Quantum Information Technology

Quantum information technology (QIT) is a rapidly advancing field that leverages quantum mechanics to manipulate information in novel ways, leading to vast improvements over classical technologies. QIT exploits quantum phenomena like superposition and quantum entanglement to overcome the challenges of information storage, processing, transmission, and security. The field of QIT encompasses a spectrum of applications that leverage quantum systems, including quantum computing, quantum sensing, and quantum communications.

We are on the brink of an information technology (IT) revolution enabled by quantum mechanics. Achieving it will require collaborative research to address the challenges inherent in trying to control the properties of the quantum realm. Every breakthrough deepens the understanding of quantum properties and leads to new architectures and innovations that bring us closer to achieving quantum supremacy and commercially viable applications.

Qubits

Quantum bits, or qubits, are the building blocks of QIT. They are the most fundamental units of quantum information. Classical data bits exist in two states, written as 0 and 1. While they can take many forms and perform a vast array of functions, a bit must always be in either a 0 state or a 1 state.

If you picture a sphere, the surface represents all of the possible states of a qubit. Classical bits are like points located only at the poles of the sphere, fixed in either the 0 or 1 state. Qubits can occupy any point along the sphere’s surface, representing an infinite number of possible states. This freedom allows qubits to encode and process exponentially more information than classical bits do.

Qubits do this through superposition and entanglement. Superposition supplies their remarkable flexibility and power. Entanglement enables qubits to become correlated in such a way that the state of one qubit instantly influences another, regardless of the distance between them. These unique properties shatter the constraints of classical bits and allow qubits to exist in a rich, complex state space. They can encode and process quantum information in ways that promise meaningful advancements in computation, communication, and cryptography.

Superposition

Going back to the sphere analogy, superposition lets qubits carry information between the 0 and 1 states at the same time. In classical physics, objects have definitive properties at any given time. In quantum physics, however, particles like electrons, photons, and atoms can exist in a combination of states simultaneously, known as superposition.

Schrödinger’s cat is a well-known thought experiment that describes how the quantum superposition of non-decayed and decayed radioactive nuclei can influence superposition on a macroscopic level. In this gruesome scenario, a scientist places a cat in a box with a radioactive mechanism that controls a vial of poison. The moment of the poison’s release is unknown; thus, the cat occupies a state of both life and death until we open the box and observe its fate.

Quantum mechanics cannot predict the fate of Schrodinger’s cat. Instead, it provides us with the probability of which state we will find the cat in when we open the box across a large set of identical experiments.

Superposition is what enables quantum bits (qubits) to perform highly complex functions by representing multiple states simultaneously. Going back to the sphere analogy, superposition states have amplitudes and enable qubits to carry information between the 0 and 1 states at the same time. As the number of qubits increases, the number of available probability amplitudes increases exponentially. It is this property that gives quantum technology the probability to dramatically accelerate the speed of computational tasks relative to their classical counterparts.

Quantum entanglement

Entanglement is the key to harnessing quantum information processing. Quantum entanglement occurs when the states of two or more particles become correlated such that the state of one particle instantaneously influences the state of another, irrespective of the distance between them.

This correlation persists when the entangled particles separate, even by long distances. Without entanglement, each qubit exists independently on the sphere, in its own world with its own possible states. When qubits become entangled, however, their positions become intricately linked, enabling them to explore a broader range of states than they could individually.

The coordinate movements of entangled qubits on the surface of the sphere maximize the flexibility enabled by superposition. If you perform a measurement on one entangled qubit and collapse it to a specific state, the state of the other qubit is determined instantaneously as well, regardless of where on the sphere it resides. Entanglement enables qubits to leverage their collective quantum nature to unlock the potential for highly efficient computation, secure communication, and novel cryptographic protocols.

This graph depicts the two observers, Alice and Bob, sharing an entangled state.

Quantum decoherence

While the previously explained properties enable qubits to hold much more computational power than classical bits, a big challenge for quantum computers is a phenomenon called quantum decoherence.

Quantum decoherence occurs when a system interacts with elements in the environment, such as air, thermal radiation, or a non-demolition measurement apparatus. To understand decoherence, you must first understand quantum coherence. In its simplest form, coherence refers to the measure of the relationship between different possible states within the quantum system. This information is necessary for the quantum computer to function. Isolation maintains coherence.

Quantum decoherence occurs when the system interacts with its internal and external environment. When this happens, the system’s information and the environmental information comingle, making it difficult to distinguish quantum system signals from environmental noise. Qubit states are unstable, and different environmental factors can affect the state of a qubit in an unknown way, similar to noise in scientific measurements. A stray photon or a fluctuation in temperature can cause an energy shift in the qubit. As a result, the state moves closer to collapse.

Quantum interference

In addition to the influence of environmental factors, qubits in an entangled system can affect the state of other qubits. Quantum interference occurs when the probability amplitudes of different states a qubit can take interfere with each other, causing either constructive or destructive interference effects. Each trajectory corresponds to a different sequence of operations applied to the qubit.

Constructive interference occurs when the probability amplitudes of trajectories align and reinforce each other, increasing the likelihood of finding a qubit in certain states. Destructive interference occurs when the trajectories’ probability amplitudes cancel each other out, reducing the probability of finding the qubit in certain states.

Quantum interference plays a crucial role in quantum information processing. By manipulating the quantum states of qubits and controlling the interference effects, quantum algorithms can exploit interference to enhance computational efficiency and solve problems considered intractable for classical systems.

Quantum error correction

In order to combat quantum decoherence and noise in the system, quantum computers use entanglement and interference to help correct errors. Classical error correction is easier to work with. A bit can be only 0 or 1, and an error can only be a bit flip. To combat this, classical computers encode their bits redundantly. Instead of using 1 bit to represent 0 or 1, they use multiple, such as 000 or 111. An error might look like 010, where we can assume that this means 0, and the middle bit just had an error. This also works because bit flips in classical bits are rare.

Qubits are different. While their state can deviate from expectations, we are unable to observe the shift because of the measurement collapse that would result from observations. The way we work with this is similar to classical bits in that we encode the qubits redundantly but put them in an entangled state with other ancillary qubits. In the entangled state, each qubit’s state connects to the other qubits’ states. This way, we can extract information from an ancillary qubit and infer the state shift in the computational qubit and thus correct it.