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  <front>
    <journal-meta />
    <article-meta>
      <title-group>
        <article-title>Content-Based Features into Quantum Knowledge Graph Embeddings</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Jonas Hendl</string-name>
          <email>jonas.hendl@student.kit.edu</email>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Michael Färber</string-name>
          <email>michael.faerber@tu-dresden.de</email>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="editor">
          <string-name>Knowledge Graphs, Link Prediction, Quantum Machine Learning</string-name>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>Karlsruhe Institute of Technology (KIT)</institution>
          ,
          <addr-line>Karlsruhe</addr-line>
          ,
          <country country="DE">Germany</country>
        </aff>
        <aff id="aff1">
          <label>1</label>
          <institution>ScaDS.AI, TU Dresden</institution>
          ,
          <addr-line>Dresden</addr-line>
          ,
          <country country="DE">Germany</country>
        </aff>
      </contrib-group>
      <pub-date>
        <year>2026</year>
      </pub-date>
      <abstract>
        <p>Link prediction in relational data structures, such as knowledge graphs, plays a crucial role in maintaining up-to-date and accurate information. While classical approaches typically leverage either the graph's connectivity or associated textual descriptions (e.g., labels, definitions), recent quantum models have focused predominantly on structural aspects, leaving the potential of textual information largely unexplored. This paper presents the ifrst quantum link prediction framework that combines both structural and textual modalities. We first generate classical text embeddings, apply dimensionality reduction, and encode them into quantum circuits using two complementary strategies: an Amplitude Encoding Model for high-dimensional fidelity and an Angle Encoding Model optimized for gate eficiency. Experimental results on standard benchmark datasets demonstrate that incorporating textual features in quantum architectures is not only feasible but also enhances the predictive performance of quantum link prediction models.</p>
      </abstract>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>1. Introduction</title>
      <p>
        Knowledge graphs (KGs) are structured representations of entities and the relations between them (e.g.,
Mount Everest –named after –George Everest ) and serve as the foundation for numerous applications,
including semantic search and question answering [
        <xref ref-type="bibr" rid="ref1 ref2">1, 2</xref>
        ]. While these graphs are crucial for organizing
and retrieving knowledge, real-world KGs are almost always incomplete [
        <xref ref-type="bibr" rid="ref3">3</xref>
        ], leading to an active
research focus on link prediction – i.e., inferring missing edges (triples) to keep the graph current and
accurate.
      </p>
      <p>
        Classical knowledge graph embedding (KGE) methods have traditionally addressed link prediction
by modeling the topological structure of the graph. However, advances in natural language processing
have shown that incorporating textual attributes—such as labels and descriptions—into KGE models
significantly boosts predictive performance [
        <xref ref-type="bibr" rid="ref3 ref4 ref5 ref6">4, 3, 5, 6</xref>
        ]. Notably, four of the top five models on the
      </p>
      <p>CEUR
Workshop</p>
      <p>ISSN1613-0073
setup not only yields improved predictive accuracy but also highlights the practical feasibility of
quantum-enhanced, content-driven link prediction.</p>
      <p>To summarize, our contributions are as follows:
• We introduce the first quantum link prediction framework that incorporates natural language
descriptions alongside structural information, bridging a critical gap between classical and
quantum approaches.
• We design a robust classical preprocessing pipeline using state-of-the-art language models and
dimensionality reduction to generate entity and relation embeddings for quantum processing.
• We develop two complementary quantum architectures – amplitude encoding and angle encoding
– that significantly reduce resource requirements and improve performance.
• We evaluate our models on the widely used link prediction benchmarks WN18RR and FB15k-237,
showing up to 15% performance gains compared to baselines and highlighting the eficiency of
our approach.</p>
      <p>The paper is structured as follows: Sec. 2 reviews related work on link prediction and quantum KGE
models. Sec. 3 outlines our methodology. Sec. 4 presents the evaluation, and Sec. 5 concludes the paper.</p>
    </sec>
    <sec id="sec-2">
      <title>2. Related Work</title>
      <p>In this section, we first present language models leveraging natural language descriptions (NLD) for link
prediction (Sec. 2.1), followed by a review of quantum KGE models (Sec. 2.2), distinguishing between
quantum-inspired and true quantum approaches.</p>
      <sec id="sec-2-1">
        <title>2.1. Language Models for Link Prediction</title>
        <p>
          Integrating natural language descriptions (NLD) into link prediction has considerably improved KGE
performance. KG-BERT [
          <xref ref-type="bibr" rid="ref14">14</xref>
          ] first demonstrated that treating structured knowledge graph data as natural
language allows pre-trained language models to generate contextual embeddings for link prediction,
achieving state-of-the-art results by fine-tuning BERT to score triples.
