Photonic Quantum Walk Advances Quantum Computing, Enhances Multi-Qubit Gate Operations

Photonic quantum walk, a model based on discrete-time quantum walk, is proving instrumental in the development of quantum computing. It provides a structured framework for photonic quantum computation, which is key for building quantum processors. The model has been used to experimentally demonstrate a three-qubit universal quantum computation and configure a six-qubit system using entangled photon pairs. The work marks significant progress towards using photonic quantum walk for quantum computing, offering a scalable framework that uses fewer photons to increase the success rate of multi-qubit gate operations.

What is the Significance of Photonic Quantum Walk in Quantum Computing?

Quantum computing is a rapidly evolving field that holds the potential to solve problems too complex for classical computers. It harnesses and controls quantum mechanical principles such as superposition, interference, and entanglement. While a practical quantum computer of sufficient size is yet to be constructed, numerous quantum algorithms have been developed and demonstrated in various quantum computing platforms.

One of the key elements in quantum computing is the quantum walk, which forms the basis for various quantum algorithms and quantum simulation schemes. Quantum walks, which have both continuous-time and discrete-time variants, have played a fundamental role in the development of quantum algorithms and schemes for quantum simulations. One-dimensional discrete-time quantum walks have been employed to engineer various high-dimensional quantum states, showcasing their versatility.

Photonic platforms are being developed using multiple approaches where discrete-variable, continuous-variable measurement-based, and fusion-based approaches are prominent. Photons are robust against noise compared to other systems, but they have their own disadvantages. For example, in the discrete-variable approach, the multi-qubit gate operations are probabilistic in nature due to natural restriction in photon-photon interaction, reducing the success probability of a computational task. Any improvement to increase multi-qubit gate operations will help in boosting the potential of the discrete-variable approach for photonic quantum computation.

How Does Photonic Quantum Walk Contribute to Quantum Computing?

Photonic quantum walk, a model based on discrete-time quantum walk, offers a more tangible and logical foundation for photonic quantum computing architecture. It maps position-based states to qubit states and in addition to that, polarization state is also a qubit state. This helps in implementing polarization controlled operations, thus conducts quantum computations by emulating gates through unitary evolution. This approach provides a physical and structured framework for photonic quantum computation, which is instrumental for building quantum processors.

One of the main criteria for a system to be considered for universal quantum computation is its ability to implement a universal set of quantum gates. The universal gate set typically includes single-qubit gates like phase (P) and Hadamard (H) gates along with the two-qubit controlled-NOT (CNOT) gate. The focus of this work is to harness the power of single-particle photonic discrete-time quantum walk to experimentally demonstrate a three-qubit universal quantum computation and configure a six-qubit system using entangled photon pairs in combination of three-qubit systems.

What are the Experimental Realizations of Photonic Quantum Walk?

The experimental realization of universal set of quantum gates using photonic quantum walk has been reported. Taking cue from the discrete-time quantum walk formalism, multiple qubits were encoded using polarization and paths degree of freedom for photon and demonstrated realization of universal set of gates with 100% success probability and high fidelity as characterized by quantum state tomography.

For a 3-qubit system, the first qubit was encoded with H and V polarization of photon and path information for the second and third qubit closely resembling a Mach-Zehnder interference setup. To generate a 6-qubit system and demonstrate 6-qubit GHZ state, entangled photon pairs were used as source to two 3-qubit systems.

What is the Future of Photonic Quantum Walk in Quantum Computing?

The work marks a significant progress towards using photonic quantum walk for quantum computing. It also provides a framework for photonic quantum computing using a lesser number of photons in combination with path degree of freedom to increase the success rate of multi-qubit gate operations.

At a three qubit level involving only single photon and its interference along paths, all multi-qubit gate operations are definite and not probabilistic in nature making it a robust scheme. High fidelity of all universal quantum gate operations as characterized by quantum state tomography involving about 28 gate operations including quantum state preparation on a 3-qubit system was reported.

At 6-qubit level, controlling the entanglement between the two-photons each 3-qubit systems can be engineered to demonstrate control on the composite 6-qubit system and used to generate GHZ state. This work provides a scalable framework for photonic quantum computing using lesser number of photons in combination with path degree of freedom to increase the success rate.