Editing Prepare-and-Send Verifiable Universal Blind Quantum Computation (section)

==Outline==
This protocol is a modified version of [[Prepare-and-Send Universal Blind Quantum Computation]], which is based on [[Supplementary Information#Measurement Based Quantum Computation (MBQC)|(MBQC)]]. Here a powerful adversarial server is delegated with quantum computation which maintains the privacy of the computation. Any computational deviations by this server are detected by high probability. This is achieved by insertion of randomly prepared and blindly isolated single qubits in the computation, which act as a trap (trap qubits), hence assisting the client in verification.

MBQC required a set of the initial state for computation. The [[Glossary#Brickwork States|brickwork states]] used in [[Prepare-and-Send Universal Blind Quantum Computation]] are modified to [[Glossary#Cylinder Brickwork States|cylinder brickwork states]] which enables the client to embed a trap qubit surrounded by multiple dummy qubits without disrupting the computation. This state is universal and maintains the privacy of the client's preparation. The dummy qubits here do not take part in the actual computation as they are disentangled from the rest of the qubits of the graph state. Hence by adding them to the neighboring nodes of the trap qubits, they are blindly isolated and thus do not interfere with the actual computation. The dummy qubits are added next to the trap qubit in a tape format as seen in [[Glossary#Cylinder Brickwork States|cylinder brickwork states]]. 

This protocol is divided into four stages: Client's preparation, server's preparation, interaction and measurement, verification.

* '''Client's preparation''': The partially quantum client prepares the quantum states with embedded traps qubits and sends them to the server for creation of the cylinder brickwork state. 
** For the server to create a cylinder brickwork state, the client prepares <math>m</math> single qubit states. The first <math>n</math> qubit input states are specially encoded and [[Supplementary Information#Quantum One Time Pad|Quantum one time pad]] is applied to these states with randomly chosen keys.
** Then the client randomly selects one qubit as the trap qubit and corresponding to the graph of cylinder brickwork state, all the other qubits in the tape are set as the dummy qubits. The trap qubit is prepared with the local phase angle set to <math>0</math>. The dummy qubits isolate the trap qubit from the graph state.
** The remaining non-input qubit states (not including the dummy states and trap qubit) are prepared with randomly chosen local phase angles.
** The client then sends all the prepared qubits in the respective order to the server so the graph state can be constructed.

* '''Server's Preparation''': The server receives the qubits in a sequential order of rows and columns till all <math>m</math> qubits are received. The server then entangles them according to the cylinder brickwork state (using CZ gate).

* '''Interaction and Measurement''': This step is exactly the same as for [[Prepare-and-Send Universal Blind Quantum Computation]].
** For a specific computation, MBQC decides which measurement angle is selected along with some extra Pauli X, Z corrections for every qubit. The correction sets are unique for every graph state and depend on the previous measurement. These can obtained from '''[[Supplementary Information#Flow Construction-Determinism|flow construction]]'''. The qubits have a randomly chosen local phase angle and hence the same local phase angle is used for computation as well as for output correction. To hide the state, a randomly chosen <math>\pi</math> rotation which may or may not be added. From all the above-mentioned conditions, a final measurement angle is formed and the client sends a classical message to the server to inform the server about the final measurement basis (in (X, Y) plane) in which they should measure the corresponding qubit. Thus it reveals no information about the underlying computation.
** The server sends the classical output of each non-input qubit's measurement to the client. The client considers the <math>\pi</math> rotation to get the corrected output. The client also uses this to calculate the measurement angle for the next qubit and thus repeats the process until the last output qubits are reached.

* '''Verification''': The verification is carried on by the client by comparing the outcome of the trap qubit measurements with the expected outcome.
**'''For Quantum outputs''':
*** The server sends all the output qubits to the client.
*** From these output qubits, the client performs a measurement on the trap qubit. If the output is equal to the expected outcome, the computation is verified. Otherwise, it is rejected.
*** If the computation is accepted, output correction is performed on the other output qubits (except the trap qubit).
**'''For Classical outputs''':
*** The server continues performing measurements on the output qubits with the measurement angles sent by the server.
*** The client compares the output of the trap qubit with the expected output. If it is equal, computation is verified. Otherwise, it is rejected. If the computation is accepted, the client accepts the other output measurement results as the computation result.