Quantum Computing Flashcards
What it the biggest problem facing quantum computing?
Errors. Quantum gates produce erroneous output at a high rate (0.1% to 1%)
How is the high error rate of quantum gates handled?
Running the same programs multiple times to see which output is most common
What makes quantum bits so much more powerful than classical bits?
Superposition. It allows for a kind of parallel computing, where the quantum computer can perform operations that process all possible states of the qubit at once
What is superposition for a quantum bit?
It describes how the bit can exist in both states simultaneously
What is this formula describing?
|ψ⟩ = α|0⟩ + β|1⟩
|ψ⟩ is the state of the qubit
|0⟩ and |1⟩ are the basis states (or the computational basis states)
α and β are complex numbers called probability amplitudes
What are probability amplitudes?
They determine the probability of measuring the qubit in either base state, once that measurement is made
What are the Pauli gates?
Analogous to the NOT gate in classical computing, it rotates the state about the Bloch sphere X, Y, or Z axis by 180 degrees.
What does the Pauli-X gate map |0> to?
|1>, and vice versa
What does the Pauli-Z gate map |+> to?
|->, and vice versa
What is the matrix representation of the Pauli-X gate?
What is the matrix representation of the Pauli-Z gate?
What is the “bit-flip” gate?
The Pauli-X gate
What is the “phase-flip” gate?
The Pauli-Z gate
What is the “bit-and-phase-flip” gate?
The Pauli-Y gate
What does the Hadamard Gate do?
Creates superposition from standard basis gates. Maps |0> to |+> and |1> to |->
What is the matrix calculation for a Hadamard gate?
What is the Identity gate?
Does nothing.
What is a Controlled gate?
One or more qubits act as a control for some operation
What is the CNOT gate?
Performs X gate on second qubit iff the first qubit is |1>
What is a SWAP gate?
Swaps the state between two qubits?
What is a SWAP gate made of?
Three CNOT gates
What is a Toffoli / Controlled CNOT / CCNOT gate?
Performs a NOT operation on the target qubit only if both control qubits are in state |1>
What is the Fredkin / Controlled SWAP / CSWAP gate?
Swaps the second and third bit iff the first (control) bit is in state |1>
