Multi-level time-bin encoded cluster states transmitted over 29 km of optical fiber

Scientists have successfully transmitted time-bin encoded quantum cluster states over 29 kilometers of partially deployed optical fiber. This achievement represents a significant step towards building robust and scalable quantum communication networks. Time-bin encoding, which uses the arrival of photons at different times to represent information, is less susceptible to decoherence and signal loss than other techniques, making it ideal for long-distance transmissions. Cluster states are a particular type of quantum entanglement considered a key resource for one-way quantum computing and quantum error correction. The ability to generate and distribute these states over significant distances is fundamental for the development of the quantum internet. The experiment used a configuration that allowed for the encoding of multiple levels of information (qudits) in each photon, increasing the transmitted information density and system efficiency. The optical fiber used in the experiment was not pristine laboratory fiber but rather existing, partially deployed infrastructure, which adds realism to the achievement and demonstrates the feasibility of integrating these technologies into current communication networks. This result opens new avenues for the practical implementation of quantum communication protocols and the interconnection of distributed quantum processors, laying the groundwork for future applications in quantum cryptography and quantum cloud computing.

Hybrid Nanophotonic Trap Boosts Atomic Storage and Coherence Times

Scientists have developed a hybrid nanophotonic trap that enables the storage of neutral rubidium (⁸⁷Rb) atoms with exceptionally long coherence times. This breakthrough is crucial for the advancement of quantum computing and precision metrology, as the ability to maintain the quantum coherence of atoms for extended periods is a fundamental requirement for these technologies. The trap combines the advantages of traditional optical traps with those of nanophotonic devices, achieving more efficient light-matter interaction and greater control over atomic states. The trap's design is based on a silicon nitride (Si₃N₄) nanophotonic waveguide that generates an evanescent field to trap atoms at a distance of approximately 100 nm from the surface. This proximity allows for strong light interaction, while the device's geometry minimizes heating and decoherence effects. Researchers achieved storage times of up to 10 seconds and coherence times of up to 20 milliseconds for the spin states of ⁸⁷Rb atoms, significantly surpassing results obtained with purely optical or nanophotonic traps. The key to success lies in the integration of the nanophotonic waveguide with a state-of-the-art atomic cooling and trapping system. This hybrid approach enables the manipulation and readout of atomic quantum states with high fidelity, opening new avenues for building scalable quantum processors and highly sensitive quantum sensors. The results demonstrate the potential of nanophotonic platforms to control and exploit the quantum properties of neutral atoms in the strong interaction regime. This advance represents a significant step towards the realization of neutral atom-based quantum computers, as well as the development of more precise atomic clocks and magnetometers. The ability to maintain quantum coherence in a nanophotonic environment paves the way for integrating these systems onto chips, which could lead to compact and robust quantum devices. Future research will focus on scaling the number of trapped atoms and exploring new architectures for quantum computing and quantum simulation.

All Quantum Unitaries Can Have Constant Depth Circuits

A new study has revealed that any n-qubit unitary operation can be approximated with ε precision by a quantum circuit of depth polynomial in n and log(1/ε), using an exponential number of ancilla qubits. This finding is surprising, as it was traditionally assumed that implementing general unitaries required exponential depth. The ability to "parallelize" these operations to polynomial depth opens new avenues for quantum algorithm design. The advance is based on a novel connection between the unitary synthesis problem, previously explored by Aaronson and Kuperberg, and concepts from complexity theory and cryptography, such as locally-decodable codes and private information retrieval. This interconnection has allowed for the development of a construction that challenges previous assumptions about the complexity of quantum circuits. Furthermore, the researchers demonstrated that if unbounded fan-out gates are permitted (i.e., one output can be the input to multiple gates), these circuits can be further reduced to constant depth. This implies that, under certain hardware conditions, the execution of complex quantum operations could be performed in a computationally very short time, regardless of the number of input qubits, which has profound implications for large-scale quantum computing.

