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Qci Software Engineer Interview Guide

Qci Software Engineer interview guide: reported rounds, questions and how candidates describe the process.

The 60-second brief

At Quantum Computing Inc. (Qci), a Software Engineer plays a pivotal role in bridging the gap between cutting-edge quantum hardware and accessible, real-world applications. Qci is an innovative, integrated photonics company that delivers room-temperature, low-power quantum machines designed to solve complex challenges in high-performance computing, cybersecurity, artificial intelligence, and remote sensing. As a Software Engineer, you will not just write isolated code; you will design and implement the vital software interfaces, APIs, and client libraries that allow external systems and cloud platforms to interact seamlessly with quantum and photonic hardware. This position demands a unique blend of systems-level thinking and high-level software craftsmanship.

This guide is scoped to a Software Engineer candidate at Qci.

Focus areas

  • C++
  • Python
  • Hardware/Software Integration

The Qci interview process

Qci candidates report 3 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Initial Phone Screen

Informal conversation with a recruiter or engineering representative focused on background and career goals.

What to demonstrate

  • Informal conversation with a recruiter or engineering representative focused on background and career goals
  • Depth in C++

How to prepare

  • Be able to walk your CV end to end in two minutes, and say why this company specifically.
  • Have your salary expectations, notice period and location constraints ready, and ask for the rest of the loop in writing.

Technical Screening

Involves online coding challenges, discussions about programming languages, and review of past projects.

What to demonstrate

  • Involves online coding challenges, discussions about programming languages, and review of past projects
  • Depth in C++

How to prepare

  • Answer aloud and timed: Describe the process of serialization and deserialization when sending data over gRPC or REST APIs.
  • Answer aloud and timed: What are the main differences between C++11 smart pointers (shared_ptr, unique_ptr, weak_ptr), and when would you use each?

Comprehensive Interview

Final stage featuring technical whiteboarding, system design discussions, and behavioral interviews with senior engineers.

What to demonstrate

  • Final stage featuring technical whiteboarding, system design discussions, and behavioral interviews with senior engineers
  • Depth in C++

How to prepare

  • Answer aloud and timed: How do you handle concurrency and thread safety in a multi-threaded C++ backend application?
  • Answer aloud and timed: Write a function to reverse a linked list, and discuss its time and space complexity.

Where candidates lose points

Emphasize your hardware-adjacent experience

During your resume walkthrough, highlight any projects where your software interacted with physical systems, sensors, microcontrollers, or complex network hardware. This immediately signals your suitability for Qci's unique engineering environment.

Talk through your whiteboard code

When writing code on a whiteboard or virtual editor, do not code in silence. Explain your strategy, discuss edge cases before you write them, and explicitly state your time and space complexity. The interviewers care as much about your problem-solving process as they do about your syntax.

If you get stuck during a coding exercise, ask clarifying questions

Qci interviewers appreciate candidates who seek to understand requirements thoroughly before writing code, as this reflects real-world engineering practices.

Questions & practice

Explain the difference between stack and heap memory allocation in C++, and how you prevent memory leaks in a

Medium · Systems & Low-Level Programming

Explain the difference between stack and heap memory allocation in C++, and how you prevent memory leaks in a long-running system.

Approach

  1. Say what the runtime actually does before reasoning about the code.
  2. Name what is shared across threads and what owns each piece of state.
  3. Identify the window where an invariant is briefly untrue.
  4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

  • What happens if two callers reach this at the same time?
  • Where could this allocate more than you expect?

How do communication protocols like UART and SPI differ in terms of speed, wiring, and data transmission?

Medium · Systems & Low-Level Programming

Approach

  1. Say what the runtime actually does before reasoning about the code.
  2. Name what is shared across threads and what owns each piece of state.
  3. Identify the window where an invariant is briefly untrue.
  4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

  • What happens if two callers reach this at the same time?
  • Where could this allocate more than you expect?

What are the main differences between C++11 smart pointers (shared_ptr, unique_ptr, weak_ptr), and when would

Medium · Systems & Low-Level Programming

What are the main differences between C++11 smart pointers (shared_ptr, unique_ptr, weak_ptr), and when would you use each?

Approach

  1. Say what the runtime actually does before reasoning about the code.
  2. Name what is shared across threads and what owns each piece of state.
  3. Identify the window where an invariant is briefly untrue.
  4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

  • What happens if two callers reach this at the same time?
  • Where could this allocate more than you expect?

How do you handle concurrency and thread safety in a multi-threaded C++ backend application?

Medium · Systems & Low-Level Programming

Approach

  1. Say what the runtime actually does before reasoning about the code.
  2. Name what is shared across threads and what owns each piece of state.
  3. Identify the window where an invariant is briefly untrue.
  4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

  • What happens if two callers reach this at the same time?
  • Where could this allocate more than you expect?

Write a function to reverse a linked list, and discuss its time and space complexity.

