An Introduction to Quantum Computing
What is different in quantum computers?
The modern processors of classical computers are made of transistors. The basic building block of modern
computers is the transistor. We use the idea of an electrical voltage to ‘encode’ bits of information into these physical devices. Unlike this superconducting Qubit (SQUID), is the basic building block of a quantum computer. The name SQUID comes from the phrase Superconducting Quantum Interference Device. Interference is electrons behaving as waves inside quantum waves, which give rise to the quantum effects. SQUIDs are made of a metal called niobium, when this is cooled down, it becomes what is known as a superconductor, and it starts to exhibit quantum mechanical effects.
The superconducting qubit structure encodes 2 states as tiny magnetic fields, which either point up or down. We call these states +1 and -1. We can control this object so that we can put the qubit into a superposition of these two states.
Couplers are used to connect multiple single qubits and create a multi-qubit processor. The couplers are also made from superconducting loops. By putting many such elements together, we can start to build up a fabric of quantum devices that are programmable. Qubits and couplers are surrounded by a framework of switches which addresses each qubit and stores that information in a magnetic memory element local to each device. Switches are formed from Josephson junctions. The majority of the Josephson junctions in a D-Wave processor are used to make up this circuitry. Additionally, there are readout devices attached to each qubit. During the computation these devices are inactive and do not affect the qubits’ behavior. After the computation has finished, and the qubits have settled into their final (classical) 0 or 1 states, the readouts are used to query the value held by each qubit and return the answer as a bit string of 0’s and 1’s to the end user.
Architecture of a quantum processor
The architecture of quantum processors are very different from conventional computers. The processor has no large areas of memory (cache), rather each qubit has a tiny piece of memory of its own. In fact, the chip is architected more like a biological brain than the common ‘Von Neumann’ architecture of a conventional silicon processor. One can think of the qubits as being like neurons, and the couplers as being like synapses that control the flow of information between those neurons.
D-Wave is one of the companies that builds quantum computers. The largest processor has 128 qubits connected together with 352 connection elements between them. It has a superconducting fabrication facility capable of yielding superconducting processors of large complexity.
In D-Wave, to control the behaviors of electrons in the quantum level the temperature of the computing environment has to be lowered below approximately 80mK, generally performance increases as temperature is lowered. The processor operates near-absolute zero temperatures. To reach this temperature refrigerators that use liquid Helium as a coolant are used.
Key components of the I/O subsystem include the processor mount and wirebonding to it. Much of the subsystem must be made using superconducting metals, such as tin, which are typically non-standard for manufacturers. None of the materials close to the processor can be magnetic.
The way that D-Wave programs quantum processors is using a cloud-based model. This means that the systems can be programmed remotely from any location with an internet connection.
For the past 8 years, the number of qubits on D-Wave’s processors has been steadily doubling each year. To go beyond ten thousand into hundreds of thousands or millions of qubits will require processor redesign, but there are certainly ways in which this can be achieved and it is not seen as a fundamental obstacle to improving the hardware.
