The University of Tokyo: Picosecond Light Pulses Boost Ultrafast Quantum Computing Potential

A key advancement in optical quantum computing has been achieved by generating Schrödinger cat states in picosecond temporal modes, a sharp leap beyond previous nanosecond limitations. Mamoru Endo and colleagues at The University of Tokyo, in collaboration with National Institute of Advanced Industrial Science and Technology, NTT Inc, Institute of Physics, Palacky University, and 3 other institutions, accomplished this through multi-photon generalised photon subtraction, enabling a higher rate of operation previously hindered by low heralding probabilities. Reconstructing Wigner functions with up to four negative regions and an effective cat-state amplitude of 1.69, the work provides a platform compatible with scalable, time-multiplexed photonic architectures and brings fault-tolerant cat-code architectures closer to practical realisation.

High-amplitude cat states generated via picosecond multi-photon subtraction

A cat-state amplitude of 1.69 has been achieved, a substantial increase from previous limitations restricted to values below one, and nearing the threshold required for practical fault-tolerant quantum computing architectures. This breakthrough overcomes a longstanding bottleneck imposed by the use of nanosecond-scale optical wave packets, previously hindering the creation of complex quantum states at sufficiently high rates for scalable systems. The University of Tokyo team, collaborating across multiple institutions, demonstrated multi-photon generalised photon subtraction within picosecond pulses, establishing a new capability for generating non-Gaussian states essential for advanced quantum computation. Non-Gaussian states, unlike those describable by Gaussian wavefunctions, are crucial for functionalities unattainable with Gaussian states alone, such as universal quantum computation and robust quantum error correction.

The team generated non-Gaussian quantum states with up to four distinct negative regions within their Wigner functions, a visual representation of quantum coherence. The Wigner function, a quasi-probability distribution, allows visualisation of quantum states in phase space; negative regions signify non-classicality and are indicative of entanglement and superposition. High-speed transition-edge sensors and pulsed homodyne detection, matched to these ultrashort pulses, operated at a pump repetition rate of 5MHz. Transition-edge sensors (TESs) are superconducting detectors renowned for their sensitivity and speed, crucial for resolving the picosecond pulses. Pulsed homodyne detection is a technique used to measure the quadratures of the electromagnetic field, providing information about the quantum state. This advancement builds upon existing methods such as photon subtraction from squeezed states and cavity-based approaches, offering a pathway towards larger-amplitude states favoured by fault-tolerant quantum computing. Squeezed states reduce quantum noise in one quadrature of the electromagnetic field, enhancing sensitivity. Photon subtraction, the removal of a photon from a state, is a common method for generating non-Gaussian states. However, the generated states currently lack the long coherence times and error correction capabilities required for fully scalable quantum processors. Maintaining quantum coherence, the preservation of quantum superposition, is a significant challenge due to environmental interactions.

Picosecond light pulses enable faster quantum computation with Schrödinger cat states

Schrödinger cat states, superpositions where a quantum system exists in two opposing states simultaneously, are vital for building strong optical quantum computers. These states are particularly useful in cat-code quantum error correction, a promising approach to protecting quantum information from decoherence. The method enables generation of these states at a rate compatible with scalable, time-multiplexed photonic architectures, addressing a longstanding limitation imposed by slower, nanosecond operations. Time-multiplexing allows multiple quantum operations to be performed within the same physical time slot by encoding information in the time of arrival of photons, increasing the effective processing speed. The previous reliance on nanosecond pulses limited the repetition rate and thus the potential for time-multiplexing. Although the Wigner function reconstruction, a visual representation of the quantum state, was performed without loss correction, a process accounting for photons lost during the experiment, this does not invalidate the significant advance. Loss correction is essential for accurate state characterisation but is not fundamental to the state generation itself.

An effective cat-state amplitude approaching levels needed for practical, error-resistant quantum processing opens questions regarding adaptive breeding techniques for generating logical qubits and the potential for building more durable quantum processors. Logical qubits, encoded using multiple physical qubits, are more resilient to errors than single physical qubits. Adaptive breeding involves iteratively refining the quantum state to increase its amplitude and improve its robustness. Reaching an amplitude of 1.69, nearing levels suitable for practical fault-tolerance, highlights the potential of this approach for creating larger-amplitude states. Larger amplitudes generally correspond to greater separation between the logical levels of the qubit, making it easier to distinguish and manipulate. Further research will focus on improving coherence times and implementing error correction, key steps towards fully scalable quantum processors and exploring the possibilities of time-multiplexed photonic architectures. Extending coherence times requires minimising interactions with the environment and developing more robust quantum materials. Implementing effective error correction schemes will necessitate complex control and measurement protocols, alongside significant computational resources for decoding.

The ability to generate high-amplitude cat states at a 5MHz repetition rate represents a substantial step towards realising practical quantum computation. The temporal-mode bottleneck, previously restricting the rate of state generation, has been effectively addressed by utilising picosecond pulses. This work paves the way for exploring more complex quantum algorithms and architectures, potentially leading to breakthroughs in fields such as drug discovery, materials science, and cryptography. The development of scalable, fault-tolerant quantum computers remains a significant scientific and engineering challenge, but this research provides a crucial building block for future progress.

The researchers successfully generated multi-photon states with an effective amplitude of 1.69 within picosecond optical pulses at a 5MHz repetition rate. This achievement overcomes a previous limitation in the speed of quantum state generation, offering a pathway towards faster and more scalable quantum computing. By demonstrating this capability, the study establishes a foundation for exploring adaptive breeding techniques to create more robust logical qubits. Further work will concentrate on improving the stability of these states and implementing error correction to advance the development of fully scalable quantum processors.