Inside a quantum computer

Quantum computing has quantum bits called qubits (pronounced cue-bit) as their fundamental unit. In the classical computing world, bits take 0 and 1 states. Qubits exist in these two states, but also in a linear combination of both these states called superpositions.

Superpositions can solve some problems faster than the deterministic and probabilistic algorithms that we commonly use today. A key technical difference is that while probabilities must be positive (or zero), the weights in a superposition can be positive, negative, or even complex numbers.

The other important quantum mechanics principle that is fundamental to understanding quantum computers is Entanglement. Two particles are said to display entanglement if one of the two entangled particles behaves randomly and informs the observer how the other particle would act if a similar observation were made on it.

This property can be detected only when the two observers compare notes. The property of entanglement gives quantum computers extra processing powers and allows them to perform much faster than classical computers.

Quantum computers have similarities and differences compared to traditional transistors that classical computers use. Research in quantum computers is moving forward to find new forms of qubits and algorithms. For example, optical quantum computers using photons have seen significant progress in the research world since 2017. Optical quantum computers using photonic qubits work at room temperatures.

A quantum computer should satisfy the following requirements:

• Qubits need to be put into a superposition
• Qubits should be able to interact with each other
• Qubits should be able to store data and allow readout of the data

Quantum computers also demonstrate some features (typically):

• Tend to operate at low temperatures, and are very sensitive to environment/noise
• Tend to have short lifetimes – the reasons are explained below

We encode qubit states into subatomic particles; electrons in the case of semiconductor quantum computers. There are several methods to create qubits and each method has advantages and disadvantages. The most common and stable type of qubits is created using a superconducting loop. A superconductor is different from a normal conductor because there is no energy dissipation (no resistance) as the current passes through the conductor. Superconductor circuits operate at close to absolute zero temperatures (that is, 0 Kelvin, or -273 degree Celsius) in order to maintain the states of their electrons.

Another qubit architecture where transistor-based classical circuits are used is called SQUIDs. SQUID stands for Superconducting Quantum Interference Device. They are used to track and measure weak signals. These signals need to only create changes in energy levels as much as 100 billion times weaker than the energy needed to move a compass needle. They are made of Josephson junctions. One of the key application areas for SQUIDs is in measuring magnetic fields for human brain imaging. Source: https://whatis.techtarget.com/definition/superconducting-quantum-interference-device

Superconducting qubits (in the form of SQUIDs) have pairs of electrons called Cooper pairs as their charge carriers. In this architecture, transistor-based classical circuits use voltage to manage electron behavior. In addition, a quantum electrical circuit is defined by a wave function. SQUIDs are termed artificial atoms, and in order to change the state of these atoms, lasers are used. As described earlier in this chapter, based on the principles of quantum mechanics, only light with specific frequency can change the state of subatomic particles. Therefore, lasers used to change the state of qubits will have to be tuned to the transition frequency of the qubits.

A superconducting qubit can be constructed from a simple circuit consisting of a capacitor, an inductor, and a microwave source to set the qubit in superposition. However, there are several improvements of this simple design, and the addition of a Josephson junction in the place of a common inductor is a major upgrade. Josephson junctions are non-linear inductors allowing the selection of the two lowest-energy levels from the non-equally spaced energy spectrum. These two levels form a qubit for quantum-information processing. This is an important criterion in the design of qubit circuits – a selection of the two lowest energy levels. Without the Josephson junction, the energy levels are equally spaced, and that is not practical for qubits. Source: https://web.physics.ucsb.edu/~martinisgroup/classnotes/finland/LesHouchesJunctionPhysics.pdf

Like the gate concept in classical computers, quantum computers also have gates. However, a quantum gate is reversible. A common quantum gate is the Hadamard (H) gate that acts on a single qubit and triggers the transition from its base state to a superposition.

Qubit types and properties

There are several variations of qubit circuits based on the properties here. The key properties that need consideration in the design of these circuits are:

• Pulse time: This is the time taken to put a qubit into superposition. The lower the pulse time, the better.
• Dephasing time: This is the time taken to decouple qubits from unwanted noise. The lower the dephasing time, the better. Higher dephasing times lead to a higher dissipation of information.
• Error per gate: As gates are used to create a transition in states of qubits when there is a faulty gate, the error can propagate onto qubits that were originally correct. Hence, error per gate needs to be measured regularly.
• Decoherence time: This is the time duration for which the state of the qubit can be maintained. Ionic qubits are the best for coherence times as they are known to hold state for several minutes.
• Sensitivity to environment: While semiconductor qubits operate in very low temperatures, the sensitivity of the particles involved in the construction of the circuit to the environment is important. If the circuit is sensitive to the environment, the information stored in the qubit is corrupted easily.

Figure 3: Qubit circuits

IBM recently launched the 50-qubit machine, and also provides a cloud-hosted quantum infrastructure that programmers can go and code in. There are also several advances in quantum assembly language that will act as the interface between these machines and the code that developers write. Figure 3 shows different qubit circuit types.

We've now covered the fundamentals of quantum computing, so let's move on to look at the other technology in focus for this book: Blockchain.