Joe Report

Exploring Qubit Superposition and Measurement

Project Discoveries

We discovered how a qubit operates through emulation software, but how does this relate to our overall understanding, specifically in relation to quantum ciphers? We are going to have to touch on several topics to really let these concepts sink in, because the physical reality of quantum computing and the emulated one are two things entirely.

First, we created a software qubit. Then, we passed it through a Hadamard gate to influence probability amplitude collapse. After that, we ran shots (or measurement iterations) of the state collapse of the qubit to infer patterns of collapse. We plotted those results on a histogram to see how gates impact the classical state. Next, we plotted that qubit on a Bloch Sphere to see how it might exist prior to state collapse into a classical bit by using the state vector function to view it in a superposition state.

So, what the heck does all this mean? A qubit is nothing like a classical bit. A gate in classical programming is absolutely nothing like a gate in quantum programming, so let’s begin with those two concepts. Then, we can delve into how they exist in the physical plain.

A Qubit in Reality

A qubit is less of a ‘thing’ and more of a state of potential things before it happens. Think of it like an environment created where there is a lot of potential to shape a finalized bit, but we need to introduce the proper ingredients first and a trigger that ignites everything.

It could be thought of as a studio with a canvas, a painter’s palette, and the room that the canvas sits inside of, with infinite possibilities before us. We then introduce a painter, comparable to a gate or excitation, who interacts with the environment and causes an output. We measure the art output to gain information about the art, the canvas, the painter, and even the room itself. That entire environment forms the art signature, which is comparable to a quantum fingerprint. By shaping these variables, we can influence what the art becomes. We can also examine the setup of the room itself to infer what was happening before the output occurred, comparable to quantum sensing.

In reality, though? A qubit isn’t mystical. It may well exist as the undetermined spin state of an electron sitting in a nitrogen vacancy created inside of a diamond, or many other material possibilities. One thing it will involve is a controlled way of exciting that state to produce an output that tells us about, potentially, the entire system.

The code we wrote and the project we ran is just an emulation of that system, but there’s much more to grasp that happens behind the code.

Material Impact

In the painting analogy, we all know that the palette an artist uses impacts the art output, as do the artist and the room they are in. Material use and tools in quantum computing have a tremendous impact on everything. It is much more than merely the difference between using copper wiring and fiber optics for data transmission in networking, for example. The reason for this is that copper wiring in classical computing doesn’t really change anything beyond speed of delivery and possibly some packet loss or noise if it is not properly shielded. In quantum computing, the materials and fields used can determine the physical constraints and dynamics that change qubit behavior and collapse outcomes. In other words, they do not merely act as carriers of predetermined data or bits. Quantum systems are much more organic and interconnected.

In fact, we can manipulate material properties and applied fields to influence quantum behavior and also the outcomes of measurement. This is known as Hamiltonian engineering, and this is exactly how gates work in quantum computing to impact outcomes.

Material and Gate Example

A gate in quantum computing is a controlled change to a physical state that influences a system. In code, we see only probability amplitudes, but in practice, we are manipulating real physical conditions to shape outcomes.

A Hadamard gate is a practical example that puts a qubit into superposition. Using a diamond as an example, the diamond contains an electron that can freely occupy different spin states. Initially, this electron is in the ground state, stable and carrying no measurable information.

To create a qubit, a carefully tuned microwave pulse excites the electron and rotates its spin, producing a superposition of |0⟩ and |1⟩ states. Different pulse shapes, durations, or frequencies allow us to implement various quantum gate operations. This mirrors our project work, such as when we plotted a qubit on a Bloch Sphere and used statevector() to view superposition without collapsing it.

The qubit remains in superposition until measurement occurs. In the diamond NV center, a laser excites the electron, which relaxes and emits one or more photons, revealing its spin state. Detecting these photons is when measurement occurs, and it collapses the qubit to a 1 or 0 classical bit.

In short, the microwave pulse creates the gate and influences probability amplitudes, while the laser allows us to measure the classical state. After measurement, the electron returns to a post-measurement state, ready for the next calculation or ‘shot’ to occur.

Types of Materials and Their Uses

Different types of materials have distinct advantages and limitations. For this tutorial, we will focus on three material types to maintain conceptual clarity and brevity.

In the cases of diamond NV centers and quantum dots, the qubits are stationary, localized within the material. In contrast, photonic qubits are mobile, and they travel through optical systems rather than remaining fixed in place.

We will expand on each material category to provide more context and detail.

Diamonds with Nitrogen Vacancy Centers (Storage) [solid state]

Creation:

These are specially engineered diamonds that contain a nitrogen atom next to a vacant carbon site. This vacant area is where the electron can spin freely and what creates the NV (Nitrogen Vacancy)

Pros:

Stable and reliable for memory or storage. It can operate at room temperature. It can be integrated into small scale optical systems for experimentation or QKD (Quantum Key Distribution) applications.

Cons:

Very costly to create. Scaling to large numbers of qubits is a major challenge. Gate operations are slower compared to other types of materials.

Best Application:

Because it has long coherence times compared to other materials (it can hold its superposition state longer), it is a solid option for storage and stability. The NV center is well insulated from noise and reduces error rates. Photon-based measurement and readout allows the qubit’s state to be read without destroying it.

Special Considerations:

NV Centers allow for optical readouts by measuring emitted photons. It can be used in quantum sensing (detecting tiny changes in magnetic fields, temperature, or strain with high precision), which is unique compared to other materials.