Medium · General Coding & Algorithms

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Given an array of integers, find the two numbers that add up to a specific target sum (Two Sum).

Medium · General Coding & Algorithms

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Explain how a hash table works internally, and how you resolve collisions.

Medium · General Coding & Algorithms

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Write a recursive algorithm to solve a classic dynamic programming problem, such as the Fibonacci sequence or

Medium · General Coding & Algorithms

Write a recursive algorithm to solve a classic dynamic programming problem, such as the Fibonacci sequence or the climbing stairs problem, and then optimize it using memoization.

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Describe the event loop in JavaScript and how asynchronous operations are handled.

Medium · Web & Database Technologies

Approach

  1. Say what the runtime actually does before reasoning about the code.
  2. Name what is shared across threads and what owns each piece of state.
  3. Identify the window where an invariant is briefly untrue.
  4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

  • What happens if two callers reach this at the same time?
  • Where could this allocate more than you expect?

Explain the difference between an inner join, left join, and outer join in SQL, and how indexes improve query

Medium · Web & Database Technologies

Explain the difference between an inner join, left join, and outer join in SQL, and how indexes improve query performance.

Approach

  1. Name the grain you start from and join outward from it.
  2. Check whether any join is one-to-many before aggregating, or the sums inflate.
  3. Say which index the query would use, and what makes it unusable.
  4. Handle the rows that do not match: that is usually the actual question.

Follow-up

  • How does the query change if that join becomes one-to-many?
  • What happens to this when the table is ten times larger?

Keep soft-deleted accounts from blocking re-registration

Easy · soft delete · unique constraints · engine differences

app_user holds user_id, tenant_id, email CITEXT, password_hash (NULL for SSO principals), email_verified_at, auth_version, status ('invited','active','suspended','deactivated'), created_at, updated_at, deleted_at. Two live accounts for one address inside a tenant must be impossible, but an address freed by a soft delete must be reusable, and the same tenant may delete and re-register it repeatedly. Write the uniqueness DDL for PostgreSQL 16, then the equivalent for MySQL 8 where partial indexes do not exist, and say what each permits once three deleted rows already hold that address.

Approach

  1. Start from what is actually unique: not (tenant_id, email), but (tenant_id, email) among live rows. PostgreSQL says that directly — CREATE UNIQUE INDEX app_user_live_email ON app_user (tenant_id, email) WHERE deleted_at IS NULL. A full constraint over the same two columns burns the address permanently the first time someone deletes an account.
  2. Keep case-insensitivity in the type or the index, never in the application: CITEXT as given, or UNIQUE (tenant_id, lower(email)) as an expression index where the extension is unavailable. A case-sensitive unique column is exactly how two accounts for one human appear.
  3. For MySQL 8 the predicate has to move inside the key: add a discriminator column that is a constant 0 while the row is live and is set to user_id on delete, with UNIQUE (tenant_id, email, deleted_marker). Live rows share the constant and still collide; deleted rows differ from each other and stop colliding.
  4. State the NULL variant and its dependency: leaving the marker NULL for deleted rows also works, because a unique index treats NULLs as distinct — true in MySQL, and true in PostgreSQL only under the default NULLS DISTINCT, which PostgreSQL 15 lets you reverse. Check the polarity against the three existing deleted rows: constant-on-live is what preserves the collision you want, and reversing it silently admits duplicate live accounts.

Follow-up

  • A deleted account re-registers with the same address the next day. Do the old resource rows follow the new user_id, and how does the API keep the two principals apart?
  • How do you honour an erasure request while resource_revision.actor_user_id still references this table?

Describe the process of serialization and deserialization when sending data over gRPC or REST APIs.

Medium · Systems & Low-Level Programming

Approach

  1. Say who the caller is and what they do when the call fails halfway.
  2. Define the identity of a request so a retry cannot double-apply it.
  3. Separate accepted, pending, failed and confirmed; they are different facts.
  4. Design the error taxonomy before the success shape; callers branch on it.

Follow-up

  • What happens if the caller retries after a timeout?
  • How does a client discover it is on an old version of this contract?

How would you design an algorithm to detect a cycle in a directed graph?

Medium · General Coding & Algorithms

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

What is the difference between RESTful APIs and GraphQL, and in what scenarios would you choose one over the o

Medium · Web & Database Technologies

What is the difference between RESTful APIs and GraphQL, and in what scenarios would you choose one over the other?

Approach

  1. Say who the caller is and what they do when the call fails halfway.
  2. Define the identity of a request so a retry cannot double-apply it.
  3. Separate accepted, pending, failed and confirmed; they are different facts.
  4. Design the error taxonomy before the success shape; callers branch on it.

Follow-up

  • What happens if the caller retries after a timeout?
  • How does a client discover it is on an old version of this contract?

How do CSS frameworks and responsive web design principles impact frontend performance?