        </p>
        <p>
          Subsequent models further optimized this integration. SimKGC [
          <xref ref-type="bibr" rid="ref6">6</xref>
          ] introduced contrastive learning
with diverse negative sampling strategies. This approach improved MRR on the WN18RR dataset by
+19%. KERMIT [
          <xref ref-type="bibr" rid="ref3">3</xref>
          ] further refined the integration of textual features by generating more coherent
descriptions using large language models, leading to further gains in Hits@1.
        </p>
        <p>
          Other approaches, such as KEPLER [
          <xref ref-type="bibr" rid="ref15">15</xref>
          ] and BLP [16], employ independent embedding architectures.
KEPLER jointly optimizes a KGE scoring function and a masked language modeling loss, while BLP
encodes entity descriptions using BERT to learn inductive embeddings without fine-tuning, achieving
strong generalization to unseen entities.
        </p>
        <p>SimKGC and KERMIT currently provide the strongest results of language models for link prediction.
Their success highlights the efectiveness of natural language description-based embeddings, motivating
our exploration of integrating textual information into quantum knowledge graph embedding models.</p>
      </sec>
      <sec id="sec-2-2">
        <title>2.2. Quantum Knowledge Graph Embedding Models</title>
        <p>We can diferentiate between quantum-inspired and true-quantum models.</p>
        <sec id="sec-2-2-1">
          <title>2.2.1. Quantum-Inspired Models</title>
          <p>Quantum-inspired models incorporate principles from quantum mechanics into KGE without requiring
quantum hardware or claiming a quantum advantage. They primarily leverage quantum logic and
algebraic structures to improve link prediction and reasoning.</p>
          <p>
            Embed to Reason (E2R) [
            <xref ref-type="bibr" rid="ref11">11</xref>
            ] introduced a quantum logic-based embedding approach that maps entities
and predicates into quantum-inspired vector spaces, preserving ontological hierarchies and relational
constraints. IQCE [
            <xref ref-type="bibr" rid="ref13">13</xref>
            ] extended this by improving generalization to unseen data and optimizing training
eficiency. Further refinements merged quantum logic with classical embedding techniques: QLogicE
[
            <xref ref-type="bibr" rid="ref10">10</xref>
            ] integrated E2R with TransE, achieving unprecedented results, particularly on the FB15k-237 dataset,
while QIQE [
            <xref ref-type="bibr" rid="ref12">12</xref>
            ] combined E2R with quaternion embeddings, further improving MRR by 81.5%. Despite
these gains, quantum-inspired models remain purely classical, relying on mathematical abstractions
rather than quantum computation.
          </p>
        </sec>
        <sec id="sec-2-2-2">
          <title>2.2.2. True Quantum Knowledge Graph Embedding Models</title>
          <p>
            Among true quantum models, only two architectures have been proposed: Tensor Singular Value
Decomposition (Tensor SVD) [
            <xref ref-type="bibr" rid="ref9">9</xref>
            ] and a variational circuit-based approach [
            <xref ref-type="bibr" rid="ref8">8</xref>
            ]. The latter is further
divided into Quantum Circuit Embedding (QCE) and Fully Parameterized Quantum Circuit Embedding
(FQCE). The key diference is that FQCE parameterizes all gates in the quantum circuit, which in
principle allows for greater expressiveness, while QCE relies partly on classical memory writes. Both
models construct quantum states for subject and object entities and measure their similarity via the
SWITCH-Test (see below). In contrast, Tensor SVD applies quantum tensor singular value decomposition
to utilize the graph structure.
          </p>
          <p>
            All existing true quantum models operate in simulation, with only FQCE later tested on a quantum
backend Kurokawa et al. [17]. Further refinements introduced quantum-specific training strategies,
such as quantum negative sampling [18]. However, no fundamental advancements in architecture have
been proposed beyond these two models. Notably, none of the implementations are publicly available,
though Ma et al.[
            <xref ref-type="bibr" rid="ref8">8</xref>
            ] provided access to FQCE upon request.
          </p>
          <p>
            Performance on FB15k-237 and WN18RR shows that true quantum models yield the lowest Hits@10
scores (32.3–37.8%), falling short of classical KGE models (40–50%) and far behind quantum-inspired
models like QLogicE, which report implausibly high gains [
            <xref ref-type="bibr" rid="ref10">10</xref>
            ]. Unlike previous quantum models that
encode only graph structure, our approach integrates natural language descriptions into quantum
embeddings. To date, no true quantum KGE model has processed textual input—a gap already addressed
in quantum GNNs [19]. This shows the need for hybrid quantum-classical KGE methods to combine
relational and semantic signals.
