[SCHEDULE 13D] Horizon Quantum Holdings Ltd. Major Shareholder Acquisition (>5%)

Rhea-AI Filing Summary

Horizon Quantum Holdings Ltd. shareholder Joseph Francis Fitzsimons reports major ownership and control following the SPAC business combination. He beneficially owns 19,744,585 Class B Ordinary Shares, representing approximately 38.3% of the Ordinary Shares on an as-converted basis and 65.0% of the issuer’s total voting power as of March 19, 2026.

His Horizon shares were converted into Class B Ordinary Shares at closing of the business combination with dMY Squared Technology Group, Inc. These Class B shares are convertible one-for-one into Class A Ordinary Shares but carry three votes per share. Fitzsimons, the company’s CEO and Board Chairman, is subject to a two-year lock-up on these Class B shares and is party to a Registration Rights Agreement covering resales of underlying Class A Ordinary Shares.

Insights

Equity ownership & governance analyst

Fitzsimons holds a large economic stake and clear voting control at Horizon Quantum.

Dr. Joseph Francis Fitzsimons beneficially owns 19,744,585 Class B Ordinary Shares, equating to about 38.3% of Horizon Quantum’s Ordinary Shares on an as-converted basis. Because each Class B share has three votes, he controls about 65.0% of total voting power as of March 19, 2026.

This filing confirms that the SPAC-related business combination concentrated voting control with the CEO and Chairman. A two-year lock-up limits his ability to sell these shares, while a Registration Rights Agreement commits the company to register resales of underlying Class A Ordinary Shares after the lock-up and registration processes described are completed.

Key Figures

Beneficial ownership: 19,744,585 Class B Ordinary Shares Ownership percentage: 38.3% of Ordinary Shares Voting power: 65.0% of total voting power +5 more

8 metrics

Beneficial ownership 19,744,585 Class B Ordinary Shares Shares directly held by Fitzsimons, as of March 19, 2026

Ownership percentage 38.3% of Ordinary Shares As-converted basis, based on 51,578,134 Ordinary Shares

Voting power 65.0% of total voting power Total issuer voting power held by Fitzsimons as of March 19, 2026

Total Ordinary Shares outstanding 51,578,134 Ordinary Shares 31,833,549 Class A and 19,744,585 Class B as of March 25, 2026

Class A Ordinary Shares outstanding 31,833,549 Class A Ordinary Shares Outstanding as part of total Ordinary Shares as of March 25, 2026

Lock-up duration Two years Transfer restrictions on Fitzsimons’ Class B Ordinary Shares after business combination closing

Prior Horizon shareholding 8,108,696 Horizon ordinary shares Held by Fitzsimons immediately prior to the Amalgamation on March 19, 2026

Registration statement timing Within 30 days of closing Deadline for filing resale registration statement under Registration Rights Agreement

Key Terms

Business Combination Agreement, Lock-Up Agreement, Registration Rights Agreement, Class B Ordinary Shares, +2 more

6 terms

Business Combination Agreement financial

"the Issuer entered into a Business Combination Agreement (the "Business Combination Agreement") with Horizon Quantum Computing Pte. Ltd."

A business combination agreement is a detailed contract that lays out the terms for two companies to join together—covering price, how ownership will be split, the steps needed to close the deal, and what each side promises to do or avoid before closing. For investors it matters because the agreement determines potential changes in value, control, timing, and risk exposure—think of it like the playbook for a merger that shows who wins, who pays, and what could still derail the plan.

Lock-Up Agreement financial

"the Reporting Person entered into a Lock-Up Agreement with the Issuer (the "Lock-Up Agreement")"

A lock-up agreement is a contract that prevents company insiders and early investors from selling their shares for a fixed period after a stock sale, often after an initial public offering. It matters to investors because it temporarily limits the number of shares that can hit the market, which can keep the share price steadier; when the lock-up ends, a sudden increase in available shares can create extra volatility, revealing insiders’ confidence or lack thereof.

Registration Rights Agreement financial

"the Issuer entered into a Registration Rights Agreement with certain shareholders, including the Reporting Person (the "Registration Rights Agreement")"

A registration rights agreement is a contract that gives investors the option to have their ownership stakes officially registered with the government, making it easier to sell their shares later. This agreement matters because it provides investors with a clearer path to cash out their investments if they choose, offering more liquidity and confidence in their ability to sell their holdings when desired.

