How To Create Custom Operator With Qiskit
Stop Writing Quantum Spaghetti Code
Simply stacking quantum gates on one another is the fastest way to turn your code into a confusing mess
You probably think that it's okay to simply stack A quantum gate is a basic operation that changes the state of one or more qubits, similar to how a logic gate operates on bits in classical computing. It uses unitary transformations, meaning it preserves the total probability (the state’s length in complex space). Quantum gates enable superposition and entanglement, allowing quantum computers to perform computations that classical ones cannot efficiently replicate.
Learn more about Quantum Gate one after the other in Qiskit is an open-source Python framework for programming and simulating quantum computers. It lets users create quantum circuits, run them on real quantum hardware or simulators, and analyze the results. Essentially, it bridges high-level quantum algorithms with low-level hardware execution.
Learn more about Qiskit until your A quantum circuit is a sequence of quantum gates applied to qubits, representing the operations in a quantum computation. Each gate changes the qubits’ state using quantum mechanics principles like superposition and entanglement. The final qubit states, when measured, yield the circuit’s computational result probabilistically.
Learn more about Quantum Circuit works.
That's what most people do when they start out. They insert a The Hadamard operator (often denoted **H**) is a single-qubit quantum gate that creates an equal superposition of |0⟩ and |1⟩. It transforms basis states as **H|0⟩ = (|0⟩ + |1⟩)/√2** and **H|1⟩ = (|0⟩ − |1⟩)/√2**. Mathematically, it’s represented by the matrix **(1/√2)·[[1, 1], [1, −1]]**, which both rotates and reflects the qubit state on the Bloch sphere.
Learn more about Hadamard Operator here, a The CNOT (Controlled-NOT) operator is a two-qubit quantum gate that flips the **target qubit** (applies an X gate) only if the **control qubit** is in the state (|1⟩). In matrix form, it leaves (|00⟩) and (|01⟩) unchanged, but swaps (|10⟩) and (|11⟩). It’s essential for creating entanglement between qubits.
Learn more about Controlled-NOT Operator there, maybe copy a block from another file, click Run, and consider the job done.
This post is accompanied by a PDF file summarizing the key points.
But this habit quickly turns your code into a confusing mess. You'll find yourself scrolling through endless gate sequences, trying to remember what each section was supposed to do. When you change one thing, something completely unrelated breaks. Debugging becomes a guessing game.
Fortunately, there is a cleaner way. A way to group parts of your A quantum circuit is a sequence of quantum gates applied to qubits, representing the operations in a quantum computation. Each gate changes the qubits’ state using quantum mechanics principles like superposition and entanglement. The final qubit states, when measured, yield the circuit’s computational result probabilistically.
Learn more about Quantum Circuit that belong together. A way to give your code structure instead of chaos. It's not complicated, but it changes everything.
When all your logic lives in one huge flat A quantum circuit is a sequence of quantum gates applied to qubits, representing the operations in a quantum computation. Each gate changes the qubits’ state using quantum mechanics principles like superposition and entanglement. The final qubit states, when measured, yield the circuit’s computational result probabilistically.
Learn more about Quantum Circuit, you lose track of where one idea ends and the next one starts. Every new part carries the risk of affecting something far away.
However, if you bundle a meaningful sequence of gates into its own reusable unit, your logic becomes modular. Only then, your A quantum circuit is a sequence of quantum gates applied to qubits, representing the operations in a quantum computation. Each gate changes the qubits’ state using quantum mechanics principles like superposition and entanglement. The final qubit states, when measured, yield the circuit’s computational result probabilistically.
Learn more about Quantum Circuit consists of chapters instead of one long paragraph. You can view each part separately and combine them like building blocks, instead of taping together lists of gates.
What I am describing here are user-defined operators. In Qiskit is an open-source Python framework for programming and simulating quantum computers. It lets users create quantum circuits, run them on real quantum hardware or simulators, and analyze the results. Essentially, it bridges high-level quantum algorithms with low-level hardware execution.
Learn more about Qiskit you can convert any sequence of A quantum gate is a basic operation that changes the state of one or more qubits, similar to how a logic gate operates on bits in classical computing. It uses unitary transformations, meaning it preserves the total probability (the state’s length in complex space). Quantum gates enable superposition and entanglement, allowing quantum computers to perform computations that classical ones cannot efficiently replicate.