Medium · Web & Database Technologies

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

Explain the Model-View-Controller (MVC) architectural pattern and how it is implemented in modern frameworks l

Medium · Web & Database Technologies

Explain the Model-View-Controller (MVC) architectural pattern and how it is implemented in modern frameworks like Spring Boot or Express.

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

Walk me through the most complex software architecture you have designed and implemented.

Medium · Behavioral & Experience-Based

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

One log partition stops advancing while the others drain

Easy · poison message · head-of-line blocking · consumer lag

Search results for a subset of tenants are hours stale; the rest are current. The projection consumer reports lag of zero on 15 of 16 partitions and 400,000 on one. Its error rate is flat and its CPU is idle. outbox_event has no pending rows older than a second, so the relay has published everything it holds. Identify the mechanism, give the ordered checks, and state what you do in the first ten minutes versus what you change permanently.

Approach

  1. Read the lag distribution first. A slow consumer lags everywhere; zero on fifteen partitions and 400,000 on one is not throughput. Idle CPU on the stuck partition means the consumer is not advancing its offset at all, which points at one message it cannot get past rather than at a rate problem.
  2. Exonerate the producer before touching the consumer. No pending outbox rows older than a second means the relay published, so the event exists in the log. This separates never sent from sent and never applied, which are different code paths and usually different owners.
  3. Read the message at the stuck offset and the handler's log lines for its event_id. A flat error rate with no progress has two explanations and you must distinguish them: the handler is throwing and the retry loop is swallowing it, or the handler is blocking on something and never returning. Idle CPU with no error lines favours the second.
  4. Mitigate before diagnosing further. Move the offending event to a dead-letter store and commit the offset past it. Adding consumers does nothing here, because a partition is consumed by exactly one member of the group, and the blast radius is every aggregate hashed to that partition, not only the aggregate that produced the bad event.

Follow-up

  • The dead-lettered event carried aggregate_version 7 and the projection had applied 6. What must the replay do differently if 8 and 9 landed in the meantime?
  • How do you show staleness to the user while the partition is behind, given the API already returns the projection's watermark?

Suggested practice checklist

Built from the rounds and topics Qci candidates report.

Day 1 — Map the Qci loop

  • Write out the reported sequence: Initial Phone Screen, Technical Screening, Comprehensive Interview.
  • For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.

Deliverable: A one-page map of the 3 reported rounds, with the weakest marked.

Day 2 — Work C++

  • Spend the session on C++, which Qci candidates report being tested on.
  • Write one worked example in C++ and time yourself on it.

Deliverable: One timed worked example in C++.

Day 3 — Work Python

  • Spend the session on Python, which Qci candidates report being tested on.
  • Write one worked example in Python and time yourself on it.

Deliverable: One timed worked example in Python.

Day 4 — Work Hardware/Software Integration

  • Spend the session on Hardware/Software Integration, which Qci candidates report being tested on.
  • Write one worked example in Hardware/Software Integration and time yourself on it.

Deliverable: One timed worked example in Hardware/Software Integration.

Day 5 — Answer out loud: Systems & Low-Level Programming

  • Answer aloud, timed: Explain the difference between stack and heap memory allocation in C++, and how you prevent memory leaks in a long-running system.
  • Answer aloud, timed: How do communication protocols like UART and SPI differ in terms of speed, wiring, and data transmission?

Deliverable: Spoken answers to 2 reported Systems & Low-Level Programming question(s), under time.

Day 6 — Answer out loud: General Coding & Algorithms

  • Answer aloud, timed: Write a function to reverse a linked list, and discuss its time and space complexity.
  • Answer aloud, timed: Given an array of integers, find the two numbers that add up to a specific target sum (Two Sum).

Deliverable: Spoken answers to 2 reported General Coding & Algorithms question(s), under time.

Day 7 — Answer out loud: Web & Database Technologies

  • Answer aloud, timed: Explain the difference between an inner join, left join, and outer join in SQL, and how indexes improve query performance.
  • Answer aloud, timed: Describe the event loop in JavaScript and how asynchronous operations are handled.

Deliverable: Spoken answers to 2 reported Web & Database Technologies question(s), under time.

Behavioural preparation

Behavioural rounds judge the decision you made and what it cost.

Describe a time when you had to collaborate with a hardware or firmware team to resolve a challenging software

Medium · Behavioral & Experience-Based

Describe a time when you had to collaborate with a hardware or firmware team to resolve a challenging software bug.

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

How do you handle a situation where requirements change mid-way through a development sprint?

Medium · Behavioral & Experience-Based

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Tell me about a time you had to optimize a slow-performing system. What was your process, and what were the re

Medium · Behavioral & Experience-Based

Tell me about a time you had to optimize a slow-performing system. What was your process, and what were the results?

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Why are you interested in working in the quantum computing and photonics industry, and how does your backgroun

Medium · Behavioral & Experience-Based

Why are you interested in working in the quantum computing and photonics industry, and how does your background align with Qci's mission?

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?