7 Key Requirements for Quantum Computing

1. Long Coherence Time

Long qubit coherence times are a prerequisite for quantum computing. Coherence time is the time duration over which the qubit states is considered to be not varying. How long can a quantum superposition state survive? That length of time is called the coherence time. As long as there exists a definite phase relation between different qubit states, the system is said to be coherent. When, the pahse relation is broken, the system is said to be decoherent. 

Many solid-state schemes suffer decoherence from external noises and thermal processes. Making them as cold as possible increases coherence time enormously. A little destructive noise can reduce the quantum coherence of the qubit. Generally, photonic quantum computing technologies have least susceptible to decoherence. However, charge qubits suffer from decoherence due to the fluctuations of voltage sources and fluxes. 

2. High Scalability

In quantum computing scalability refers to ability to handle increased computing demands. The quantum computers must be able operate in a Hilbert space whose dimensions may be grown exponentially without an exponential cost in resources (such as time, space or energy). Successful development of quantum computers will require not only further quantum computing hardware development, but also the continued theoretical development of algorithms and quantum error correction codes, and the architecture connections between the theory and the hardware Normally, architecture with solid-state qubits provides the route of scalability. 

3. High Fault Tolerance and Quantum Error Correction

By nature, qubits are fragile. They require a precise environment and state to operate correctly, and they’re highly prone to outside interference. This interference is referred to as ‘noise’, which is a consistent challenge and a well-known reality of quantum computing. As a result, error correction plays a significant role. The ability to correct errors using error-prone resources is called fault-tolerance.  Fault tolerance has been shown to be theoretically possible for error rates beneath a critical threshold that depends on the computer hardware, the sources of error, and the protocols used for QEC. Quantum error correction (QEC) is used in quantum computing to protect quantum information from errors due to decoherence and other quantum noise.  Quantum error correction is challenging because measurements of a quantum state in general disrupt the delicate superpositions that they are supposed to protect. However, many new codes, techniques, and methodologies have been developed to implement error correction for large scale quantum algorithms.  This can be overcome by using Peter Shor’s quantum error correcting code. Essentially the use of entanglement helps detect and correct errors while keeping the state of a qubit intact.

4. Ability to Initialize Qubits

As a computation begins, the initial set of qubits in the quantum computer are referred to as ‘physical qubits’.  . Initialization refers to the ability to quickly cool a quantum system into a low-entropy state; for example, the polarization of a spin into its ground state. Normally, the models of quantum computation are based on performing some operations on a state of qubit and finally measuring/reading out the result, a procedure that is dependent on the initial state of the system. In most of the cases the approach to initialise a state is to let the system anneal into the ground state and then we can start the computation. This is of particular importance when you consider Quantum Error Correction, a procedure to perform quantum processes that are robust to certain types of noise, that requires a large supply of fresh initialised qubits. 

5. Universal quantum gates

The  system must have available a universal set of quantum logic gates. The large Hilbert space must be accessible using a finite set of control operations; the resources for this set must also not grow exponentially.  In the case of qubits, it is sufficient to have available any “analog” single qubit gate (e.g. an arbitrary rotation of a spin-qubit), and almost any “digital” two-qubit logic operation, such as the controlled-NOT gate. Quantum logic gates are reversible, unlike many classical logic gates. The universal set of quantum gates are  {CNOT, H⊗I, I⊗H, T⊗I, and T⊗I}, where H is the Hadamard gate, T is the π/8 gate, and I is the identity gate. 

6. Efficient Qubit-state Measurement Capability

Measurement is at the corner of all quantum algorithms. Measurement refers to the ability to quickly determine the state of a qubit with the accuracy allowed by quantum mechanics.  After the measurement, the system is in the measured state! That is, further measurements will always yield the same value. We can only extract one bit of information from the state of a qubit.

7.  Faithful transmission of flying qubits and  interconversion between stationary and “flying” qubits

These two conditions are necessary when considering quantum communication protocols such as quantum key distribution that involve exchange of coherent quantum states or exchange of entangled qubits. When creating pairs of entangled qubits in some experimental set up usually these qubits are ‘stationary’ and cannot be moved from the laboratory. If these qubits can be teleported to flying qubits such as encoded into the polarisation of a photon then we can consider sending entangled photons to a third party and having them extract that information, leaving two entangled stationary qubits at two different locations. The ability to transmit the flying qubit without decoherence is major problem. Several attempts have been made to produce a pair of entangled photons and transmit one of the photons to some other part of the world by reflecting off of a satellite or by use optical fibres.

Read : 7 Main Qubit Technologies for Quantum Computing, 

Summary:

Advances in materials design and processing have enabled enormous increases in performance of quantum computing. Lov Grover demonstrated how a quantum data-base could enable quadratic speedup in search queries. Shor’s algorithm provides an  exponential speedup in the factoring of large integers. Photonic qubits, quantum dots and Josephson junctions are used for developing quantum computers.   However, the information stored in these qubits is delicate. The quality of the qubit is primary requirement for quantum computing. The seven criteria discussed here, is the main guiding principles for designing efficient quantum computing systems.