Learn more about Quantum Gate into its own named operator. It becomes a user-defined building block that behaves exactly like a built-in building block.
? shows how it works.
- In this code listing, we
- the required
QuantumCircuitclass from Qiskit; - Create a
QuantumCircuitinstance (custom_qc) to hold our . Essentially, a A quantum circuit is a sequence of quantum gates applied to qubits, representing the operations in a quantum computation. Each gate changes the qubits’ state using quantum mechanics principles like superposition and entanglement. The final qubit states, when measured, yield the circuit’s computational result probabilistically.
Learn more about Quantum Circuit is the scaffolder for the A quantum gate is a basic operation that changes the state of one or more qubits, similar to how a logic gate operates on bits in classical computing. It uses unitary transformations, meaning it preserves the total probability (the state’s length in complex space). Quantum gates enable superposition and entanglement, allowing quantum computers to perform computations that classical ones cannot efficiently replicate.
Learn more about Quantum Gate to be applied; - Apply an arbitrary number of A quantum gate is a basic operation that changes the state of one or more qubits, similar to how a logic gate operates on bits in classical computing. It uses unitary transformations, meaning it preserves the total probability (the state’s length in complex space). Quantum gates enable superposition and entanglement, allowing quantum computers to perform computations that classical ones cannot efficiently replicate.
Learn more about Quantum Gate to our custom operation. Any A quantum gate is a basic operation that changes the state of one or more qubits, similar to how a logic gate operates on bits in classical computing. It uses unitary transformations, meaning it preserves the total probability (the state’s length in complex space). Quantum gates enable superposition and entanglement, allowing quantum computers to perform computations that classical ones cannot efficiently replicate.
Learn more about Quantum Gate is a A **unitary operator** is a linear operator ( U ) on a complex vector space that satisfies ( U^\dagger U = UU^\dagger = I ), meaning it preserves inner products. In simpler terms, it preserves the **length** and **angle** between vectors—so it represents a **reversible, norm-preserving transformation**. In quantum mechanics, unitary operators describe the evolution of isolated systems because they conserve probability.
Learn more about Unitary Operator. Applying multiple A **unitary operator** is a linear operator ( U ) on a complex vector space that satisfies ( U^\dagger U = UU^\dagger = I ), meaning it preserves inner products. In simpler terms, it preserves the **length** and **angle** between vectors—so it represents a **reversible, norm-preserving transformation**. In quantum mechanics, unitary operators describe the evolution of isolated systems because they conserve probability.
Learn more about Unitary Operator subsequently corresponds to multiplying them. And the product of A **unitary operator** is a linear operator ( U ) on a complex vector space that satisfies ( U^\dagger U = UU^\dagger = I ), meaning it preserves inner products. In simpler terms, it preserves the **length** and **angle** between vectors—so it represents a **reversible, norm-preserving transformation**. In quantum mechanics, unitary operators describe the evolution of isolated systems because they conserve probability.
Learn more about Unitary Operator is again a A **unitary operator** is a linear operator ( U ) on a complex vector space that satisfies ( U^\dagger U = UU^\dagger = I ), meaning it preserves inner products. In simpler terms, it preserves the **length** and **angle** between vectors—so it represents a **reversible, norm-preserving transformation**. In quantum mechanics, unitary operators describe the evolution of isolated systems because they conserve probability.
Learn more about Unitary Operator and therefore a A quantum gate is a basic operation that changes the state of one or more qubits, similar to how a logic gate operates on bits in classical computing. It uses unitary transformations, meaning it preserves the total probability (the state’s length in complex space). Quantum gates enable superposition and entanglement, allowing quantum computers to perform computations that classical ones cannot efficiently replicate.
Learn more about Quantum Gate again; - the composed
custom_qcinto a A quantum gate is a basic operation that changes the state of one or more qubits, similar to how a logic gate operates on bits in classical computing. It uses unitary transformations, meaning it preserves the total probability (the state’s length in complex space). Quantum gates enable superposition and entanglement, allowing quantum computers to perform computations that classical ones cannot efficiently replicate.