          </p>
        </sec>
      </sec>
    </sec>
    <sec id="sec-3">
      <title>3. Approach: Integrating Knowledge Graph Content into Quantum</title>
    </sec>
    <sec id="sec-4">
      <title>Models</title>
      <p>Our approach introduces a hybrid quantum-classical framework for link prediction that efectively
integrates textual information from knowledge graphs into a quantum knowledge graph embedding
(KGE) model. It comprises two key components: (i) a classical preprocessing pipeline that extracts
and encodes natural language descriptions (NLDs) into embeddings, and (ii) a quantum module that
process and further scores these embeddings. In the following, we provide a detailed explanation of
each component.</p>
      <sec id="sec-4-1">
        <title>3.1. Classical Preprocessing Pipeline</title>
        <p>For the preprocessing stage, we employed a classical preprocessing pipeline rather than a quantum
alternative, primarily due to current resource limitations and the significant performance disparity
between classical and quantum language models. As discussed in Section 2, classical models presently
ofer superior maturity, scalability, and accuracy, making them a more practical choice for generating
high-quality textual embeddings. As illustrated in Figure 1, the preprocessing pipeline consists of the
following steps:
1. Retrieve the natural language description for each triple.
2. Transform these descriptions into vectors using a Transformer model.
3. Reduce the dimensionality of these vectors to match the quantum model’s input constraints.</p>
        <p>Classical Preprocessing Pipeline</p>
        <p>Triple
NLD(subject)</p>
        <p>NLD(predicate) NLD(object)
 
෥</p>
        <p>Embed:  ∈ ℝ1024
Reduce: ෥ ∈ ℝ64
 
෥
Aggregate:  ,  ∈ ℝ64</p>
        <p>Quantum Model
 
෥</p>
        <p>4. Aggregate the subject, predicate, and object vectors into two vectors, conforming to the quantum
model’s architecture.</p>
        <p>The two resulting vectors are then passed to the quantum model, which scores their similarity to
evaluate the triple. Each of the aforementioned steps is described in detail below.</p>
        <sec id="sec-4-1-1">
          <title>3.1.1. Retrieve Natural Language Descriptions from the Knowledge Graph</title>
          <p>For each triple, we retrieve the associated label or description from the knowledge graph. Depending on
availability, we either use the label alone or concatenate the label and description into a single string for
embedding. Figure 2 shows an example from the WN18RR dataset, where the ID “146138” corresponds
to the label “change state” and the description “undergo a transformation or a change of position or action.”</p>
        </sec>
        <sec id="sec-4-1-2">
          <title>3.1.2. Embedding Vector Generation</title>
          <p>To capture semantic similarity, the natural language descriptions are embedded into vectors such that
semantically similar texts yield similar vectors. This task of semantic text similarity is well-studied,
as shown by the Massive Text Embedding Benchmark [20], where the top-performing model on the
English sentence similarity leaderboard is jina-embeddings-v3 with an embedding dimension of 1,024
[21]. Therefore, we adopt jina-embeddings-v3 as our embedding model.</p>
          <p>The model builds upon the XLM-RoBERTa architecture [22] by incorporating several key adaptations.
Rotary Position Embeddings extend its input capacity from 512 to 8192 tokens, while five LoRA adapters
ifne-tune lightweight, low-rank parameters for task-specific performance – one of which is trained
for semantic text similarity. Furthermore, the model employs Matryoshka Embeddings, which allow
lfexible output dimensions (ranging from 32 to 1,024).</p>
          <p>However, due to the constraints of our quantum models, which require low-dimensional inputs
(typically between 6 and 32 dimensions), we cannot use the full or simply truncated embeddings directly;
instead, we next perform a dimensionality reduction step.</p>
        </sec>
        <sec id="sec-4-1-3">
          <title>3.1.3. Reduction of the Vector Dimensionality</title>
          <p>The original 1,024-dimensional embeddings must be reduced to match the input size of the quantum
model. Although we are not theoretically limited to a specific dimension, simulation time doubles with
each additional qubit. From preliminary runs, we found that embedding sizes (i.e., dimensions) between
6 and 64 are feasible for a comprehensive hyperparameter search.</p>
          <p>We reduce the embeddings to the desired dimensions using UMAP, which better preserves the local
and global structure than PCA or t-SNE [23] and yields denser clusters of similar points [24].</p>
        </sec>
        <sec id="sec-4-1-4">
          <title>3.1.4. Aggregation Functions</title>
          <p>Based on the steps described so far, we create three embedding vectors s, p, o ∈ ℝ for the subject,
predicate, and object of a knowledge graph triple. However, our quantum model requires exactly two
input vectors. To resolve this, we aggregate the three vectors into two vectors a, b ∈ ℝ . The object
vector is directly used as the second input, i.e., b = o. For the first input vector a, which combines the
subject and predicate, we explore several techniques:
Addition (Add ) Element-wise addition of the subject and predicate vectors:
where s, p, a ∈ ℝ .</p>
          <p>Weighted Addition (WAdd )
and predicate:
with   ,   ∈ ℝ.</p>
          <p>a = s + p,
a =   ⋅ s +   ⋅ p,
