Microwave Photons Directly Linked for Robust Quantum Data Processing
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A new method directly entangles harmonic oscillators, a key step towards advanced information processing and strong quantum computing. Adrian Copetudo and colleagues at the Centre for Quantum Technologies, National University of Singapore, engineer a Raman-assisted cross-Kerr interaction between microwave photons to overcome this hurdle. Their work, a collaboration between the Centre for Quantum Technologies and the Department of Physics at the National University of Singapore, achieves a direct controlled-phase gate between these photons without relying on traditional methods that introduce unwanted dissipation and decoherence. This breakthrough expands the capabilities of bosonic circuit electrodynamics, offering a coherence-preserving pathway for photon-photon interactions and a key building block for fault-tolerant bosonic computing. High-fidelity controlled-phase gates enable strong bosonic quantum error correction A controlled-phase (CZ) gate, exhibiting an average infidelity of just 2.6% and 3% depending on the control state, marks a key step towards practical, fault-tolerant bosonic quantum computing. This was achieved by engineering strong interactions between microwave photons and actively suppressing decoherence arising from nonlinear ancilla excitations, a common limitation in quantum systems. Critically, the gate operates entirely within the bosonic code space, preventing excursions that compromise error correction. High-fidelity controlled-phase gates enable strong bosonic quantum error correction. Critically, the gate operates entirely within the bosonic code space, preventing excursions that compromise error correction. Parity checks, essential for error detection, were implemented using purely bosonic interactions, paving the way for fully-contained bosonic quantum error correction schemes. Further validation of the gate’s performance came from extending tests to a biased-erasure encoding, yielding an initial infidelity of 3.9% per gate. Implementing standard parity checks reduced this to 2.0% per gate, demonstrating the effectiveness of error mitigation strategies. Encoding states with two photons, instead of zero, resulted in an infidelity of 0.9% per gate, confirming the system’s durability across different quantum states. These parity checks, however, still require entanglement with external elements, limiting the potential for a completely self-contained, fault-tolerant system. Raman-assisted photon entanglement bypasses nonlinearity limitations in superconducting circuits Microwave photons within superconducting cavities have been directly coupled, achieving a controlled-phase gate with average fidelities exceeding 95 percent. This represents a genuine advance in bosonic quantum computing by circumventing the need for nonlinear elements typically used to entangle harmonic oscillators, a limitation previously causing decoherence and fidelity issues.
The team’s approach utilises a Raman-assisted cross-Kerr interaction, effectively entangling photons without exciting the ancillary nonlinear element and preserving the photon number within each cavity. Scalability to larger Hilbert spaces, essential for complex computations, remains an open question, and the durability of this method beyond the initial tests is unclear. The experiment relies on carefully fabricated 3D superconducting cavities and transmon qubits for state preparation and measurement, introducing constraints not fully detailed in the publication. Previously, entanglement between harmonic oscillators necessitated parametrically driven dynamics mediated by nonlinear couplers, inevitably introducing decoherence. This work directly addresses this issue, offering a coherence-preserving alternative that expands the set of tools available within bosonic circuit electrodynamics, the study of superconducting circuits interacting with light. Achieving parity checks on a storage cavity, using the cross-Kerr interaction and an auxiliary oscillator, further protects the storage mode from measurement-induced decoherence. For real-world impact, extending this technique beyond the limited photon number states demonstrated is vital. Demonstrating consistent performance with increased complexity and exploring integration with existing quantum error correction protocols will be essential steps towards building a scalable, fault-tolerant bosonic quantum computer. Raman-assisted entanglement of microwave photons enhances bosonic quantum computation Researchers from Singapore have demonstrated a new method for entangling microwave photons within superconducting cavities, bypassing a longstanding limitation in bosonic quantum computing. Their approach engineers a Raman-assisted cross-Kerr interaction, directly coupling photons without relying on nonlinear elements that typically introduce errors. This direct entanglement represents a significant advance over previous techniques which parametrically activated interactions, effectively creating a link via an excited nonlinear element. Previously, achieving entanglement between harmonic oscillators, systems exhibiting wave-like behaviour, necessitated the use of these nonlinear elements, introducing dissipation and limiting the fidelity of quantum states. Instead, this new method utilises a Raman process to directly interact photons, improving coherence and compatibility with quantum error correction protocols.
The team successfully implemented a controlled-phase gate within limited subspaces of the system. Several research groups globally, including those at Quantinuum and the University of Bristol, are pursuing bosonic quantum computing, largely focusing on encoding quantum information in harmonic oscillators such as microwave photons or the vibrational modes of mechanical resonators. While the Singapore team’s work does not directly compete with these efforts, it offers a complementary approach to enhancing the quality of entanglement. Scaling this technology to larger, more complex quantum systems remains a considerable challenge. The current demonstration operates only within the single- and two-photon subspaces, and it is unclear how well this method will perform with a greater number of photons or more intricate quantum circuits. However, successful scaling could unlock new architectures for fault-tolerant quantum computers, potentially offering advantages in encoding and error correction compared to qubit-based systems. Real-world deployment is likely several years away, contingent on overcoming these scalability hurdles and achieving sustained high-fidelity operation. By engineering interactions between microwave photons without relying on traditional, error-inducing nonlinear components, the team has circumvented a longstanding limitation in quantum information processing. The resulting controlled-phase gate, operating entirely within the encoded quantum space, sharply reduces decoherence, the loss of quantum information to the environment. This achievement extends beyond gate fidelity; the method successfully maps parity information, crucial for detecting errors, onto an auxiliary oscillator instead of a conventional nonlinear element. 👉 More information🗞 A direct controlled-phase gate between microwave photons🧠 ArXiv: https://arxiv.org/abs/2603.15587 Tags: