How to build a quantum-entangled blockchain that travels through time
This article is one in a series of my 🍌Banana Papers — blockchain whitepapers re-written in an easy to digest (like bananas!) manner. My goal is to help readers quickly understand and evaluate complex blockchain ideas with minimal pain.
It’s difficult to write about quantum physics.
First, it’s complicated. Much more complicated than, say, blockchain. In blockchain, if needed, I can open the code in a text editor, or spend an afternoon reading about game theory, and the whitepapers make sense. But quantum physics is a magnitude more complicated, explaining abstract theories in mathematical terms rather than in visible realities (in the sense that code is a visible reality). Quantum physics requires a depressingly-wide foundation of knowledge in order to offer up more than a recapitulation of what others have said. And to make it more difficult, even the experts can’t agree on why quantum behaves the way it does.
Second, quantum physics exists within a knowledge cult. Now, I know— blockchain exists within a knowledge cult, too. We can admit this, right? We enjoy being “one of the select few,” capitalizing on insider, specialized knowledge and using our micro-status for economic and personal-brand gains. It’s OK. We temper our cult with altruistic motives of decentralization, banking the unbanked, and cries of “screw the man.” And with blockchain we have a business-driven cult, driving ultimately towards real profits and real products. Blockchain is a thing already realized. But quantum physics reminds me more of the knowledge cult around nanotechnology in the 90’s where laymen (probably from reading K. Eric Drexler’s book) latched on to massive concepts that solve humanity’s biggest problems — nano robots that could continuously scan and fix our cells, extending an average lifetime to centuries, for example. Possible, yes, and someday probable, yes. But often hypotheses read as truth, with actual products much further out in the future than justified by the excitement.
The third problem with quantum physics? It’s fundamentally at odds with the way we perceive, and interact with, reality. Just as we can’t truly envision a five-dimensional space, we can’t truly envision quantum. Our brains are trapped in the experiential world of classical physics, where outcomes are deterministic, and if something spins clockwise … well, then it spins clockwise. But quantum? It tells us about particles that simultaneously exist in multiple states. Photons that influence other photons instantaneously across light-years. Particles that are aware they are being measured. Those are sentences easy to read, but difficult to accept.
Combine these three issues — very few understand the science, a cult of knowledge that obfuscates current state, and a reality different from the one we experience —and you have a whole lot of speculation by a whole lot of laymen, and a space where even intelligent, diligent, committed minds have difficulty staying prudent and factual.
In a recently released paper, Del Rajan and Matt Visser of the Victoria University of Wellington, propose a design for a blockchain built on a foundation of quantum principles. This is not a quantum layer across the top of an existing blockchain, or a novel defense against quantum computer hacks, but a blockchain whose underlying design is a network of entangled states. One that is deployed, not though physical space, but through time. In effect, they propose a quantum-networked time machine.
The Double Slit Experiment
The foundations of quantum mechanics start with The Double Slit Experiment — a test first run in the 1920’s, and refined, reproduced, and proven many times since.
Imagine we have two boards — one (board A) sits a few feet in front of the other (board B). Board A, in front, is a solid board with a slit cut just large enough for a single grain of sand to pass through. Board B, in the back, has a special coating that registers impact.
We also have a special gun that shoots single grains of sand. If we stand several feet back and shoot single grains of sand towards this setup, we’d expect the following pattern to eventually show on board B:
And that’s exactly what happens. Now we change things up a little, and cut a second slit in the first board as shown, and again shoot single grains of sand. We would expect this pattern, as grains (we’ll assume randomly) go through one slit or the other.
So far, so good. That’s exactly what happens. But now, let’s switch from our sand gun to a photon gun. We close the second slit, leaving just one slit open, and shoot a single photons at the boards. We observe this pattern:
Just as expected. But now we open the second slit, expecting the same pattern as the double-slit sand test. But things get interesting here, as we observe this pattern instead:
This pattern is called an interference pattern, and is typically seen when working with waves (sound waves, for example). You can envision the effects as waves in water — if you threw two rocks into separate locations in a small lake, eventually the ripples from each rock would collide and interfere.
