However, it is possible to transfer the state of one quantum bit to another, and this operation is known as quantum teleportation. Quantum teleportation involves entangling two qubits and then measuring one of them (and another one decided by this measurement).
The lines of quantum circuit representing the state of quantum bits never branch or merge.
Branching operations can be achieved using the Toffoli gate.
The Toffoli gate is a quantum version of the NAND gate, and it allows the creation of quantum circuits for classical logical circuits (but using Toffoli gate alone does not make the quantum circuit efficient).
The Toffoli gate flips the target qubit when both control qubits are in state |1>.
Merely using superposition to perform parallel computations does not guarantee that quantum computers are faster than classical ones.
In quantum computing, with three types of quantum gates (Hadamard, T, CNOT), any quantum computation can be performed by combining them. Such a set of gates is called a universal gate set.
Any single-qubit gate U can be approximated with sufficient accuracy using a combination of Hadamard and T (phase shift) gates, as guaranteed by the Solovay-Kitaev theorem.
Many quantum operations involve multiple auxiliary qubits. If one wants to reuse these auxiliary qubits for other calculations, they need to be initialized to states like |0>. However, during the computation, these auxiliary qubits are in an entangled state with the register qubits. Carelessly measuring the auxiliary qubits can lead to changes in the state of the register qubits. To avoid this, a technique called “uncomputation” exists
Encoding
Once the amplitude vector is encoded, direct access to data from classic computer becomes impossible.
To decode the amplitude-encoded result, statistical processing such as multiple measurements is required
A quantum analog-to-digital conversion method has been proposed to interchange between basis encoding and amplitude encoding. In this approach, the conversion from amplitude encoding to basis encoding is deterministic, while the reverse conversion is probabilistic.
It is called Quantum Random Access Memory (QRAM), which consists of quantum bit memories for both the address-register and the data-output register.
There is a way to efficiently perform amplitude encoding by preparing classical data in a tree structure.
