After Sending the Circuit?-?Quantum Computing
After sending a quantum circuit to its hardware, it goes through four steps: transpliation, gates as pulses, readout, and display. Each step is crucial to make sure your circuit gets returned back to you with adequate results.
Transpilation
This first step converts gates or translates them into the gates that can be implemented on the specified hardware you are using. These are also known as the basis gates. Without this step, the hardware will not know what the gate is supposed to do. For instance, IBM’s Belem quantum system has the basis gates CX, ID, RZ, SX, and X.
Gates as?Pulses
This step is to convert the gates to a sequence of energy pulses, in which different gates are implemented as pulses of different duration and energy. IBM, in fact, allows developers to create circuits at this level with their Qiskit Pulse library. This allows generating greater performance from quantum hardware.
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Readout &?Display
Next, is to measure this circuit. In this step, the quantum circuit as energy pulses is measured, and a final state is determined for each of the qubits. After which, these results are sent back to the developer to analyze the results.
Now comes the question, that if these steps are in place to return the correct results, why are they not always correct. Unfortunately, environments are noisy due to heat, radiation, stray electromagnetic field, mechanical vibrations, and more. This noise can cause qubit relaxation and decoherence.
Firstly, relaxation is a property of any high energy state system that wants to generally be at its lowest energy or its relaxed state. In qubits, this is no different. Qubits want to be in the lowest energy state, causing them to relax from the 1 state to the 0 state. On quantum hardware, this is also known as T1. Secondly, decoherence, or T2, is the loss of quantum information stored in qubit states due to noise. What makes this more threatening to running quantum circuits is that the qubit will lose its phase information as well as its general state.?
Due to these two properties of qubits, it is important to maintain a high gate fidelity to make sure the gates we apply to quantum circuits are reliable and are not causing unnecessary errors.
Other errors can be caused by measuring the quantum circuit. Since the quantum circuit has to temporarily open qubits to the environment, they are more prone to errors. Fortunately, there are several methods to try to prevent these errors. One popular method is through quantum error correction, which in one implementation uses repetition codes to essentially have repeated qubits in the circuit and check if they all match.
In the future, quantum computers will get closer to having many logical qubits, which are each a collection of error-prone physical qubits together to reduce the effect of noise. This will help to create fault-tolerance, the goal of quantum hardware.