A quantum network can explain many things but our networks need protocols to work, so how can it avoid transfer errors, when data is sent but not received? Network transfers are like a ball thrown from a pitcher to a catcher. If the pitcher throws when the catcher is busy the ball is missed, and if two pitchers throw to a catcher at the same time, catching one loses the other. Likewise on a network, if one point sends data to another when it is busy, the transfer is lost, and if two points send transfers at the same time, one is also lost.
But does it matter if a transfer is lost as after all, it’s just information? Unfortunately, the money in your bank account is also just information, so a failed bank transfer could lose it all! If two people call you at once, it may not matter that one gets a busy signal, but on a network, losing a transfer loses what it represents. In a virtual world like Sim City, objects are represented by information, so a failed transfer could make the sword in your hand suddenly disappear. Obviously, networks can’t afford to lose transfers.
Our universe has, as far as we know, conserved energy for billions of years. If the movement of every photon in our vast universe is a transfer, none have been lost, or we would notice an energy loss. If our world is virtual, the quantum network must avoid transfer errors, but how? Our networks avoid transfer errors by protocols like:
1. Locking. Makes a receiver exclusively available before sending the transfer.
2. Synchrony. Synchronizes all transfers to a common clock.
3. Buffers. Stores transfer overloads in a buffer memory.
Could a quantum network then use any of these methods?
Locking. Locking makes a receiver dedicate itself to a sender before the transfer is sent. For example, if I edit this document, my laptop locks it exclusively, so any other edit attempt fails, with a message that it is in use. Otherwise, if the same document is edited twice, the last save will overwrite the changes of the first, which are lost. Locking avoids this by making every transfer two steps not one, but also allows transfer deadlock (Figure 2.13), where point A is waiting to confirm a lock from B, that is waiting for a lock from C, that is waiting for a lock from A, so they all wait forever. If the quantum network used locking, we would occasionally find dead areas of space unavailable for use, but we never have, so the quantum network can’t use locking to avoid transfer losses.
Synchrony. On a computer motherboard, when a fast central processing unit (CPU) sends data to a slower memory register, it must wait before resending. If it sends data again too soon, the transfer fails because the register is still busy with the first transfer. On the other hand, if it waits too long, this wastes valuable CPU cycles. It can’t check if the memory is free before sending because that is a new command that also needs checking! Using the double-send of locking slows the motherboard down, so it uses a common clock to synchronize events. The CPU sends data to memory when the clock ticks, then sends more data when it ticks again. This avoids transfer losses if the clock is set to the speed of the slowest component, plus some slack. One can lower a motherboard’s clock rate to make it run faster, but too much over-clocking can also crash it. Synchrony requires a common time but according to relativity, our universe doesn’t have that. If it did have a central clock, it would have to cycle at the rate of its slowest part, say a black hole, which would be massively inefficient, so the quantum network can’t use synchrony to avoid transfer losses either.
Buffers. The Internet uses memory buffers to avoid transfer errors so it doesn’t have to be centralized. Protocols like Ethernet (Note 1) distribute control, to let each point run at its own rate, and let buffers handle the overloads. If a point is busy when a transfer arrives, a buffer stores it to be processed later. Buffers let fast devices work with slow ones, so if a laptop sends a document to a printer, it goes to a buffer that feeds the printer in slow time, so you can carry on working while the document prints. But buffers require planning, as too big buffers waste memory and too small buffers can overload. Internet buffers are matched to load, so backbone servers like New York have big buffers, but backwaters like New Zealand have small ones. In our universe, a star is like a big city while empty space is a backwater. If the quantum network used buffers, where stars occur would have to be predictable, which according to quantum theory, wasn’t so. Equally, same buffers in the vastness of space would be a massive waste of memory, so the quantum network can’t use buffers to avoid transfer losses either.
Our networks avoid transfer losses by locking, synchrony, or buffers, but quantum events act one step at a time, so it can’t use locking, relativity has no common time, so it can’t use synchrony, and quantum processing has no memory storage, so it can’t use buffers. The quantum network is discrete, decentralized, and dynamic, so how can it avoid transfer errors?
Note 1 . Or CSMA/CD – Carrier Sense Multiple Access/ Collision Detect. In this decentralized protocol, multiple clients access the network carrier if they sense no activity but withdraw gracefully if they detect a collision.