Class B Ordinary Shares financial

"Represents 19,744,585 Class B Ordinary Shares, with no par value per share"

Class B ordinary shares are a type of ownership stake in a company that typically come with different voting rights or privileges compared to other share classes. For investors, they represent a way to hold part of the company’s value and influence its decisions, often with fewer voting rights than Class A shares. Understanding these shares helps investors assess their level of control and potential returns within a company.

Amalgamation financial

"Merger Sub 1 amalgamated with Horizon, with Horizon surviving as the amalgamated company"

Amalgamation is the combining of two or more companies into a single new business, where assets, liabilities and ownership are merged rather than one firm simply buying another. For investors it matters because an amalgamation can change the value and risk of holdings by creating scale, cutting costs, diluting or concentrating ownership, and introducing integration or regulatory risks—like mixing ingredients to bake a new cake that may taste better or worse than the originals.

SPAC Merger financial

"Merger Sub 2 merged with and into DMY, with DMY surviving the merger as a wholly-owned subsidiary of the Issuer (the "SPAC Merger")"

A SPAC merger is when a private company combines with a SPAC, a publicly traded shell company created to find and buy a business, so the private company becomes publicly listed without a traditional initial public offering. Investors should care because this shortcut can speed up market access but often brings greater uncertainty about valuation, potential share dilution, and reliance on investor votes and future performance, which can increase stock volatility.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

Horizon Quantum Holdings Ltd.

(Name of Issuer)

Class A Ordinary Shares

(Title of Class of Securities)

(CUSIP Number)

Joseph Francis Fitzsimons
29 Media Cir., #05-22,
Singapore, U0, 138565
65 6591 8840

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

03/19/2026

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.

The information required on the remainder of this cover page shall not be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

schemaVersion:

SCHEDULE 13D

CUSIP No.

1

Name of reporting person

Joseph Francis Fitzsimons

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

OO

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

IRELAND

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 19,744,585.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 19,744,585.00
10	Shared Dispositive Power 0.00

11

Aggregate amount beneficially owned by each reporting person

19,744,585.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

38.3 %

14

Type of Reporting Person (See Instructions)

IN

Comment for Type of Reporting Person:

Note to Rows 7 and 11: Represents 19,744,585 Class B Ordinary Shares, with no par value per share (the "Class B Ordinary Shares"), directly held by Dr. Fitzsimons. Each Class B Ordinary Share is entitled to 3 votes per share, whereas each Class A ordinary share, with no par value (the "Class A Ordinary Shares," and together with the Class B ordinary Shares, the "Ordinary Shares") is entitled to one vote per share. Dr. Fitzsimons holds 65.0% of the total voting power of the Issuer as of March 19, 2026. Note to Row 13: Based on 51,578,134 outstanding Ordinary Shares as a single and as-converted class, being the sum of 31,833,549 Class A Ordinary Shares and 19,744,585 Class B Ordinary Shares issued and outstanding as of March 25, 2026.

