qubit

8 Articles

MIT Spins Qubits Round And Round

February 16, 2023 by 2 Comments

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.

Twist Promises Easier Quantum Programming

February 5, 2022 by 17 Comments

We keep trying to learn more about quantum computers. But the truth is, the way we program quantum computers — or their simulators — today will probably not have much in common with how we program them in the future. Think about it. Programming your PC is nothing like programming the ENIAC. So we expect we’ll see more and more abstractions over the “bare metal” quantum computer. The latest of these is Twist, from MIT.

According to the paper (and the video, below), Twist expresses entangled data and processes in a way that traditional programmers can understand. The key concept is known as “purity” of expressions which helps the compiler determine if data is actually entangled with another piece of data or if any potential entanglement is extraneous. A pure expression only depends on qubits it owns, while a mixed expression may use qubits owned by other expressions.

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Schrodinger’s Cat Lives

June 10, 2019 by 31 Comments

If quantum physics always sounded a little squirrelly to you, take heart. Yale researchers have announced that they can do what quantum physics claimed to be impossible: they can determine the state a quantum system will collapse to before it happens. This contradicts Schrodinger’s famous hypothetical cat that is superimposed as 50% alive and 50% dead at the same time. The research appears in Nature.

Schrodinger argued that until you open the box, the cat is half alive and half dead in the same way that a qubit can be in 50% of one state or another. When you observe it, you force the system to one state. Researchers at Yale, however, have found a way to use microwaves to indirectly monitor qubits to determine their state prior to the system making a jump. Unlike a normal observation which occurs too late, the Yale technique allows researchers to change the future state to their choice.

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You’re Listening To Quantum Radio

March 18, 2019 by 7 Comments

Researchers at Delft University of Technology have created a detector that enables the detection of a single photon’s worth of radio frequency energy. The chip is only 10 mm square and the team plans to use it to explore the relationship of mass and gravity to quantum theory.

The chip has immediate applications in MRI and radio astronomy. Traditionally, detecting a single photon at radio frequencies is difficult due to the significance of thermal fluctuations. At lower frequencies, cryogenic cooling can reduce the issue, but as frequency increases the fluctuations are harder to tame.

The trick requires a qubit that samples the radio frequency energy. While the radio source is at 173 MHz, the qubit is at 1 GHz, allowing a fine time resolution. Coupling of the two is via an LC circuit that uses a Josephson junction which, of course, requires very cold temperatures. Continue reading “You’re Listening To Quantum Radio” →

Quantum Computing For Computer Scientists

July 13, 2018 by 16 Comments

Quantum computing is coming, so a lot of people are trying to articulate why we want it and how it works. Most of the explanations are either hardcore physics talking about spin and entanglement, or very breezy and handwaving which can be useful to get a little understanding but isn’t useful for applying the technology. Microsoft Research has a video that attempts to hit that spot in the middle — practical information for people who currently work with traditional computers. You can see the video below.

The video starts with basics you’d get from most videos talking about vector representation and operations. You have to get through about 17 minutes of that sort of thing until you get into qubits. If you glaze over on math, listen to the “index array” explanations [Andrew] gives after the math and you’ll be happier.

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Google Ups The Ante In Quantum Computing

March 9, 2018 by 36 Comments

At the American Physical Society conference in early March, Google announced their Bristlecone chip was in testing. This is their latest quantum computer chip which ups the game from 9 qubits in their previous test chip to 72 — quite the leap. This also trounces IBM and Intel who have 50- and 49-qubit devices. You can read more technical details on the Google Research Blog.

It turns out that just the number of qubits isn’t the entire problem, though. Having qubits that last longer is important and low-noise qubits help because the higher the noise figure, the more likely you will need redundant qubits to get a reliable answer. That’s fine, but it does leave fewer qubits for working your problem.

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Quantum Computing Hardware Teardown

January 22, 2018 by 59 Comments

Although quantum computing is still in its infancy, enough progress is being made for it to look a little more promising than other “revolutionary” technologies, like fusion power or flying cars. IBM, Intel, and Google all either operate or are producing double-digit qubit computers right now, and there are plans for even larger quantum computers in the future. With this amount of inertia, our quantum computing revolution seems almost certain.

There’s still a lot of work to be done, though, before all of our encryption is rendered moot by these new devices. Since nothing is easy (or intuitive) at the quantum level, progress has been considerably slower than it was during the transistor revolution of the previous century. These computers work because of two phenomena: superposition and entanglement. A quantum bit, or qubit, works because unlike a transistor it can exist in multiple states at once, rather than just “zero” or “one”. These states are difficult to determine because in general a qubit is built using a single atom. Adding to the complexity, quantum computers must utilize quantum entanglement too, whereby a pair of particles are linked. This is the only way for any hardware to “observe” the state of the computer without affecting any qubits themselves. In fact, the observations often don’t yet have the highest accuracy themselves.

There are some other challenges with the hardware as well. All quantum computers that exist today must be cooled to a temperature very close to absolute zero in order to take advantage of superconductivity. Whether this is because of a reduction in thermal noise, as is the case with universal quantum computers based on ion traps or other technology, or because it is possible to take advantage of other interesting characteristics of superconductivity like the D-Wave computers do, all of them must be cooled to a critical temperature. A further challenge is that even at these low temperatures, the qubits still interact with each other and their read/write devices in unpredictable ways that get more unpredictable as the number of qubits scales up.

So, once the physics and the refrigeration are sorted out, let’s take a look at how a few of the quantum computing technologies actually manipulate these quantum curiosities to come up with working, programmable computers. Continue reading “Quantum Computing Hardware Teardown” →