Quantum Computing Technology and Networking

 Quantum Computing: an Ongoing Process

- Quantum Computing

Quantum computers take the unique behaviors of quantum physics -- such as superposition, entanglement, and quantum interference -- and apply them to computation. This introduces new concepts to traditional programming methods. 

A quantum computer has three main parts: the area that houses the qubits; a way to transmit signals to the qubits; and a classical computer to run programs and send instructions. 

For some qubit storage methods, the cells that hold the qubits are kept at a temperature just above absolute zero to maximize their coherence and reduce interference. Other types of qubit enclosures use vacuum chambers to help minimize vibration and stabilize the qubit. Signals can be sent to qubits using a variety of methods, including microwaves, lasers and voltages. 

Superposition and entanglement are two hallmarks of the quantum physics on which these supercomputers are based. This enables quantum computers to process operations at much higher speeds and with less energy consumption than conventional computers.

- The Fundamental Physics Driving Quantum Computing

Quantum computers aren’t the next generation of supercomputers - they’re something else entirely. Before we can even begin to talk about their potential applications, we need to understand the fundamental physics that drives the theory of quantum computing.

A quantum computer is a device performing quantum computations. It manipulates the quantum states of qubits in a controlled way to perform algorithms. A universal quantum computer is defined as a machine that is able to adopt an arbitrary quantum state from an arbitrary input quantum state. The development of a quantum computer is currently in its infancy, systems consist of a few to a few tens of quantum bits (qubits). Main challenges in further development are to make the quantum computer scalable and to make it fault-tolerant. This means that it will be able to perform universal quantum operations using unreliable components. The production of a working quantum computer has become a real possibility, thanks to recent developments in the nanotechnology field, but there is still a long way to go. 

- Quantum Computing and Quantum Theory

Quantum computing focuses on the principles of quantum theory, which involves modern physics that explain the behavior of matter and energy at the atomic and subatomic level. Quantum computing exploits quantum phenomena such as qubits, superposition and entanglement to perform data manipulations.

Quantum computers can handle more than just the binary information which conventional computers operate on. Quantum computers can also handle data in between a 0 or 1 bit, which should, in turn, provide new types of simulation and calculations. 

In quantum computations the spin direction, which is either up or down, serves as the basic information unit which is similar to the 0 or 1 bit in a classical computing system. Electron spin can assume both 0 and 1 simultaneously, as a result of quantum entanglement, which greatly enhances the ability to perform complex computations.

[Hamamatsu Castle, Japan]

- Quantum Communications and Networking

Earlier in June 2022, a group of research labs in Chicago unveiled a 124-mile extended quantum network that runs from the suburbs of Lemont through the city of Chicago to near Hyde Park and back. This total length accounts for the newly added 35-mile fiber-optic segment that recently connected to an 89-mile quantum loop launched by the U.S. Department of Energy's Argonne National Laboratory in 2020, connecting the Chicago Quantum Exchange and the University of Chicago. 

The purpose of building such a network is to allow researchers to experiment with new types of quantum communications, security protocols and algorithms, with the goal of advancing towards a preliminary quantum internet (which will likely look like an early version of the classical internet). Currently, Toshiba is using it to Test their distributed quantum encryption key, which is subject to factors such as noise, weather and temperature fluctuations, to see how robust this approach is and what potential problems can arise. 

So far, researchers have been able to send information at a rate of 80,000 qubits (or qubits -- more on that below) per second. Such experimental keys could be useful in a future where powerful quantum computers have the potential to disrupt classical encryption.

As larger quantum computers begin to appear, researchers are actively exploring ways to use the laws of quantum physics to create tamper-proof and hacker-proof communication channels. This type of communication channel could also be a way to "wire" quantum devices together.

- Quantum Key Distribution (QKD)

QKD involves sending encrypted data over a network as classical bits, while the keys to decrypt the information are encoded and transmitted in quantum states using qubits. 

Various methods or protocols have been developed to implement QKD. A widely used BB84 works like this. Imagine there are two people, Alice and Bob. Alice wants to send data to Bob securely. To do this, she created an encryption key in the form of qubits, whose polarization states represent the individual bit values of the key. 

The qubits can be sent to Bob over a fiber optic cable. By comparing the state measurements of some of these qubits -- a process called "key screening" -- Alice and Bob can determine that they hold the same key. 

When the qubit reaches its destination, some of its fragile quantum states collapse due to decoherence. To get around this, Alice and Bob next run a process called "key distillation," which involves calculating whether the error rate is high enough to indicate that a hacker is trying to intercept the key. 

If so, they discard the suspect key and keep generating new keys until they are confident they share a secure key. Alice can then encrypt the data with her key and send it in classical bits to Bob, who uses his key to decode the message. 

We are already starting to see more QKD networks emerge. The longest is in China, with a 2,032-kilometer (1,263-mile) ground link between Beijing and Shanghai. Banks and other financial companies are already using it to transfer data. In the US, a startup called Quantum Xchange has struck a deal that will allow it to use 500 miles (805 kilometers) of fiber-optic cable running along the East Coast to create a QKD network. The initial line will connect Manhattan and New Jersey, where many banks have large data centers. 

While QKD is relatively secure, it would be even more so if it could rely on quantum repeaters.

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