A quantum in music.

What does a quantum computer sound like?

Quantum mechanics describes a physical reality whose behaviour at the smallest scales differs fundamentally from classical intuition. A quantum system does not possess definite properties prior to measurement; rather, it exists in a superposition of possible states whose probabilities are described by a complex-valued wavefunction (Nielsen and Chuang, 2010). The act of measurement is therefore not an act of discovery but of selection: it forces the system to resolve into one of its possible outcomes, with a probability distribution determined by the prior state. This formulation, although counter-intuitive, has been confirmed by a century of experimental evidence and underpins technologies including the transistor, the laser, and magnetic resonance imaging (Griffiths, 2018).

Quantum computers are the first machines designed to exploit these principles directly for computation. Rather than encoding information in classical bits, they operate on qubits, which can exist in superposition and become entangled, producing correlations that cannot be reproduced by any classical system (Nielsen and Chuang, 2010). Significant resources are now being directed toward the development of quantum hardware and software. IBM, Google, and others have produced cloud-accessible quantum processors and predict substantial advances in the coming decade across cryptography, materials science, drug discovery, and machine learning (IBM Quantum, 2024).

The question this project addresses is one that the dominant application areas do not consider: if a quantum computer's native language is probability, and music is the structured organisation of events through time, what occurs when the first is allowed to articulate itself through the second?

The work presented here suggests that the result is neither random in the colloquial sense nor algorithmically conventional. It is, instead, what may be described as structured uncertainty: musical material whose generation is governed by the mathematics of quantum measurement, biased by the correlations that physicists term entanglement (Einstein, Podolsky and Rosen, 1935; Bell, 1964), rendered as audible MIDI events and shaped in performance through a hardware controller.

"Prior to measurement, every outcome is, in a precise mathematical sense, equally real. Measurement is the act that forces a single outcome to emerge."

The piece presented in the following section is generated by a three-qubit circuit producing eight possible measurement outcomes, each mapped onto a note of a musical scale. When entanglement gates are applied to produce a Greenberger-Horne-Zeilinger (GHZ) state (Greenberger, Horne and Zeilinger, 1989), only two of the eight outcomes occur with non-zero probability. When the qubits are independent, all eight outcomes occur with equal probability. The audible difference between these two cases constitutes the project's central artistic claim.

The architecture.

The system comprises five layers operating in series. A quantum circuit produces a measurement; the measurement is mapped onto a musical scale; the resulting MIDI is transmitted to a digital audio workstation; the same data is broadcast via Open Sound Control to a real-time visual engine; and the performer modulates the resulting output through a hardware controller during performance.

The stack.

Live, from the terminal.

The Python source is intentionally compact; the value of the project lies in its routing rather than the complexity of any individual component. The recording below presents the source on the upper portion of the screen and the live terminal output on the lower. Each pad press on the MPK Mini produces a corresponding line of output, for example scale → Blues, mode → GHZ, or PANIC: all notes off, demonstrating that the loop is operating, listening for controller input, and switching state in real time during performance.

The core loop.

Reduced to its essential elements, the engine of the system can be expressed in fewer than twenty lines of Python.

# quantum_live.py - the heart of the system
from qiskit import QuantumCircuit
from qiskit_aer import AerSimulator
from pythonosc.udp_client import SimpleUDPClient
import mido, time

SCALE = [60, 62, 64, 65, 67, 69, 71, 72]   # C major
simulator = AerSimulator()
port = mido.open_output('IAC Driver Bus 1')
osc  = SimpleUDPClient("127.0.0.1", 9000)

while True:
    qc = QuantumCircuit(3)
    qc.h(0)                  # superposition
    qc.cx(0, 1)              # entangle 0 → 1
    qc.cx(1, 2)              # entangle 1 → 2 (GHZ state)
    qc.measure_all()

    binary = list(simulator.run(qc, shots=1).result().get_counts().keys())[0]
    note   = SCALE[int(binary, 2)]

    osc.send_message("/quantum/note", note)
    osc.send_message("/quantum/binary", binary)

    port.send(mido.Message('note_on', note=note, velocity=120))
    time.sleep(0.4)
    port.send(mido.Message('note_off', note=note, velocity=0))

Three modes of quantum behaviour.

