Quantum Computing Technologies Compared

Quantum computing (QC) went from theory to experiments, and from experiments to R&D labs. If quantum computers can be scaled up, we might reach a state of quantum supremacy. Where quantum computers outperform good old computers. At least for certain tasks. This makes this a field of interest for startup and scaleup investors.

What makes quantum computing fundamentally different from the old stuff? We’re used to run computers on 0s and 1s, a bit. A bit is either a 0 or a 1, not both at the same time. That’s the fundamental difference with a quantum bit or qubit: it can be a 0 and a 1 at the same time. This ability to simultaneously be in multiple states is called ‘superposition’. To put qubits into superposition, researchers manipulate them using precision lasers or microwave beams.

Furthermore, a qubit can be entangled, meaning that the quantum states of two qubits are correlated, independent of their physical distance to each other. This allows for instant quantum communication or complex logical operations taking advantage of this parallelism.

To take advantage of quantum chips, new software needs to be written to run multidimensional quantum algorithms. Their processing power increases exponentially as qubits are added. This contrasts with a classical processor using bits. Their power increases linearly as more bits are added. For this reason, the computer power of classical computers is far less scalable than that of quantum computers. At least in theory.

Towards 1 million qubit chips

Although quantum computing is still very much a research thing, undeniably a race is going on to produce the most powerful qubit computer. Currently, 1180 qubits is the record, set by Atom Computer last year. Also IBM has recently revealed a quantum chip with more than 1000 qubits. IBM is on a roadmap to roughly double the number of qubits on a chip every year. With this speed it takes another 10 years to reach a 1 million qubit chip. This is a level that is assumed to be minimally needed to be able to reach quantum supremacy. A quantum computer that is superior to classical computing, and which is fault-tolerant, assuming that many qubits are needed for error correction.

Scalability challenges of quantum computing technologies

Quite some different techniques are used. Each with their advantages and drawbacks. Currently, there’s no single clear winner yet for what the most scalable quantum technology is, but below are some promising contenders with their pros and cons regarding scalability.

1. Superconducting Qubits (Current Leader)

Supercooling to almost 0 Kelvin is used to bring particles in a superconductive state to loose their electrical resistance and maintain quantum information for a bit longer as atoms are moving less.

Pros: It’s the most mature technology, with companies like IBM and Google already achieving over 1,000 qubits in their machines.

Cons: Supercooling requirements and complex wiring limit scalability as the qubit count increases.

2. Diamond Nitrogen-Vacancy (NV) Centers

Little imperfections in diamonds – vacancies – are exploited by giving the present nitrogen atoms a spin. A spin “up” or a spin “down”. With light their spin states can be read out.

Pros: Operate at relatively higher temperatures (around cryogenic but not millikelvin range) and have long coherence times (qubit state stability).

Cons: Manufacturing challenges and difficulty integrating them into large-scale architectures.

3. Silicon Qubits

In silicon the spin of electrons is manipulated to represent the 0 and 1 state of a qubit. The electrons are trapped in a special structure called a “dot”.

Pros: Leveraging the well-established silicon chip manufacturing infrastructure could lead to highly scalable and cost-effective production. Recent breakthroughs in removing impurities from silicon are promising.

Cons: Still in early stages of development, with challenges in achieving long coherence times and reliable manipulation of qubits. Still operates under cryogenic temperatures.

4. Topological Qubits

Topological qubits utilize exotic particles called anyons. These particles braid around each other in unique ways, and the braiding patterns themselves encode the quantum information. Manipulating these braids allows for performing quantum operations. These anyons can be woven into the “fabric of reality”.

Pros: Theoretically very robust against errors due to their unique properties, potentially leading to lower error correction needs and easier scaling.

Cons: Extremely challenging to fabricate and manipulate with current technology.

5. Photonic Quantum Computing

Last but not least, photonic quantum computing or linear optical quantum computing (LOQC), is a promising approach that harnesses the power of light (photons) to perform quantum computations. LOQC uses photons, the fundamental particles of light, as their qubits.

Pros: Operates at room temperature, eliminating the need for bulky and expensive cryogenic systems.
Information is encoded in light pulses, which can be easily transmitted over long distances using fiber optic cables, potentially enabling modular architectures.

Cons: Maintaining quantum coherence (fidelity of qubit states) over long distances in optical fibers remains a challenge. Generating and manipulating the specific entangled states needed for certain quantum algorithms can be complex with current technology.

Scalability Potential: Photonic quantum computing holds significant promise for scalability due to its room-temperature operation and potential for modular architectures. However, researchers are still working on overcoming challenges related to coherence and state manipulation.

Universal quantum computing

What technology or combination of technologies will bring about universal quantum computing? Supercooling seems to be challenging, in this respect. The energy needs for supercooling or cryonic cooling are just too much for many business or home applications. On the other hand, photonic QC, maybe in combination with diamond NV and silicon-based techniques, seems to be the most feasible candidate. Still, photonic QC has to do catchup regarding the number of qubits per chip setup. This seems to be merely a matter of years, not something fundamental.