a =
s + p
2</p>
          <p>.</p>
          <p>Here, we assign weights to control the relative importance of the subject</p>
        </sec>
        <sec id="sec-4-1-5">
          <title>Average</title>
          <p>We compute the element-wise average of the subject and predicate vectors:
When using amplitude encoding, normalizing a with the  2-norm renders averaging equivalent to
addition.</p>
          <p>|a</p>
          <p>U( )
|b</p>
          <p>U( )
H</p>
          <p>H</p>
          <p>Neural Combinator This method applies a single afine transformation followed by a non-linear
activation to combine the vectors:
s
a =  (W ⋅ [p] + b) ,
where [s] ∈ ℝ2 , W ∈ ℝ×2 , b ∈ ℝ , and  ∶ ℝ  → ℝ is an activation function. Since we do not
p
optimize W and b (as optimization is confined to the quantum model parameters), this approach is not
feasible for our use case.</p>
          <p>Dot Product (DotProd ) Inspired by DistMult [25], we can combine s and p by computing their dot
product:
where s, p ∈ ℝ .</p>
          <p>Concatenation (Concat ) Unlike previous methods that aggregate s and p, concatenation preserves
their individual components. In this approach, we form:
a = ⟨s , p⟩,
a = s ⊕ p,
b = o ⊕ p,
where s, o, p ∈ ℝ , a, b ∈ ℝ2 , and ⊕ denotes concatenation. We use concatenation to ensure that both
input vectors to the quantum model have the same dimensionality.</p>
          <p>The average and concatenation methods were suggested by Garten et al. [26], while the addition and
weighted addition approaches were introduced by Mitchell et al.[27]. A neural aggregator similar to
ours was proposed by Maurya et al. [28].</p>
        </sec>
      </sec>
      <sec id="sec-4-2">
        <title>3.2. Quantum Model</title>
        <p>In the following, we describe the loading, development, and scoring of quantum states. Our approach
connects established encoding methods with the FQCE model, enabling the integration of classical
content into a quantum KGE model. The overall circuit is shown in Fig. 3.</p>
        <sec id="sec-4-2-1">
          <title>3.2.1. Encoding a Vector as a Quantum State</title>
          <p>To process the aggregated embedding vectors a, b ∈ ℝ in a quantum circuit, we need to encode them as
quantum states in a way that preserves their geometric properties while minimizing quantum resource
requirements. Given our model constraints – ideally fewer than 6 qubits for feasible simulation and a
circuit depth of approximately log() – the encoding needs to be eficient. Additionally, the mapping
from embeddings to quantum states must be injective to ensure that semantically distinct vectors remain
distinguishable.</p>
          <p>We adopt both amplitude and angle encoding techniques. Amplitude encoding eficiently represents
high-dimensional embeddings. However, it requires 2+2 − 5 rotational and 2+2 − 4 − 4 C-NOT gates,
where  = log() . For instance, a 64-dimensional vector would need 6 qubits with 251 rotational and
228 C-NOT gates. In simulation, the state is prepared directly, circumventing the explicit gate count. In
contrast, angle encoding, though less qubit-eficient, uses a single gate per embedding dimension. To
maintain injectivity, we scale the vectors appropriately.</p>
        </sec>
        <sec id="sec-4-2-2">
          <title>3.2.2. Variational Quantum Circuit</title>
          <p>We implement two variational circuit designs: strongly entangling layers for their expressiveness and a
simplified 2-design for their robustness against barren plateaus and eficient circuit depth usage. The
number of variational layers is treated as a hyperparameter during training.</p>
        </sec>
        <sec id="sec-4-2-3">
          <title>3.2.3. The SWITCH-Test</title>
          <p>We adopt the SWITCH-Test architecture from the FQCE model over alternatives such as Tensor SVD
or a pure variational design with concatenated inputs. This choice is driven by how we integrate the
natural language descriptions, as the selected integration method directly influences the input structure
and subsequent design considerations.</p>
          <p>Integration of Natural Language Embeddings To incorporate content eficiently, embedding
vectors must be generated from the natural language descriptions ahead of time. Existing
languagebased knowledge graph embedding models generally fall into one of three categories: triple embedding
architectures, translational embedding architectures, and independent embedding architectures.</p>
          <p>Triple embedding architectures concatenate the natural language descriptions (NLD) of a triple into
a single token sequence and embed it in one call. However, if a triple is corrupted, a new embedding
must be computed, leading to significant overhead – for example, a single true triple in the WN18RR
dataset can yield over 80,000 corrupted triples. Similarly, translational embedding architectures combine
subject-predicate and predicate-object natural language descriptions and face the same computational
burden when handling corrupted triples. In contrast, independent embedding architectures embed
the subject, predicate, and object separately, allowing each entity and relation to be embedded only
once. This approach minimizes computational overhead and is ideal for precomputation. We follow this
paradigm for generating our embedding vectors, which seamlessly integrates with the SWITCH-Test
architecture.</p>
          <p>SWITCH-Test vs. Tensor SVD We aim for a true quantum model that can run on quantum hardware.
