as the complementary Bell-pair partner (Coecke, 3 Sep 2025). In all iterations, the unique ability to effect and mix quantum and classical sonic layers in real time has been highlighted as particularly advantageous for improvisational flexibility and dynamic musical storytelling. The documented control schema—where both hands engage the fretboard while the feet modulate quantum parameters and sound regimes—enables a multidimensional performance akin to complex drumming in its kinetic requirements.

The instrument’s reliability and the repeatability of quantum–to–classical interpolation schemes were demonstrated in various venues, further supporting its standing as a robust platform for practice-based research and creative realization.

6. Conceptual and Mathematical Framework

Table: Core Quantum Operations in Quantum Guitar Performance

Operation Type	Formula / Description	Control Mechanism
Qubit initialization	$|\psi\rangle = \alpha|0\rangle + \beta|1\rangle$	Pluck/bow technique
Unitary rotation (Bloch Y)	$U_y(\theta) = \exp(-i\theta\sigma_y/2)$	Expression pedal
Projective measurement	$|0\rangle$ or $|1\rangle$ outcome	Foot switch
Crossfade quantum/classical	Blend two output channels	Dual volume pedals
Bell-pair protocol	$$|\Phi^+\rangle = (|00\rangle + |11\rangle)/\sqrt{2}$$	Paired instruments/measurement

This formalism provides a rigorous vocabulary for system description, mapping physical gestures and control protocols directly onto quantum state evolution and measurement theory.

7. Artistic Implications and Outlook

The Quantum Guitar paradigm marks a transdisciplinary convergence between quantum information science and contemporary music technology. Its architecture enables the performer and composer to:

• Treat the mapping from instrumental gesture to sound as a dynamically variable, quantum–classical probability distribution.
• Leverage measurement-induced randomness—and more generally quantum statistics—for stochastic or indeterministic musical forms.
• Model nontrivial correlations and entanglements among musical components or even between distributed performers.

The experimental demonstration of live, reliable, and musically salient quantum–classical interpolation expands the palette of expressive techniques available to composers, improvisers, and research practitioners seeking to probe the sonic domain at the quantum–technological interface (Coecke, 3 Sep 2025).

A plausible implication is that as quantum synthesis engines and controller integration mature, the Quantum Guitar may form the basis of a new class of hybrid instruments—both as a proving ground for quantum–inspired musical cognition and as an experimental platform for mapping abstract quantum principles into tangible, performable sound.