Photonic Qubits (Networking) [transporting qubit]

Creation:

Photonic qubits are created using single photons generated by lasers or nonlinear optical processes, often encoded in properties such as polarization, phase, or time-bin. Optical circuits, beam splitters, and waveguides are used to manipulate and route these photons.

Pros:

Photons travel long distances with minimal loss, making them ideal for quantum communication and networking. They are naturally resistant to decoherence and can operate at room temperature. Quantum gate operations using photons can be extremely fast due to the speed of light.

Cons:

Interacting photons for gate operations is challenging, often requiring probabilistic methods or nonlinear materials. Scaling multi-photon operations is difficult due to photon loss and detector inefficiencies. Storing photons long-term is harder compared to solid-state qubits.

Best Application:

Photonic qubits excel at transmitting quantum information over distance, making them ideal for quantum networking, distributed computing, and Quantum Key Distribution (QKD). They are also useful in certain quantum computing protocols where speed and coherence over distance are critical. They are particularly suited for Bell State Measurements (BSM), which are required for entanglement-based protocols and quantum teleportation.

Special Considerations:

Photonic qubits can be entangled easily across distances and are compatible with fiber-optic infrastructure. They can also implement measurement-based quantum computing schemes and are used in advanced quantum sensing techniques that exploit interference patterns.

Quantum Dots in Semiconductors (Processing) [solid state]

Creation:

Quantum dots are nanoscale semiconductor structures that confine electrons or holes in all three spatial dimensions. They are typically created using techniques like lithography, self-assembly during epitaxial growth, or chemical synthesis. This confinement gives them discrete, atom-like energy levels.

Pros:

Can be fabricated using established semiconductor manufacturing techniques. Good for fast gate operations due to strong interaction with applied electric or magnetic fields. Can be integrated into larger arrays, offering a path toward scalability. Can be manipulated electrically, which makes control circuits simpler.

Cons:

Coherence times are generally shorter than NV centers in diamonds. Sensitive to environmental noise, requiring cooling or shielding for stable operation. Optical readout is more complex compared to NV centers.

Best Application:

Quantum dots excel in processing and manipulation of qubits, where fast gate operations and controllability are more critical than long-term storage. They are ideal for computation-focused tasks, high-speed quantum logic gates, and experiments requiring tunable qubit interactions.

Special Considerations:

Quantum dots can be engineered to couple with photons for optical interconnects or entanglement. Their tunable energy levels allow flexible gate design and can be adapted for use in quantum error correction schemes. They’re also good for PUFs (physically unclonable functions), which can be incredibly useful in cryptography and creating a cipher.

Solid State vs Mobile Qubits

Solid State Qubits

These qubits exist in a stationary place, such as in the NV center of a diamond or in semiconductor quantum dots as mentioned above. Their state is well controlled and contained, typically within a lattice or engineered structure. In the case of diamond NV centers, the qubit corresponds to the spin state of an electron within the carbon lattice. Solid state qubits can be manipulated locally, maintain coherence for memory and computational tasks, and allow direct gate operations. The main limitation is that transmitting quantum information over long distances requires additional protocols, such as entanglement swapping or optical interconnect workarounds that mobile qubits can achieve more naturally.

Mobile Qubits

These are not confined to a physical lattice and can travel through free space, such as in optical fibers. They can transmit quantum data over distances efficiently while maintaining coherence, making them ideal for quantum networking. They do not reside in a stable lattice, but photonic qubits are encoded into the properties of the photon itself. They require specialized optical circuits, or “junction boxes”, where gate operations are applied as the qubits travel along their path.

In essence, solid state qubits are anchored and ideal for local computation and storage, while mobile qubits are designed for communication and linking distant nodes.

Conclusion: Qubits, Fingerprints, QPUFs, and BSM

We explored what physically is happening behind the scenes when quantum code executes for a reason. It’s important to understand how all of these parts work together to build a complete picture of the system. Emulation helps us grasp the principles, while knowing the hardware and science behind the code opens entirely new doorways to insight.

Gates, materials, and measurement all matter: they interact to produce outcomes. Gates not only impact the probabilities of outcomes but can also entangle qubits, such as through a Bell State Measurement (BSM). Materials can leave behind signatures in the quantum state, creating Quantum Physical Unclonable Functions (QPUFs) and enabling quantum fingerprinting, which makes states unique and extremely difficult to replicate. Measurement determines the qubit’s lifecycle, collapsing the superposition and resetting it for future operations.

In the next section, we will delve more deeply into different gate types and their functions.

References

Solid‑state qubits (NV centers in diamond, long coherence, multi‑qubit memory) https://arxiv.org/abs/1905.02094

Quantum Physical Unclonable Functions (QPUFs) / Quantum fingerprinting https://arxiv.org/abs/1910.02126

QPUF: Quantum Physical Unclonable Functions for Security‑by‑Design of Industrial Internet‑of‑Things, a recent practical proposal applying QPUFs to hardware security and fingerprinting https://www.mdpi.com/2410-387X/9/2/34

Quantum sensing (NV‑center based sensing, material / interface engineering) Enhancement of quantum coherence in solid‑state qubits via interface engineering, shows how interface engineering can improve coherence of shallow NV centers, relevant for sensing and stable qubit operation https://www.nature.com/articles/s41467-025-61026-3

Hybrid or network‑ready quantum nodes (color‑centers + photonics, quantum networks) Solid‑state quantum nodes based on color centers and rare‑earth ions coupled with fiber Fabry–Pérot microcavities, recent review of solid‑state emitters + photonic integration as building blocks for quantum networks https://www.sciencedirect.com/science/article/pii/S2709472323000448