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Quantum Leap: Scientists Measure a Universal Quantum Processor's Mysterious "Central Charge"z

The successful demonstration of the experimental measurement of central charge using IBM's universal quantum computers is an important step in the research of conformal field theories. Using a classically tuned variational quantum circuit, the scientists produced high-fidelity ground states for key 1+1D spin models, including the Transverse Field Ising and XXZ chains. The study used probabilistic error cancellation to overcome hardware noise and reliably extract information-theoretic quantities.

Notably, the researchers created periodic boundary conditions using the special heavy-hex configuration of the quantum circuitry to reduce border effects. With relative errors as low as 5%, the derived central charge values showed that digital quantum computers are effective tools for identifying universality classes in many-body systems. This work establishes a solid basis for studying complex quantum critical phenomena using advanced post-processing and local projective measurements.

The Journey of Forty Years The central charge is an important quantity in two-dimensional systems that acts as a signature to identify the universality class of the system's important sites. In high-energy physics, particularly string theory, conformal symmetry is a crucial property of the "worldsheet," or the surface a string tracks as it moves through spacetime.

Despite its theoretical importance, measuring the central charge in a lab setting has long proved difficult. Earlier methods often required the simultaneous determination of the system's sound velocity, adding a layer of complexity that prevented experimental progress for decades. However, the advent of quantum information measures such as entanglement and classical entropies has made it possible to overcome these challenges.

Simulation of Quantum Critical Points Under the guidance of Swarnadeep Majumder, Nazlı Uğur Köylüoğlu, and colleagues from Harvard University and IBM Quantum, the research team modeled complex quantum spin chains using universal quantum computers, specifically the 65-qubit Hummingbird and the 27-qubit IBM Falcon.

The experiment began with the scientists setting up the high-fidelity ground states of 1+1D quantum spin chains at their critical places. They focused on two distinct models: the Transverse Field Ising (TFI) model with Z2 symmetry and the XXZ model with U(1) symmetry. To do this, they employed a variational quantum circuit that was classically optimized. Circuit parameters were numerically modified using a "checkerboard ansatz" in a Variational Quantum Eigensolver (VQE) algorithm in order to reduce the energy of the objective Hamiltonian.

Getting Past Hardware Restrictions Two significant problems in digital quantum simulation that might produce inaccurate findings are boundary effects and hardware noise. The researchers used the unique heavy-hex structure of IBM's CPUs to construct periodic boundary conditions (PBC) in order to get around these problems. The core charge could be more accurately extracted from smaller chains of 12 qubits thanks to the discovery that PBCs were more resistant to system-size effects than open boundary conditions.

Moreover, the researchers employed advanced error mitigation strategies to compensate for the intrinsic "noise" of current quantum equipment. They used Probabilistic Error Cancellation (PEC) based on a sparse Pauli-Lindblad noise model. By sampling many circuit instances and rebuilding a "error-mitigated" version of the data, this process effectively removes gate errors.

Assessing Criticality's DNA After preparing and mitigating the crucial ground states, the researchers extracted the core charge by analyzing the scaling behavior of sub-leading components in Rényi extensions of classical Shannon entropy. The researchers demonstrated that Shannon-Rõi entropies, which are solely computed from local bitstring probabilities, may accurately expose the central charge when measured in specific "conformal bases" (σz and σx), despite the fact that entanglement entropy is a commonly used metric.

The results were astonishingly accurate. The experiment yielded a central charge for the TFI chain that matched the determined value of c = 0.5. With relative errors as low as 5%, they were able to acquire a value for the XXZ chain that was consistent with c = 1.

A Novel Approach to Physics The implications of this study extend far beyond the specific models that were looked at. By proving that universal quantum processors can determine the core charge with great accuracy, the researchers have opened the door to investigating more exotic aspects of matter. The group looked at the tricritical spin chain with supersymmetry, and they found that "barren plateaus" in optimization can make scaling to larger systems difficult.

The researchers noted in the commentary that their results demonstrated that a variety of factors, including symmetries, hardware noise, and finite-size effects, can significantly affect the retrieved value of the core charge. Their protocol provides a solid basis to mitigate these effects, highlighting the potential of quantum computers as useful tools for exploring the intricate theoretical landscape of conformal field theory.