Only three true quantum models – Quantum Circuit Embedding (QCE), FQCE, and Tensor SVD – meet
this criterion. Both QCE and FQCE utilize the SWITCH-Test to compare two quantum states, and we
consider them as SWITCH-Test architectures. The primary diference between QCE and FQCE lies in
the encoding of the initial quantum states of the subject and object, which we adapt for integrating
language embeddings. Therefore, we focus on comparing Tensor SVD and FQCE, the latter ofering a
quantum advantage over QCE.</p>
          <p>Notably, Tensor SVD has not been implemented with quantum computing libraries (e.g., Qiskit or
Pennylane), limiting its execution to classical simulation and lacking concrete circuit depictions – only
mathematical formulations exist, which adds significant implementation overhead. In contrast, FQCE
has well-documented quantum circuits that have been implemented using Pennylane, demonstrating
practical feasibility on quantum hardware. Moreover, FQCE has garnered attention from the research
community, resulting in additional studies that further validate its design.</p>
        </sec>
        <sec id="sec-4-2-4">
          <title>SWITCH-Test vs. a Pure Variational Quantum Classifier Design We choose the SWITCH-Test</title>
          <p>architecture, which generates the superposition of two quantum states and scores their similarity (e.g.,
ℜ⟨|⟩ ). Although this design scores only two states – while a triple contains three elements – it
remains more resource-eficient than a pure variational quantum classifier.</p>
          <p>Table 1 shows that the pure variational classifier requires significantly more gates due to the additional
factor from encoding the concatenated vector. Since 2log2(3) &gt; 21, the gate count is substantially higher.
For these reasons, the SWITCH-Test architecture is preferred for its flexibility in encoding methods and
overall resource eficiency.</p>
        </sec>
        <sec id="sec-4-2-5">
          <title>3.2.4. Scoring the Similarity of Quantum States</title>
          <p>Using amplitude encoding and strongly entangling layers, we prepare two quantum states that represent
the aggregated embedding vectors. Their similarity is evaluated via the SWITCH-Test [29], as illustrated
in Fig. 3. This method measures the similarity by sampling a single ancilla qubit.</p>
          <p>We begin by applying a Hadamard gate to the ancilla qubit initially in |0⟩ :
The remaining  qubits are initialized in the zero state |0⟩ and then evolved by unitary operations  2
and  1 conditioned on the ancilla state, yielding:
A second Hadamard is applied to the ancilla, transforming the state to:
Substituting |⟩ = 
2 |0⟩ and |⟩ = 
1 |0⟩ gives:
[ |0⟩ ( 2 |0⟩ +  1 |0⟩ ) + |1⟩ ( 2 |0⟩ −  1 |0⟩ )].</p>
          <p>1 [ |0⟩ (|⟩ + |⟩) + |1⟩
2
 (|⟩ − |⟩)
].</p>
          <p>The probability of measuring the ancilla in state |0⟩ is then:
  (|0⟩  ) = ‖ 1 (|⟩ + |⟩)</p>
          <p>2
= 1 (⟨|⟩ + ⟨|⟩ + ⟨|⟩ + ⟨|⟩</p>
          <p>4
= 1 (2 + ⟨|⟩ + ⟨|⟩</p>
          <p>4
= 1 + 1 ℜ⟨|⟩,
2 2
2
‖
)
)
represents the real part of the inner product between |⟩
and |⟩ .</p>
          <p>Defining the similarity score as  
= ℜ⟨|⟩</p>
          <p>, we obtain the scoring function:</p>
          <p>= 2   (|0⟩  ) − 1.</p>
          <p>This function estimates the similarity between the two quantum states by sampling the ancilla qubit.</p>
        </sec>
        <sec id="sec-4-2-6">
          <title>3.2.5. Proposed Models</title>
          <p>We propose two quantum knowledge graph embedding models: (1) the Amplitude Encoding Model and
(2) the Angle Encoding Model. The Amplitude Encoding Model employs amplitude encoding to load the
embedding vectors and uses strongly entangling layers in the variational circuit. Because amplitude
encoding requires a high number of gates, strongly entangling layers are preferred over the simplified
2-design – which, as shown in Tab. 2, almost doubles the gate count – to keep the total number of gates
manageable.</p>
          <p>In contrast, the Angle Encoding Model requires only  gates for encoding, which allows us to use
the simplified 2-design for the variational circuit. The Amplitude Encoding Model can encode a real
vector from ℝ</p>
          <p>2 and utilizes more expressive gates that rotate around all available axes, making it
more expressive. Meanwhile, the Angle Encoding Model encodes a vector from ℝ and relies solely on
Y-rotational gates, which reduces its expressiveness but enhances robustness against barren plateaus
and minimizes the overall gate count.</p>
          <p>For our experiments, we combine each of these models with various aggregation functions and
diferent levels of content integration, efectively yielding two groups of models.</p>
        </sec>
      </sec>
    </sec>
    <sec id="sec-5">
      <title>4. Evaluation</title>
      <sec id="sec-5-1">
        <title>4.1. Evaluation Settings</title>
        <sec id="sec-5-1-1">
          <title>4.1.1. Baseline</title>
          <p>
            In this section, we evaluate the performance of our proposed methods. We begin by introducing the
baseline model and datasets in Sec. 4.1, followed by training considerations and strategies in Sec. 4.2.
