Quantum Hardware in a Nutshell
In quantum computing, the development of quantum algorithms is much more progressive. The advancement of these algorithms is very rigorous, involving extensive analysis seeking to determine the time it would take to run the algorithm and if it would be an advantage over its classical counterpart. But the problem is that all of this effort is only being put into the purely theoretical aspects of quantum computing and computational complexity theory. However, quantum algorithms that tackle real-world problems cannot yet be run on a scale that offers actual speedup over their classical counterparts since we don’t currently have the physical hardware to run them. Therefore, the development of efficient quantum hardware is just as crucial as developing efficient quantum algorithms.
There are many proposals and ideas for the development of quantum computers, the most common ones being superconducting qubits, neutral atoms, and photonics. But these are not the only two ways of building a quantum computer. We’ll go into some much more diverse possibilities (such as topological qubits), but before we get into that craziness, let’s first take a step back and put ourselves in the shoes of the pioneers of classical computing. I bet many of them thought that transistors had no future in the computing industry, but now approximately 400 billion of them are within the cellphones we use every day. Therefore, we shouldn’t overlook any proposal of building a quantum computer. Since you never know what technology will be the “transistor” of this fascinating technology.
This article will attempt to briefly overview the most studied methods of building quantum computers. Which by no means a complete explanation of every technology mentioned here, but it should give you a good idea of how these work. Some things may not be as easy to get the first time, so I advise you to reread any parts that seem confusing and do further research on any technology that interests you. With that, let’s get started!
Nuclear Magnetic Resonance
In this technology, an NMR spectrometer is used to measure and control the magnetic field in the nuclei of molecules. The spectrometer has a powerful magnet cooled at a shallow temperature using liquid Nitrogen and Helium. Inside the magnet, there is a crystal of Malonic acid CH2(COOH3)2, whose Carbon nuclei act as the qubits. These nuclei behaving like a tiny magnet, align with the magnet’s magnetic field placed inside the spectrometer to provide the initial computation stage. Electromagnetic pulses are used to manipulate these qubits. They align with the magnetic field to create the state |0> and against the magnetic field to create the state |1>. States between |0> (aligned) and |1> (against) are superposition states. Due to having magnetic properties, nuclei affect one another, which results in entanglement. Any desired quantum algorithms can be implemented by controlling the pulses and interaction between nuclei. After implementing an algorithm, a coil wrapped around the crystal measures the magnetic fields of the nuclei to complete the computation.
Key Features:
1. Computation can be done at room temperature.
2. Long decoherence time (almost 1 second).
3. Quantum computation is implemented using a pseudo-pure state instead of a pure one.
4. With the current NMR technology, error correction techniques cannot be appropriately implemented.
5. Computation process is slow.
Trapped Ion
Calcium and Strontium are primarily used in ion-trapped quantum computers because they are favorable for multiple quantum computing architectures. A steel vacuum chamber is used to trap ions, which contains a chip whose electrodes are chilled to nearly 5.37222 Kelvin. Ca and Sr atoms are streamed into the chamber. Multiple lasers knock electrons from the atoms turning Ca and Sr atoms into ions. The electrodes generate electric fields that catch the ions and hold the ions 50 micro-meter above the chip surface. Other lasers cool the ions to maintain the temperature. Ca+ and Sr+ crystals are formed by bringing the ions together. Sr+ ions act as the qubits, and Ca+ ions take away extra energy from Sr+ ions to keep them cool and help to maintain their quantum properties. Laser pulses nudge the two ions into entanglement, forming a gate through which Sr+ ions can transport their quantum information into Ca+ ions. For measurement, a laser of a definite wavelength interacts only with the electrons of Ca+ ions without affecting the Sr+ ions. Suppose a laser has an actual wavelength that only manipulates electrons in the ground state and is used for measurement. Then if the electron is in the ground state, it will emit photons; otherwise, it will remain dark.
Key Features:
1. Computation can be done at room temperature.
2. Long decoherence time (almost 1 second).
3. Quantum computation is implemented using a pseudo-pure state instead of a pure one.
4. With the current NMR technology, error correction techniques cannot be appropriately implemented.
5. Computation process is slow.
Trapped Ion
Calcium and Strontium are primarily used in ion-trapped quantum computers because they are favorable for multiple quantum computing architectures. A steel vacuum chamber is used to trap ions, which contains a chip whose electrodes are chilled to nearly 5.37222 Kelvin. Ca and Sr atoms are streamed into the chamber. Multiple lasers knock electrons from the atoms turning Ca and Sr atoms into ions. The electrodes generate electric fields that catch the ions and hold the ions 50 micro-meter above the chip surface. Other lasers cool the ions to maintain the temperature. Ca+ and Sr+ crystals are formed by bringing the ions together. Sr+ ions act as the qubits, and Ca+ ions take away extra energy from Sr+ ions to keep them cool and help to maintain their quantum properties. Laser pulses nudge the two ions into entanglement, forming a gate through which Sr+ ions can transport their quantum information into Ca+ ions. For measurement, a laser of a definite wavelength interacts only with the electrons of Ca+ ions without affecting the Sr+ ions. Suppose a laser has an actual wavelength that only manipulates electrons in the ground state is used for measurement. Then if the electron is in the ground state, it will emit photons; otherwise, it will remain dark.
