Quantum Non-Classicality Witnessed via Temporal Entanglement and Qubit Probes.

Researchers demonstrate a method to identify non-classical behaviour in physical systems by utilising a qubit probe and examining violations of temporal Bell inequalities. Experimental emulation, using a three-qubit Nuclear Magnetic Resonance computer, confirms the robustness of this approach, applicable across diverse scientific fields including biology.

The fundamental limits of physical theory are currently under intense scrutiny, prompting physicists to devise novel methods for identifying genuinely non-classical behaviour in physical systems. A team led by researchers at the University of Oxford and the Dana-Farber Cancer Institute now presents a protocol for detecting such non-classicality by utilising the concept of temporal entanglement. In this phenomenon, quantum correlations exist not in space, but in time.

In their work, published under the title ‘Temporal Entanglement and Witnesses of Non-Classicality’, Giuseppe Di Pietra, Gaurav Bhole, James Eaton, Andrew J. Baldwin, Jonathan A. Jones, Vlatko Vedral, and Chiara Marletto demonstrate how probing a system with a single qubit and observing violations of specific temporal Bell inequalities can serve as a robust witness to its non-classical nature. Their approach, validated through simulations on a three-qubit Nuclear Magnetic Resonance computer, offers a potentially versatile tool applicable across diverse fields, extending beyond fundamental physics into areas such as biology.

Researchers have established a demonstrable connection between temporal entanglement and non-classical behaviour within hybrid quantum systems, presenting both a comprehensive theoretical framework and corroborating experimental evidence. The work details a protocol for detecting non-classicality by probing a system with a qubit, confirming that violations of temporal Bell inequalities directly indicate the non-classical nature of the system under investigation. This approach, minimising underlying assumptions, broadens applicability across diverse fields, including potentially biological systems, and allows for the study of how one system (M) induces temporal entanglement within another (Q).

At the heart of this research lies Equation S3, a mathematical description of a hybrid quantum system (Q ⊕ M) evolving under conditions of global observable conservation and adherence to the principles of quantum mechanics. Temporal entanglement, a non-classical correlation existing between a quantum system at different points in time, forms the basis for the non-classicality witness. The equation’s parameters – denoted as a, b, c, α, γ, and β – define specific physical scenarios, enabling tailored analysis of the system’s behaviour.

Experimental validation employs a three-qubit Nuclear Magnetic Resonance (NMR) computer. NMR utilises the magnetic properties of atomic nuclei to perform quantum computations. The researchers meticulously detail the NMR spectrometer (Agilent Inova 600 MHz), specifying the resonance frequencies and spin-spin coupling constants (JMQ, JMA, JQA) crucial for designing the quantum gates that manipulate the qubits. Relaxation times (T1 and T2), which characterise the coherence – the ability to maintain quantum information – of the spins, are also reported. Pulse shaping, achieved through Gradient Ascent Pulse Engineering (GRAPE), optimises the fidelity and robustness of the applied pulses. GRAPE is an algorithm used to design optimal control pulses for quantum systems. Phase-only control simplifies the optimisation process and mitigates the effects of radiofrequency (RF) inhomogeneity through averaging. Crucially, pseudo-pure states are prepared —a necessary step in NMR quantum information processing —by applying tailored pulses and crush gradients to overcome the mixed nature of the initial thermal equilibrium state, thereby establishing a strong link between the theoretical framework and experimental implementation.

Future research should focus on extending this protocol to more complex hybrid quantum systems and exploring its potential for characterising non-classicality in systems where direct access to the internal state is limited. Investigating the impact of decoherence – the loss of quantum coherence resulting from interaction with the environment – on observed temporal entanglement and developing error mitigation strategies represent crucial avenues for advancing this research. Exploring the practical implications of this non-classicality witness in quantum technologies, such as quantum sensing and metrology, also warrants further attention. Expanding the experimental platform beyond NMR to include systems such as trapped ions or superconducting circuits would broaden the scope of this research and demonstrate its versatility. A theoretical investigation into the fundamental limits of this non-classicality witness and its relationship to other measures of quantumness would provide a deeper understanding of the underlying physics.