Learn more about Quantum Gate instance; - another
QuantumCircuitobject to serve as our main circuit; - IBM, 2025, , the
custom_gateto the main circuit. We feed theappendfunction with two parameters. The first is the instructioncustom_gatethat tells the circuit what operation to apply. The second paramter denotes theqargs. These are the A qubit is the basic unit of quantum information, representing a superposition of 0 and 1 states.
Learn more about Quantum Bit to apply the instruction to. Here, we give a simple list of the A qubit is the basic unit of quantum information, representing a superposition of 0 and 1 states.
Learn more about Quantum Bit positions. The order is important and it corresponds to the A qubit is the basic unit of quantum information, representing a superposition of 0 and 1 states.
Learn more about Quantum Bit in ourcustom_qc. So, in this example, we map the first qubit (at position) of the main A quantum circuit is a sequence of quantum gates applied to qubits, representing the operations in a quantum computation. Each gate changes the qubits’ state using quantum mechanics principles like superposition and entanglement. The final qubit states, when measured, yield the circuit’s computational result probabilistically.
Learn more about Quantum Circuit to the first qubit of thecustom_qcA quantum circuit is a sequence of quantum gates applied to qubits, representing the operations in a quantum computation. Each gate changes the qubits’ state using quantum mechanics principles like superposition and entanglement. The final qubit states, when measured, yield the circuit’s computational result probabilistically.
Learn more about Quantum Circuit.
When we look at our A quantum circuit is a sequence of quantum gates applied to qubits, representing the operations in a quantum computation. Each gate changes the qubits’ state using quantum mechanics principles like superposition and entanglement. The final qubit states, when measured, yield the circuit’s computational result probabilistically.
Learn more about Quantum Circuit now, we can see that there is only one A quantum gate is a basic operation that changes the state of one or more qubits, similar to how a logic gate operates on bits in classical computing. It uses unitary transformations, meaning it preserves the total probability (the state’s length in complex space). Quantum gates enable superposition and entanglement, allowing quantum computers to perform computations that classical ones cannot efficiently replicate.
Learn more about Quantum Gate in it. Our CustomGate as depicted in ?.
At first glance, this may seem excessive. Why wrap something that you could just write inline?
Because wrapping it does more than just clean up your code. It defines meaning. It tells you, your teammates, and Qiskit is an open-source Python framework for programming and simulating quantum computers. It lets users create quantum circuits, run them on real quantum hardware or simulators, and analyze the results. Essentially, it bridges high-level quantum algorithms with low-level hardware execution.
Learn more about Qiskit what this group of A quantum gate is a basic operation that changes the state of one or more qubits, similar to how a logic gate operates on bits in classical computing. It uses unitary transformations, meaning it preserves the total probability (the state’s length in complex space). Quantum gates enable superposition and entanglement, allowing quantum computers to perform computations that classical ones cannot efficiently replicate.
Learn more about Quantum Gate represents. It is no longer a random bunch of instructions, but a unit of logic with a name, a purpose, and clear boundaries.
When you use custom operators, your A quantum circuit is a sequence of quantum gates applied to qubits, representing the operations in a quantum computation. Each gate changes the qubits’ state using quantum mechanics principles like superposition and entanglement. The final qubit states, when measured, yield the circuit’s computational result probabilistically.
Learn more about Quantum Circuit are no longer raw lists of operations, but become structured programs. They are easier to read, easier to test, and easier to maintain.
This is particularly important if you use the same subcircuit multiple times in your main A quantum circuit is a sequence of quantum gates applied to qubits, representing the operations in a quantum computation. Each gate changes the qubits’ state using quantum mechanics principles like superposition and entanglement. The final qubit states, when measured, yield the circuit’s computational result probabilistically.
Learn more about Quantum Circuit as filling it in manually each time can lead to inconsistencies. One typo, and you'll be chasing invisible errors for days.
However, if you turn this subcircuit into a reusable component, you can test it once and then reuse it safely. You can check its behavior in isolation. Does it produce the correct state, the correct phase shifts? And you don't have to worry about rechecking it every time. Your code becomes predictable. You can focus on the higher-level algorithm instead of worrying about the details.