Quantum Foundations – Physics Unleashed!
Let’s get our physics on and explore some key quantum foundations.
Here is the next post in our series on Quantum Technologies. Here we dive a bit deeper into the core foundational properties and characteristics of Quantum Technologies including superposition, entanglement, decoherence and others. Yes, much of this stuff comes from the mind bending realm of quantum physics and the big brains of Albert Einstein, Max Planck, Niels Bohr, Werner Heisenberg, Erwin Schrödinger, Paul Dirac, and Wolfgang Pauli to name a few. Let’s dig in and simplify!
Superposition
Quantum bits or qubits, are the quantum version of classic (binary ones and zeros) bits. Similar to classic bits, a qubit can be in a state of a one or a zero, OR, a superposition of both simultaneously. In superposition a qubit representing a quantum particle can be in all possible quantum states between one and zero. This is foundational and a critical aspect of quantum technologies/computing because groups of qubits in superposition can create complex, multidimensional computational spaces and process exponentially more information than classical computers. NOTE: Superposition of states in a qubit is not just theory – it has been experimentally demonstrated.
A Pendulum Swinging Between Two Points:
Here is a useful analogy for understanding superposition, the idea that a quantum system can be in multiple states at once until measured.
Think of a pendulum swinging from left to right. At any moment during the swing, the pendulum is not just in a left or right position, but somewhere in between, effectively covering all possible positions simultaneously, until we measure its position, at which point it resolves to a single identifiable position.
With superposition, a qubit system with quantity n qubits can encode 2n possible states in parallel. This property enables quantum computers to perform many calculations simultaneously—a phenomenon known as quantum parallelism—which is the basis for the exponential faster speed delivered by quantum computing relative to classic computing. This means complex problems can be represented in new ways in quantum computing and this in part explains why researchers are racing to scale quantum computing systems to operate thousands, tens of thousands, or even larger quantities of qubits.
The more qubits operating (that are stable), the more powerful the system. Current “state of the art” (as of Sept 2025) for volume of operational qubits is slightly more than 1,000, with many platforms operating tens or 1-2 hundreds of qubits. (This is a rapidly evolving space 😊)
Superposition also plays a crucial role in quantum error correction, where the superposition of states can contribute to the detection and correction of errors without disturbing the information stored in the quantum state, which is essential for the practical realization of large-scale quantum computers.
Superposition is also integral to creating quantum cryptography where cryptographic keys are encoded in the superposition states of photons, providing a level of security that is impossible with classical cryptography methods.
While counterintuitive, the principle of superposition is a powerful tool in quantum computing, enabling parallelism, quantum simulation, quantum error correction, and quantum cryptography. As our understanding and control of quantum systems continue to improve, we can expect to see even more practical applications of superposition in the future.
Tradeoffs? Superposition states are highly sensitive to their environment - a phenomenon called Decoherence, more on this below. Maintaining superposition states long enough for meaningful computation is one of the core engineering challenges for building reliable quantum computers.
Striping out all the physics talk, quantum computing is nothing more than a technological system for creating and manipulating qubit superpositions in a controlled way – aka harnessing superposition is the first step to achieve quantum computing.
Entanglement
Now let’s tackle entanglement – which is somewhat even MORE mind bending than superposition.
Quantum entanglement is a phenomenon in which two qubits (or any two or more quantum particles) intertwine in such a way that the state of one particle cannot be described independently of the state of the other, regardless of the distance between them.
When two qubits are entangled, they both exist in a superposition until either is measured. Once observed, the quantum superposition of both is collapsed and whichever qubit isn’t observed assumes the opposite position of the one that was observed. Net-net, entanglement is the ability of qubits to correlate their state with other qubits. Entangled systems are so intrinsically linked that when quantum processors measure a single entangled qubit, they can immediately determine information about other qubits in the entangled system.
As with superposition, entanglement is real. Experiments in the 1980’s confirmed that quantum entanglement is a real phenomenon, and it is now a cornerstone of quantum mechanics.
