If you are a retrocomputer fan, you might remember when serial ports were a few hundred baud and busses ran at a few megahertz at the most. Today, of course, we have buses and fabric that can run at tremendous speeds. Quantum computing, though, has to start from scratch. One major problem is that jockeying quantum states around for any distance is difficult and slow. Part of it is that qubits decay rapidly, so you don’t have much time. They are also generally susceptible to noise and perturbation by outside forces. So many quantum machines today are limited by how much they can cram on one chip since there isn’t a good way to connect to another chip. The University of Sussex thinks it has improved the outlook for quantum interconnects with a technique they claim can move qubits around at nearly 2,500 links per second.
The technique, called UQ Connect, uses electric field links to connect multiple chips using trapped ions for qubits. If you want to read the actual paper, you can find it in Nature Communications.
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Quantum computers are coming, but there are still many problems with realizing practical machines. One is finding a reliable and affordable way to encode qubits — the basic unit for quantum computers. MIT researchers have a proposal. By using two slightly different colored lasers, they can manipulate nuclear spin. This isn’t the first time someone’s tried to use light to impact spin, but according to MIT, the other methods use an indirect coupling which is more prone to noise, something that limits the viability of quantum computers. They published a recent paper on the process if you want to read more.
Nuclear spin has weak interactions, but the new method doesn’t require intermediate steps, so it may be much more practical than previous methods. MIT mentions that typical quantum elements have coherence time limits, which means data stored in them becomes useless in less than a second. The new method promises to have coherence times measured in hours.
The method is known as the optonuclear quadrupolar effect or ONQ. From the paper:
[The ONQ effect] is second order in the electric field and nuclear spin I, as mediated by the quadrupole electric coupling, and is thus one of the nonlinear optical (NLO) responses of materials present in perfect crystals. Via the ONQ effect, nuclear spins can be coherently controlled by two-color photons, without electron spins as the media.
If you understood that, you should probably head over and read the rest of the paper. Meanwhile, the rest of us are waiting for our quantum Arduino.
We think of data storage as a modern problem, but even ancient civilizations kept records. While much of the world used stone tablets or other media that didn’t survive the centuries, the Incas used something called quipu which encoded numeric data in strings using knots. Now the ancient system of recording numbers has inspired a new way to encode qubits in a quantum computer.
With quipu, knots in a string represent a number. By analogy, a conventional qubit would be as if you used a string to form a 0 or 1 shape on a tabletop. A breeze or other “noise” would easily disturb your equation. But knots stay tied even if you pick the strings up and move them around. The new qubits are the same, encoding data in the topology of the material.
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Want to try a big quantum computer but don’t have the cash? Google wants to up your simulation game with their “Quantum Virtual Machine” that you can use for free.
On the face of it, it sounds like marketing-speak for just another quantum simulator. But if you read the post, it sounds like it attempts to model effects from a real Sycamore processor including qubit decay and dephasing along with gate and readout errors. This forms what Google calls “processor-like” output, meaning it is as imperfect as a real quantum computer.
If you need more qubits than Google is willing to support, there are ways to add more computing using external compute nodes. Even if you have access to a real machine of sufficient size, this is handy because you don’t have to wait in a queue for time on a machine. You can work out a lot of issues before going to the real computer.
This couldn’t help but remind us of the old days when you had to bring your cards to the central computer location and wait your turn only to find out you’d made a stupid spelling mistake that cost you an hour of wait time. In those days, we’d “desk check” a program carefully before submitting it. This system would allow a similar process where you test your basic logic flow on a virtual machine before suffering the wait time for a real computer to run it.
Of course, if you really need a quantum computer, the simulation is probably too slow to be practical. But at least this might help you work out the kinks on smaller problems before tackling the whole enchilada. What will you do with a quantum computer? Tell us in the comments.
Google, of course, likes its own language, Cirq. If you want a leg up on general concepts with a friendly simulator, try our series.