SCHEDULE 13D

Item 1.	Security and Issuer
(a)	Title of Class of Securities: Class A Ordinary Shares
(b)	Name of Issuer: Horizon Quantum Holdings Ltd.
(c)	Address of Issuer's Principal Executive Offices: 29 Media Cir., #05-22, Singapore, SINGAPORE, SINGAPORE , 138565.
Item 1 Comment: This statement on Schedule 13D relates to the Class A Ordinary Shares and Class B Ordinary Shares of Horizon Quantum Holdings Ltd. ("Issuer"). The Issuer's Ordinary Shares consist of Class A Ordinary Shares and Class B Ordinary Shares. Each holder of Class A Ordinary Shares is entitled to one vote per share and each hold of Class B Ordinary Shares is entitled to three votes per share. The Class B Ordinary Shares are convertible at any time into Class A Ordinary Shares on a one-for-one basis, while Class A Ordinary Shares are not convertible into Class B Ordinary Shares under any circumstances. The Issuer's Class A Ordinary Shares are listed on the Nasdaq Stock Market under the symbol "HQ."
Item 2.	Identity and Background
(a)	Joseph Francis Fitzsimons (the "Reporting Person").
(b)	29 Media Cir., #05-22, Singapore, SINGAPORE, 138565.
(c)	Dr. Fitzsimons serves as Chief Executive Officer and Chairman of the Board of Directors of the Issuer
(d)	During the last five years, the Reporting Person has not been convicted in any criminal proceedings (excluding traffic violations or similar misdemeanors).
(e)	During the last five years, the Reporting Person has not been a party to a civil proceeding before a judicial or administrative body of competent jurisdiction that resulted in such person being subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.
(f)	Dr. Fitzsimons is a citizen of Ireland.
Item 3.	Source and Amount of Funds or Other Consideration
	To the extent required by Item 3, the information contained in Item 4 is incorporated herein by reference.
Item 4.	Purpose of Transaction
	As previously disclosed on the Issuer's Form 20-F filed with the Securities and Exchange Commission on filed on March 25, 2026,, on September 9, 2025, the Issuer entered into a Business Combination Agreement (the "Business Combination Agreement") with Horizon Quantum Computing Pte. Ltd. ("Horizon"), dMY Squared Technology Group, Inc. ("DMY"), Rose Acquisition Pte. Ltd. ("Merger Sub 1"), and Horizon Merger Sub 2, Inc. ("Merger Sub 2"), pursuant to which, and subject to the satisfaction or waiver of certain conditions set forth therein, on March 19, 2026, the following occurred: (1) the Issuer converted from a Singapore private company limited by shares to a Singapore public company limited by shares and, in connection therewith, adopted an amended and restated constitution (the "Holdco A&R Constitution"); (2) Merger Sub 1 amalgamated with Horizon, with Horizon surviving as the amalgamated company and a wholly-owned subsidiary of the Issuer (the "Amalgamation"); (3) Merger Sub 2 merged with and into DMY, with DMY surviving the merger as a wholly-owned subsidiary of the Issuer (the "SPAC Merger"); and (4) completion of the other transactions contemplated by the Business Combination Agreement and documents related thereto (such transactions, together with the Amalgamation and the SPAC Merger, the "Business Combination"). Prior to the Business Combination, Dr. Fitzsimons was the Chief Executive Officer, Chairman and a shareholder of Horizon. Immediately prior to the Business Combination and related transactions described below, the Reporting Person beneficially owned 8,108,696 ordinary shares of Horizon. On March 19, 2026, the Amalgamation occurred. Upon the completion of the Amalgamation, the Horizon ordinary shares held by Dr. Fitzsimons were converted into an aggregate of 19,744,585 Class B Ordinary Shares of the Issuer. Also on March 19, 2026, in connection with the closing of the Business Combination and as contemplated by the Business Combination Agreement, the Reporting Person entered into a Lock-Up Agreement with the Issuer (the "Lock-Up Agreement"), pursuant to which the Class B Ordinary Shares beneficially owned by the Reporting Person will be subject to transfer restrictions for two years after the closing of the Business Combination. Also on March 19, 2026, in connection with the closing of the Business Combination and as contemplated by the Business Combination Agreement, the Issuer entered into a Registration Rights Agreement with certain shareholders, including the Reporting Person (the "Registration Rights Agreement"), pursuant to which the Issuer agreed to register for resale certain Class A Ordinary Shares and Class A Ordinary Shares issuable upon the exercise or conversion of certain securities of the Issuer held by the parties thereto from time to time, including the Class A Ordinary Shares underlying the Class B Ordinary Shares held by Dr. Fitzsimons. The foregoing descriptions of the Business Combination Agreement, Lock-Up Agreement, and Registration Rights Agreement are qualified in their entirety by reference to such agreements, which are filed as exhibits 1 through 3 to this Schedule 13D. Plans or Proposals Dr. Fitzsimons is the founder, Chief Executive Officer and Chairman of the board of directors ("Board") of the Issuer. In this capacity, he may communicate with other members of management, other members of the Board, and/or other stockholders from time to time with respect to operational, strategic, financial or governance matters or otherwise work with management and the Board with a view to maximizing stockholder value. Such discussions and actions may be preliminary and exploratory in nature, and may not rise to the level of a plan or proposal. Except as described in this Schedule 13D, the Reporting Person does not have any present plans or proposals that relate to, or would result in, any of the actions described in Item 4 of Schedule 13D, although the Reporting Person, at any time and from time to time, may review, reconsider and change such position, change his purpose, or develop such plans, and may seek to influence management of the Issuer or the Board with respect to the business and affairs of the Issuer, and may from time to time consider pursuing or proposing such matters with advisors, the Issuer or other persons.