We then present our evaluation results in Sec. 4.3 and discuss their implications in Sec. 4.4.
As a baseline, we implement a multi-layer perceptron (MLP) that processes concatenated
subjectpredicate and object-predicate embeddings (x ∈ ℝ2 , with  = 2  ) through fully connected layers
with batch normalization and non-linear activations, ultimately yielding a scalar output in the range
[
            <xref ref-type="bibr" rid="ref1">−1, 1</xref>
            ]consistent with our quantum circuit. Each layer employs variance-scaled weight initialization to
maintain stable gradients. Batch normalization [30] is applied after each linear transformation. Dropout
is implemented to prevent overfitting. The ReLU activation function is used in all hidden layers, while
the final layer employs tanh to constrain outputs to [
            <xref ref-type="bibr" rid="ref1">−1, 1</xref>
            ].
          </p>
        </sec>
        <sec id="sec-5-1-2">
          <title>4.1.2. Datasets</title>
          <p>It is essential to compare our approach against the FQCE model, which was evaluated on the kinship,
GDELT, WN18RR, and FB15k-237 datasets. Our goal is to determine whether the additional input
from natural language descriptions improves performance. Since the kinship dataset comprises family
member names with minimal exploitable natural language descriptions (NLD), and only QCE and FQCE
have been evaluated on the GDELT dataset, we focus on WN18RR and FB15k-237 for their widespread
use and compatibility with our approach.</p>
          <p>The two datasets are semantically distinct and thus complementary for evaluation. WN18RR
emphasizes lexical and hierarchical relations (e.g., hypernymy, meronymy), reflecting structured linguistic
knowledge. In contrast, FB15k-237 comprises diverse factual relations (e.g., nationality, profession,
location) across a wide schema of 237 relation types. Their combination enables a comprehensive
assessment of a model’s ability to handle both abstract semantic structures and concrete real-world
facts. As shown in Tab. 3, WN18RR includes 92,583 triples, distributed over 40,599 unique entities and
11 relations. Meanwhile, FB15k-237 consists of 310,079 triples spanning 14,505 unique entities and 237
relations.</p>
          <p>We enhanced WN18RR by extracting natural language labels and descriptions from the NLTK library
based on entity IDs. Since the 11 relations originally had only labels, we generated descriptions using a
prompt-based approach with manual refinement. For FB15k-237, we used descriptions from [ 31] and
retained only the original relation labels.</p>
        </sec>
      </sec>
      <sec id="sec-5-2">
        <title>4.2. Model Training Settings</title>
        <p>Training Procedure We train our models by corrupting positive triples using negative sampling. Both
positive and negative samples are shufled and grouped into mini-batches. Parameters are updated
using the Adam optimizer.</p>
        <p>Hyperparameter Optimization General hyperparameters (e.g., batch size, learning rate, dropout
rate, negative samples per positive) and quantum-specific parameters (e.g., number of qubits, circuit
depth) are tuned using Optuna with Tree-structured Parzen Estimator (TPE) sampling. Preliminary
experiments tested batch sizes in {64, 128, 256, 512, 1024} (with 128–256 performing best) and qubit
counts in {3, 4, 5}, which were increased to 6 for the main study. Early stopping is applied with a
patience of 5 epochs [32].</p>
        <p>Initialization of Variational Quantum Circuits Rotational gates are initialized using Xavier-based
sampling [33]. Additionally, paired block initialization is employed, where two consecutive blocks are
initialized such that their composition equals the identity (i.e.,  † =  ) [34].</p>
        <p>
          Loss Functions and Dropout We compare Binary Cross-Entropy (BCE) and Mean Squared Error
(MSE) losses. The BCE loss maps scores to probabilities via a sigmoid function, while MSE treats scores
continuously in [
          <xref ref-type="bibr" rid="ref1">−1, 1</xref>
          ]. Rotation Dropout replaces a fraction of the rotational gates with the identity.
The dropout rate is optimized as a hyperparameter.
