Spooky Action Could Help Boost Quantum Machine Learning

Mysterious quantum links could help lead to exponential scale-up

3 min read
A drawing of two laptops. The left laptop, labeled classical data, shows 1s and 0s on its screen, and then interlinked lines, and finally an identified cat. The right laptop, labeled entangled data, shows the same but the third screen identifies the cat as alive.

Both conventional and quantum machine-learning approaches are hampered by the “no free lunch” theorem, which suggests that ultimately any approach is hampered by the amount of data (or entanglement) available.

Los Alamos National Laboratory

Machine learning, which now powers speech recognition, computer vision, and more, could prove even more powerful when run on quantum computers. Now scientists find the strange quantum phenomenon known as entanglement, which Einstein dubbed “spooky action at a distance,” might help remove a major potential roadblock to implementing quantum machine learning, a new study finds.

Quantum computers can theoretically prove more powerful than any conventional computer on a number of tasks, such as finding a number’s prime factors—the mathematical foundation of the modern encryption currently protecting banking and other secure data. The more components known as qubits that are linked together in a quantum computer through entanglement—wherein multiple particles can influence each other instantaneously regardless of how far apart they are—the greater its computational power can grow, in an exponential fashion.

Scientists are still researching the specific problems for which quantum computing might have an advantage over classical computing. Recently, they have begun exploring whether quantum computing might help boost machine learning, the field of AI that investigates algorithms that improve automatically through experience.

One potential application of quantum machine learning is simulating quantum systems—for instance, chemical reactions that might yield insights leading to next-generation batteries or new drugs. This might entail creating models of the molecules of interest, having them interact, and using experiments of how the actual compounds interact as training data to help improve the models.

A potential major stumbling block that quantum machine learning may face is the so-called “no free lunch” theorem. The theorem suggests any machine learning algorithm is as good as, but no better than, any other when their performance is averaged over many problems and sets of training data.

A consequence of the no-free-lunch theorem is that a machine-learning algorithm’s average performance depends on how much data it has, suggesting the amount of data ultimately limits machine learning’s performance. This raised the possibility that in order to model a quantum system, for example, the amount of training data that a quantum computer might need would grow exponentially as the modeled system became larger. This could potentially eliminate the edge that quantum computing could have over classical computing.

Now scientists have discovered a way to eliminate this exponential overhead using a newfound quantum version of the no-free-lunch theorem. Their findings, verified using quantum-hardware startup Rigetti’s Aspen-4 quantum computer, suggest that adding more entanglement to quantum machine learning can lead to exponential scale-up.

Specifically, the researchers suggested entangling additional qubits with the quantum system that a quantum computer aims to model. This extra set of “ancilla” qubits can help the quantum machine-learning circuit interact with many quantum states in the training data at the same time. As such, a quantum machine learning circuit may experience a speedup even with relatively few ancillas.

“Trading entanglement for training states could give huge advantages for training certain types of quantum systems,” says study coauthor Andrew Sornborger, a physicist at Los Alamos National Laboratory, in New Mexico.

Sornborger cautions that it can prove extremely difficult entangling ancilla qubits with the quantum systems used in the experiments needed to supply training data. Still, “as long as it is not exponentially difficult in some sense to create entanglement, then we stand to benefit,” he says.

One potential futuristic application of this work is what the researchers call “black box uploading.” “Wouldn’t it be cool to be able to learn a model of a quantum experiment, then study it on a quantum computer without having to do repeated experiments?” Sornborger says.

For example, if the atom smashers at CERN, the largest particle physics lab in the world, entangled the protons being collided together with the detectors used to analyze them and with an extraordinarily powerful quantum computer (on the order of a billion billion qubits), scientists could have a way to directly analyze the Standard Model, currently the best explanation for how all the known elementary particles behave.

“This is the sort of possibility that we’ve begun to contemplate in the quantum machine-learning context,” Sornborger says.

The scientists detailed their findings 18 February in the journal Physical Review Letters.

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The Inner Beauty of Basic Electronics

Open Circuits showcases the surprising complexity of passive components

5 min read
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A photo of a high-stability film resistor with the letters "MIS" in yellow.
All photos by Eric Schlaepfer & Windell H. Oskay
Blue

Eric Schlaepfer was trying to fix a broken piece of test equipment when he came across the cause of the problem—a troubled tantalum capacitor. The component had somehow shorted out, and he wanted to know why. So he polished it down for a look inside. He never found the source of the short, but he and his collaborator, Windell H. Oskay, discovered something even better: a breathtaking hidden world inside electronics. What followed were hours and hours of polishing, cleaning, and photography that resulted in Open Circuits: The Inner Beauty of Electronic Components (No Starch Press, 2022), an excerpt of which follows. As the authors write, everything about these components is deliberately designed to meet specific technical needs, but that design leads to “accidental beauty: the emergent aesthetics of things you were never expected to see.”

From a book that spans the wide world of electronics, what we at IEEE Spectrum found surprisingly compelling were the insides of things we don’t spend much time thinking about, passive components. Transistors, LEDs, and other semiconductors may be where the action is, but the simple physics of resistors, capacitors, and inductors have their own sort of splendor.