Key Features:
1. Decoherence is more than superconducting quantum computers and NV-Center Quantum computers.
2. Stable qubits can be developed.
3. Easy to form an entangled state.
4. Difficult to work with large numbers of qubits in this system.
5. Extremely difficult to implement a complete quantum algorithm.
Majorana fermions
The Majorana fermion is a quasi-particle (an excitation of a multi-particle system that has an energy-momentum relationship like a particle) of anyons (particles that are neither fermions nor bosons, appear online in the 2D design showing exchange statistics) which is a particle and its anti-particle at the same time. These Majorana fermions act as qubits in topological quantum computing. Here, information is encoded with how Majorana fermions interact and are braided, not in the particles themselves. Measurement is taken by fusing the particles, which results in the annihilation (a reaction in which a particle and its anti-particle collide and disappear, releasing energy) of some particles.
Key Features:
1. Local perturbations and noise cannot impact the system’s state unless they are strong enough to create a new braid.
2. Probability of quantum error per gate is low.
3. Long decoherence.
4. Topological quantum computing technology is still theoretical.
Super-conducting Chip
Quantum mechanics can be directly applied in micron-sized electrical circuits (or qubits) using superconductivity. A Josephson-junction combined with a pair of lithium superconductors creates a qubit that behaves like an atom with two quantum energy levels — — |0> represents the lower energy state and |1> represents the higher energy state. Quantum operations are performed by sending microwave pulses to the resonator coupled to the qubit. The duration of pulses creates superposition states. For entanglement, two neighboring qubits are connected with a separate resonator. Microwave pulses are sent to the resonators, and the reflected signals are analyzed for measurements. The amplitude and phase of the signal depending on the qubit state.
Key Features:
1. It is easy to build qubits in this system.
2. Qubits can be measured quickly and correctly.
3. Qubits can be operated at a nanosecond time scale.
4. Short decoherence time.
5. Qubits need to be cooled at nearly zero temperature to operate.
6. Computation is susceptible to quantum noise.
Diamond Nitrogen Vacancy-Center
NV consists of a substitutional nitrogen atom on the diamond lattice next to a missing carbon atom creating a vacancy where some electrons are trapped. These trapped electrons from an electron spin of (+1, 0, -1) are used as a qubit. After applying microwave pulses, the spin rotates up and down coherently. Exactly halfway in that rotation is the quantum superposition of spin up (|1>) and down (|0>). In NV centers, different optical transitions are associated with varying spin states. If a laser phase changes to spin up is applied to an NV-Centre whose spinning up, the NV-Centre will get excited. Otherwise, it will stay dark. This helps in the measurement of the qubit state. Photons create entangled states between two or more neighboring diamond NV.
Key Features:
1. Computation can be carried out at room temperature.
2. Exhibit universal quantum gates with fault-tolerant control fidelity.
3. Stable qubits can be developed.
4. Difficult to work with a large number of qubits.
Neutral atom
Here, computation is based on an array of single-atom qubits in optical or magnetic traps. The variety is loaded from a reservoir of laser-cooled atoms at micro-Kelvin temperature. In these atoms, a hyperfine-Zeeman ground substrate is prepared with optical pumping (a process where light is used to raise electrons from a lower energy level in an atom or molecule to a higher one) and are used as qubits. Quantum gates are performed with some combinations of optical and microwave pulses. The results are measured with resonance fluorescence — -a process in which a two-level atom system interacts with the quantum magnetic field of the field and is driven at a frequency near to the natural frequency of the atom.
Key Features:
1. Possible to work with a large number of qubits.
2. Fault-tolerant model and error correction are much more accessible.
3. Powerful computational problems can be solved using this platform.
4. Absolute temperature is required to carry out the computation.
Photonics
A photonic chip is made using silicon and silicon-nitride; the chip has three primary modules. Bright laser light is distributed to an array of squeezers (microscopic devices made of small ring resonators) which generates a squeezed state (a particular quantum state of light that works as a qubit). These squeezed states involve a quantum superposition of a different number of photons. After generation, the squeezed states are coupled into an array of bus waves which carries them to an interferometer (a network of beam splitters and phase shifters; these can be thought of as a sequence of quantum gates). Programs can be loaded electrically into the chip, then translated into a set of electrical voltages by the control system. These voltages are applied to different components of the chip. The squeezed states interact with one another to produce entanglement. The interferometer’s output is a highly entangled quantum state encoding the quantum information obtained by the program. A transition edge sensor (a photon detector that counts the number of photons in each output) yields an array of integers processed as output.
Key Features:
1. Qubits in this technology are much more stable.
2. A large number of photons can be entangled.
3. Computation can be done at room temperature.
4. This technology has achieved quantum supremacy.
5. Less fault-tolerant and difficult to correct errors.
You have probably encountered several fancy terms and technologies that you may have never heard about and have no idea their function. Don’t be worried!. Quantum hardware is a highly specialized field, and many things will not make sense the first time you read about them. These things require a lot of time to understand intuitively, but it is gratifying when you do so. My advice is to pick one technology that you found the most interesting and look for more resources about them. Read a lot, look at lectures, and do anything that works best for your learning. I hope this article has helped you find something you are interested in and given you a good introduction.
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