## 1 Introduction  
Computers are omnipresent in almost every aspect of the music industry nowadays [1, 2]. Emerging new quantum computing technologies are likely to continue this trend. They are bound to impact how we create, perform, listen, and commercialize music in the future.  
Researchers and practitioners in the newborn field of _Quantum Computer Music_ have begun to explore ways to leverage the quantum-mechanical nature of quantum computing for applications in music [3, 4, 5]. It is expected that this new technology will lead to new instruments and approaches to creating music. This paper introduces examples of this.  
The majority of quantum computer music research to date has been focused on using quantum computers to generate music algorithmically. For instance, ref. [6] adapted a quantum natural language processing (QNLP) method to implement a system for creating music exploring the relationship between music and language. Additionally, ref. [7] introduced the Basak-Miranda generative music algorithm. This algorithm leverages a property of quantum mechanics known as constructive and destructive interference to compose tunes. Furthermore, ref. [8] introduced methods to generate music using a type of quantum computing known as adiabatic quantum computing.  
We are interested, however, in developing new musical instruments. Research on this front has been less common. This is probably due to the fact that the quantum representation of audio is not trivial. There is no agreed method to date for representing audio quantumly [9]. Moreover, at present, there is no hardware available for the fully-fledged implementation of quantum audio.  
Nevertheless, there have been some initiatives to synthesise sounds using quantum computing. For instance, ref. [10] developed a system for controlling the parameters of a digital sound synthesiser with results from quantum computations. Another approach was proposed in [11]. In this case, data acquired from the workings of a quantum processor were converted into audio signals in real time during a performance.  
This paper introduces QISynth and an example of a performance using it. QISynth is a novel software-based musical instrument that renders sounds from _quantum state vectors_ representing the properties of a _qubit_ and its _measurement_.  
QISynth is presented on the computer screen as a Bloch sphere, which represents a qubit (Figure 1). The performer plays the instrument by rotating and measuring the qubit. It can be rotated using the computer's mouse or an external MIDI controller. While the qubit is rotated, a continuously changing sound is produced. Additionally, when the qubit is measured, the system also produces a dynamic sound.  
The paper begins with a brief introduction to quantum computing, focusing on the qubit and Bloch's sphere. This introduction is limited to what is deemed necessary to understand how QISynth works. For more detailed explanations of quantum computing, please refer to [12; 13]. Then, it explains how the instrument synthesises sound. It focuses on just one of its synthesis techniques: frequency modulation. Next, it shows how we networked three QISynths to form a trio for a live performance. The paper ends with a discussion on ongoing developments.