Item 5.	Interest in Securities of the Issuer
(a)	The information set forth in the facing pages of this Schedule 13D with respect to the beneficial ownership of the Reporting Person is incorporated by reference into this Item 5. Dr. Fitzsimons' aggregate percentage of beneficial ownership is approximately 38.3% of the Issuer's Class A Ordinary Shares, assuming the full conversion of Dr. Fitzsimon's Class B Ordinary Shares into Class A Ordinary Shares. References to percentage ownership of Ordinary Shares in this Schedule 13D are based 51,578,134 outstanding Ordinary Shares as a single and as-converted class, being the sum of 31,833,549 Class A Ordinary Shares and 19,744,585 Class B Ordinary Shares issued and outstanding as of March 25, 2026.
(b)	The number of shares as to which there is sole power to vote or to direct the vote, shared power to vote or to direct the vote, sole power to dispose or to direct the disposition, or shared power to dispose or to direct the disposition are set forth on rows 7 through 11 and row 13 of the cover pages of this Schedule 13D and are incorporated herein by reference.
(c)	Except for the transactions described in Item 4 of this Schedule 13D, the Reporting Person has not engaged in any transaction during the past 60 days involving the Ordinary Shares of the Issuer.
(d)	To the best knowledge of the Reporting Person, no one other than the Reporting Person is known to have the right to receive, or the power to direct the receipt of, dividends from, or proceeds from the sale of, the Ordinary Shares reported herein as beneficially owned by the Reporting Person.
(e)	Not applicable.
Item 6.	Contracts, Arrangements, Understandings or Relationships With Respect to Securities of the Issuer
	Item 4 above summarizes certain provisions of the contracts, arrangements, understandings and relationships the Reporting Person has with respect to Ordinary Shares and is incorporated herein by reference. Copies of the agreements summarized in Item 4 and in this Item 6 are attached as exhibits to this Schedule 13D and are incorporated herein by reference. Other than as described in Items 3, 4 and 5, which disclosure is incorporated into this Item 6 by reference, to the Reporting Person's knowledge, there are no contracts, arrangements, understandings or relationships (legal or otherwise) between the Reporting Person and any person with respect to any securities of the Issuer. Indemnification Agreements As a director of the Issuer, Dr. Fitzsimons entered into the Issuer's standard form of Indemnification Agreement (the "Indemnification Agreement"). The Indemnification Agreement provides for indemnification and advancement by the Issuer of certain expenses and costs relating to claims, suits, or proceedings arising from service by Dr. Fitzsimons to the Issuer or, at its request, service to other entities, as a director to the maximum extent permitted by applicable law. Registration Rights Agreement On March 19, 2026, in connection with the closing of the Business Combination and as contemplated by the Business Combination Agreement, the Issuer entered into the Registration Rights Agreement with certain shareholders, including the Reporting Person. Pursuant to the Registration Rights Agreement, the Issuer agreed that, within 30 days after the closing of the Business Combination, the Issuer will file with the SEC (at the Issuer's sole cost and expense) a registration statement registering the resale of certain securities held by or issuable to the parties thereto, including the Reporting Person, and the Issuer will use its reasonable best efforts to have the such registration statement declared effective as soon as reasonably practicable after the filing thereof. Such holders will also be entitled to customary piggyback registration rights and demand registration rights pursuant to the terms of the Registration Rights Agreement.
Item 7.	Material to be Filed as Exhibits.
	1. Business Combination Agreement, by and among the Issuer, Horizon, DMY, Merger Sub 1 and Merger Sub 2, dated as of September 9, 2025 (incorporated by reference to Exhibit 2.1 to the Issuer's Registration Statement on Form F-4 (File No. 333-292737)). https://www.sec.gov/Archives/edgar/data/2088256/000121390026004420/ea0261018-03.htm#T202 2. Form of Lock-Up Agreement (incorporated by reference to Exhibit 10.3 to the Issuer's Registration Statement on Form F-4 (File No. 333-292737)). https://www.sec.gov/Archives/edgar/data/2088256/000121390026004420/ea0261018-03.htm#T206 3. Form of Restated Registration Rights Agreement (incorporated by reference to Exhibit 10.4 to the Issuer's Registration Statement on Form F-4 (File No. 333-292737)). https://www.sec.gov/Archives/edgar/data/2088256/000121390026004420/ea0261018-03.htm#T207 4.* Indemnification Agreement by and between the Issuer and Joseph Fitzsimons, dated March 19, 2026. *Filed herewith.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Joseph Francis Fitzsimons

Signature:	/s/ Joseph Francis Fitzsimons
Name/Title:	Joseph Francis Fitzsimons, Self
Date:	03/31/2026