        </p>
      </sec>
      <sec id="sec-5-3">
        <title>4.3. Evaluation Results</title>
        <p>As summarized in Tab. 4, quantum approaches exhibit a substantial performance gap compared to
both the purely language–based KERMIT model and even a minimal neural baseline. On FB15k-237,
Add
WAdd
Concat
DotProd
Add
WAdd
Concat
Add
WAdd
Concat
DotProd
Add
WAdd</p>
        <p>Concat
Model
Angle
Angle
Angle
Angle
Angle
Angle
Angle
Angle
Amplitude
Amplitude
Amplitude
Amplitude
Amplitude
Amplitude
Amplitude
Amplitude
Neural Base Add
Neural Base</p>
        <p>WAdd
Neural Base Concat
Neural Base DotProd
Neural Base Add
Neural Base</p>
        <p>WAdd
Neural Base Concat
FQCE
QCE
Tensor SVD –
–
–
Performance of link prediction models: Angle, Amplitude, neural baseline, and quantum models (FQCE, QCE,
Tensor SVD). L indicates label only, LD indicates label plus description.</p>
        <p>FB15k-237</p>
        <p>WN18RR
Aggreg. NLD</p>
        <p>MR</p>
        <p>MRR</p>
        <p>MR</p>
        <p>MRR
Neural Base DotProd LD 11132 0.0002
KERMIT achieves an MRR of 0.359 and Hits@10 of 54.7 %, whereas the best quantum variant yields
only 0.003 MRR and 0.07 % Hits@10. This disparity persists on WN18RR, with KERMIT reaching 0.700
MRR and 83.2 % Hits@10, while our top quantum model delivers just 0.0093 MRR and 1.64 % Hits@10.
Notably, the simple neural reference model already outperforms every quantum approach on WN18RR
– attaining 0.048 MRR and 11.35 % Hits@10 – highlighting that quantum embeddings currently cannot
compete with either classical embedding techniques or language-driven methods.</p>
        <p>A closer look at each dataset reinforces this conclusion. On FB15k-237, random guessing yields an
expected mean rank (MR) of approximately 14,500; our best quantum configurations –</p>
        <p>(MR 13,895, Hits@10 0.07 %) – improve only marginally over chance and
fall far short of the FQCE benchmark (MR 236). The neural baseline ( 

- , MR 7,582) narrows the
gap but remains over an order of magnitude behind. On WN18RR, quantum models achieve lower MRs
(best:</p>
        <p>- , MR 7,016) yet still trail classical quantum methods – QCE (MR 3,655) and FQCE (MR
2,160) – and the neural baseline (MR 1,549). These results make clear that, despite exploring amplitude
versus angle encodings and various aggregation schemes, quantum models under current designs fail
to approach the efectiveness of both simple neural and advanced language-based knowledge-graph
completion methods.
product methods performed only marginally better than random guessing, with MRs of 50,833 and
MRs of 40,281 and 36,425. Overall, the plain addition aggregation is consistently outperformed by
weighted addition and concatenation. In fact, for the neural baseline, the dot product aggregation yields
the best performance (e.g.,</p>
        <p>achieved an MR of 1,549).
the</p>
        <sec id="sec-5-3-1">
          <title>Comparison Between Angle and Amplitude Architectures</title>
          <p>On average, the</p>
          <p>models
architecture reached 1.64% compared to 0.048% for the best 
model. However, despite
these diferences, the best quantum model by MR is 
(7,016) versus 12,979 for the best
model (
  −
). Furthermore, the performance range across diferent aggregation
functions is wider for the 
models than for the more consistent 
models.
11,973, 
outliers (e.g., 


from 16,705 to 12,979, and</p>
        </sec>
        <sec id="sec-5-3-2">
          <title>Impact of Adding Descriptions</title>
          <p>Adding natural language descriptions generally improves MR
across approximately 83% of the models. For instance, 
improved from an MR of 19,003 to
from 8,284 to 1,549. However, some
) exhibited an increase in MR when descriptions were added. Conversely, the
impact on Hits@10 is mixed: for quantum models, Hits@10 typically decreased, while for the neural
baseline, certain aggregation functions benefited (e.g.,  
improved).</p>
          <p>In summary, while adding descriptions clearly benefits MR, its efect on Hits@10 varies across models.
Overall, the neural baseline outperforms the quantum models, and among the quantum variants,
weighted addition and concatenation prove to be the most efective aggregation methods.