The performer is able to switch between three different quantum circuits in real time, each producing a distinct musical character:

• GHZ mode - three qubits prepared in a maximally entangled GHZ state. Only two measurement outcomes occur with non-zero probability: 000 or 111. The musical result is sparse and binary, alternating between two notes at the extremes of the chosen scale.
• BELL mode - two qubits entangled in a Bell state, with the third qubit independent. Four outcomes occur with equal probability. This produces what is, in practice, the most musical setting, in which paired notes ascend or descend together.
• FREE mode - three independent superpositions, with no entanglement applied. All eight outcomes occur with equal probability. This produces the most melodically varied output, but the least overtly quantum-correlated structure.

The choice of circuit thereby becomes a compositional parameter. Switching from GHZ to FREE during performance is, in functional terms, equivalent to changing key in a conventional musical setting; the difference is that what changes is not the harmonic centre but the degree to which quantum correlations are shaping the output.

Music technology meets the next computational paradigm.

The history of music technology is, in significant part, the history of physical principles being adopted from other fields and adapted to musical use. The vacuum tube, originally developed for radio communication, made the electric guitar amplifier possible (Théberge, 1997). The transistor, invented at Bell Laboratories in 1947, enabled the synthesisers of Robert Moog and Donald Buchla in the 1960s (Pinch and Trocco, 2002). Digital signal processing, which arose from research in telecommunications and military signal analysis, became the foundation of sampling, software-based instruments, and the contemporary digital audio workstation (Roads, 1996). In each instance, the technology originated outside music and entered it through the work of practitioners willing to repurpose it for creative ends.

Quantum computing constitutes a candidate for the next such transition, although its uptake within music has so far been limited to a small body of pioneering work. Miranda (2022a) provides the most comprehensive treatment of the field to date, presenting both theoretical foundations and practical compositional methods. His own compositions, including Zeno (2022b), have used real quantum hardware as a generative source. Hamido (2020) has developed the QAC Toolkit for Max/MSP, allowing composers to design quantum circuits within an environment already familiar to electronic musicians. Weaver et al. (2023) at IBM Research have produced the Quantum Music Playground, a web-based step sequencer in which each row of the interface corresponds to a qubit. These works represent early experiments in what is likely to develop into a more substantial sub-field of music technology.

The contribution of the present project is not in the use of quantum measurements as a generative source, which has been demonstrated previously (Miranda, 2022a; Hamido, 2020), but in the integration of the quantum layer with a real-time audio engine, an immersive visualisation system, and a hardware controller, all of which operate as a single continuous performable instrument. The performer is not simply triggering a pre-rendered output: they are operating a quantum process during performance, with direct, gestural control over its musical and visual expression.

"The question is not whether quantum computing can produce interesting musical material - that has been demonstrated. The question is what music will become as the structures of quantum mechanics enter the working vocabulary of practising musicians."

Quantum hardware is becoming progressively more accessible. Cloud-based platforms operated by IBM, Google, IonQ, and others now permit researchers, students, and artists to submit circuits to physical quantum processors and receive results within seconds, subject to queueing (IBM Quantum, 2024). Free and educational tiers exist; the technical barrier has reduced significantly within the past five years (Cross, 2018). What has not yet developed at comparable pace is the artistic vocabulary: the techniques, conventions, and aesthetic instincts through which a new computational paradigm is translated into musical practice. As with previous technological transitions in music, this vocabulary is more likely to be developed by musicians than by physicists or engineers (Théberge, 1997).

The present project is offered as one contribution toward that emerging vocabulary. It demonstrates that quantum-driven music can be made expressive, controllable, and immediate, and that it can function not only as an academic curiosity but as a performable instrument suitable for live presentation.

Process and methodology.

The project originated during my professional placement, at which I attended a music technology event that introduced quantum computing in a creative context. The conceptual implications of that introduction prompted further investigation, including attendance at related talks and exhibitions in London and a sustained period of self-directed study using the Qiskit framework, in order to develop practical familiarity with quantum systems.

