Quantum Computing : Hardware at High Level
Quantum Computers have been around for more than a decade now, and we, as a general audience is getting to know more and more about them. An ever-increasing number of images are coming up, and we wonder what they consist of and why they are challenging to develop and maintain.
They are still a mystery for most of the part because the front runner companies have decided to keep their recipes secret. But what we know is that they follow a typical architecture that consists of 4 major components.
- Classical Computer and Control Processor
- Cooling and Vacuum Casing
- Control and Measurement Plane
- Qubit Assembly
In this article, we will look at these four major components and their responsibilities in the assembly at a very high level. We will start at the most outer layer and analyze different components as we zoom in.
Let’s begin!
Classical Computer and Control Processor
Although quantum computers are foretold to dominate classical computers, they are still in their infancy and do not provide everything we need to create a fully quantum system. Quantum computers can help with the computation, but they do not provide networking, storage, display, etc. For all this, we still depend on classical computers.
The classical computer is placed outside the shielding of the quantum assembly. It is usually running a conventional operating system with software and libraries required to operate a quantum system. The user interacts with the quantum system using the standard computer peripherals of this classical computer. It referred to as the host processor.
Many quantum computers allow cloud access and need to connect to existing networking infrastructure; hence, classical computers furnish the system’s networking capabilities. The classical computer also contains the whole system’s storage, including the storage required during the execution of a quantum program.
The other component of a quantum system is a control processor. A control processor could be part of the classical computer or could be a separate unit. It does two essential tasks: error correction and converting quantum programs into quantum interpretations that control, and measurement planes can execute.
Because of the way quantum computers operate and are implemented, the occurrence of errors is unavoidable. Also, as quantum computers are scaled up and more qubits are added, it is likely to increase the error rate. Therefore control processor performs error correction on the results from the quantum assembly.
Next, the quantum algorithms are written in programming languages such as Python, Q#, etc. they are yet to be converted to quantum operations. Hence, the control processor is responsible for taking the compiled code and transforming it into low-level commands to be processed by the control and measurement plane. They also act as accelerators by simplifying the execution of some of the quantum applications.
Cooling and Vacuum Casing
When we talk about qubits, we talk about particles at the atomic level, and these particles are fragile and sensitive to their surroundings. Even a small uncontrolled change in the environment, called interference or noise, can randomly change the qubit’s state, resulting in corrupt outcomes.
Fundamentally, these interferences or noises are forms of extra energy. This unwanted energy might come from the heat exchange with the surrounding atoms. The solution is to make the quantum system very cold. Here, the aim is to get the temperature as close as possible to the absolute zero (0 K / -273.15 C / -459.67 F). At this temperature, the atoms have exhausted all their heat, and hence there is no more heat to exchange.
Note that the cooling unit aims to achieve absolute zero; in reality, that temperature is a few decimal degrees above the absolute zero. Therefore the atoms are still able to exchange some heat. Also, the use of shielding does not nullify the effect of vibrations completely. These issues result in the occurrence of errors.
Next, apart from thermal energy, other forms of energy, such as electromagnetic energy and vibrational energy, create interference and stops qubit from stabilizing. To overcome this, the quantum systems are put in a vacuum casing, which minimizes interference from the external world.
Different quantum systems have varying implementations for cooling and vacuum casing, but none can completely nullify the effect of interferences from surrounding. This is a big hurdle in implementing large quantum computers and hence a significant research area.
Control and Measurement Plane
The control and measurement plane is the component that contains all the circuitry required to operate on qubits and carry out quantum operations. This is the only component in the quantum assembly that directly interacts with the qubits.
The control and measurement plane receives the quantum operations in digital signals from the control processor. It converts them into analog operations to be performed on the qubits. Depending on the type of qubit control plane can operate on qubits by various methods such as microwaves, lasers, voltage, etc. It also reads the analog output from the individual qubits and converts them into binary data, which gets transmitted back to the control processor.
Because of the tiny size of qubits, the control and measurement equipment have to be precise and operate in a minimal area. Imperfections in the separation of qubits lead to the introduction of small errors. Also, the irregularities in shielding affect analog signals. Therefore a few quantum computers have some error correction mechanisms embedded in the control and measurement plane.
Generally, quantum computers have more than one qubit; therefore, those qubits have to be connected to exchange information. The control and measurement plane provide this communication channel. Along with this, many operations are performed on the values obtained from qubits; hence it also contains the gates, switches, memory, and other circuitry to execute quantum algorithms.
You can think of the control and measurement plane as a grid, where each intersection is connected to a qubit and has the equipment to operate and read qubits. Between each intersection, there is circuitry that performs operations on values obtained from the qubits.
Qubit Assembly
This is the elephant in the room. This is where all the qubits live along with the necessary assembly to hold them in place and perform analog operations.
Since the discovery of Shor’s algorithm rigorous efforts were put into the physical implementation of qubits. In recent years two technologies have shown promising developments, trapped-ion qubits, and superconducting qubits. There are ongoing efforts on other approaches, photonic qubits, neutral atom qubits, semiconductor qubits, topological qubits are some of those candidates.
A trapped-ion qubit consists of an ion and an electromagnetic trap to hold that ion. The qubit could be operated by the use of either laser or microwave. The equipment for these lives on the control and measurement plane and photon detectors measure the state of the qubit by detecting the photons the ions scatter. These equipment have to be precise not to affect the quantum state of ions other than the one intended.
A superconducting qubit is made up of an inductor, a capacitor, and a non-linear inductive element called Josephson junction. The inductor and capacitor create a superconducting resonator, and with the help of Josephson junction to differentiate distinct energy levels, it forms an atom, this atom is the qubit. The atoms are called artificial atoms because, unlike the natural atoms, which are all identical, artificial atoms are the product of circuit elements; therefore, manufacturing variations induce different properties in them.
Emphasizing again that this is a basic architecture, the specifics are going to vary per each implementation. The architecture itself is evolving as a result of increasing efforts into the research. Quantum computers are yet to prove all the promises it made. Big industry players see quantum computers as a massive market in the future, but right now, we are far from there and even more to reach the general public. Till then, we rely on our classical computers to make this world a better place.
References and Further Readings
- The National Academies Press : Essential Hardware Components of a Quantum Computer
- Illinois Science Council : A DIY Guide to Building a Quantum Computer
- D-Wave : Introduction to the D-Wave Quantum Hardware
- Hackaday : Quantum Computing Hardware Teardown
- Microsoft : Quantum computers and Quantum Simulators Overview
- IBM : What is Quantum Computing?
