## The Promise of Quantum Computing

Quantum computing utilizes quantum mechanical phenomena like superposition and entanglement to perform computations in powerful new ways. While traditional computers encode information as binary digits called bits that can have a value of either 0 or 1, quantum computers encode information in quantum bits or “qubits” that can exist in a superposition of 0 and 1 simultaneously. By leveraging superposition and entanglement, quantum computers can solve certain complex problems exponentially faster than classical computers. This is known as “quantum advantage” or “quantum supremacy“.

### Moving from Classical to Quantum

However, building a large-scale, error-corrected quantum computer capable of demonstrating unambiguous quantum advantage over classical supercomputers is an immense technological challenge. As quantum pioneer John Preskill stated, quantum computing must proceed through various milestones before we reach the ultimate goal of fault-tolerant, scalable quantum computing.

## Why 50 Qubits Matters

Experts believe that around 50 high quality qubits represents an important milestone on the path towards more advanced quantum information processing. This is why there is tremendous interest around the 50 qubit mark from companies like IBM, Google, Microsoft, Rigetti and others who are all racing to be the first achieve this goal. But why does this specific qubit count matter so much?

### Modeling Useful Quantum Systems

For one, 50 qubits is large enough to model interesting quantum systems and physics that are impossible to simulate on classical computers. For example, modeling the nitrogenase enzyme which fixes atmospheric nitrogen for fertilizer production requires around 50 qubits with sufficient connectivity. Chemical simulation of useful enzymes and proteins are important potential applications for quantum computers.

### Sampling Complexity Growth

Additionally, the complexity of random quantum circuit sampling grows exponentially with qubit count. Sampling even a simple 50 qubit quantum circuit is estimated to be beyond the reach of the most powerful existing supercomputers. Demonstrating an unambiguous quantum advantage over classical computing typically requires sampling much larger circuits than this. Thus scaling further to 100, 500 or 1000+ high quality qubits is key to unlocking more definitive quantum advantages.

## Technological Challenges at 50 Qubits

While the 50 qubit mark itself does not guarantee quantum advantage, it represents reaching a regime where quantum effects begin to dominate and classical simulation may become intractable. However, many immense technological obstacles still remain at this scale before practical quantum computing is achieved.

### Qubit Fidelity and Stability

Maintaining 50+ high fidelity qubits stable for long enough coherence times to perform useful computations is extremely challenging. This requires complex control systems, calibration processes and error correction to detect and correct for decoherence noise and other errors. Missing even one or two fault tolerant physical qubits could undermine a logical qubit needed for computation. Much higher raw physical qubit counts are likely needed to produce 50+ usable logical qubits.

### Connectivity and Control

Enabling connections between individual qubits to exchange information as well as precise control over multi qubit quantum gate operations is also hugely difficult. Most existing prototype quantum processors have limited qubit connectivity and restrictive control capabilities that may work for 10-20 qubits but rapidly break down at larger scales without major innovations.

## Path Forward Beyond 50 Qubits

Once a system with 50+ high quality, stable qubits is achieved, researchers must continue pushing to even larger qubit counts to realize advanced quantum applications.

### Targeting Applications

Specific quantum advantage applications can be developed and optimized to run using a few hundred or few thousand logical qubits. Algorithms like quantum chemistry simulations or machine learning models may be designed around near-term system limitations and still offer usefulness over classical approaches.

### Investing in Infrastructure

Significant investment into ancillary systems like cryogenics, cabling, and electronics is also needed to control and read out signals from large grids of physical qubits. Emerging modular architectures could help solve this by connecting multiple smaller quantum processors into a larger integrated system once individual chips reach 50-100 qubits.

### Pursuing Alternative Approaches

Companies are also investigating alternative qubit modalities like trapped ions, photonics, and neutral atoms that may prove more scalable than superconducting qubits currently dominating the field. Hybrid architectures combining different qubit types may unlock additional paths towards practical, commercial quantum computing in the years ahead.

## Conclusion

Achieving 50 high quality qubits is a major stepping stone along the road towards useful, practical quantum computers. At this scale, we begin probing complex quantum phenomena intractable for classical simulation. However, many challenges around stability, connectivity, error correction and control still remain at 50 qubits and beyond before the full promise of quantum computing can be realized. Pushing substantially past this qubit count while developing robust algorithms and infrastructure tailored to available near-term quantum hardware capabilities will help drive quantum computing forward towards delivering transformative applications and economic value.

### Frequently Asked Questions

**What is quantum advantage?**

Quantum advantage refers to a quantum computer’s ability to solve certain problems exponentially faster or more efficiently than the most powerful classical supercomputers. This milestone demonstrating unambiguous quantum advantage is considered by many experts to be key to achieving practical, commercially valuable quantum computing.

**How many physical qubits are needed for 50 logical qubits?**

Due to noise, errors and imperfections, many more physical qubits are likely needed per usable logical qubit perhaps 5x, 10x or more. So at least 250-500 physical qubits may be required for 50 useful logical qubits depending on the quantum processor’s architecture and error correction capabilities.

**What comes after 50 qubits?**

After achieving 50 high quality qubits in a single integrated system, researchers will continue expanding towards targets like 100, 500 and 1000+ qubits while also optimizing algorithm development, control systems, connectivity and error correction to unlock useful applications before reaching the ultimate end goal of a full-scale fault tolerant quantum computer.

**What are the leading qubit modalities right now?**

Currently, superconducting qubits built using Josephson junctions have demonstrated the highest performance and counts, with companies like IBM, Google and Rigetti leading here. However trapped ion and photonics platforms are also promising for future scalability going forward.

**When will quantum computers surpass classical computers?**

While definitionally a quantum computer with “quantum advantage” has surpassed classical capabilities for some specific problems already, experts predict it may still be 5-10+ years before universal, error corrected quantum computers reliably and decisively surpass classical supercomputers across a range of practical applications. Much further progress is still needed in algorithm, software and hardware development to reach this point.