7 Core Qubit Technologies for Quantum Computing

Here we discussed the advantages and limitations of seven key qubit technologies for designing efficient quantum computing systems. The seven qubit types are: Superconducting qubits , Quantum dots qubits , Trapped Ion Qubits , Photonic qubits , Defect-based qubits , Topological Qubits , and Nuclear Magnetic Resonance (NMR) . They are the seven pathways for designing effective quantum computing systems. Each one of them have their own limitations and advantages. We have also discussed the hierarchies of qubit types. Earlier, we have discussed the seven key requirements for designing efficient quantum computers. However, long coherence time and high scalability of the qubits are the two core requirements for implementing effective quantum computing systems.

Quantum computing is the key technology for future artificial intelligence. In our Compassionate AI Lab, we are using AI based quantum computing algorithms for human emotion analysis, simulating human homeostasis with quantum reinforcement learning and other quantum compassionate AI projects. This review tutorial is for the researchers, volunteers and students of the Compassionate AI Lab for understanding the deeper aspects of quantum computing qubit technologies for implementing compassionate artificial intelligence projects. We followed a scalable layered hybrid computing architecture of CPU, GPU, TPU and QPU, with virtual quantum plugin interfaces.

Earlier we have discussed Spin-orbit Coupling Qubits for Quantum Computing and AI, Quantum Computing Algorithms for Artificial Intelligence, Quantum Computing and Artificial Intelligence , Quantum Computing with Many World Interpretation Scopes and Challenges and Quantum Computer with Superconductivity at Room Temperature. Here, we will focus on the primary qubit technologies for developing efficient quantum computers.

Building a quantum computer differs greatly from building a classical computer. The core of quantum computing is qubits. Unlike classical bits, qubits can occupy both the 0 and 1 states simultaneously and can also be entangled with, and thus closely influenced by, one another. Qubits are made using single photons, trapped ions, and atoms in high finesse cavities.

Superconducting materials, semiconductor quantum dots are promising hosts for qubits to build scalable quantum processor. However, other qubit technologies have their own advantages and limitations. The details of the seven primary qubit systems are as below:

1. Superconducting qubits Superconductors are materials that conduct electricity with no resistance. Thus superconductors can carry large amount of current without loss. Electrical current can flow forever in a closed loop, making a superconductor the closest thing to a perpetual motion machine. The key theory behind superconductivity is that the basic charge carriers are pairs of electrons (known as Cooper pairs), rather than the single electrons in a normal conductor. Generally, superconducting materials are metals, ceramics, organic materials, or heavily doped semiconductors that conduct electricity without resistance. In superconductive material the total spin of a Cooper pair is an integer number, thus the Cooper pairs are bosons (while the single electrons in the normal conductor are fermions). Several types of superconducting circuits have been used to implement qubits and quantum logic gates with different properties and potential uses. There are three primary types of superconducting qubits: charge qubits, phase qubits and flux qubits. In superconducting quantum computing, a charge qubit is formed by a tiny superconducting island (also known as a Cooper-pair box) coupled by a Josephson junction to a superconducting reservoir. The state of the qubit is determined by the number of Cooper pairs which have tunneled across the junction.

2. Quantum dot qubits A quantum dot is a nanoparticle made of any semiconductor material such as silicon, cadmium selenide, cadmium sulfide, germanium or indium arsenide. Typically, a quantum dot is a isolated spherical volume, typically with a diameter of one ten-thousandth of a millimeter, and located inside a solid. A free electron, which is not bound within an atom is “locked inside” the sphere. The surrounding solid is built up in layers made of two semiconducting materials (such as silicon and germanium) and cooled to a very low temperature. Just one tenth of a degree above absolute zero – and the free electron is held in place using electrical fields. In this configuration, the electron spin can be switched “up” and “down” electrically – and can therefore be used to store one of the smallest units of information (0/1). In theory, tiny quantum dots can be used to create computers that fit hundreds of millions of qubits onto chips.

3. Trapped Ion qubits “Ions” are just atoms which have lost or gained one or more electrons, thus acquiring an electrical charge. The ion trap are used for building the quantum registers. The number of qubits in the register is equal to the number of ions trapped. For calcium ion, one electron is removed. They are therefore positively charged. The ions pick up and lose kinetic energy when they absorb and emit single photons. Ion traps can store quantum information with long coherence times and support universal quantum computations. Trapped ions possess a high degree of stability when in a superposition state. They have the advantage of being able to be entangled with each other. Measuring the state of the qubit stored in an ion is quite simple. Typically, a laser beam is applied to the ion that couples only one of the qubit states. When the ion collapses into this state during the measurement process, the laser will excite it, resulting in a photon being released when the ion decays from the excited state. After decay, the ion is continually excited by the laser and repeatedly emits photons. These photons can be collected by a photo-multiplier tube (PMT) or a charge-coupled device (CCD) camera. If the ion collapses into the other qubit state, then it does not interact with the laser and no photon is emitted. By counting the number of collected photons, the state of the ion may be determined with a very high accuracy (>99.9%).

