Major Advances in Quantum Memory Make Quantum Networks Increasingly Probable. What Comes Next?

 

By James Myers

With significant progress being made in creating stable quantum circuits, development of quantum computing memory is also advancing. Once fully functional, quantum computers will require new methods for memory storage and retrieval, since they calculate at a far greater speed than today’s most powerful machines and produce even more massive volumes of data.

Networking many computers would require even greater memory capacity, and the small number of quantum computer prototypes now operating are not networked. When a reliable quantum memory mechanism is created, it could lead to a quantum internet connecting many of the powerful machines. A major step has recently been taken to reaching that turning point, and scientists are closer to assembling an efficient and scalable quantum memory mechanism.

 

 

In February 2024, physicists at Stony Brook University demonstrated a network of quantum memories that performed identically at room temperature. If their method can operate on a larger scale, it would relieve the cost burden of the super-cooling that many of today’s quantum computers require for protection against environmental interference. In their paper published in the journal Quantum Information, the researchers concluded that their demonstrations “lay the groundwork for future applications using large-scale memory-assisted quantum networks.”

Nobody can foresee all potential applications that can be developed with a single quantum computer, let alone a network of the machines.

We have a history of underestimating our own technological future that often turns out either far better or far worse than we imagined.

 

A brief, humorous clip of a 1995 night show discussion between Microsoft founder Bill Gates and host David Letterman is an example of how much some very knowledgeable people underestimated the young internet’s power and versatility. Nobody then imagined the world we have now created, with the internet.

 

The internet began to become widely available about 30 years ago, around 1995.

A brief, humorous clip of a 1995 night show discussion between Microsoft founder Bill Gates and host David Letterman is an example of how much some very knowledgeable people underestimated the young internet’s power and versatility. Nobody then imagined the world we have now created, with the internet.

One possible use for networked quantum computers has been under discussion ever since Nobel Physics laureate Richard Feynman proposed in a 1981 speech that physics could be simulated in a “universal computer.”  Feynman’s proposal was published in 1982 under the title “Simulating Physics with Computers,” (pdf available), in which he stated that for such a device it would be “necessary that everything that happens in a finite volume of space and time would have to be exactly analyzable with a finite number of logical operations.”

The universality of quantum computers remains an unproven conjecture, but if it turns out that they can flawlessly simulate the physics of space and time then how much of the quantum universe could we decode, and what great depth of information might the decoding reveal to us about the fundamental workings of the universe?  Could a network of quantum computers  eventually (although likely many years down the road) give us control of physical reality, as Feynman theorized might be possible. ” I therefore believe,” he stated in his speech, “it’s true that with a suitable class of quantum machines you could imitate any quantum system, including the physical world.”

Why would that control be so important, and so valuable for the first people to hold that power? It’s the power of change: the quantum is the smallest amount of energy in the universe that can either cause change, or be affected by change. Quantum control might, theoretically, convey power of cause and effect over long distances and sequences of time at the very quantum scale of the universe – its minimum scale. It’s not difficult to imagine that changing the foundation of anything – whether it’s a house or a universe – would ripple through the entire structure. 

 

On November 7, 1940, the Tacoma Narrows Bridge collapsed dramatically after sections of the suspension and roadbed began to twist in strong winds. The difference in the torque among sections built up a wave of force strong enough to overwhelm the bridge’s foundation and collapse the structure into the Puget Sound. To what extent could a quantum “butterfly effect” propagate?

 

And now, with quantum repeaters, the technology to transmit waves of human-generated quantum information across vast distances may be close at hand. 

A fundamental principle of the universe is its conservation of information (see our feature, Unraveling Secrets of Black Holes: Are We Holograms?). As Stony Brook University reports, “The field of quantum information essentially combines aspects of physics, mathematics, and classical computing to use quantum mechanics to solve complex problems much faster than classical computing and to transmit information in an unhackable manner.” 

The university adds that, “While the vision of a quantum internet system is growing and the field has seen a surge in interest from researchers and the public at large, accompanied by a steep increase in the capital invested, an actual quantum internet prototype has not been built.”

The researchers used a widely-accepted process to detect interference between two memory cells in which they had created identical quantum states. Then they tested the outputs of both to ensure that they functioned identically. Their success, the university states, “allows for memory-assisted entanglement swapping, a protocol to distribute entanglement over long distances and the key to building operational quantum repeaters.”  (Entanglement is the word used for the connection of two quanta sharing the same quantum state).

Since amplifiers would have to replicate the quantum to boost its transmission, and it’s theoretically impossible to clone or copy a quantum state, repeaters, rather than amplifiers, are required for transmission in a quantum network. Repeaters would distribute the outputs of all connected quantum computers throughout the entire network, enabling a quantum version of the internet, but one that will have far greater computing power.

 

 

How a Quantum Repeater Doubles the Output: A quantum repeater transmission requires two sources of entangled photon pairs separated by distance (we’ll call the distance “L”). One photon from each pair is sent toward a central measurement hub, where they are stored in quantum memories. Their partner photons are sent in opposite directions, also stored in quantum memories separated by a distance equal to two times “L.”  A measurement certifying that two photons arriving at the central node are identical can be used to interconnect (“entangle”) the distantly located photons. – (Adapted from Stony Brook University)

Quantum repeaters would effectively amplify the interconnections among entangled quantum pairs to twice the distance that each quantum could achieve on its own. “Stitching together several of these repeater hops, it is possible to spread entanglement over hundreds of kilometers,” the university notes.

