Researchers at Sandia National Laboratory, in conjunction with Harvard University, have developed a method of joining quantum computers together on an atomic scale. What does this mean for future quantum computers and their capabilities?

Quantum Computer Joining

Quantum computers have been featured in sensationalized news stories so often that announcements made about them are becoming stale. While quantum computers have extraordinary potential for future computing, it is unlikely that they will see commercial use for a decade or two.

However, researchers at Sandia National Laboratory working with Harvard University have developed and demonstrated a quantum bridge whereby two small quantum computational devices can communicate with each other on an atomic scale.

A Brief Explanation of Quantum Computing

In classical computation, data is represented as either off (0) or on (1) in binary units called bits. This fact leads to the creation of logic gates which manipulate these bits to perform operations ranging from mathematical functions to bitwise comparisons. For example, a microcontroller may compare two input pins and turn on a motor if the inputs are equal. This requires the controller to perform a logical AND on the inputs and bind the result to the motor control.

Quantum computers, however, do not use discrete bits to represent information. Instead, quantum computers rely on the quantum nature of a particle (such as an electron, photon, or even an atom) to store a qubit. The information that is stored in a qubit depends on its qubit state which is represented as a linear region with probabilities of either being a 0 or a 1. It is easier to think of a qubit as having three states:

0 - Off

1 - On

S - Superposition

The Bloch Sphere is used to represent a qubit. Image courtesy of Glosser.ca (own work) [CC BY-SA 3.0]

One issue with qubits is that when they are directly read, their superposition state is lost. In other words, if an atom is in a superposition state n and is read it will change to either a 1 or 0.

One method for reading qubits is by exploiting quantum entanglement where two particles are linked with each other. The state of one particle is dependent on the state of the other and vice versa so, by reading one particle, you can infer what state the other particle is in.

Quantum Emitters and Diamonds

One problem in quantum computing is getting two separate quantum computational devices to interact and transfer data. Quantum emitters are atoms that are “pumped” with photons which excite their electrons. Eventually, the excited electrons return to their previous energy state and emit a photon when they undergo this energy change. The energy change represents quantum information that the quantum emitter contains which does not affect the quantum state of the emitter.

So now we have a qubit that can store quantum information and be read without losing its quantum state. The next issue is how to fabricate and control individual qubits.

Researchers at Sandia in conjunction with Harvard have developed a method of creating qubit in a way never seen before which exhibit exciting quantum properties.

For a qubit to be reliable, it must not be affected by external influences such as charge, fields, and forces. The qubit must also be able to retain its quantum superposition when read so that information is not lost during operation. Lastly (from a manufacturing perspective), the qubit creation should be possible in controlled situations (instead of relying on chance).

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To solve these issues, the researchers use a particle-ion beam to replace carbon atoms in a diamond with individual silicon atoms. When a carbon atom in diamond is replaced with a single silicon atom, two neighbour carbon atoms begin to move away from the valance and eventually leave. This gap enables the silicon to essentially float as if it were in a gas as well as being buffered from stray electrical current thanks to the non-conductive neighbour vacancies. As the silicon is well below the surface of the diamond the silicon atom is kept within a specific location and is easily accessed by external photons.

Representation of holes in diamond holding the silicon emitter. Image courtesy of Sandia National Laboratories.

The advantage of the ion beam is that normally the quantum emitters (silicon in diamond) are in the form of random defects that scientists had to locate. Not only are such emitters hard to find but not all work as expected. The ion beam method produces thousands of reliable quantum bits which all yield working devices.

When these silicon atoms are pumped with photons, the emitted photons from the silicon atoms contain quantum information in their frequency, polarisation, and intensity.

Future Of Quantum Computers

The researchers behind the silicon emitters believe that quantum computers in the future will not consists of large powerful quantum devices but many simple quantum devices working in parallel. Such a device may be a better solution for the current rise in power demand from computers especially when making large generic q-bit quantum computers is difficult.

This use of parallelism is no surprise when considering modern processors. In the past (Intel 4004 onward), CPUs became more powerful as they had more instructions and the ability to compute them faster than their predecessor.

But now, processor speed has slumped and is essentially no longer increasing (with an upper limit near 5GHz) yet computation is still becoming more powerful. This is thanks to the use of parallel processing in modern computers where different processors handle different tasks simultaneously. If large multi-qubit quantum computers are too difficult to produce on scale with current technology, then creating more simple devices but connecting many of them together may be a better alternative.

Either way, there is no doubt that these quantum emitters could be the key to future quantum computational devices.