What Is Quantum Physics? Research Areas, Funding, and Career Paths

Quantum physics (also called quantum mechanics) is the branch of physics that describes how matter and energy behave at the smallest scales — atoms, subatomic particles, and the light and forces that connect them. At those scales, nature stops behaving like the smooth, predictable world of everyday experience and instead follows probabilistic rules: particles can exist in a superposition of multiple states at once, pairs of particles can become entangled so that measuring one instantly constrains the other, and quantities like energy and angular momentum come in discrete, indivisible steps rather than a smooth continuum. Quantum physics is not a fringe or speculative theory — it is one of the two pillars of modern physics (alongside relativity) and the best-tested framework science has ever produced, underlying technologies from lasers and MRI machines to semiconductors and, increasingly, quantum computers and quantum sensors. CASRAI’s companion guide on what physics is covers the parent discipline in full; this guide focuses specifically on the quantum sub-field — what it studies, its major branches, who funds the research, and how researchers actually train and work in it.

What quantum physics actually studies

At its core, quantum physics studies a small number of counterintuitive but rigorously tested phenomena that appear only at very small scales or very low temperatures:

• Quantization — certain physical quantities (energy levels in an atom, the spin of an electron, the number of photons in a light field) can only take specific discrete values rather than any value in a continuous range.
• Superposition — a quantum system can exist in a combination of multiple distinct states simultaneously until it is measured, at which point it is found in only one of them, with a probability determined by the mathematics of the superposition.
• Entanglement — two or more particles can become correlated so strongly that measuring one instantaneously determines the outcome of measuring the other, no matter how far apart they are, a phenomenon Einstein famously (and skeptically) called “spooky action at a distance.”
• Wave-particle duality — entities like electrons and photons display both particle-like and wave-like behavior depending on how they are measured, rather than being cleanly one or the other.
• The uncertainty principle — certain pairs of physical properties (most famously position and momentum) cannot both be known to arbitrary precision simultaneously; this is a fundamental feature of nature, not a limitation of measurement instruments.

Quantum physics is, like the rest of physics, fundamentally empirical: its predictions (energy levels of atoms, interference patterns, entanglement correlations) are stated as precise probabilities and have been confirmed experimentally to extraordinary precision, repeatedly, for over a century. What makes quantum theory distinctive among physical theories is the “measurement problem” it raises — the mathematics describes a system evolving smoothly through superpositions of possibilities, but every actual measurement returns one definite outcome, and physicists and philosophers of science still debate exactly how to interpret that transition (the Copenhagen interpretation, many-worlds, and pilot-wave theories are among the best-known competing interpretations). Notably, the interpretive debate does not affect the theory’s predictive success or its practical applications — researchers can and do build working quantum technologies without first resolving it.

A brief history of how quantum physics developed

Quantum theory began in 1900, when Max Planck proposed that energy is emitted and absorbed in discrete packets (“quanta”) to correctly explain the spectrum of light radiated by a heated object — a problem classical physics could not solve. Albert Einstein extended the idea in 1905, showing that light itself behaves as discrete quanta (later called photons) to explain the photoelectric effect, work that later earned him the Nobel Prize. Niels Bohr applied quantization to atomic structure in 1913, proposing that electrons orbit the nucleus only at specific allowed energy levels, correctly predicting the hydrogen spectrum.

The mid-1920s produced the modern mathematical theory: Louis de Broglie proposed in 1924 that matter itself has wave-like properties; Werner Heisenberg developed matrix mechanics in 1925; Erwin Schrödinger developed the equivalent wave-mechanics formulation (the Schrödinger equation) in 1926; and Paul Dirac unified quantum mechanics with special relativity in 1928, predicting the existence of antimatter. Niels Bohr and Werner Heisenberg’s “Copenhagen interpretation” became the dominant way of understanding what the theory meant, though it was contested from the start — most famously by Einstein, Boris Podolsky, and Nathan Rosen, whose 1935 “EPR paper” argued that quantum mechanics must be incomplete because of the entanglement correlations it predicted.

That debate remained largely philosophical until 1964, when physicist John Stewart Bell showed mathematically that it produced a testable difference: any theory in which particles carry hidden, predetermined properties (as Einstein favored) must obey a statistical limit that genuine quantum entanglement can violate. Experiments through the 1970s-80s (notably by Alain Aspect) and increasingly rigorous “loophole-free” Bell tests in 2015 confirmed that nature does violate the limit — entanglement is real, not a description of pre-existing hidden properties. In parallel, physicist Richard Feynman proposed in 1981-82 that a computer built from quantum systems could simulate quantum physics far more efficiently than any classical computer, an idea that, combined with Peter Shor’s 1994 discovery of a quantum algorithm for efficiently factoring large numbers, launched the modern field of quantum information science and, eventually, the current effort to build practical quantum computers.

Major branches and sub-disciplines of quantum physics

Quantum physics is the foundational framework underneath a set of more specialized modern sub-fields, each with its own instrumentation, methods, and funding pathways:

• Quantum mechanics (non-relativistic) — the foundational mathematical framework itself: wavefunctions, operators, the Schrödinger equation, and how they describe atoms, molecules, and simple systems.
• Quantum field theory — extends quantum mechanics to be consistent with special relativity and to describe particles as excitations of underlying fields; it is the mathematical language of the Standard Model of particle physics.
• Quantum information science — studies how information can be encoded, processed, transmitted, and protected using quantum states (qubits), superposition, and entanglement; the theoretical foundation behind quantum computing, quantum cryptography, and quantum communication.
• Quantum computing — building and programming computers that use qubits rather than classical bits, exploiting superposition and entanglement to solve certain problems (like Shor’s factoring algorithm or quantum simulation) faster than any known classical algorithm.
• Quantum optics — the study of light at the quantum level, including single photons and squeezed light, and how light interacts with atoms and matter; a core toolkit for both fundamental research and many quantum-technology applications.
• Atomic, molecular, and optical (AMO) physics — closely related to quantum optics, this sub-field studies individual atoms and molecules using techniques like laser cooling and trapping, and underlies precision quantum sensors and atomic clocks.
• Quantum condensed matter physics — studies collective quantum phenomena in solids and engineered materials, including superconductivity, quantum magnetism, and topological phases of matter; closely tied to materials science.
• Quantum sensing and metrology — uses quantum phenomena (entanglement, superposition) to make measurements — of time, magnetic fields, gravity, or position — more precise than is possible with classical techniques; atomic clocks are the best-known example.
• Quantum chemistry — applies quantum mechanics to predict molecular structure, chemical bonding, and reaction behavior computationally; a major overlap point with chemistry.
• Quantum gravity — a still-unresolved theoretical effort to reconcile quantum mechanics with general relativity; string theory and loop quantum gravity are the two most prominent (and still experimentally unconfirmed) approaches.

How quantum physics relates to neighboring disciplines

Quantum physics is a sub-field of the broader discipline of physics, and it increasingly sits at the center of research that spans several other fields:

• Physics — quantum mechanics is one of the two foundational pillars of modern physics (alongside relativity), and it underlies most other physics sub-fields, from particle physics to condensed matter; see CASRAI’s parent guide on what physics is for the full discipline.
• Chemistry — quantum chemistry applies quantum mechanics directly to predict molecular structure, bonding, and reaction energetics; see CASRAI’s companion guide on what chemistry is.
• Materials science — the design and characterization of quantum materials (superconductors, topological insulators, engineered quantum devices) draws directly on quantum condensed matter physics; CASRAI’s companion guide on what materials science is covers that neighboring field.
• Thermodynamics and statistical mechanics — quantum statistical mechanics extends classical thermodynamic concepts (entropy, temperature, equilibrium) into the quantum regime, and the emerging field of quantum thermodynamics studies how those concepts behave for very small, individually-addressable quantum systems; see CASRAI’s guide on what thermodynamics is.
• Computer science and engineering — quantum computing and quantum information science sit directly at the intersection of physics, computer science, and electrical engineering, drawing on algorithms, error correction, and control-systems expertise from computing disciplines.
• Mathematics — quantum theory is expressed in the language of linear algebra and functional analysis (Hilbert spaces, operators, eigenvalues), and quantum information theory draws heavily on probability theory and computational complexity.

Quantum physics is one of many disciplines mapped in CASRAI’s overview guide to the branches of science, which shows how it relates to the full landscape of scientific disciplines rather than just its closest neighbors.

Who funds quantum physics research

Quantum physics research spans everything from tabletop theory groups to expensive, specialized experimental infrastructure (dilution refrigerators, cleanrooms for qubit fabrication, ultra-stable lasers), and its funding landscape reflects that range. In the United States, the core federal funders are:

• The National Science Foundation (NSF), primarily through its Physics Division within the Directorate for Mathematical and Physical Sciences (MPS), which funds fundamental quantum research at universities, alongside dedicated cross-agency programs such as the Quantum Leap Challenge Institutes, large multi-institution awards aimed at foundational quantum-science breakthroughs. NSF’s newer NSF X-Labs initiative has also specifically targeted quantum systems: its first funding topics at launch included “Quantum Systems: Interconnects and Integrated Photonics,” funded through milestone-based Other Transactions rather than standard research grants.
• The Department of Energy (DOE), through its Office of Science — the largest single federal sponsor of basic physical-science research in the US — which funds quantum research across several of its program offices (Advanced Scientific Computing Research, Basic Energy Sciences, High Energy Physics, and Nuclear Physics all fund quantum-relevant work) and, since 2020, through a set of dedicated National Quantum Information Science Research Centers hosted at several of its national laboratories, focused on quantum computing, communication, and sensing.
• The National Institute of Standards and Technology (NIST), which conducts and funds quantum research directly tied to measurement science, including atomic clocks, quantum-based standards, and quantum information science.
• Defense and intelligence agencies — the Department of Defense, through basic-research offices such as AFOSR, ARO, and ONR, and the Intelligence Advanced Research Projects Activity (IARPA), fund substantial quantum information science research given its long-term dual-use and national-security relevance, particularly in quantum computing and quantum sensing.

These agencies’ efforts are coordinated at a national level under the National Quantum Initiative Act, signed into law in December 2018, which directed NSF, DOE, and NIST to expand and align their quantum research and workforce programs and established a National Quantum Coordination Office to oversee the effort. Outside the US, national funders play an equivalent role — the UK’s Science and Technology Facilities Council (STFC) and Engineering and Physical Sciences Research Council (EPSRC) both fund UK quantum-physics research, including through the UK’s National Quantum Technologies Programme. A small number of major private foundations are also genuinely active in quantum physics specifically: the Gordon and Betty Moore Foundation funds quantum-materials research through its EPiQS (Emergent Phenomena in Quantum Systems) initiative, and the Simons Foundation supports quantum information theory and its connections to theoretical physics, including collaborations bridging quantum information science and quantum gravity research. Because eligibility rules, program names, and funding priorities change from year to year, always confirm current solicitation details directly with the funding agency or foundation before relying on them for a specific proposal.

Research methods, tools, and equipment

Like the rest of physics, quantum-physics research combines theory, experiment, and computation, but each takes a distinctive form in the quantum regime:

• Theory — developing and solving the mathematical models (wavefunctions, Hamiltonians, quantum field theories) that describe quantum systems and make testable predictions, ranging from pencil-and-paper derivations to large symbolic and numerical calculations.
• Experiment — building and controlling physical quantum systems with extreme precision. Common experimental platforms include superconducting qubits (cooled to near absolute zero in dilution refrigerators), trapped ions and neutral atoms controlled with laser cooling and optical tweezers, and photonic systems using single-photon sources and detectors. Common supporting instrumentation includes ultra-high-vacuum chambers, cryostats, precision lasers, and low-noise microwave and RF control electronics.
• Computation and simulation — classical high-performance computing is used to simulate quantum systems too complex to solve analytically (a technique with inherent scaling limits that is itself part of the motivation for building quantum computers), and increasingly, small-scale quantum computers and simulators are used as research tools in their own right to study quantum many-body physics.

Because much of this instrumentation is expensive and highly specialized, quantum-physics experimental work is often carried out in shared university cleanrooms and cryogenics facilities, or at national-laboratory user facilities, rather than in single-investigator labs — a research-infrastructure pattern research administrators should factor into budgeting and facility-access planning for quantum-physics grants.

Careers and training in quantum physics

Quantum physics research is an advanced-degree field, following the same broad training path as physics generally: a bachelor’s degree in physics (or a closely related quantitative field), followed by a PhD — commonly around 5-6 years in the US graduate-school model — involving coursework in quantum mechanics and a specialized sub-field, qualifying examinations, and original dissertation research. Many quantum-physics PhD holders complete one or more postdoctoral research positions at universities or national laboratories before moving into a permanent faculty, national-laboratory, or industry research role. Because quantum information science and quantum computing have become active commercial fields, quantum-physics PhD holders also have a substantially wider set of industry research options than in many other physics sub-fields, at companies building quantum computing hardware and software as well as in the broader technology sector.

The American Physical Society (APS), the principal professional society for physicists in the US, includes a dedicated Division of Quantum Information recognizing the field’s growth as a distinct research community, alongside its longstanding divisions and its Physical Review journals; the American Institute of Physics (AIP) is a federation of physical-science societies that publishes Physics Today and conducts workforce and education research across the discipline, including quantum. Because quantum research spans several disciplines, professional societies in adjacent fields are also active in the space — for example, optics- and photonics-focused societies engage directly with quantum-optics and photonic-quantum-computing research, and computing-focused professional bodies increasingly cover quantum algorithms and quantum software.

Frequently asked questions

What is the simplest definition of quantum physics?

Quantum physics is the branch of physics that describes how matter and energy behave at the smallest scales — atoms and subatomic particles — where phenomena like superposition, entanglement, and quantization replace the smooth, predictable behavior of everyday classical physics.

What is the difference between quantum physics and quantum mechanics?

In practice the terms are used interchangeably. “Quantum mechanics” more precisely refers to the specific mathematical theory (wavefunctions, the Schrödinger equation, operators) developed in the 1920s; “quantum physics” is the broader umbrella covering that theory plus its many modern sub-fields, such as quantum field theory, quantum information science, and quantum optics.

What are the main branches of quantum physics?

The major branches are non-relativistic quantum mechanics, quantum field theory, quantum information science, quantum computing, quantum optics, atomic/molecular/optical (AMO) physics, quantum condensed matter physics, quantum sensing and metrology, quantum chemistry, and quantum gravity — the last of which remains an open, unresolved research problem rather than a settled field.

How is quantum physics different from classical physics?

Classical physics describes objects as having definite properties (position, speed) at all times, changing smoothly and predictably. Quantum physics shows that at small scales, systems can exist in superpositions of multiple states at once, some properties can only be known probabilistically until measured, and certain quantities are quantized (restricted to discrete values) rather than continuous.

Who funds quantum physics research in the US?

The core federal funders are the National Science Foundation (mainly through its Physics Division and cross-agency programs like the Quantum Leap Challenge Institutes and NSF X-Labs), the Department of Energy’s Office of Science (through several program offices and its National Quantum Information Science Research Centers), the National Institute of Standards and Technology, and defense/intelligence agencies including AFOSR, ARO, ONR, and IARPA. These efforts are coordinated nationally under the 2018 National Quantum Initiative Act. A smaller number of private foundations, including the Gordon and Betty Moore Foundation and the Simons Foundation, also fund quantum research.

What career paths are available with a background in quantum physics?

Typical paths include academic and national-laboratory research (usually via a PhD and one or more postdoctoral positions), and, increasingly, industry research and engineering roles at companies building quantum computing hardware and software — a wider set of industry options than many other physics sub-fields currently offer, reflecting the field’s growing commercial activity.

Related CASRAI resources

Quantum physics is one of many scientific disciplines and sub-fields covered in CASRAI’s overview guide to the branches of science, and it is a sub-field of the parent discipline covered in CASRAI’s guide to what physics is. For quantum physics’s closest neighboring disciplines, see CASRAI’s companion guides on chemistry, materials science, and thermodynamics. On the funding side, see CASRAI’s guide to how STFC funds UK physics and its guide to Simons Foundation funding for mathematics and the physical sciences.