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Complexifying Quantum States

Alice Flarend, Bob Hilborn

Published Apr 15, 2022• 0 citations
Publisher

Abstract

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.

DOI
10.1093/oso/9780192857972.003.0015

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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.
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.
One attractive interpretation of quantum mechanics is the ensemble interpretation, where Quantum Mechanics merely describes a statistical ensemble of objects and not individual objects. But this interpretation does not address why the wave-function plays a central role in the calculations of probabilities, unlike most other interpretations of quantum mechanics. We prove: 1) the wave-function is a parametrization of any probability distribution of a statistical ensemble: there is a surjective map from an hypersphere to the set of all probability distributions; 2) for a quantum system defined in a 2-dimensional real Hilbert space, the role of the (2-dimensional real) wave-function is identical to the role of the Euler's formula in engineering, while the collapse of the wave-function is identical to selecting the real part of a complex number; 3) the collapse of the wave-function of any quantum system is a recursion of collapses of 2-dimensional real wave-functions. The wave-function plays a central role because it is a good parametrization that allows us to represent a group of transformations using linear transformations of the hypersphere. It is precisely the fact that the hypersphere is not the phase-space of the theory that implies the collapse of the wave-function. Without collapse, the wave-function parametrization would be inconsistent.
Based on the concept of classical phase,we formulate a new explanation for the quantum phase from the quantum mechanical point of view. The quantum phase is the canonically conjugate variable of an angular momentum operator,which corresponds to the angular position θ in an actual physical space with a classical reference frame,but it takes a complex exponential form eiθ ≡cosθ +i sinθ in the abstract Hilbert space of a quantum reference frame. This formulation is simply the famous Euler formula in a complex number field. In particular,when θ = π/2,the correlative quantum phase is a unitary pure imaginary number eiπ/2 ≡cos(π/2)+i sin(π/2) ≡ i. By using a photon state-vector function that is the general solution of photon Schrdinger equation and can completely describe a photon's behavior,we discuss the relationship between the angular momentum of a photon and the phase of the photon; we also analyze the intrinsic relationship between the macroscopic light wave phase and the microscopic photon phase.
> 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.”
Abstract Real polarizations play an important part in linking classical and quantum mechanics. We shall see that picking a polarization in a classical phase space amounts to choosing a representation in the underlying quantum theory-’representation’ in the sense of a position or momentum space representation. We shall also come across complex representations, which in many ways are easier to handle. An obvious example is the Fock space treatment of the harmonic oscillator, in which the quantum Hilbert space is realized as a space of holomorphic functions Complex representations arise from complex polarizations, which are natural generalizations of real polarizations: one simply relaxes the condition that the Hamiltonian vector fields spanning P should be real.
Quantum states have complex probability amplitudes that are sometimes represented by positive real numbers multiplied by complex exponentials.Although the overall phase of a superposition state does not affect the probabilities, the relative phases between the component basis states can have measurable effects.A thorough grasp of relative phase is needed for students to understand various key ideas in quantum mechanics, including quantum interference and time dependence.We present preliminary results from an investigation into student understanding of the measurable effects of relative phases that was conducted in sophomore-and junior-level quantum mechanics courses at the University of Washington (UW).The findings suggest that many students do not recognize that relative phases have measureable effects and tend to overlook the important role that complex numbers play in quantum mechanics. I.