Here we report on the production and tomography of genuinely entangled Greenberger-Horne-Zeilinger states with up to ten qubits connecting to a bus resonator in a superconducting circuit, where the resonator-mediated qubit-qubit interactions are used to controllably entangle multiple qubits and to operate on different pairs of qubits in parallel. The resulting 10-qubit density matrix is probed by quantum state tomography, with a fidelity of 0.668±0.025. Our results demonstrate the largest entanglement created so far in solid-state architectures and pave the way to large-scale quantum computation.
Here we report on the production and tomography of genuinely entangled Greenberger-Horne-Zeilinger states with up to ten qubits connecting to a bus resonator in a superconducting circuit, where the resonator-mediated qubit-qubit interactions are used to controllably entangle multiple qubits and to operate on different pairs of qubits in parallel. The resulting 10-qubit density matrix is probed by quantum state tomography, with a fidelity of 0.668±0.025. Our results demonstrate the largest entanglement created so far in solid-state architectures and pave the way to large-scale quantum computation.
Abstract A single-shot entangling gate plays a crucial role in quantum information processing due to its high fidelity. This operation gate is fast to create a maximally entangled state and forms a universal gate set for quantum computing. Currently, the preparation and demonstration of multi-qubit entanglement are achieved based on sequences of single- and two-qubit operations, yielding lower fidelity and requiring longer execution time. Here, we demonstrate by numerically simulating the use of quantum process tomography to fully characterize the performance of a single-shot three-qubit entangling gate. This gate is used to create a Greenberger–Horne–Zeilinger entangled state in Sakhouf et al (2021 J. Phys. B: At. Mol. Opt. Phys. 54 175501), directly generated by three transmon-type superconducting qubits which are mediated by a resonator with the assistance of a microwave field. Comparing ideal and simulated quantum process tomography, we characterize the entangling gate performance by calculating the mean fidelity achieving a high value ≻ 0.93 .
Abstract Quantum technology, leveraging non-classical characteristics, enables breakthroughs in computation, communication, and metrology. Entanglement generation is fundamental. However, efficiently creating large-scale entanglement remains challenging. Using 2 n physical modes within a single particle has been exploited for encoding n -qubit entanglement, yet this approach is believed to be unscalable. Here, using sequentially quantum correlations on a single particle, we present a Ququart Encoding scheme to encode and process multi-qubit entanglement within ququart. We implement a programmable integrated-photonic entanglement generator on a silicon chip, encoding up to 5-qubit entanglement and quantum circuits within 2 ququarts. Within a ququart, we generate high-fidelity 3-qubit W and Greenberger-Horne-Zeilinger states and verify quantum teleportation and phase estimation. Encoding a pair of entangled photons as ququarts yields a 5-qubit Greenberger-Horne-Zeilinger state at 688 Hz, with fidelity 0.956 ± 0.053, confirming genuine 5-qubit entanglement. While the 2 n physical-mode overhead persists for general purposes, our scheme reduces it from exponential to linear under certain circumstances, potentially accelerating large-scale quantum systems.
Entanglement is the quintessential quantum phenomenon and a key enabler of quantum algorithms. The ability to faithfully entangle many distinct particles is often used as a benchmark for the quality of hardware and control in a quantum computer. Greenberger-Horne-Zeilinger (GHZ) states, also known as Schrödinger cat states, are useful for this task. They are easy to verify, but difficult to prepare due to their high sensitivity to noise. In this Letter we report on the largest GHZ state prepared to date consisting of 120 superconducting qubits. We do this via a combination of optimized compilation, low-overhead error detection and temporary uncomputation. We use an automated compiler to maximize error-detection in state preparation circuits subject to arbitrary qubit connectivity constraints and variations in error rates. We measure a GHZ fidelity of 0.56(3) with a post-selection rate of 28%. We certify the fidelity of our GHZ states using multiple methods and show that they are all equivalent, albeit with different practical considerations.
Going beyond the entanglement of microscopic objects (such as photons, spins, and ions), here we propose an efficient approach to produce and control the quantum entanglement of three macroscopic coupled superconducting qubits. By conditionally rotating, one by one, selected Josephson-charge qubits, we show that their Greenberger-Horne-Zeilinger (GHZ) entangled states can be deterministically generated. The existence of GHZ correlations between these qubits could be experimentally demonstrated by effective single-qubit operations followed by high-fidelity single-shot readouts. The possibility of using the prepared GHZ correlations to test the macroscopic conflict between the noncommutativity of quantum mechanics and the commutativity of classical physics is also discussed.
Quantum entanglement is a cornerstone of quantum information science, enabling advancements in quantum computing, cryptography, and metrology. We report a comprehensive experimental study of quantum entanglement in a superconducting circuit comprising two transmon qubits coupled via a tunable bus resonator, operating at ultra-low temperatures (10 mK) in a dilution refrigerator. Using precise microwave control, we generate Bell states with a fidelity of 0.94 ± 0.02 and a concurrence of 0.92 ± 0.03, confirming robust entanglement. Quantum state tomography and coherence measurements yield energy relaxation (T1) times of 92 ± 5 µs and 88 ± 4 µs, and dephasing (T2) times of 85 ± 6 µs and 82 ± 5 µs for qubits Q1 and Q2, respectively, at 10 mK. We systematically investigate the impact of temperature (10 mK, 50 mK, 100 mK) and environmental noise on entanglement persistence. Real-time analysis of recent literature and social media discussions highlights ongoing challenges in scaling superconducting qubit systems due to 1/f noise and material limitations. Our findings demonstrate the viability of superconducting circuits for high-fidelity quantum operations and provide critical insights into optimizing coherence at millikelvin temperatures.
Important tasks in cavity quantum electrodynamics include the generation and control of quantum states of spatially separated particles distributed in different cavities. An interesting question in this context is how to prepare entanglement among particles located in different cavities, which are important for large-scale quantum information processing. We here consider a multi-cavity system where cavities are coupled to a superconducting (SC) qubit and each cavity hosts many SC qubits. We show that all intra-cavity SC qubits plus the coupler SC qubit can be prepared in an entangled Greenberger–Horne–Zeilinger (GHZ) state, by using a single operation and without the need of measurements. The GHZ state is created without exciting the cavity modes; thus greatly suppressing the decoherence caused by the cavity-photon decay and the effect of unwanted inter-cavity crosstalk on the operation. We also introduce two simple methods for entangling the intra-cavity SC qubits in a GHZ state. As an example, our numerical simulations show that it is feasible, with current circuit-QED technology, to prepare high-fidelity GHZ states, for up to nine SC qubits by using SC qubits distributed in two cavities. This proposal can in principle be used to implement a GHZ state for an arbitrary number of SC qubits distributed in multiple cavities. The proposal is quite general and can be applied to a wide range of physical systems, with the intra-cavity qubits being either atoms, NV centers, quantum dots, or various SC qubits.