The unit that represents information in a quantum computer. It can be in a superposition α|0⟩ + β|1⟩ of |0⟩ and |1⟩, and a measurement gives 0 with probability |α|² and 1 with probability |β|². Qubits are realized with superconducting circuits, ions, neutral atoms, electron spins in silicon, photons and more.
Japanese name: 量子ビット
Qubit appears in 34 practice questions across 1 topic, including Quantum Computing Hardware.
Field: Quantum Computing Hardware
The state of a single qubit can be written α|0⟩ + β|1⟩, where α and β are complex numbers (probability amplitudes) satisfying |α|² + |β|² = 1. A measurement gives 0 with probability |α|² and 1 with probability |β|², and the state changes to match the result. A single measurement does not reveal the probabilities themselves, so the same circuit is repeated many times (each repetition is called a shot) and the probabilities are estimated from the fraction of each result. For a probability of 0.5, even 10,000 repetitions leave a standard deviation of about 0.5% in the estimate. The state can be shown as a point on the Bloch sphere, and quantum gate operations correspond to rotations of that sphere.
The state of n qubits has 2ⁿ probability amplitudes, about 10¹⁵ for 50 qubits. Storing them as double-precision complex numbers on a classical computer takes about 18 PB, and each additional qubit doubles that. However, a measurement yields only n bits, and the amplitudes cannot all be read out. Quantum algorithms are designed so that interference enlarges the amplitude of the correct answer and cancels the amplitudes of wrong ones.
Superconducting circuits (transmons) use two energy levels about 5 GHz apart as a qubit and are operated after cooling to about 10 mK. Trapped ions and neutral atoms use atomic energy levels, silicon spin qubits use the spin direction of electrons confined in quantum dots, and photonic approaches use properties of photons such as polarization or path. In every platform, performance is set not only by the number of qubits but also by gate error rates, coherence times and how the qubits are connected. In error correction, many physical qubits are combined into one logical qubit.
Answer: B. It can be in a superposition of |0⟩ and |1⟩, and measuring it gives either 0 or 1 at random
The state of a qubit can be written α|0⟩ + β|1⟩, where α and β are complex numbers (probability amplitudes) that satisfy |α|² + |β|² = 1. A measurement gives 0 with probability |α|² and 1 with probability |β|², and the state changes to match the result. A superposition is not a value halfway between 0 and 1: a measurement always gives either 0 or 1. A single measurement yields only one bit of information, and the values of α and β cannot be read out directly.
Answer: A. 25%
The probability of getting 1 is the squared magnitude of the amplitude of |1⟩: (1/2)² = 1/4 = 25%. The probability of getting 0 is (√3/2)² = 3/4 = 75%, and the two add up to 100%. Taking the amplitude 1/2 itself as the probability leads to the wrong answer of 50%.
At most n bits of classical information can be obtained by measurement. Even though the qubits hold 2ⁿ amplitudes, they cannot all be read out. The speed of a quantum computer comes not from reading out more information but from using interference to reduce the number of steps needed to reach the answer.
A physical qubit is an individual qubit actually built into a device. A logical qubit is a single, more error-resistant qubit made by combining many physical qubits with quantum error correction. With the surface code, one logical qubit can require several hundred or more physical qubits.
The state of a qubit can be written α|0⟩ + β|1⟩, where α and β are complex numbers (probability amplitudes) that satisfy |α|² + |β|² = 1.
Which statement about a qubit is correct?The Bloch sphere represents a single-qubit state cos(θ/2)|0⟩ + e^{iφ} sin(θ/2)|1⟩ as a point on a sphere, with the angle θ playing the role of latitude and φ that of longitude.
When the state of a single qubit is represented on the Bloc…The state of n qubits is a superposition of the 2ⁿ basis states from |00…0⟩ to |11…1⟩, each with its own probability amplitude.
How many probability amplitudes (complex numbers) are neede…Each additional qubit doubles the memory: 30 qubits need only about 17 GB, but 40 qubits need about 18 TB, and with 50 qubits even the largest supercomputers have difficulty holding the state vector as is.
The state vector (all the probability amplitudes) of 50 qub…Looking at either qubit alone, 0 and 1 each appear 50% of the time, but the results are correlated in a way that two independent bits cannot explain; this is quantum entanglement.
Two qubits are in the entangled state (|00⟩ + |11⟩)/√2. Whi…As an Amazon Associate I earn from qualifying purchases.
Quantum superposition Transmon Josephson junction Quantum entanglement Dilution refrigerator Mobility Surface code Ion trap Quantum gate Time constant