New Quantum Algorithm Accelerates Equilibrium Correlation Function Calculation

Researchers have developed a novel quantum algorithm that significantly optimizes the calculation of equilibrium correlation functions, a fundamental quantity for describing the dynamic properties of physical systems. Unlike conventional quantum methods, which require a distinct quantum circuit for each time point, this new approach allows all time points to be computed simultaneously with a single circuit. This represents a substantial improvement in computational efficiency, drastically reducing the required quantum resources. Previous quantum algorithms for these functions, such as the Hadamard test or variational methods, scaled with a complexity of at least O(Nₜ) circuit shots and O(Nₜ²) quantum gates, where Nₜ is the number of time points. The new algorithm, however, uses O(log Nₜ) ancilla qubits to encode time as a quantum variable. This effective parallelization allows the circuit to be run only O(log Nₜ) times, resulting in a total complexity of O(Nₜ log Nₜ) quantum resources for large systems. The authors demonstrate that this scaling is optimal and validate their method by calculating the Green's function of a one-dimensional spinless Hubbard model. The ability to efficiently calculate equilibrium correlation functions is crucial for understanding and predicting the behavior of complex quantum materials and systems. This advance could accelerate research in fields such as materials science, quantum chemistry, and the design of new devices. The optimization in quantum resource usage brings closer the possibility of performing simulations of quantum systems that are currently intractable for classical computers, opening new avenues for scientific discovery.

New Quantum de Finetti Theorems Improve Error Bounds in Channels

Researchers have developed new versions of quantum de Finetti theorems, applicable to both quantum states and channels, and valid for any distance measure. These theorems, which traditionally approximate the k-system marginals of permutation-invariant states of n-systems by mixtures of independent and identically distributed (iid) states, are now presented in a stronger form of operator inequalities. This implies that the error bounds are valid for every Schatten norm and for every quantum Rényi divergence satisfying data processing. In the standard case, for a fixed local dimension, the new k/n error bound in max-relative entropy significantly improves upon the previously known k²/n scaling, even in the classical setting. The operator-inequality approach is particularly suited to study channel de Finetti representations because operator order between Choi states is equivalent to completely positive (CP) order between the underlying channels. For permutation-covariant channels N⁽ⁿ⁾:Aⁿ→Bⁿ, with d_A=dim A, it is proven that the k-system reduced channel is CP-dominated by a mixture of tensor-power channels with an error of O(k/√n) and a polynomial dependence on the local dimensions. This addresses a question previously raised by Berta et al. Furthermore, under the no-signalling condition, an exponential channel de Finetti theorem is proven, where the approximating mixture consists of Choi-almost-iid channels, whose normalized Choi states are almost-iid in the sense of Mazzola-Sutter-Renner. In the case of r defects, the representation error is at most poly(n)(2d_A⁴k³/(nr²))⁽ʳ⁺¹⁾/² and decays exponentially with n for a suitable choice of parameters r and k.

Single-Copy Quantum Bandit Problem Collapses to Classical

A new study demonstrates that the single-copy quantum bandit problem, where each "arm" supplies an unknown quantum state and rewards are defined by a known quantum effect, can be optimally solved using classical strategies. Researchers have proven that the measured relative entropy from an arm state to its "confusing reward half-space" is equivalent to the classical Burnetas-Katehakis functional, which is based on the spectral statistics of the quantum effect. This implies that the inherent quantum complexity of the problem collapses to a classical description in this specific scenario. The work establishes that a spectral measurement of the quantum effect, combined with a classical KL-UCB (Kullback-Leibler Upper Confidence Bound) type algorithm, is asymptotically instance-optimal. This means that neither adaptive or randomized measurements, nor storing copies for later measurements, can improve the leading logarithmic regret coefficient. This result holds for any finite dimension and for every reward effect under the stated nondegeneracy assumptions, providing a self-contained finite-time bound for general effects and boundary distributions. The key implication is that any further improvement in solving these types of quantum bandit problems must come from measurements outside the single-copy class. For consistent policies with arbitrary quantum memory, an amortized relative-entropy argument provides a converse with the Umegaki half-space divergence. The authors note that the per-arm regret floors, 1/Kᵢₙ𝖿 and 1/Dᵢₙ𝖿, coincide if and only if the arm commutes with the effect F. Whether the Umegaki floor is attainable remains an open problem, with two-copy numerical evidence suggesting it may require using two copies.