But wait — we sent a single photon through the board. It’s impossible for one single photon to interfere with itself. How can a single photon going through a single slit cause this type of pattern?
But hold on — it gets even stranger. Let’s say we add something new to the experiment— we add a detector just after board A that can detect which slit the photon traveled through. Once we know which slit the photon traveled through, we can prove it didn’t somehow go through both slits. We run the experiment again, and get a second seemingly impossible result:
We are back to the pattern we first expected. The photon suddenly stops showing the interference pattern, and starts showing the sand-like pattern we expected all along. When we started detecting the photon, it altered its behavior!
This experiment has been run and proven many times, in many ways. In short, in the slit test:
- A single photon appears to travel through both slits at the same time, interfering with itself and creating the interference pattern.
- If we try to detect the photon, the photon suddenly picks a single slit and changes the observed pattern.
How is this all possible? This is the foundation of quantum physics. The particle is traveling all the possible paths at the same time, and interfering with itself. The particle exists along both paths, until we try to detect it, at which time the particle picks just one.
The particles are in a superposition of possible states (at least until they are measured). This doesn’t mean the particles could be either A or B and we don’t know until we measure. This means they literally are both A and B, at the same time, until they are measured, at which time they pick either A or B.
That goes against everything our minds observe.
If you think you understand quantum mechanics, you don’t understand quantum mechanics. — Richard Feynman
To understand our new blockchain, we have to understand one more concept of quantum physics: quantum entanglement. Quantum entanglement occurs when you entangle two particles so that the particles stay correlated as one single system, even across great distances.
Let’s say we have two particles A and B. We take particle A, entangle it with particle B, and then separate them by many light years. Now remember, from the above, that both particles are in superposition of possible states — meaning for example they are both spinning both clockwise and counterclockwise at the same time. Now that we have separated the particles, if we measure the spin of A, and it picks clockwise, and someone else an instant later measures B — B will always choose counterclockwise.
Entanglement means that somehow B knew, instantaneously, what A picked, regardless of the distance between A and B.
Einstein called this “spooky action at a distance.”
(How do you entangle photons? Shine a laser on a nonlinear optical crystal and one out of every billion photons will split and emerge as two entangled photons.)
Back to our proposed blockchain. It turns out that not only can two particles remain entangled over great distances, but they can also remain entangled in time. Particle B can remain entangled to particle A, even when particle A no longer exists. (more on that later)
Based on this, Rajan and Visser propose a blockchain whose blocks are not blocks of data that co-exist on the chain at the same time, but rather photons, entangled in time with each other, of which the older photons no longer exist, but are still entangled with the current state photons.
In a traditional blockchain, a group of transactions from the past, that all happened around the same time, are collected into blocks of data, timestamped, written to the chain, and linked to the previous block. If an attacker tries to alter a block, hash functions make it very hard to succeed. The older the block, the more difficult it is to successfully hack: every block leading back to that block has to be hacked as well. Validator nodes on the network, through incentives and consensus algorithms, ensure data is true, and thus keep the network decentralized.
In the proposed quantum blockchain, the functions of the chain (blocks, data, timestamps) have the same functions as a traditional blockchain, but operate through quantum methods. The goals of the chain are the same — valid blocks of data stored on a decentralized network.
In our new chain, instead of data being coded using bits, data is encoded using qubits. Qubits can be a variety of underlying particles and physical states— and are the basic unit of quantum information. Unlike bits, which are either a 1 or a 0, qubits are both 1 and 0 at the same time. So a qubit could be an electron that has both an up and down spin. This is the superposition we learned above. In the case of our new blockchain, we are going to use photons as our qubits.
How is the actual data written to a block? This is done based on a process called superdense coding. Superdense coding is a method of sending two traditional bits of information (00, 01, 10,or 11) using a single qubit.
Imagine Alice wants to send information to Bob. Two entangled qubits are sent, one each, to Alice and Bob. Alice, depending on what two-bit message she wants to send, applies a quantum gate to her qubit which sets the entanglement between the two qubits to a certain Bell state (Bell states are a way to measure the entangled quantum states between two photons). There are four possible Bell states, which maps very well to the four possible 2-bits of information Alice is trying to send.