−</p>
        </sec>
      </sec>
      <sec id="sec-5-4">
        <title>4.4. Discussion</title>
        <sec id="sec-5-4-1">
          <title>4.4.1. Underparameterization of the Quantum Models</title>
          <p>The baseline model outperformed random guessing on the FB15k-237 dataset and even surpassed the
QCE and FQCE models on the MR metric, indicating that our preprocessing pipeline is efective. The
limited performance of our quantum models therefore stems less from the preprocessing and more from
the model architecture itself and its interaction with the pipeline. In our view, this highlights a central
tension in current quantum machine learning: while conceptual designs can anticipate long-term
advantages, today’s hardware and simulation constraints prevent these models from competing with
strong classical baselines. We see our work primarily as a step toward exploring how textual content
could be represented in future quantum knowledge graph embeddings once more expressive circuits
become practical.</p>
          <p>A key diference is that our model learns only two sets of parameters (  1 and  2), whereas FQCE
optimizes distinct parameters for each subject, predicate, and object. For FB15k-237, this translates to
roughly 104 diferent parameter sets for subjects and objects and 237 for predicates. This disparity is
particularly problematic given the higher node degree in FB15k-237.</p>
        </sec>
        <sec id="sec-5-4-2">
          <title>Model Performance on the FB15k-237 Dataset</title>
          <p>The poor performance of text-based methods on
FB15k-237 is well-documented in the literature. Wang et al. [35] attribute this to the dataset’s density –
each subject has an average out-degree of approximately 21, compared to only 2 in WN18RR – and Cao
et al. [36] note that some triples cannot be inferred solely from the training data. These factors explain
the minimal learning observed in our quantum models, particularly on FB15k-237, where the neural
baseline also sufers from insuficient parameter scaling relative to the dataset’s complexity.</p>
        </sec>
        <sec id="sec-5-4-3">
          <title>4.4.2. Model Diferences</title>
        </sec>
        <sec id="sec-5-4-4">
          <title>Robustness of the Amplitude Model</title>
          <p>We observe that amplitude models perform more consistently
across aggregation functions and are less sensitive to changes in input content. Their ability to handle
higher-dimensional vectors appears beneficial, though this doesn’t necessarily lead to better peak
performance compared to angle models.</p>
          <p>Additional Content Improves Performance We can see that adding descriptions improves MR
across all model types – angle, amplitude, and baseline. For instance,    improves from 19,003
to 11,973, and   from 16,705 to 12,979. The neural    model drops from 8,284 to
1,549. While Hits@10 sometimes decreases for quantum models, the consistent MR improvements
suggest that even small amounts of added content are useful—despite the limited input sizes (6 dim. for
angle models, 64 for amplitude).</p>
          <p>Impact of Aggregation Functions The choice of aggregation function clearly influences
performance. Simple addition consistently underperforms, while weighted addition and concatenation yield
better results. In weighted addition, we typically see the subject receiving more weight, resulting in
minimal modification to its vector. Concatenation also performs well, likely due to preserving the
original inputs. The dot product works poorly for quantum models but remains strong in the neural
baseline.</p>
          <p>Quantum vs. Baseline Models Across the board, the neural baseline outperforms the quantum
models. This is likely due to its larger number of trainable parameters and non-linear activations, which
help model complex, non-linear interactions between entities and relations.</p>
        </sec>
        <sec id="sec-5-4-5">
          <title>4.4.3. Quantum Speed-up: Analysis and Limitations</title>
          <p>
            We build on work by Ma et al. [
            <xref ref-type="bibr" rid="ref8">8</xref>
            ], who show that FQCE achieves a quantum training speed-up under two
assumptions: (1) data loading via QRAM scales as  ( log2()), and (2) circuit depth does as well. While
such scaling is theoretically feasible [37], its practicality remains uncertain [38], and no implementation
currently exists. If such a QRAM were available, similar speed-ups would apply to our models using
amplitude encoding.
          </p>
          <p>FQCE circuits require a depth of approximately 3 log2() , based on three variational layers. Our
models scale similarly, with hyperparameter tuning between 2 and 10 layers. In the best cases, fewer
layers sufice; in the worst, more depth is used. This raises the question of whether such quantum
speedups are worth the hardware overhead, especially since typical KGE embeddings use 102–104 dimensions.
However, if embedding sizes or KG complexity grow, the advantage becomes more compelling. For
example, 31 qubits already support an embedding space exceeding 109 dimensions.</p>
        </sec>
      </sec>
    </sec>
    <sec id="sec-6">
      <title>5. Conclusion and Outlook</title>
      <p>In this paper, we integrated the textual content of a knowledge graph into a quantum embedding model
by embedding text into vectors and encoding them as quantum states – introducing a new way to
incorporate rich semantic information into quantum representations. We proposed two quantum models
with distinct properties and developed a classical preprocessing pipeline that efectively combines labels
and descriptions. Evaluation showed that while our models were under-parameterized and performed
worse than established quantum models, integrating additional descriptive content positively impacted
performance – demonstrating the potential of content-enriched quantum embeddings.</p>
    </sec>
    <sec id="sec-7">
      <title>Declaration on Generative AI</title>
      <p>During the preparation of this work, the authors used ChatGPT in order to: Grammar and spelling
check, Paraphrase and reword. After using this tool, the authors reviewed and edited the content as
needed and take full responsibility for the publication’s content.
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