You may have wondered where the term Qubit comes from. Or maybe not. But its origins are pretty logical. We walk through the history of the development of Qubits. Quite literally a work in progress as scientists from around the globe develop Quantum Computers, and there is no single technology as yet. The principles and the origin of the Qubit are exciting and not always covered in reading material, but here we’ll start with the basics.
Before Qubit, The Bit
The term “bit” is fundamental in computing and digital communications. The word itself is a portmanteau derived from the phrase “binary digit.” A bit represents a basic unit of information in computing and digital communications, characterized by a binary nature, meaning it has two possible states: traditionally represented as 0 and 1.
The concept of binary digits was discussed in a 1936 paper by the mathematician and computer scientist Alan Turing (famous for his work at Bletchley Park), who referred to them as “binary units.” However, the term “bit” in the context of information theory was first introduced by Claude Shannon in his landmark 1948 paper, “A Mathematical Theory of Communication.” In this work, which laid the groundwork for the field of information theory, Shannon used the term “bit” to denote a binary decision or digit.
The concept of the “bit” has been fundamental to developing digital electronics and computing, serving as the foundational building block of digital data.
Qubit: The Quantum Bit
A “qubit” is a short term for “quantum bit,” a quantum computer’s fundamental unit of quantum information. Unlike classical bits, which are either in a state of 0 or 1, qubits are governed by the principles of quantum mechanics, which allow them to exist in multiple states simultaneously. This property is known as superposition. Additionally, qubits can be entangled, a quantum phenomenon where the quantum states of different qubits are interdependent regardless of the distance separating them. These two properties – superposition and entanglement – give quantum computers their potential power.
Properties of the Qubit
The deterministic nature of bits is a defining feature of classical computing. When you input data and instructions, you can predict the exact state of each bit at any point in the processing sequence. Operations in classical computing are clear-cut, meaning that given an input and a process, the outcome is always the same, ensuring consistency and reliability in computing tasks. This completely contrasts the behavior of a Qubit, which can exist in a state known as a superposition. That gives the Qubit some pretty unique properties, giving it certain behaviors that programmers can take advantage of. It doesn’t stop there, too; Multiple Qubits can be entangled, which is another property that programmers can exploit in their Quantum circuits. Qubits operate in the realm of quantum mechanics, granting them abilities that are both powerful and challenging to harness as of current understanding.
Measuring a Qubit isn’t deterministic. The state can vary and is essentially probabilistic, and measuring a Qubit is a destructive process that generally destroys the state of the Qubit. So unlike traditional 1’s and 0’s that can be read non=destrucivgely, quantum programmers must be smart about interacting with Qubits.
Physical Qubits
Classical bits can be stored in any medium, from CDs (Compact Discs) to silicon devices or even paper (historically punch cards and paper tape). The concept remains the same. There are just two classical states. But a Qubit can exist in the superposition of states and, therefore, requires a medium that can allow this is crucial. Qubits must also be physically realized to perform quantum computing. However, creating a physical qubit that maintains quantum properties long enough for computation is a significant challenge due to the fragile nature of quantum states.
Below, we detail how some types of physical qubits are created and manipulated, along with the companies investing and developing each technology.
Superconducting Qubits
Superconducting qubits, often used in circuits called Josephson junctions, leverage the quantum properties of superconducting materials. These materials, usually alloys or compounds, exhibit zero electrical resistance and expulsion of magnetic fields at very low temperatures. The circuits allow current to flow unimpeded, enabling the electrons to behave coherently and occupy quantum states.
The creation of superconducting qubits involves fabricating nanoscale circuits on a silicon or sapphire substrate, often using techniques similar to those in the semiconductor industry. The circuit includes a Josephson junction, which consists of a thin insulator sandwiched between two superconductors. When cooled to temperatures near absolute zero, the current flows without resistance and the circuit can exhibit quantum behaviors, representing a qubit. These qubits are manipulated via microwave pulses that control their quantum states, allowing for operations necessary for quantum computing.
One challenge of superconducting qubits is their sensitivity to external disruptions, leading to a phenomenon known as quantum decoherence. Researchers isolate the qubits and use error correction algorithms to work around this. Despite these challenges, superconducting qubits are among the most promising for scalable quantum computing due to their strong resemblance to traditional computing components. Companies using this approach include IBM, Google, and Rigetti Computing.
Trapped Ion Qubits
Trapped ion qubits employ individual ions as quantum bits. These ions are charged atoms that are isolated in a vacuum and manipulated using electromagnetic fields. The ions’ electron states form the basis for the qubit’s state, with different energy levels representing the binary information.
To create a trapped ion qubit, ions are first isolated in a trap created by electromagnetic fields. This process involves using lasers and magnetic fields to cool atoms until they form ions and then trapping them in a field. The quantum state of these ions is manipulated using laser pulses, which can excite the ions to higher energy levels or cause them to emit photons and drop to lower energy levels, effectively allowing the ions to represent the 0s and 1s of binary code.
