Physical quantum implementation

Question 1

What are the Di Vincenzo criteria for (don’t name them)

Answer 1

Proposed necessary conditions for constructing a quantum computer. The first 5 pertain to quantum computation, whereas the latter 2 are for quantum communication.

Some are contradictory, such as maintaining a long decoherence time requires isolation from the environment. but 3 other criteria necessitate interaction with qubits which means they are not isolated

Question 2

What are the 7 di Vincenzo criteria

Answer 2

1) Physical system must be well understood and scalable to implement multiple well-characterized qubits representing at least (preferably exactly) 2 states

2) Be able to initialize all qubits to simple fiducial state e.g. |00…0>, other states can be created if universal set of unitary operators is available

3) Long decoherence time, must be longer than total time taken for qubits to pass through gates and be measured

4) Universal set of unitary gates must be available

5) Qubit specific measurement capability

6) Be able to convert between flying qubits (qubits in process of being sent from area A -> B) and stationary qubits (those present in quantum computer)

7) Be able to faithfully transmit flying qubits between specified locations

Question 3

How are some of the di Vincenzo criteria in contradiction

Answer 3

Long decoherence times require isolation from outside environment. But to initalising qubits, implementing universal set of quantum gates or measuring the qubits requires interaction

Question 4

What is trapped Ion quantum computing, and what are it’s advantage / disadvantages

Answer 4

-Uses charged particles e.g. electrons and positrons trapped in magnetic fields and shuttled by them
-State preparation is done via laser
-Quantum gates implemented by microwaves or lazers
-2 qubits gates utilise the fact two ions in motion affects neighbours

Advantages:
-long decoherence times
-High fidelity of quantum gates
-All ions are same particle (same manufacturing)
-small number of states, less error

Disadvantages:
-requires perfect vacuum at low temperatures
-not many qubits (max 20)
-ions may be lost over long periods
-gate speeds are slow

Question 5

What are superconductubng quantum computers, and what are it’s advantage / disadvantages

Answer 5

solid state electrical circuits cooled to near 0k for superconductivity

advantages:
-qubits have many parameters
-easy to facilitate multiqubit interactions
-manufacturing solid state chips is feasible at scale
-qubits controlled by microwaves

disadvantages:
-short coherence times
-low temp required for operation, building at scale not too feasible

Question 6

What is photonic Ion quantum computing, and what are it’s advantage / disadvantages

Answer 6

Uses polarized photons as qubits. It is not gate based but rather uses Gaussian Boson Sampling so universal unitary gates

advantages:
-photons don’t interact with environment so it can run in regular conditions
-photons easily used for communication via optical fibres and through air
-operations on photon can be done at high clock speeds (low gate time)

disadvantages:
-photons don’t interact with each other so two qubit entangled states are very hard to achieve
-optical components must be arranged along their path sequentially (due to their light speed movement) -> large scale QC would be massive
-Photonic circuits are usually purpose built and non-programmable (not universal)