If there proves to be no limit to the number of repeater interconnections that can be made, could hundreds of kilometers expand to hundreds of parsecs on the astronomical scale? (A parsec is approximately 3.26 light-years, which is nearly 31 trillion kilometres).

How soon will we have quantum memory banks?

Using the principles of quantum mechanics, researchers have already identified a theoretically workable method to amplify by 1,000 times the storage capacity of today’s optical memory discs.

The process, which has been called “wavelength multiplexing,” would combine a large number of memory cells, each containing a photon-emitting rare earth metal that’s embedded in a crystal of magnesium oxide. 

The photons firing out of the rare earth metal are captured in cavities within the crystal. These cavities are activated by the inflow of the photon’s light energy, which changes the cavity from its ground state (the state of rest that it’s in when nothing’s happening) into a spin state, where it can transmit and produce torque with its angular momentum. Spin states are difficult to reverse, making them potentially reliable mechanisms for quantum memory storage.

Existing optical memory devices are limited in their capacity by the fact that each piece of data cannot be smaller than the wavelength of the laser that reads and writes the data. By combining, or multiplexing, data in memory cells using this crystal structure, researchers believe that a far greater volume of data can be stored much more densely in the same size of memory device. Their findings were published this August in the journal Physical Review Research.

Another method that shows promise for storing a high volume and density of quantum information uses a material called AIE-DDPR, the acronym for “dye-doped photoresist with aggregation-induced emission luminogens.” The method involves the use of two chemicals, one of which initiates a reaction when exposed to photons of light, causing a response in the other chemical. The combination of the two allows for much denser data storage than today’s optical memory devices can provide.

Using AIE-DDPR in 100 layers, each separated by only one-thousandth of a millimetre, researchers created a disc that could store 1 petabit of data, which is more than 125,000 gigabytes.

How different will the quantum internet be, from the one that’s already become essential to our functioning?

“The quantum internet represents a paradigm shift in how we think about secure global communication,” Dr. David Awschalom told the Uchicago News.  

Dr. Awschalom is Professor of Molecular Engineering and Physics at the University of Chicago, director of the Chicago Quantum Exchange, and director of Q-NEXT, a Department of Energy Quantum Information Science Center at Argonne. The University of Chicago states that Dr. Awschalom is “one of the world’s leading scientists in spintronics and quantum information engineering. His research involves understanding and controlling the spins of electrons, ions, and nuclei for fundamental studies of quantum systems, as well as potential applications in computing, imaging, and sensing.”

As Dr. Awschalom stated, “Being able to create an entangled network of quantum computers would allow us to send unhackable encrypted messages, keep technology in perfect sync across long distances using quantum clocks, and solve complex problems that one quantum computer might struggle with alone–and those are just some of the applications we know about right now. The future is likely to hold surprising and impactful discoveries using quantum networks.”

The memory capacity of a quantum network could, for example, yield significantly more information on quantum systems. 

In an April 2024 paper, Dr. Sitan Chen, Assistant Professor of Computer Science at Harvard University, and co-authors demonstrated the exponential power of quantum memory for reconstructing a series of quantum events.

Reconstruction is necessary since changes in quantum states cannot be measured directly. This measurement limitation is due to the Uncertainty Principle that says the more we know of a particle’s position in space the less we know about its direction in time, and vice-versa. Measuring changes in quantum states therefore requires repeated measurements of their outcomes to reconstruct the properties of the original system, which is a tedious task of stitching together many measurement “snapshots.”  The job made easier by machine learning.

Quantum computers operating with far greater speed hold the advantage of requiring fewer measurements to reconstruct a system, which was demonstrated in a paper published in 2021 by Hsin-Yuan Huang, Richard Kueng, and John Preskill. In their April 2024 paper, Dr. Chen and co-authors established that quantum computers could theoretically use as little as two outcome measurements to exponentially reduce the number of snapshots required to reconstruct a system.

Machines in a quantum network could, in theory, operate in tandem to assemble reconstructions of complex systems, piece by piece, at great speed.

What comes next?

In its feature The quantum internet, explained, the Uchicago News observed that, “Few could imagine 60 years ago that a handful of interconnected computers would one day spawn the sprawling digital landscape we know today. The quantum internet presents a similar unknown, but a number of applications have been theorized and some have already been demonstrated.”

It’s not expected that quantum networks will replace the existing internet, rather that both will exist in tandem, with the quantum networks processing tasks the quantum computer is particularly well-suited for – like computations with many variables, combinations, and permutations. Because of their extreme cost, it’s also not expected that personal quantum computers will be available anytime soon, and therefore the quantum networks will likely connect only large companies, universities, and governments with the necessary financial resources.

 

 

Time and money will be required to resolve the problem of creating stable quantum circuits, and then to develop reliable repeaters and build the network infrastructure. In the meantime, researchers are already developing algorithms to take advantage of the power of a quantum network and its memory.

There has been little public discussion on the rules that should govern a quantum internet, including control over domain names and access, security protocols, and many rules of today’s internet that we have come to take for granted. In designing a quantum network we can, however, draw on a number of important and sometimes painful lessons learned during the nearly 30 years that the internet has been evolving. 

Do we want to follow the same path with a quantum network, or can we think of better ways?

With the European Union and other jurisdictions increasing their focus on AI regulations and consumer protection, it seems like a question well worth considering sooner rather than later.