Alice now sends her qubit to Bob, who can measure the Bell state between the two qubits, and decode which two-bit message Alice sent. For example, if the measured Bell state is X, he knows Alice is sending the bits 00. If the Bell state is Y, Alice is sending the bits 01. And so forth.
This is pretty good explanation. In practice, our blockchain is based on slightly newer theories, where Bob doesn’t actually need the qubits to measure the Bell state — he can determine the Bell state from just the entanglement itself. But the concept is the same.
A block of data in our new chain is called a GHZ state. Think of a GHZ state as the collection of entanglements between all the photons in the block. In a GHZ state, if any of the photons are tampered with, the entire GHZ state falls apart.
Now let’s walk through the process of writing and verifying data to the quantum blockchain one step at a time.
- A new block of data (remember, the data is encoded in qubits inside a GHZ state) is proposed to the network by some untrusted node. This new block is shared with every node on the network.
- We don’t know if we can trust this node, so a verifier node is chosen randomly from the network using a quantum random number generator.
- Using the theta-protocol, the network verifies that the proposed new block is valid. The theta-protocol verifies the block by what I call fancy math that measures whether or not the block contains genuine multipartite entanglement (GME) — a type of entanglement that can only exist if all qubits in the GHZ state were involved in the creation of the state. To accomplish this proof, the verifier generates a set of random angles and sends them to the network. Each node measures the qubits against the angles using what I can only call complicated math that validates the data. (If you want to understand both the fancy and complicated math, you can read about it here)
- Once the block of data is accepted as true, the other nodes entangle the qubits in the new GHZ state to the qubits in the current GHZ state, effectively “absorbing” all the entanglements into one single GHZ state. The entire history of the blockchain is now encoded into the most recent GHZ state.
- And finally, here’s some magic. The qubits from the previous block of data are now destroyed, leaving only the most current qubit. But the entanglement to the previous qubits — and all prior qubits throughout the history of the chain — remain. We can extract that information, because with the entanglements comes all the encoded data in the chain — so we have access to the entire history of data. But — and here’s the key — since the older qubits no longer exist, you can’t change the older blocks or the history of entanglement. And if you try to change the current block, the entanglement unravels, and the entire chain falls apart. This chain is locked down and secure.
In a classic blockchain, changing old blocks is expensive and difficult. In a quantum blockchain based on a physical space, changing old blocks becomes ever harder, as the blocks are entangled, and changing an old block breaks the entanglement and invalidates the entire chain.
In our new time-based quantum blockchain, the protection is even better — the old nodes can’t be changed because they no longer exist. At best, an attacker can try to modify the current block. But any attempt to modify the current block involves looking at that block, which immediately invalidates the entire chain by breaking the entanglement.
This time-based blockchain state exists in the entanglement between photons that never existed at the same time, yet they still share an entanglement that exists at the current moment.
In other words, and shockingly, this blockchain links the current block not to a record of the past, but links it to the actual record in the past that no longer exists. If you were to measure the current existing photon, you would change the photon that no longer exists.
Let me write that one more time —
The blocks are linked, not to blocks from the past that still exist, but are linked, through time, to the actual blocks in the past, which no longer exist.
To say it more boldly, the entanglement travels backwards in time.
This blockchain is, quite literally, influencing and changing events that have already happened. That’s counterintuitive, and that’s time travel magic. The authors call this new blockchain a “quantum networked time machine.”
Is this all real?
This new blockchain is a concept, and has yet to be built. But “all the subsystems of this design have already been shown to be experimentally realized,” say Rajan and Visser. So yes, the science is real. But no, the chain doesn’t yet exist.
Back to my preamble, we have to be careful not to get wrapped up into the knowledge cult here. And this is a difficult subject — I welcome community feedback on any details I may have gotten wrong. But I’m excited to see what new possibilities exist once we add time into the blockchain variables. Like quantum computers, we’re still some number of years from seeing this concept a reality. Just one of the hurdles is that we don’t yet have a global quantum network capable of transmitting quantum data. But smaller quantum networks do already exist, and we’re perhaps only months, maybe years, from having a global network. So the time is coming.
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