Quantum Computing & Qubit Coherence

Quantum computing relies on a quantum chip, the core processing unit, which uses qubits (quantum bits) to perform computations.

Quantum computing is based on quantum mechanics, harnessing the behavior of subatomic particles through properties like superposition, entanglement, and interference to perform computations.

Quantum chips are not yet in widespread commercial use like silicon chips, they are being used in research labs by companies like Google , IBM , Microsoft , NVIDIA , and many other companies.

When it comes in practical, all these three foundational principles of quantum mechanics are facing technical challenges.

In-case qubits superposition, quantum computer uses qubits (quantum bits) that can exist in a superposition of both 0 and 1 simultaneously.

The main challenge in Quantum-based computing is maintaining the coherence of superposition qubits, as they are extremely fragile and susceptible to environmental noise from heat, vibrations, and electromagnetic fields. It means a qubit loses its quantum properties due to interaction with its environment.

What is Qubit Coherence?

Qubit coherence refers to a qubit's ability to maintain a stable, well-defined quantum state, specifically its superposition and entanglement. It's a measure of how long a qubit can hold onto the delicate quantum information it contains before it's lost to the environment.

Example: Imagine a perfectly balanced spinning top.

Coherence: When the top is spinning fast and smoothly, it appears stable and is in a superposition of all possible rotational positions. This is a qubit in a coherent state, ready for a quantum calculation. The longer it can spin smoothly without wobbling, the more "coherent" it is.

Note: It means, once a qubit enters into decoherence state due to environmental or surroundings noise, which is qubit's fragile quantum state which is again non-measurable state. In other words, the environment destroys the property/state of the qubit. Once the state/property is destroyed, The quantum advantage is also gone.

After a qubit entered into decoherence state the qubit is no longer connected in a quantum way, and they begin to behave as individual, isolated classical bits. This disconnect makes the quantum algorithm's multi-qubit operations fail, leading to errors and rendering the entire calculation useless. This is where quantum-based computing is more error prone as of now.

After qubits entered into Decoherence state, it is becoming a classical bit. The hypothetical questions or thought experiments are...

1. Is It Possible to Reposition a Classical Bit Back into an Eligible Qubit Coherent State After Decoherence?. The key reasoning of this question is - Decoherence isn't a complete "destruction" of quantum information. It seems like a scrambling or entanglement with the environment, which makes reversal challenging but not impossible in principle.

2. Can These Isolated Classical Bits Be Used to Help the Main Quantum Computing Process?. The key reasoning of this question is - These classical data(bits) can be processed by a classical computer to apply corrections to the quantum qubits to preserve the qubit superposition coherence.

If further research on the above points take place, we may find ways to make hybrid quantum qubits and classical bits integrated system.

Nevertheless, Researchers are actively working to extend qubit coherence time, which is a significant bottleneck in current quantum computing systems.

In the context of quantum-based computing, an research benchmark RCS test is performed which is a specific type of testing benchmark called Random Circuit Sampling. It's a method used to stress-test a quantum processor to test how quantum processor maintains coherence in the presence of noise. In other words, RCS testing helps to demonstrate the raw power of a quantum computer's and it's quantum computing.

In 2019, Google showed that 53-qubit Sycamore processor could perform a specific Random Circuit Sampling(RCS) calculation in 200 seconds that would have taken the fastest classical supercomputer at the time an estimated 10,000 years.

Last year 2024, Google has demonstrated again another RCS testing with their 105-qubit Willow processor under 5 minutes. It means the qubits maintained it's coherent state for 5 minutes.

From the above 2nd RCS test, Google's claimed Quantum Advantage is the same task would have taken the world's fastest supercomputers at the time an estimated 10 septillion (10[power.of.25]) years to complete. This is a significantly more claim than Google's initial 2019 result.

Quantum-based computing faces significant technical challenges related to scalability, error correction, and maintaining the qubits' delicate quantum coherent state.

Many companies are focused on building fault-tolerant quantum computing with millions of qubits, which will require significant advancements in hardware and software. The ultimate hope is that quantum computing will not be a replacement for classical computing.

Nevertheless, there are technical challenges in the current quantum system, in the near future, quantum computing will serve as a transformative accelerator, unlocking unprecedented computational power to tackle humanity's most complex and pressing challenges, in drug discovery, climate modeling, simulating materials & chemicals, solving complex optimization problems, cryptography & cybersecurity, AI & ML, Network & Communication and beyond.