Entangled Photons Gain New Verification Method for Quantum Computing Accuracy
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Certification of quantum device performance is crucial for realising the potential of quantum technologies. Riko Schadow, Naomi Spier and Stefan N. van den Hoven, working with colleagues from the Dahlem Center for Complex Quantum Systems, Freie Universitat Berlin, and the MESA+ Institute for Nanotechnology, University of Twente, present a novel approach to verifying the preparation of linear optical quantum states.
This research addresses a significant challenge in photonic quantum computing, where the indistinguishability of photons, essential for creating powerful entangled states, renders standard fidelity measurements inadequate. By introducing a new fidelity measure and demonstrating its experimental implementation, the team, including contributions from Malaquias Correa Anguita, Redlef B.G. Braamhaar, Sara Marzban, Jens Eisert, Jelmer J. Renema and Nathan Walk, certifies the fidelity of multi-photon states, paving the way for more reliable and robust photonic quantum technologies and advancing applications in metrology and linear optical quantum computing. Certification of quantum devices is paramount to guaranteeing reliable functionality, and a central task is verifying the accurate preparation of desired output states. This work addresses this challenge within photonic platforms, where single photons are manipulated through linear optical interferometers to generate complex, entangled states for applications ranging from precision metrology to linear optical quantum computing (LOQC). The computational power of these systems relies critically on the indistinguishability of the photons used. Conventional methods for assessing fidelity, typically designed for distinguishable quantum particles like qubits, are inadequate because they only confirm closeness to a predetermined state. Researchers have developed a new measure of fidelity specifically suited to these photonic systems and demonstrated multiple ways to experimentally verify it, building upon previous work on multi-photon indistinguishability.
The team argues that a witness based on the discrete Fourier transform provides an optimal approach for this certification process. By implementing this witness, standard fidelity assessments struggle when photons aren’t perfectly identical, often focusing on external properties while neglecting crucial internal characteristics governing interference.
This research circumvents these limitations by focusing on the permutation symmetries of detected photons, revealing how the level of interference is directly linked to the indistinguishability of the photons involved. The developed fidelity witness, leveraging standard LOQC components, provides a practical and conceptually sound method for certifying state preparation in these complex quantum systems. This advancement is not merely a refinement of existing techniques but a conceptual shift in how certification is approached in photonic quantum computing. By acknowledging and addressing the inherent challenges of photon indistinguishability, the researchers have created a more accurate and reliable method for validating the performance of these increasingly sophisticated quantum devices. Certification of multi-photon states reached a fidelity of 0.86, as determined by implementing a discrete Fourier transform-based witness. This value represents the highest fidelity achieved for these specific states, demonstrating a robust method for verifying their quality. The research focused on photonic platforms where indistinguishable photons are crucial for applications like metrology and LOQC. Consequently, the study developed a fidelity measure tailored to these indistinguishable particle systems, moving beyond standard qubit fidelity witnesses. Experimental results demonstrate the ability to certify the fidelity of multi-photon states, with the 0.86 fidelity achieved representing a milestone in photonic quantum technology. Further analysis detailed the theoretical underpinnings of the fidelity witness, comparing its performance against alternative approaches and considering various levels of assumptions regarding the input state and practical limitations of the experimental setup. By carefully controlling and characterising the photons’ internal degrees of freedom, the research team established a clear link between the observed probabilities and the underlying fidelity. A discrete Fourier transform (DFT)-based witness forms the core of the methodology for certifying multi-photon states. This approach directly addresses the challenges inherent in photonic platforms where photons must exhibit indistinguishability to function effectively in LOQC. The experimental setup began with the creation of multi-photon states using linear optical interferometers, devices that split and recombine light beams to manipulate photon properties. To implement the DFT witness, the team precisely controlled the phase and amplitude of each photon within the interferometer using motorized stages and electro-optic modulators. Photon counting detectors, highly sensitive devices that register individual photons, were then employed to measure the output state of the interferometer, with calibration performed to account for detection efficiencies and dark count rates. A key methodological innovation was the adaptation of techniques originally developed for measuring genuine multi-photon indistinguishability, allowing construction of a robust witness that effectively captures the fidelity of the prepared states. The choice of the DFT witness was motivated by its optimality in distinguishing between states with high and low fidelity, providing a clear and unambiguous certification of performance, and circumventing the need for full state tomography. The relentless pursuit of scalable photonic quantum technologies has long been hampered by the challenge of proving that light sources and circuits are working as intended. Certification of quantum states, verifying that a device reliably produces the desired entangled photons, is a fundamental bottleneck. Existing methods, borrowed from the qubit world, fall short because they assume photons are identical, a condition crucial for photonic quantum computation but not accounted for in standard verification protocols. What distinguishes this advance is not simply a more accurate number, but a practical pathway towards robust certification. By adapting techniques for measuring multi-photon indistinguishability, the researchers demonstrate a discrete Fourier transform-based witness that offers an optimal balance between accuracy and experimental feasibility. While limitations remain, and the complexity of scaling this witness to even larger numbers of photons is considerable, the demonstrated fidelity levels represent a tangible step forward. Looking ahead, this work will likely spur development of more streamlined and automated certification procedures, and could inform the design of more resilient photonic circuits, less susceptible to imperfections that degrade indistinguishability. Ultimately, reliable certification is a necessary condition for unlocking the full potential of photonic quantum technologies, from secure communication to powerful new forms of computation. 👉 More information 🗞 Certification of linear optical quantum state preparation 🧠 ArXiv: https://arxiv.org/abs/2602.12269 Tags: