Complex numbers are broadly used in physics, normally as a calculation tool that makes things easier due to Euler's formula. In the end, it is only the real component that has physical meaning or the two parts (real and imaginary) are treated separately as real quantities. However, the situation seems to be different in quantum mechanics, since the imaginary unit i appears explicitly in its fundamental equations. From a learning perspective, this can create some challenges to newcomers. In this article, four conceptually different justifications for the use/need of complex numbers in quantum mechanics are presented and some pedagogical implications are discussed.
Complex numbers are broadly used in physics, normally as a calculation tool that makes things easier due to Euler's formula. In the end, it is only the real component that has physical meaning or the two parts (real and imaginary) are treated separately as real quantities. However, the situation seems to be different in quantum mechanics, since the imaginary unit i appears explicitly in its fundamental equations. From a learning perspective, this can create some challenges to newcomers. In this article, four conceptually different justifications for the use/need of complex numbers in quantum mechanics are presented and some pedagogical implications are discussed.
Quantum mechanics seems to use complex numbers in a fundamentally different way when compared to classical physics. Selected episodes from early stages of the theory are presented to identify different approaches to the use of complex numbers. If for Heisenberg it seemed quite natural to use complex exponentials, Schrödinger clearly struggled with the complex nature of his wave function and tried to assign physical meaning only to its real component. Dirac was probably the first to stress that complex quantities in quantum mechanics cannot be split up into pure real and imaginary parts. Possible educational implications are outlined.
> In standard quantum mechanics, the imaginary part of the wavefunction is treated as a mathematical convenience. This work shows that it corresponds to a real internal coordinate in a strictly real two-component representation Ψ = (R, I). The quantum phase is identified as a rotation angle in this internal plane, and Schrödinger dynamics becomes a continuous rotation between R and I. Crucially, the I-component — though not directly measurable in a single projective measurement — can be fully reconstructed via controlled phase shifts and interferometry against a known reference. This explains why real-only models fail: they lack the second degree of freedom required to produce the sinθ term in interference patterns (P(θ) = A + B cosθ + C sinθ, with C ≠ 0). The model is empirically equivalent to complex quantum mechanics but provides a clear physical interpretation of the “imaginary” part as an internal information coordinate. > This document is self-contained, uses no complex notation, and is fully compatible with established quantum theory. It includes a testable experimental signature and connects to quantum tomography, interferometry, and foundational questions about the role of complex numbers in physics. This work is conceptually related to recent discussions on the necessity of complex numbers in quantum theory (Renou et al. 2021; Chen et al. 2022; Weilenmann et al. 2025), but takes a different approach by giving an explicitly real two-component interpretation of the imaginary part.”
Complex numbers, i.e., numbers with a and an imaginary part, are essential for mathematical analysis, while their role in other subjects, such as electromagnetism or special relativity, is far less fundamental. Quantum is the only physical theory where these numbers seem to play an indispensible role, as the theory is explicitly formulated in terms of operators acting on complex Hilbert spaces. The occurrence of complex numbers within the quantum formalism has nonetheless puzzled countless physicists, including the fathers of the theory, for whom a version of quantum physics, where states and observables are represented by operators, seemed much more natural. In fact, previous works showed that such real quantum physics can reproduce the outcomes of any multipartite experiment, as long as the parts share arbitrary quantum states. Thus, are complex numbers really needed for a quantum description of nature? Here, we show this to be case by proving that and complex quantum make different predictions in network scenarios comprising independent quantum state sources. This allows us to devise a Bell-type quantum experiment whose input-output correlations cannot be approximated by any quantum model. The successful realization of such an experiment would disprove quantum physics, in the same way as standard Bell experiments disproved local physics.
Abstract This chapter explains why complex numbers are important for more general quantum mechanical situations. The complex exponential function, aka Euler’s formula, plays a critical role in expressing phase differences between components of a quantum state vector. These phase differences affect the outcome of quantum operations. The Bloch sphere and Bloch vector provide a visual representation of these more complex quantum states with the real and imaginary parts on different axes. The complex exponential is important in the quantum Fourier transform, which is then used to prove an important result for finding the repetition period in the Shor algorithm.
Mathematically, the imaginary unit 𝑖 has long been regarded as an algebraic generator extending the real numbers to the complex field. Yet a century after the birth of quantum mechanics (1925–2025), the answer to its oldest question is now clear: quantum mechanics is not merely a mathematical structure—it is a physical reality, and all quantities appearing in its non-commutative relations are physical. Therefore, 𝑖 is not a mere algebraic extension but the universal operator demanded by the geometric completeness of non-commutative reality between position and momentum. The imaginary unit is a physical reality. The canonical relation [𝑥, 𝑝] = 𝑖ℏ is not merely algebraic—it is a geometric law, showing that the complex structure of quantum mechanics arises inevitably from physical reality itself. It stands without contradiction, without complexity. This is nature. In the (𝑥, 𝑝) plane, 𝑖 represents a 90° rotation and ℏ quantizes the curvature of that rotation. Complex numbers thus arise as the natural geometric language of non-commutative physics, providing a geometric resolution to the century-old question of physical completeness first raised by Einstein, Podolsky and Rosen (1935).
In 1927, Nobel prize winning physicist, E. Schrodinger, in correspondence with Ehrenfest, wrote the following about the new theory: “What is unpleasant here, and indeed directly to be objected to, is the use of complex numbers. Psi is surely fundamentally a real function.” This seemingly simple issue remains unexplained almost ninety years later. In this dissertation I elucidate the physical and theoretical origins of the complex requirement. I identify a freedom/constraint situation encountered by vectors when, employed in accordance with adopted quantum representational methodology, and representing angular momentum states in particular. Complex vectors, quite simply, provide more available adjustable variables than do real vectors. The additional variables relax the constraint situation allowing the theory’s representational program to carry through. This complex number issue, which lies at the deepest foundations of the theory, has implications for important issues located higher in the theory. For example, any unification of the classical and quantum accounts of the settled order of nature, will rest squarely on our ability to account for the introduction of the imaginary unit.