The development of the project proceeded through three iterative phases.

1. System familiarisation.

The first phase comprised the construction of simple quantum circuits and the observation of their behaviour. Probability distributions were plotted, and the Bloch sphere was used as a conceptual model for the evolution of qubit states. This phase was concerned with developing intuition for a system whose underlying rules are non-classical, rather than with musical output.

2. Translation to musical material.

The second phase concerned translation. Quantum measurements were converted into MIDI events and routed into Logic Pro. The initial output was perceptually indistinguishable from random noise, which raised the methodological question of what constitutes musical material in this context.

The methodological turning point of the project occurred when I ceased to treat the quantum component as a source of randomness and began to treat it as a structural compositional element. A simple superposition produced by three independent Hadamard gates yields a uniform probability distribution and is therefore equivalent, for musical purposes, to a classical random source. The introduction of entanglement, particularly through the GHZ state with its characteristic correlation between qubits, produced output with internal structure: notes related to one another, and the result ceased to resemble pseudo-random output and began to exhibit the qualities of a system governed by an internal logic.

3. Construction of the instrument.

The third phase addressed integration and live performance. Python was connected to TouchDesigner to enable real-time visual updates. The MPK Mini was integrated as a control surface. Latency was reduced until the relationship between gesture, sound, and image was perceptible to a viewer, which constitutes the threshold separating a software demonstration from a musical instrument suitable for live use.

Each phase involved a sequence of trial, failure, and adjustment. Their cumulative effect was the development of a system in which quantum behaviour can be experienced through performance rather than only observed in the abstract.

Findings.

Four reflections emerged from the project, presented here in approximate order of significance:

1. The interface is the project. Quantum mathematics is well-documented, and music technology is well-documented; the work of the project is in the interface between them. Decisions concerning which features to surface - entanglement rather than measurement alone, circuit-mode switching as a compositional control, pitch-driven colour as a visual link - constitute the substantive contribution of the work, rather than its technical implementation.
2. Real-time responsiveness takes priority over physical authenticity. The Qiskit local simulator was selected over IBM's cloud quantum hardware because the latency associated with real hardware submission renders live performance impractical. The simulated output is statistically equivalent to that of physical hardware (Nielsen and Chuang, 2010); the difference lies in timing, not in physics. This represents a recurring tension within music technology, between fidelity and immediacy, and within the context of live performance the resolution of that tension typically favours immediacy.
3. Audience comprehension requires visual reinforcement. A purely auditory quantum-music system is acceptable for private listening. Live performance, however, requires a visual layer that does not merely decorate the audio but renders its underlying structure perceptible. Pitch-driven colour, particle clusters spawning at probability-cube corners, and the visible asymmetry between entangled and non-entangled modes were the elements that rendered the system legible as quantum to viewers without prior expertise in the field.
4. Music is, in fundamental respects, mathematics. The project reinforced an observation often intuited by musicians but rarely stated explicitly: the translation of quantum data into music is not unnatural. Rhythm is the division of time, pitch is frequency, and harmony is ratio. What quantum systems contribute is a different category of structure - probabilistic and non-classical, but no less mathematical. The challenge of the work was therefore not the conversion of mathematics into music, but the selection of which mathematical relationships are sufficiently meaningful to be worth hearing.

Future work.

Three directions for further development are identified, in increasing order of ambition:

1. Deployment as a multi-wall LED installation, in which the audience would inhabit the visual field rather than observe it from outside.
2. Integration with IBM Quantum cloud hardware for non-real-time validation, in which measurement sequences obtained from a physical quantum processor are pre-recorded and used as compositional material (IBM Quantum, 2024).
3. Polyphonic extension, in which multiple parallel circuits feed multiple voices, with entanglement applied across circuits so that the harmonic relationships between voices are themselves quantum-correlated.

Each of these is additive in nature: none requires the reconstruction of the existing system. The current implementation should therefore be understood both as a completed instrument suitable for live performance and as a foundation for further development.