An Overview for Hybrid Algorithm and Its Architecture

admin — Mon, 09 May 2022 20:01:38 +0000

What is a Hybrid Algorithm?

While you were skimming over quantum programming, you’ll probably heard of “hybrid quantum-classical computing”. Hybrid quantum-classical computing is a hot topic among quantum computing enthusiasts and it typically refers to a set of programs that runs on a quantum processor. Programs such as variational quantum algorithms and quantum approximate optimization algorithms.

But what does “hybrid” really mean?

In layman terms, “hybrid” means a program that runs both classical and quantum code. Quantum code is defined as a sequence of quantum gates that is applied to one or multiple qubits on a quantum device or quantum processing unit i.e. QPU. “Classical” refers to a program that runs on a regular computer, written in available programming language. Typically anyone can choose to easily interface with quantum computing services.

Hybrid quantum computing is the preferred industry term for a simple idea: “A quantum computer and a classical computer working together to solve a problem.” It is an approach that takes the form of a back-and-forth collaboration where different aspects of a problem are passed between the quantum and classical tools best suited for each stage. According to scientists, hybrid quantum-classical algorithms are maybe one of the best ways for users to get the most out of a current or near-term quantum computer. Keeping in mind the edge existing classical systems possess in terms of hardware, there are a lot of things in which classical computers are better, or faster at. By letting the classical computer do what it’s good at and quantum computers do what it’s good at, could be a better solution.

Hybrid architecture for quantum algorithms:

The gate-based quantum computers are not universal. On the other hand, global unitary operations like the shift operator cannot be expressed within the circuit model, cannot be equally applied to machines with unlimited and limited memory and cannot be assumed to be equally available on different quantum hardware architectures.

To overcome the above restrictions, quantum programming uses a classical universal language to define the actual sequence of elementary instructions for a quantum computer, so a program is not intended to run on a quantum computer itself, but on a (probabilistic) classical computer, which in turn controls a quantum computer and processes the results of measurements. In the terms of classical computer science, you can describe this setting as a universal computer with a quantum oracle. Figure 1 shows the hybrid architecture.

Quantum algorithms such as Shor’s algorithm are in two separate parts: First part is classical algorithm which can be done on a classical computer and second part is Quantum algorithm which can be done on a quantum computer or can be simulated on classical computer.

Fig 1. The hybrid architecture between classical and quantum computers.

Naturally quantum algorithms are the hybrid algorithms that consist of a classical and a quantum component. However, the quantum part of the manu algorithm is probabilistic, often they need multiple runs to get desired result. The complete cycle of the hybrid architecture for the quantum algorithms will be done as follows:

Pre-calculate certain classical factors (initialize and run the classical part of the algorithm)
Running the quantum algorithm by the quantum circuit
- Initialize the quantum node (Initialize quantum circuit and define all gates, switches and unitary function)
- Prepare inputs state (store inputs on target and control registers)
- Execute the quantum portion of the algorithm (Apply gates and unitary transformation on input data)
- Measure the output of Machine State (Measure the output registers of the quantum circuit)
- Evaluate Measurement (If have the desired result, then doing post-processing in step 3)
- Exit if desired result (If solution found then exit from quantum circuit, else repeat step 2)
Finish post-processing (Run the second classical part of the algorithm)

Fig 2.

Steps 1 and 3 were executed on classical computers and step 2 was executed on quantum computers by quantum circuits. Measuring and evaluating the quantum circuit in steps 2(e) and 2(f) can be done on classical computers. The diagram in Fig. 3 shows the development of a general plan of hybrid architecture for the quantum algorithms and being simulated on classical computers. The quantum circuit is simulated on a classical computer.

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Quantum Algorithms and its Advantages

admin — Thu, 24 Mar 2022 17:31:21 +0000

We have talked about quantum concepts and its applications in our previous post. Here we will be talking about applications enablement through quantum procedures, also known as quantum algorithms. We are going to write here about the workings of quantum. In computer programming, an algorithm is a set of well-defined instructions to solve a particular problem. It takes a set of input and produces a desired output. Quantum algorithms are a specific procedure for solving computational problems. These are different from protocols because they are a set of standard rules that allow multiple devices to communicate. The advantages of an effective quantum algorithm are as follows:

Accuracy – solve a problem correctly
Efficiency – solve a problem as fast as possible

Clearly the primary concern for quantum algorithms is its speed of execution. While it is important to solve every problem correctly, the long pole for the quantum algorithm is its efficiency. There is a well defined metric to measure the efficiency of the particular algorithm and it’s called Big-O-Notation. Big-O-Notation is a mathematical notation of the worst case performance of the algorithm relative to the input size (n). It is fundamentally different from runtime complexities because that later is inherently tied to hardware performance.

Using the above criteria, algorithm efficiency is represented in Big-O-Notation in the form of O(f(n)). Here O stands for Order and f(n) is a function of “n” to determine the number of operations that can be done as the input size fluctuates. Figure 1 lists some common functions of input size represented in Big-O Notation.

The main goal of quantum algorithms designers is to create algorithms that can perform any computation exponentially more efficiently on quantum computers than on conventional computers. This can be done by exploiting quantum properties such as superposition, quantum entanglement and many more. Unsurprisingly, some of the quantum algorithms have already achieved this goal. Some of the example of quantum algorithms is as follows:

The Deutsch Jozsa algorithm is a good example. It is the first algorithm that shows the separation between the quantum and classical difficulty of a problem. This algorithm demonstrates the significance of allowing quantum amplitudes to take both positive and negative values, as opposed to classical probabilities that are always non-negative
Shor’s algorithm is used for factoring integers in polynomial time. The computing ability of quantum computers could factor complex RSA encryption thereby undermining the global financial system. Shor’s Algorithm gave credence to the fact that quantum computers could have unforeseen economic impact
Grover’s algorithm is another complex implementation that enables searching of an unordered list in a square root n time. Every element is scanned in an unordered list and a quantum computer could achieve this in a function order of square root

Many algorithms have been developed using quantum concepts, but not all can comply with the requirements of accuracy and efficiency – the two pillars of quantum algorithms. Therefore, not every quantum algorithm is better than classical algorithms. So, a novel approach is to devise hybrid algorithms to take advantage of the best of both worlds of classical and quantum computing. Hybrid algorithms apply concepts of both quantum and classical computing to perform higher computations and obtain better results than on one or the other. It is assumed that hybrid algorithms will become more prevalent in future as they prudently employ both quantum and classical computational power.

We will write on hybrid algorithms soon.

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An Overview for Hybrid Algorithm and Its Architecture

What is a Hybrid Algorithm?

Hybrid architecture for quantum algorithms:

Quantum Algorithms and its Advantages