Quantum Data Storage

Data Storage Requirements

As data processing and data storage requirements continue to grow, there is a need to look for better and faster processing technology. Quantum Computing and Quantum Data Storage is one option, and if it takes off, it will change the face of computing. Quantum storage has the promise to store data in a very dense fashion and for the data to be very secure, though quite transient, with fast access times. So who is working on quantum storage and how might it work?

How might Quantum Data Storage work?

In quantum terms, 'spin' is a type of angular momentum intrisic to fundamental particles like electrons. Most of us need mental pictures to understand things, so you could visualise an electron as a spinning ball, which can spin clockwise or anti-clockwise. While this is easy to understand, the experts say it is not really like this at all, spin is just a number, and the two permutations of spin are 'spin up' and 'spin down'. Now most of us can probably live with this idea, but the Copenhagen interpretation of quantum theory, introduced 100 years ago, states that this electron does not exist in either state until someone looks at it. Instead it has both up and down spin at the same time. However, once we take a look at the electron to see which way it is spinning, we force it to chose and it will then take one spin state. Another way of looking at it is as a probability function, where there is a probability that an individual electron will be in one of the two states. Instead of a 2-state bit (0 or 1), we have a quantum bit or 'qubit'. Now traditional bits are either 0 or 1 and can be read anytime. If it is a '1', you will always read it as a '1'. Qubits aren’t like that. They have a probability of being 0 and a probability of being 1, but until you measure them, you don't know which one they are. This uncertain quantum property is called superposition.

Another way of visualising this, is to think of an electron spin state to be like a ball, with spin up at the top of the ball, and spin down at the bottom. This ball is actually called a Bloch Sphere and has three axis (Top, Bottom and Phase). All round the ball, between the three axis are spin probabilities. Now imagine these positions are data states, which can exit on any point on this sphere, and so can take several different values. A quantum computer can actually calculate probabilities on a qubit while it is in the unknown superposition state and change the probabilities in various ways through logic gates, then read out a result by measuring the qubit. Qubits typically start life at 0, although they are often then moved into an indeterminate state using a Hadamard Gate, which results in a qubit that will read out as 0 half the time and 1 the other half. Other gates are available to flip the state of a qubit by varying amounts and directions. Once a qubit is read, it is either 1 or 0 and loses its other state information.

Now I must admit that I don't understand this. I can get the idea of a probability function, but not that an object can be in two states at the same time. For me, Schrodinger's cat can be probably alive, or probably dead, but I don't see how it can be both alive and dead. In the 'explanation' above, I can kind of see how a qubit can have several probability values before you look at it, but once you look at it, it is either '1' or '0', so how do you have more than 2 data points? Hopefully this will all be clarified with time.

Electrons are not the only option for storing quantum data, other options include photons, ions and atoms. So far at least, the attempts to manipulate quantum storage is tightly linked with quantum computing.

Quantum Entanglement

Quantum entanglement is an intruiging possibility for instant data mirroring at a distance. It is possible that when two quantum particles are created, they become entangled, so that some of their properties, such as position, momentum, spin, and polarization are connected. Then, even if the two particles are physically separated, if an attribute of one particle is altered, then other particle instantly changes too. For example, if one particle has an 'up' spin and the other 'down', then if the spin of the first particle can be changed to 'down', the spin of the second particle instantly changes to 'up'.

It should then be obvious that if this can be managed and controlled so the particles are storing physical data, then two perfectly synchronous copies of the data can be maintained. At face value, this suggests that data can be propagated between two points faster than the speed of light, but this may be open to interpretation. One defininite advantage of propagating data using entanglement is that the data is virtually unhackable.

What is happening out there?

Here's a short summary of some of the activities.

The university of New South Wales in Australia is investigating storing and retrieving quantum data from the nucleus of a single phosphorus atom embedded in silicon. The team has built a proof-of-concept quantum computer where the system's two qubits are the nucleus and a single orbiting electron of the phosphorus atom. They use microwave pulses to read the spin direction of the electron, and transfer that information into the nucleus. They can hold that data for 80 milliseconds before transferring it back to the electron.
The current machine has 10 qubits, but the team intends to eventually build a system running on a million qubits. The NSW University team think they can do this economically, as they are building memory based on silicon.

A team from The Hong Kong University of Science and Technology has created such a quantum memory by trapping billions of rubidium atoms into a tiny space the size of a human hair. These atoms are cooled down to nearly absolute zero temperature using lasers and magnetic field. When with working with light photons it is difficut to distinguish a single photon from the surrounding light. The team also found a smart way to distinguish the single photon from the noisy background light sea. One possibile application of this technology would be for repeaters in a photon quantum network, which could lead to a new internet generation based on light. So far, they have only demonstrated the principle with a single qubit.

Institut national de la recherche scientifique (INRS) are developing chips that use light particles (photons) as the data medium. The INRS researchers are working with d-level quantum cluster states, cluster states based on sub-systems that have more than two dimensions. On these coin-sized chip structures, photons are generated and transformed so they can be assigned unique quantum properties. This team was the first to successfully create high-dimensional quantum computing operations using these cluster states.

The Australian National University Research School of Physics are using crystals treated with a rare-Earth element called erbium. By using erbium crystals, with their unique quantum properties, the ANU team were able to successfully store quantum information for 1.3 seconds, which is a very long time in quantum terms. As the erbium crystals operate in the same bandwidth as current fiber optic networks there is no conversion process needed to get the data onto the internet and the quantum entanglement encryption would make the data almost totally secure.

The University of Stuttgart is using Nitrogen atoms in diamond to store data. Instead of just using the spin of the electron to store data, the team have developed a technique that involves the spin of the Nitrogen nucleus in the process as well. If the data is stored by only using the electron spin, there is only one opportunity to measure the state of the qubit. By adding in the spin of the Nitrogen nucleus, the researchers discovered that they can force the state of the Nitrogen nucleus to change state twice before the information in the qubit is finally erased. This results in a more convoluted quantum mechanical process that triples the number of events that occur before information is destroyed, which in turn strengthens the signal revealing information stored in the qubit.

Technology Status

This is right at the really bleeding edge of technology. It might never really work, and if it does, it will probably be decades before we see anything on the general market. One of the aspects of using quanta for storage, is that when you read the quantum state, by definition you destroy it. This is one of the many challenges that are currently being investigated.

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