Researchers at the University of New South Wales and a startup company, Silicon Quantum Computing, published results of their quantum dot experiments. The circuits use up to 10 carbon-based quantum dots on a silicon substrate. Metal gates control the flow of electrons. The paper appears in Nature and you can download the full paper from there.
What’s new about this is that the dots are precisely arranged to simulate an organic compound, polyacetylene. This allowed researchers to model the actual molecule. Simulating molecules is important in the study of exotic matter phases, such as superconductivity. The interaction of particles inside, for example, a crystalline structure is difficult to simulate using conventional methods. By building a model using quantum techniques on the same scale and with the same topology as the molecule in question, simulation is simplified.
The SSH (Su-Schreffer-Heeger) model describes a single electron moving along a one-dimensional lattice with staggered tunnel couplings. At least, that’s what the paper says and we have to believe it. Creating such a model for simple systems has been feasible, but for a “many body” problem, conventional computing just isn’t up to the task. Currently, the 10 dot model is right at the limit of what a conventional computer can simulate reasonably. The team plans to build a 20 dot circuit that would allow for unique simulations not feasible with classic computing tech.
The dots are made with a scanning tunneling microscope and there is a Goldilocks effect regarding the size of the dots. If they are too small, the energy levels are overwhelmed by phosphorous donors. Too large, and capacitive coupling between dots makes the system unstable.
We’ll admit, the science in the paper is pretty dense. But the Methods section outlines what it takes to create something like this. You’ll need silicon, high-temperature ovens, and the ability to handle exotic gasses and perform lithography. Pretty much an IC fab in your basement. However, we did wonder if anyone homebrewing chips had ever tried STM lithography like this as an alternative to optical lithography. Seems like it might be possible.
We can’t help with some of the more exotic gear, but if you want to build an STM, it has been done. While you can make quantum dots in your kitchen, we don’t think they are going to work the same.
There are a few ways to access real quantum computers — often for free — over the Internet. However, most of these are previous-generation machines that have limited capabilities. Great for learning, perhaps, but not something you could do anything practical with. Xanadu, however, has announced what they claim to be a computer capable of reaching quantum advantage that is free for anyone to use, within limits. Borealis — the computer in question — uses photonic states and has the capability of working with over 216 squeezed-state qubits.
The company is selling time on the computer, but the free tier includes 5 million free shots on Borealis and 10 million shots on an earlier series of quantum computers. You can also buy pay-as-you go service for about $100 per million shots on Borealis.
While a few million shots may sound like a lot, we noticed that the quickstart demo consumes 10,000 shots and that’s presumably something simple. That’s still about 500 runs of that on Borealis — not bad for free on a state-of-the-art quantum computer. You will be wanting to debug with a simulator, though.
We presume the developers are Beatles fans given that you use software called Penny Lane and Strawberry Fields to access the machines. Your job is controlled by Python and there is a cloud simulator to save your shots.
We won’t pretend to understand all there is about squeezed light qubits and the Borealis architecture. But you can get some general practice in our series on quantum computing. Or there are a few lectures around including one that aims at different levels of experience.
Continue reading “Quantum Computing: The First Taste Is Free” →
Historically, the first program you write for a new computer language is “Hello World,” or, if you are in Texas, “Howdy World.” But with quantum computing on the horizon, you need something better. Like “Hello Many Worlds.” [IonQ] proposes what that looks like and then writes it in seven different quantum languages in a post you should check out.
Here’s the description of the simple program:
The basic quantum program we’ll write is simple. It creates a fully-entangled state between two qubits, and then measures this state. This state is sometimes called a Bell State, or Bell Pair, after physicist John Stewart Bell.
The measurement results for this program should give us 0 for both qubits or 1 for both qubits, in equal amounts. When running these, we’ll be able to tell that we’re running on real hardware because that’s not always what we get! These errors are what currently limit quantum computers, but the first steps to overcome this with quantum error correction have already begun.
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