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arXiv:2506.21831

Implementation and representation of qudit multi-controlled unitaries and hypergraph states by N-body angular momentum couplings

Frank E. S. Steinhoff

Published Jun 27, 2025• 0 citations
arXivHTMLPDF

Abstract

We construct a representation of qudit multi-controlled unitary operators in terms of N-body angular momentum interactions. The representation is particularly convenient for odd-dimensional systems, with interesting connections to the Pegg-Barnett phase formalism. We illustrate the main points in the special case of qutrits, where simplifications and connections to dipole-quadrupole and quadrupole-quadrupole interactions can be established. We describe the representation of the closely related set of qudit hypergraph states, identifying possible realizations and their main obstacles. Qutrit tripartite controlled unitaries are decomposed in terms of more familiar two-body angular momentum couplings, enabling their implementation in a variety of physical systems. We give then a concrete example of implementation of qutrit unitaries and hypergraph states in optical systems that employs single-photon sources, two-mode cross-Kerr interactions and linear optical operations. Moreover, we define a new set of states, called angular momentum hypergraph states, which are more directly related to the angular momentum representation.

DOI
10.48550/arxiv.2506.21831

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We construct a representation of qudit multi-controlled unitary operators in terms of N-body angular momentum interactions. The representation is particularly convenient for odd-dimensional systems, with interesting connections to the Pegg-Barnett phase formalism. We illustrate the main points in the special case of qutrits, where simplifications and connections to dipole-quadrupole and quadrupole-quadrupole interactions can be established. We describe the representation of the closely related set of qudit hypergraph states, identifying possible realizations and their main obstacles. Qutrit tripartite controlled unitaries are decomposed in terms of more familiar two-body angular momentum couplings, enabling their implementation in a variety of physical systems. We give then a concrete example of implementation of qutrit unitaries and hypergraph states in optical systems that employs single-photon sources, two-mode cross-Kerr interactions and linear optical operations. Moreover, we define a new set of states, called angular momentum hypergraph states, which are more directly related to the angular momentum representation.
We develop an angular momentum representation and implementation of the Clifford+T set of unitaries for qutrits. We show that local gates from this set can be realized by the sole use of suitable rotations and one-axis-twisting operations, which are at most quadratic in the angular momentum operators and thus can be experimentally realized in many quantum systems. Controlled rotations are shown to only require linear angular momentum couplings and, as a consequence, the full qutrit Clifford+T set is shown to be expressed solely in terms of two-body angular momentum couplings, rotations and one-axis-twisting operations. By employing the Jordan-Schwinger map, we show an analogous implementation in terms of bosonic modes, improving on the number of modes with regard to a previous scheme. Moreover, we employ the cross-Kerr interaction in order to obtain any qutrit Clifford+T gate for bosonic modes. We illustrate our findings with simple schemes for preparing entangled states of interest.
How to generate and manipulate high dimensional quantum state is a hot topic in the area of quantum information processing. In the optical system, based on the technique of linear optics and cross phase modulation, we consider the realization of universal unitary operation of single partite polarization encoded qudit. Firstly, we introduce the spatially encoded qudit as ancilla, and the entangler of polarization encoded qudit and spatially encoded qudit as core element. After that, the realization of a special unitary operation for single qutrit is discussed by one entangler and the Fourier transformation performed on the spatially encoded qutrit by a linear optical multi-port interferometer (LOMI). The corresponding success probability could be high, but this scheme is only available for a special type of unitary operations and cannot be developed to realize higher dimensional unitary operation. For general qudit universal unitary operation realization, we use qutrit unitary operation as example. The polarization encoded qutrit could be transformed to spatially encoded qutrit by one entangler and the measurement of polarization encoded qutrit, associated with the measurement results are used to control the transmission factors of mirrors placed on the spatial modes of spatially encoded qutrit. Then the desired universal unitary operation is performed on spatially encoded qutrit by a LOMI. Finally, one more entangler, a LOMI implementing Fourier transformation to spatially encoded qutrit, the measurement of spatially encoded qutrit and the phase shifts controlled by the measurement results through classical feed-forward will complete the realization of qutrit unitary operation. This approach could be developed for general case. By two entanglers, one LOMI for Fourier transformation of spatially encoded qudit and the measurement of polarization encoded qudit associated with its results are used to control the transmission factors of mirrors, the bi-directional mapping between polarization and spatially encoded qudits could be realized. Therefore, the universal unitary operation for general qudit could be realized by one more LOMI performed on the ancilla spatially encoded qutrit. In principle, our scheme is available for general single partite polarization encoded qudit and is feasible with the current experimental technology.
Abstract A simple argument is presented that explicitly shows how to construct an arbitrary quantum gate acting on orbital angular momentum (OAM) of single photons. The scheme can be applied to implement subspace multiplexing, where a single high-dimensional OAM qudit represents effectively a stack of multiple independent lower-dimensional qudits. A special subclass of unitaries composed of single-photon controlled gates is studied in detail and notable examples of the general approach are discussed. The generalization of the simple argument leads to the parallelization scheme, which results in the savings of resources. The presented schemes utilize only conventional optical elements and apply not only to single photons but also to classical light.
By exploiting a symmetric scheme for coupling NNN spin-1∕21∕21∕2 constituents (the physical qubits) to states with total angular momentum $N∕2\ensuremath{-}1$, we construct rotationally invariant logical qudits of dimension $d=N\ensuremath{-}1$. One can encode all qudit states, and realize all qudit measurements, by this construction. The rotational invariance of all relevant objects enables one to transmit quantum information without having aligned reference frames between the parties that exchange the qudits. We illustrate the method by explicit constructions of reference-frame-free qubits and qutrits and, for the qubit case, comment on possible experimental implementations.
In multi-qubit quantum control, target unitary operations are commonly specified through full-unitary target descriptions and assessed through global comparison measures. In this work, we introduce an interaction-resolved decomposition of n-qubit unitaries that provides explicit access to their many-body interaction structure through computational-basis phases collected in a diagonalizing frame. Such a frame is conveniently given by local rotations for many operationally relevant operations, including gates generated by single Pauli strings or commuting sets of Pauli strings, such as stabilizer operations, controlled-phase gates, Toffoli-type operations, and Ising interactions. We derive parity-weighted sums of these computational-basis phases that exactly and uniquely resolve k-body interaction terms supported on arbitrary qubit subsets, which we term support-selective phase invariants. These invariants provide an interaction-resolved coordinate system that organizes unitary operations according to their multipartite interaction structure, giving direct access to local, pairwise, tripartite, and general k-partite interaction content underlying entangling operations. This enables the formulation of selective quantum optimal control targets for synthesizing desired combinations of many-body interactions. We supplement this with numerical demonstrations for a representative hardware model, a realistic nitrogen-vacancy spin register, where we synthesized isolated tripartite interactions up to local equivalence within a single control pulse, guided by these invariants, for both diagonal (ZZZ) and non-diagonal (XZZ) terms.
We generalize the class of hypergraph states to multipartite systems of qudits, by means of constructions based on the ddd-dimensional Pauli group and its normalizer. For simple hypergraphs, the different equivalence classes under local operations are shown to be governed by a greatest-common-divisor hierarchy. Moreover, the special cases of three qutrits and three ququarts is analyzed in detail.