Actias Quantum Synth: Hybrid Sound

• Actias Quantum Synth is a quantum sound synthesis platform that converts classical musical signals into quantum states using qubit representations.
• It employs dual processing paths—classical audio and quantum gate operations—to enable seamless transitions and enriched sonic textures.
• The system integrates hardware and software with real-time MIDI and foot controller interfaces, offering performers advanced live manipulation and hybrid audio output.

The Actias Quantum Synth is a quantum sound synthesis and control platform that integrates quantum information processing concepts with digital musical instrument design, enabling quantum–enhanced sonic performance and real-time musical interaction. Its core methodology involves translating classical or continuous musical signals—such as those from a guitar—into quantum representations using qubits, and providing musicians with interfaces for direct manipulation of quantum states, yielding a hybrid output that allows seamless transitions between conventional and quantum-processed sound. Developed as a modular system, Actias Quantum Synth underpins experimental musical instruments such as the “Quantum Guitar,” where it actuates both timbral and state control via quantum gate operations and measurement, coordinated with live performer input (Coecke, 3 Sep 2025).

1. Conceptual Overview and Quantization Principle

At the heart of the Actias Quantum Synth is the quantization of classical wave signals from musical instruments into quantum states. A vibrating guitar string, classically described by a wave function $\psi(x, t)$, is mapped onto a quantum superposition in a two-dimensional Hilbert space spanned by the basis states $|0\rangle$ and $|1\rangle$. This procedure is captured by the mapping:

$$\psi(x, t) \longrightarrow |\psi_{\text{qubit}}\rangle = \alpha |0\rangle + \beta |1\rangle,$$

where the amplitudes $\alpha$ and $\beta$ are extracted from features of the classical vibration, such as amplitude, phase, or spectral content, via a calibration function implemented in the synth's hardware or software domain. Technically, this amounts to defining a quantum-mapping operator $Q$ such that $|\psi_{\text{qubit}}\rangle = Q[\psi(x, t)]$.

The quantized state is constantly updated in real time as the instrument is played, allowing the mapping of musical gestures onto quantum operations. These quantum states are then subject to further manipulation through the application of single-qubit gates, such as rotations around the x, y, or z axes:

$R_k(\theta) = e^{-i \theta \sigma_k / 2},$

where $\sigma_k$ is the Pauli matrix for the chosen axis, and $\theta$ encodes the control parameter, typically modifiable in real time by the performer through external interfaces.

2. System Architecture and Signal Flow

The architecture of the Actias Quantum Synth is a hybrid signal-processing system with two parallel paths:

• Classical Path: The standard audio signal from the instrument is processed using conventional effects (distortion, delay, reverberation, etc.), remaining entirely in the classical domain.
• Quantum Path: The incoming instrument signal (from analog pickups or MIDI conversion) is transcoded into qubit state(s) by the Actias module, which then applies quantum gates and, possibly, projective measurements. The resulting processed signal is subsequently “sonified”—rendered as audio—by mapping the post-operation quantum state back to a real or synthesized waveform.

This dual-path infrastructure is controlled by mixing and volume circuits or pedals, so the performer may continuously modulate the balance:

$$\text{Output} = V_{\text{classical}} \cdot \text{Sound}_{\text{classical}} + V_{\text{quantum}} \cdot \text{Sound}_{\text{quantum}},$$

where $V_{\text{classical}}$ and $V_{\text{quantum}}$ are performer-controlled weights, enabling any real-time mixture between standard and quantum-enhanced timbres.

3. Performer Interfaces and Quantum Control

Actias Quantum Synth is engineered for real-time musical performance, accommodating the constraint that players’ hands are typically occupied with instrumental technique. To enable continuous live quantum control, critical state manipulations (such as gate rotations and measurements) are mapped to foot controllers and MIDI foot pedals. These controllers send commands to the synth module to, e.g., adjust the rotation angle $\theta$ in operations like $R_y(\theta)$, or initiate a measurement event that “collapses” a superposed quantum state, selecting one of the possible sonic outcomes.

Because these manipulations can be tightly synchronized with hand gestures (such as string bends or vibrato), the result is an expanded parameter space for musical expression—akin to the multidimensional control traditionally found only in percussion performance or advanced MIDI setups.

4. Quantum–Classical Transition and Hybrid Sound Production

A distinguishing feature is the capacity for smooth, continuous morphing between purely classical and purely quantum sound characteristics. This is realized in performance scenarios by the selective routing and mixing of the two audio paths. The quantum sound embodies the stochastic and superposed nature of quantum state processing—the result of unitary evolution, interactive quantum operations, and measurement-induced “collapse”—while the classical sound maintains deterministic audio characteristics. The relative dominance of each component is under the live control of the performer.

This flexibility is utilized in performance contexts to great effect, as demonstrated in live appearances spanning genres from industrial to experimental, including situations in which the Quantum Guitar acts as a qubit within a sonified Bell pair, with the second qubit “realized” via another instrument (Coecke, 3 Sep 2025).

5. Implementation Details: From Hardware to Software

The quantization and quantum–sound synthesis pipeline is implemented across both dedicated hardware and software platforms. Hardware integration includes:

• Analog-to-qubit conversion circuits, likely incorporating high-speed digitization and parameter extraction from the instrument’s signal.
• Real-time digital gate processors that execute the quantum gates and measurements according to control inputs.
• Digital-to-audio rendering modules, which output the sonified quantum state as audio suitable for live mixing.

Software components implement the mapping $Q[\psi(x,t)]$, gate arithmetic, measurement statistics (including pseudorandom selection per quantum mechanical projection probabilities), and real-time audio buffering. MIDI input is used both for note detection and fine-tuning of quantum parameters.

The foot controller interface is standard MIDI or continuous controller (CC) protocol, allowing integration with a variety of existing stage and studio hardware. Gate operations and measurements are programmable and configurable, so the system can be tailored to the expressive requirements of a particular performance or composition.

6. Applications, Significance, and Prospects

Actias Quantum Synth’s introduction into performance practice establishes a new class of instrument that is both a tool for creative exploration and an educational platform. It provides performers with direct, tactile access to fundamental quantum processing concepts in a live, audible, and manipulable format.

Its versatility is demonstrated in live settings and in the context of ongoing recording projects, such as those undertaken by the group Black Tish. The instrument’s dual-mode control, extended expressive range, and ability to engage listeners in quantum concepts (as in “sonified Bell pairs” or “quantum–classical transitions”) position it for impact in both experimental and mainstream music settings.

A plausible implication is that the modular architecture may be extended to other instruments by adapting the signal analysis interface and mapping protocol, indicating broader applicability for “quantum enhancement” of traditional acoustic and electronic sound sources.

7. Position within the Broader Quantum Sound Synthesis Landscape

The Actias Quantum Synth is differentiated from other quantum music projects, such as Schrödinger-equation–based sonification tools (Freye et al., 1 Feb 2024) or quantum-circuit–driven sequencers (Miranda, 2020), by its direct coupling of live instrumental input, quantum state parameterization, and real-time control with a focus on performative ergonomy. It shares with these other systems the aim of translating quantum informational processes into novel musical timbres and structures; however, its explicit integration of live musical technique with hands-free quantum parameter modulation and continuous classical–quantum sound blending constitutes a unique synthesis within the emerging field of quantum musical instrument design.