Why is entanglement important and useful? While there is a LOT of work to be done, these counterintuitive principles will form the foundation of next-generation technologies. Here are a few examples:
Quantum cryptography:
Enables ultra-secure communications, as any interference instantly disrupts entangled states and reveals eavesdropping attempts.
Quantum key distribution protocols use entangled pairs to create theoretically unbreakable encryption keys.
Quantum computing:
Entanglement enables quantum computers to perform parallel computations and run algorithms exponentially faster than classical computers.
Quantum teleportation:
Allows for the transmission of particle states between distant locations using entangled pairs, crucial for future quantum networking and MAYBE even the quantum internet.
Quantum measurement and sensing:
Enables measurement precision beyond classical limits, benefiting fields like spectroscopy, gravitational wave detection, and magnetic field sensing.
Quantum sensors: Achieve extreme atomic level sensitivity and reliability for object detection and analysis for positioning, timing and navigation systems.
You may have picked up that principles of quantum entanglement and superposition are intertwined, and this relationship is a complex one. When two particles are entangled, the state of one particle is directly related to the state of the other, no matter the distance between them. This is a result of the superposition principle. When a measurement is made on one of the entangled particles, it collapses from a superposition of states into a single state. This collapse instantaneously affects the other particle’s state, causing it to collapse into a corresponding state.
Interference
Interference is another key engine of quantum computing. As mentioned above in entanglement, when a measurement is made on one of a pair of entangled particles, it collapses from a superposition of states into a single state. However, the single state that results is only one of the possible results, and this result is random. You do not know what result will emerge. The result you get is determined by wave amplitude probabilities. This is where interference plays a critical role.
An environment of qubits placed into a state of collective superposition, structures information in a way that looks like waves, with amplitudes associated with each outcome. Interference is a phenomenon where two or more waves overlap and form a new wave pattern, and the amplitudes of the waves become the probabilities of the outcomes of a measurement of the system.
Waves can build on each other - if the peak of one wave aligns with peak of another, they add up (constructive interference), or if a peak aligns with a trough, they cancel out (destructive interference). Amplifying a probability or canceling out others are both forms of interference.
Interference is present in many systems including water, light, sound, and atomic particles like electrons show the same behavior. When two stones are dropped into water, the resulting ripples spread out and meet. Where the peaks of the waves meet, the water rises higher (constructive interference). Where the peak of one meets the trough of another, the waves flatten out (destructive interference).
Interference in quantum computing: As an example, a qubit’s state has an amplitude for “0” and an amplitude for “1”. If you apply certain sequences of operations, you can cause these amplitudes to interfere. As reviewed earlier, with superposition quantum computing systems can explore many possibilities at once. We then apply interference to favor the right answer and filter out the wrong ones.
Stated another way, a well-designed quantum algorithm sets up the problem such that all the myriad ways to get a wrong answer cancel each other out, and all the ways to get the right answer line up together by leveraging interference. When you measure the qubits, the right answer leaps out with high probability – the system has engineered an interference pattern to make the right answer emerge.
Quantum circuits are engineered so that as qubits are manipulated, their probability amplitudes interfere in ways that make the correct answer more likely. In this way interference is the “secret sauce” that lets qubits work together to find answers in ways no classical computer can.
In a future post we dive deeper into interference and review some practical examples of how Grover’s Algorithm and Shor’s Algorithm take advantage of interference to execute real computations .
Coherence - Decoherence
Interference may be the “secret sauce” of quantum computing, but like superposition, interference is fragile. It requires the quantum system to maintain rock solid stability and keep qubits in well-defined states of superposition – this property is known as Coherence. The system stability created in coherence in turn supports interference, entanglement, and ultimately allows quantum algorithms to operate effectively.
