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| quantum coherence | |
|---|---|
| Name | Quantum coherence |
| Field | Quantum physics |
| Introduced | 20th century |
| Notable affiliations | Albert Einstein, Niels Bohr, Erwin Schrödinger |
quantum coherence
Quantum coherence is the property of a quantum system whereby superpositions of quantum states exhibit well-defined relative phases, enabling interference phenomena that distinguish quantum from classical behavior. It underpins phenomena such as interference, entanglement, and quantum transport, and is foundational to technologies ranging from quantum computing to quantum sensing. Coherence's preservation and manipulation are central challenges in both fundamental studies and applied efforts within Quantum physics.
Quantum coherence refers to the presence of nonzero off-diagonal elements in a system's density matrix with respect to a chosen basis, indicating phase correlations between basis states. In isolated systems described by the Schrödinger equation, coherence enables persistent quantum superpositions such as those in double-slit experiment setups and Rabi oscillations. In practical contexts coherence is basis-dependent and operationally defined relative to measurement or computational bases used in systems like superconducting qubits, trapped ions, or photonic quantum computing platforms.
The concept emerges from early quantum theory debates involving Niels Bohr and Albert Einstein and formal developments by Erwin Schrödinger and Paul Dirac. Mathematically, coherence arises from unitary evolution in Hilbert space and is captured by the density operator formalism used in statistical mechanics and open quantum systems theory. Connections exist with decoherence theory as developed by researchers like H. Dieter Zeh and Wojciech Zurek, and with resource-theoretic frameworks introduced by groups including I. Marvian and G. Gour. Coherence relates to other quantum resources such as quantum entanglement and quantum discord but is distinct: it can exist in single systems and is convertible to entanglement under certain operations studied in resource theories.
Quantification employs measures satisfying axioms analogous to entanglement monotones. Prominent measures include the l1 norm of coherence, the relative entropy of coherence, and robustness of coherence, developed in works by T. Baumgratz, M. Plenio, and collaborators. Operational metrics relate coherence to tasks: coherent states can improve quantum metrology (via the quantum Fisher information), enable algorithms in quantum algorithms like Grover's algorithm, and serve as inputs for channel discrimination. Experimental observables often estimate coherence through tomography, interference visibility, or witness operators tailored to systems in nitrogen-vacancy centers or cold atoms.
Coherence has been demonstrated across platforms: superconducting qubit circuits at institutions such as IBM and Google Quantum AI, trapped ion systems at groups like IonQ and academic labs, neutral-atom arrays by ColdQuanta, photonic implementations in groups led by Anton Zeilinger, and solid-state spins in Diamond NV center research at Harvard University and University of Oxford. Techniques for control include dynamical decoupling pulse sequences, pulse shaping, error-corrected logical qubits with surface codes, and reservoir engineering developed in Max Planck Institute for Quantum Optics and partner labs. Benchmarks such as coherence time T1 and T2 are routinely reported for device characterization.
Quantum coherence is the engine of quantum advantage in sensing, communication, and computation. In quantum metrology, coherence enhances precision limits beyond classical bounds exemplified by Heisenberg limit strategies. In quantum communication, coherent optical states enable protocols like quantum key distribution developed by groups including ID Quantique and standards bodies. In quantum computing, coherent gate operations are prerequisites for algorithms proposed by Peter Shor and Lov Grover; preserving coherence underlies efforts by institutions such as Rigetti Computing and national labs like Lawrence Berkeley National Laboratory to scale devices. Coherence also plays a role in emergent topics like quantum biology hypotheses (e.g., photosynthetic energy transfer research) and in hybrid systems integrating spintronics and photonics.
Decoherence—loss of coherent phase relations due to interactions with environments—was formalized by Wojciech Zurek and others to explain quantum-to-classical transitions. Common noise sources are electromagnetic fluctuations, phonons, and stray two-level systems in materials. Mitigation strategies include quantum error correction (codes studied by Peter Shor and Andrew Steane), dynamical decoupling protocols, materials engineering in collaboration with institutions like National Institute of Standards and Technology (NIST), and cryogenic isolation employed at facilities such as Fermilab and MIT. Policy and funding choices by governments and agencies (e.g., European Commission quantum initiatives, U.S. National Quantum Initiative), influence capacity to address decoherence at scale.
Coherence-driven technologies raise foundational and ethical questions. Foundationally, decoherence research informs debates on measurement problem interpretations and collapse models proposed by thinkers like Ghirardi–Rimini–Weber proponents. Socially, equitable access to quantum technologies depends on inclusive research funding, workforce development, and community engagement, with stakeholders including universities, minority-serving institutions, and industry consortia. Concentration of quantum capabilities in corporate or national hands can exacerbate inequalities; responsible governance—through transparent standards, open science initiatives, and public investment—can help ensure benefits from coherent-quantum technologies (e.g., sensing for climate monitoring or secure communications) serve broader social justice goals.