IBM Quantum Developer track Verified Questions,
Correct Answers, and Detailed Explanations for
Computer Science Students||Already Graded A+
1. What is the state vector of a qubit in the “|0⟩” state?
0
A. ( )
1
1
B. ( )
0
1
2
C. (√1 )
√2
1
2
D. ( √ 1 )
−
√2
…because “|0⟩” is represented by the amplitude vector [1, 0] in the
computational basis.
2. What is the effect of applying a Hadamard gate H to the state
|0⟩?
A. |0⟩
B. |1⟩
∣0⟩+∣1⟩
C.
√2
∣0⟩−∣1⟩
D.
√2
…H maps |0⟩ to an equal superposition of |0⟩ and |1⟩.
3. In Qiskit, which object is used to represent a quantum circuit?
A. QuantumRegister
B. ClassicalRegister
C. QuantumCircuit
D. PulseSchedule
…QuantumCircuit is the class that defines qubits, gates, and
measurement operations.
,4. Which of the following is a two-qubit gate that flips the second
qubit only if the first qubit is |1⟩?
A. SWAP
B. CNOT
C. Hadamard
D. Phase gate
…CNOT (controlled-NOT) uses the first qubit as control and flips the
second qubit conditional on control being |1⟩.
5. What is the matrix representation (in the computational basis) of
a Pauli-X (NOT) gate?
1 0
A. ( )
0 −1
0 1
B. ( )
1 0
1 0
C. ( )
0 1
0 −𝑖
D. ( )
𝑖 0
…Pauli-X swaps |0⟩ and |1⟩ — its matrix has off-diagonal 1s.
∣𝟎⟩+∣𝟏⟩
6. If a qubit is in state , what is the probability of measuring
√𝟐
|0⟩ (in the computational basis)?
A. 0
B. 0.25
C. 0.5
D. 1
…the amplitudes are equal (1/√2), probability is (1/√2)² = ½.
7. What does the operation qc.measure(q, c) in Qiskit do?
A. Initializes the qubit q to 0 and stores in classical bit c
B. Applies a Pauli-Z gate based on classical bit c
C. Measures the quantum bit q and stores the result in classical bit
c
, D. Creates a new classical register c
…measure maps a qubit’s state to a classical bit, collapsing the
superposition.
8. Which principle forbids making a perfect copy of an unknown
quantum state?
A. Quantum tunneling
B. Entanglement
C. Wave–particle duality
D. No-cloning theorem
…the no-cloning theorem states you cannot duplicate an arbitrary
unknown quantum state.
9. What is entanglement?
A. Two qubits both in state |0⟩
B. Two qubits both in |1⟩
C. A quantum correlation where the joint state cannot be factored
into individual qubit states
D. A classical correlation between measurement outcomes
…entangled states cannot be expressed as tensor products of
individual qubit states.
10. In Qiskit, which method runs a circuit on a simulator backend?
A. backend.run_simulation(qc)
B. execute(qc, backend)
C. qc.simulate()
D. backend.simulate(qc)
…execute(...) is the Qiskit function to run circuits on backends
(simulator or real hardware).
11. What does the Bloch sphere representation depict?
A. Energy levels of an atom
B. The probability of measuring 0 vs 1 only
C. Any pure single-qubit state as a point on a unit sphere
Correct Answers, and Detailed Explanations for
Computer Science Students||Already Graded A+
1. What is the state vector of a qubit in the “|0⟩” state?
0
A. ( )
1
1
B. ( )
0
1
2
C. (√1 )
√2
1
2
D. ( √ 1 )
−
√2
…because “|0⟩” is represented by the amplitude vector [1, 0] in the
computational basis.
2. What is the effect of applying a Hadamard gate H to the state
|0⟩?
A. |0⟩
B. |1⟩
∣0⟩+∣1⟩
C.
√2
∣0⟩−∣1⟩
D.
√2
…H maps |0⟩ to an equal superposition of |0⟩ and |1⟩.
3. In Qiskit, which object is used to represent a quantum circuit?
A. QuantumRegister
B. ClassicalRegister
C. QuantumCircuit
D. PulseSchedule
…QuantumCircuit is the class that defines qubits, gates, and
measurement operations.
,4. Which of the following is a two-qubit gate that flips the second
qubit only if the first qubit is |1⟩?
A. SWAP
B. CNOT
C. Hadamard
D. Phase gate
…CNOT (controlled-NOT) uses the first qubit as control and flips the
second qubit conditional on control being |1⟩.
5. What is the matrix representation (in the computational basis) of
a Pauli-X (NOT) gate?
1 0
A. ( )
0 −1
0 1
B. ( )
1 0
1 0
C. ( )
0 1
0 −𝑖
D. ( )
𝑖 0
…Pauli-X swaps |0⟩ and |1⟩ — its matrix has off-diagonal 1s.
∣𝟎⟩+∣𝟏⟩
6. If a qubit is in state , what is the probability of measuring
√𝟐
|0⟩ (in the computational basis)?
A. 0
B. 0.25
C. 0.5
D. 1
…the amplitudes are equal (1/√2), probability is (1/√2)² = ½.
7. What does the operation qc.measure(q, c) in Qiskit do?
A. Initializes the qubit q to 0 and stores in classical bit c
B. Applies a Pauli-Z gate based on classical bit c
C. Measures the quantum bit q and stores the result in classical bit
c
, D. Creates a new classical register c
…measure maps a qubit’s state to a classical bit, collapsing the
superposition.
8. Which principle forbids making a perfect copy of an unknown
quantum state?
A. Quantum tunneling
B. Entanglement
C. Wave–particle duality
D. No-cloning theorem
…the no-cloning theorem states you cannot duplicate an arbitrary
unknown quantum state.
9. What is entanglement?
A. Two qubits both in state |0⟩
B. Two qubits both in |1⟩
C. A quantum correlation where the joint state cannot be factored
into individual qubit states
D. A classical correlation between measurement outcomes
…entangled states cannot be expressed as tensor products of
individual qubit states.
10. In Qiskit, which method runs a circuit on a simulator backend?
A. backend.run_simulation(qc)
B. execute(qc, backend)
C. qc.simulate()
D. backend.simulate(qc)
…execute(...) is the Qiskit function to run circuits on backends
(simulator or real hardware).
11. What does the Bloch sphere representation depict?
A. Energy levels of an atom
B. The probability of measuring 0 vs 1 only
C. Any pure single-qubit state as a point on a unit sphere