Loss of Coherence – known as Decoherence, happens through interaction with environmental factors such as thermal fluctuations, electromagnetic fluctuations, and material defects that create noise which causes quantum states to lose their unique quantum behaviors. In decoherence, the quantum system collapses into a non-quantum state and reverts to classical statistics. Sources of decoherence vary by platform, so identifying, understanding and countering sources of noise is extremely complex and crucial for the success of quantum computing.
Various strategies, such as error correction codes, isolating systems, and using topological qubits, are being developed to protect qubits from decoherence and to extend and preserve coherence. Preserving coherence is a core and difficult engineering challenge in quantum hardware design. You can expect more coverage on this topic in future posts.
Quantum Teleportation – Let’s Meet Alice and Bob!
A good place to start for understanding Teleportation in quantum systems is to review why this technique is helpful.
Teleportation is useful to perform remote operations on quantum systems particularly in situations where it’s not feasible to physically move a qubit from one location to another, or where we want to perform a quantum operation on a qubit that is in an inaccessible location. Quantum teleportation is often used in error correction systems where quantum states are encoded into multiple qubits in a way that makes them more resilient to errors. Further, teleportation can be used to transmit information about the state of a qubit that has been subject to errors to a different location, where it can be corrected using error-correcting codes.
How it works – this is the Alice and Bob show!
Teleportation is a protocol in quantum computing that provides for the transfer of an unknown quantum state from one location to another without physically transmitting the state itself. The process doesn’t transport physical matter. It transfers the exact properties or state of a quantum particle - like the state of an atom or photon. Quantum teleportation uses both entanglement and classical communication to move information.
Here is simple explanation for how teleportation operates. It is often expressed as an interchange between two human characters, Alice and Bob.
Alice and Bob each have a qubit (or a photon) that is part of an entangled pair meaning their states are highly connected.
Alice (the sender) wants to send the state of a 3rd qubit – called the “message qubit” – to Bob (the receiver).
Alice takes the message qubit and entangles it with her own qubit using a specific quantum operation called a CNOT gate. This creates a three-qubit entangled state.
Alice then performs a measurement on both the message qubit and her own qubit. The result of this measurement is a classical two-bit string, which she sends to Bob using a classical communication channel.
Next, Bob performs a specific quantum operation on his own qubit from the entangled pair. This operation effectively “teleports” the state of the message qubit onto Bob’s qubit.
Bob now has a qubit that is in the same state as the original message qubit, and Alice’s qubit and the message qubit are now both in a different state.
Net-net, teleportation exploits the phenomenon of quantum entanglement to transfer an unknown quantum state via a combination of entangled qubits and classical communication. In effect, teleportation turns entanglement into a transport for quantum data, thereby “networking” quantum processors in a way analogous to how classical networks exchange bits.
Quantum teleportation has the potential to revolutionize secure communications and lead to the implementation of new quantum networks with built-in security properties that will protect data in ways that are not possible today. Teleportation also holds out promise of a fully realized quantum internet – WWWHHAAAATTT! (another topic for a future post)
Quantum Tunneling
Finally, quantum tunneling refers to the observable fact in which a quantum particle penetrates an energy barrier that is higher in energy – something it should note be able to accomplish. In the quantum world, particles are also waves, and if a wave can “leak” through a barrier, there is a chance the particle will pop out on the other side.
Quantum tunneling enables several technological wonders, including nuclear fusion that powers the sun, and devices like the scanning tunneling microscope. I mention tunneling here because it plays key roles in quantum computing platforms (vendor products), such as where superconductors are separated by a thin insulator and qubit pairs tunnel though the insulator which gives qubits their quantum behaviors.
Whew! You made it! Thanks for hanging in there. Hopefully you are starting to get a sense of how the core principles quantum mechanics are intertwined and cross-functional and are supporting and posing challenges for efforts to build out Quantum Computing Systems.
We have much more to get after – topics like Quantum Key Distribution, Quantum Algorithms, Error Correction, not to mention getting under the covers of how various quantum computing platforms operate!
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