One of the advantages of trapped ion qubits is their long coherence times, the duration over which a qubit can maintain its quantum state. However, scaling up systems of trapped ions is challenging due to the complexity of the ion traps and the need for precise control of the lasers that manipulate the ions’ states. Companies pursuing the trapped ion approach include IonQ, Honeywell, and AQT.
Quantum Dot Qubits
Quantum dots are nanoscale semiconductor particles that can trap electrons in a way that allows their quantum states to be manipulated. These quantum dots are sometimes called “artificial atoms” because they can trap electrons in discrete energy levels, similar to how electrons occupy energy levels in atoms.
The fabrication of quantum dots involves advanced nanotechnology techniques. Materials like semiconductor heterostructures are used, where layers of different semiconductors are laid atop one another. Electrons can be confined in all three spatial dimensions by applying voltages using nanoscale electrodes, creating a zero-dimensional quantum dot. The spin of an electron in a quantum dot can be used as a qubit. The electron spins are manipulated using electric or magnetic fields to perform quantum operations.
While quantum dot qubits offer advantages in potentially operating at higher temperatures than superconducting qubits and being made with well-established semiconductor technology, they pose challenges in isolating and controlling single electron spins and dealing with decoherence. Companies exploring quantum dot qubits include Intel and Qutech.
Photonic Qubits
Photonic qubits utilize the fundamental quantum properties of photons, which are light particles, to perform quantum computation. Unlike other qubit types that use matter to hold information, photonic qubits use the quantum states of light particles, making them inherently less susceptible to environmental disturbances.
Creating photonic qubits involves generating single photons or manipulating the quantum states of photons in a controlled manner. This can be achieved using several techniques, one of which is called spontaneous parametric down-conversion (SPDC), where a photon is split into two lower-energy photons using a nonlinear crystal. These photons can be entangled, a quantum property that makes them suitable for quantum computation. The quantum state of a photonic qubit can be defined in various ways, including the polarization state of a photon, the phase, or even the pathway the photon takes.
Manipulating photonic qubits requires high-precision optics. Beamsplitters, phase shifters, and waveplates are used to perform quantum operations on the photons. These components alter the path or polarization state of the photons, effectively implementing quantum gates. Measuring the state of a photonic qubit involves using detectors that can detect single photons, like avalanche photodiodes or superconducting nanowire single-photon detectors.
One of the primary challenges with photonic qubits is the difficulty in ensuring single-photon sources and efficient detectors, which are crucial for various quantum information protocols. Despite these challenges, photons have the advantage of being relatively easy to transmit over long distances, which makes photonic qubits particularly promising for quantum communication applications.
Companies working on photonic quantum computing include Xanadu and PsiQuantum.
Topological Qubits
Topological qubits represent a different approach to quantum information processing, where the qubit’s information is not stored in a specific physical location but rather in the quantum state of an entire system. This concept relies on topological quantum numbers, properties that describe the global nature of a quantum system and are not easily altered by local perturbations or noise, potentially leading to intrinsic fault tolerance.
Creating topological qubits involves creating and manipulating quasi-particles known as anyons, specifically a type of anyon called a Majorana fermion. These particles emerge in certain two-dimensional materials under strong magnetic fields at very low temperatures. The materials are typically in a state of matter called a topological superconductor. In these systems, Majorana fermions emerge at the ends of one-dimensional wires laid on the surface of these materials, and their quantum states are highly resistant to local disturbances due to their non-local nature.
The process begins by fabricating nanowires on top of specific superconducting materials. When an external magnetic field is applied, and the system is brought to a low enough temperature, a state conducive to the emergence of Majorana fermions is induced. Pairs of these quasi-particles can then be used to form qubits. The information in a topological qubit is stored in the overall state of the pair, specifically in whether or not they are entangled, rather than the state of the individual particles.
One of the main challenges in creating practical topological qubits is the stringent material and experimental requirements. Majorana fermions have been evidenced under laboratory conditions, but controlling and manipulating these particles is an area of ongoing research. Furthermore, reading out the state of a topological qubit is also non-trivial and requires further technological developments.
Microsoft is one of the primary research groups dedicated to developing topological qubits. They collaborate with universities and research institutions worldwide to investigate the necessary materials and technologies.
Generic Learning
Despite the different technologies used to embody the qubit, some universal principles apply, and one of the key ways of helping understand qubits is the Bloch-Sphere. The Bloch sphere is a geometrical representation of the quantum states of a two-level quantum system, which is the most straightforward quantum system you can have.
The Bloch-Sphere
The Bloch sphere gives you a way to visualize the state of a qubit in three dimensions and how different quantum gates operate on that state. While it can’t capture every aspect of quantum computing (like multi-qubit entanglement), it’s a potent tool for gaining intuition about how individual qubits behave.