4. Photonic qubits Photons exhibit neither mass nor charge. As a consequence they do not interact with each other. Photons are ideal candidates for many quantum information processing tasks and quantum communication. They have a long coherent time, very weak interaction with the environment, they travel at the speed of light and allow a multiple degree-of-freedom encoding. Photonic quantum state is their horizontal or vertical polarization. Photonic qubits operate equally well at cryogenic and room temperatures, suffer from virtually no decoherence, and are readily manipulated with optical components. They are implemented using universal quantum computing architecture based on phase shifters, beam splitters, and photon counters— so-called linear optical quantum computation.

5. Defect-based qubits One well-known spin defect is the nitrogen-vacancy (NV) center in diamond. Recently, among the possibilities in the solid state the diamond nitrogen-vacancy (NV-1) center has emerged as a leading qubit candidate because it is an individually addressable quantum system that may be initialized, manipulated, and measured with high fidelity at room temperature [3].

6. Topological qubits Topology is a branch of mathematics describing structures that experience physical changes such as being bent, twisted, compacted, or stretched, yet still maintain the properties of the original form. When applied to quantum computing, topological properties create a level of protection that helps a qubit retain information despite what’s happening in the environment. The topological qubit achieves this extra protection in two different ways: Electron fractionalization and Ground state degeneracy. Due to the higher fidelity of topological qubits, fewer qubits are needed for error correction, dramatically reducing the number of qubits the system requires overall. Quantum computing researchers want to use a specific kind of Majorana fermion, known as a Majorana zero mode, as a qubit. The ultimate system for quantum computing might be devices based on topological protection of information. One such system could be Majorana bound states (MBS) in Sm-S nanostructures that produce Andreev bound states (ABSs) at the interface between the normal NW semiconductor (Sm) and the superconductor (S). By applying an axial magnetic field along the S-NW device, one can make the ABSs move to zero energy with increasing magnetic field and form mid gap states.

7. Nuclear magnetic resonance (NMR) qubits Matter is made of molecules, molecules are made of atoms, and inside every atom is a nucleus. Nuclei occur in many different species, called nuclides, which are defined by the numbers of protons and neutrons they contain. The chemical nature of an atom is defined by the number of protons in its nucleus. For example, all atoms of carbon have six protons in the nucleus, and all atoms of hydrogen have a single proton in the nucleus. Some nuclides are magnetic and have a magnetic moment, which means that they interact with applied magnetic fields. Nuclear magnetic resonance (NMR) detects the bulk magnetism of the nuclei, i.e., the sum of all the microscopic magnetic moments in the sample. NMR is the experimental observation of the resonant absorption of energy by the nuclei from the radio-frequency sources. When the nuclear magnetic moment associated with a nuclear spin is placed in an external magnetic field, the different spin states are given different magnetic potential energies. In the presence of the static magnetic field which produces a small amount of spin polarization, a radio frequency signal of the proper frequency can induce a transition between spin states. This “spin flip” places some of the spins in their higher energy state. If the radio frequency signal is then switched off, the relaxation of the spins back to the lower state produces a measurable amount of RF signal at the resonant frequency associated with the spin flip. This process is called Nuclear Magnetic Resonance (NMR). Nuclear spin is robust against any stray magnetic fields. Nuclear spin state is manipulated using a Nuclear Magnetic Resonance (NMR) technique. This robustness is due to the small magnitude of the magnetic moment of the nuclear spin compared with that of the electronic spin. Nuclear magnetic resonance quantum computing (NMRQC) is one of the several proposed approaches for constructing a quantum computer, that uses the spin states of nuclei within molecules as qubits. There are two approaches. The fist approaches is to use the spin properties of atoms of particular molecules in a liquid sample as qubits – this is known as Liquid State NMR (LSNMR). The second approach uses solid state NMR (SSNMR) sample, for example a nitrogen vacancy diamond lattice rather than a liquid sample.

Hierarchy of the quantum qubit types

The hierarchy of the quantum qubit types are shown below. Solid-state , optical, atoms and NMR are the main four classes of quantum qubit types. Superconductor qubits can be classified as: charge qubits, phase qubits and flux qubits. The semiconductor qubits can be classified as: electron speed qubits, nuclear spin qubits, and orbital qubits. The optical qubits can be classified as: linear optical and cavity QED. The atoms qubits can be classified as: trapped ion and optical lattics.

Summary:

The key challenge in implementing quantum computing system is to identify isolated quantum mechanical systems with long coherence times that can be manipulated and coupled together in a scalable fashion. In this article we have discussed the seven core qubit technologies to overcome the challenges of quantum computing. We reviewed seven core qubit types namely: Superconducting qubits , Quantum dots qubits , Trapped Ion Qubits , Photonic qubits , Defect-based qubits , Topological Qubits , and NMR. We have also discussed the hierarchies of different type of qubits. Long coherence time, fidelity and scalability plays significant roles in the development of quantum computing. Whatever the future qubit type is, requirements must be met from these sides. Superconducting circuits are among the leading platforms for quantum computing. Their main building block is the Josephson tunnel junction, a non-dissipative and non-linear electrical element that enables long-coherence times. Qubits based on semiconductor quantum dots are now on the way to build scalable and fault-tolerant quantum computer in the near future. Since, the materials-science progress on nanowire-superconductor hybrids has been remarkable. Researchers can now grow extremely clean, versatile devices featuring various control and manipulation. As a result, hybrid nature of qubits are gaining momentum to fulfill these requirements.

Source Books:

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