High-Stability Film Resistor

A photo of a high-stability film resistor with the letters "MIS" in yellow.

All photos by Eric Schlaepfer & Windell H. Oskay

This high-stability film resistor, about 4 millimeters in diameter, is made in much the same way as its inexpensive carbon-film cousin, but with exacting precision. A ceramic rod is coated with a fine layer of resistive film (thin metal, metal oxide, or carbon) and then a perfectly uniform helical groove is machined into the film.

Instead of coating the resistor with an epoxy, it’s hermetically sealed in a lustrous little glass envelope. This makes the resistor more robust, ideal for specialized cases such as precision reference instrumentation, where long-term stability of the resistor is critical. The glass envelope provides better isolation against moisture and other environmental changes than standard coatings like epoxy.

15-Turn Trimmer Potentiometer

A photo of a blue chip
A photo of a blue chip on a circuit board.

It takes 15 rotations of an adjustment screw to move a 15-turn trimmer potentiometer from one end of its resistive range to the other. Circuits that need to be adjusted with fine resolution control use this type of trimmer pot instead of the single-turn variety.

The resistive element in this trimmer is a strip of cermet—a composite of ceramic and metal—silk-screened on a white ceramic substrate. Screen-printed metal links each end of the strip to the connecting wires. It’s a flattened, linear version of the horseshoe-shaped resistive element in single-turn trimmers.

Turning the adjustment screw moves a plastic slider along a track. The wiper is a spring finger, a spring-loaded metal contact, attached to the slider. It makes contact between a metal strip and the selected point on the strip of resistive film.

Ceramic Disc Capacitor

A cutaway of a Ceramic Disc Capacitor
A photo of a Ceramic Disc Capacitor

Capacitors are fundamental electronic components that store energy in the form of static electricity. They’re used in countless ways, including for bulk energy storage, to smooth out electronic signals, and as computer memory cells. The simplest capacitor consists of two parallel metal plates with a gap between them, but capacitors can take many forms so long as there are two conductive surfaces, called electrodes, separated by an insulator.

A ceramic disc capacitor is a low-cost capacitor that is frequently found in appliances and toys. Its insulator is a ceramic disc, and its two parallel plates are extremely thin metal coatings that are evaporated or sputtered onto the disc’s outer surfaces. Connecting wires are attached using solder, and the whole assembly is dipped into a porous coating material that dries hard and protects the capacitor from damage.

Film Capacitor

An image of a cut away of a capacitor
A photo of a green capacitor.

Film capacitors are frequently found in high-quality audio equipment, such as headphone amplifiers, record players, graphic equalizers, and radio tuners. Their key feature is that the dielectric material is a plastic film, such as polyester or polypropylene.

The metal electrodes of this film capacitor are vacuum-deposited on the surfaces of long strips of plastic film. After the leads are attached, the films are rolled up and dipped into an epoxy that binds the assembly together. Then the completed assembly is dipped in a tough outer coating and marked with its value.

Other types of film capacitors are made by stacking flat layers of metallized plastic film, rather than rolling up layers of film.

Dipped Tantalum Capacitor

A photo of a cutaway of a Dipped Tantalum Capacitor

At the core of this capacitor is a porous pellet of tantalum metal. The pellet is made from tantalum powder and sintered, or compressed at a high temperature, into a dense, spongelike solid.

Just like a kitchen sponge, the resulting pellet has a high surface area per unit volume. The pellet is then anodized, creating an insulating oxide layer with an equally high surface area. This process packs a lot of capacitance into a compact device, using spongelike geometry rather than the stacked or rolled layers that most other capacitors use.

The device’s positive terminal, or anode, is connected directly to the tantalum metal. The negative terminal, or cathode, is formed by a thin layer of conductive manganese dioxide coating the pellet.

Axial Inductor

An image of a cutaway of a Axial Inductor
A photo of a collection of cut wires

Inductors are fundamental electronic components that store energy in the form of a magnetic field. They’re used, for example, in some types of power supplies to convert between voltages by alternately storing and releasing energy. This energy-efficient design helps maximize the battery life of cellphones and other portable electronics.

Inductors typically consist of a coil of insulated wire wrapped around a core of magnetic material like iron or ferrite, a ceramic filled with iron oxide. Current flowing around the core produces a magnetic field that acts as a sort of flywheel for current, smoothing out changes in the current as it flows through the inductor.

This axial inductor has a number of turns of varnished copper wire wrapped around a ferrite form and soldered to copper leads on its two ends. It has several layers of protection: a clear varnish over the windings, a light-green coating around the solder joints, and a striking green outer coating to protect the whole component and provide a surface for the colorful stripes that indicate its inductance value.

Power Supply Transformer

A photo of a collection of cut wires
A photo of a yellow element on a circuit board.

This transformer has multiple sets of windings and is used in a power supply to create multiple output AC voltages from a single AC input such as a wall outlet.

The small wires nearer the center are “high impedance” turns of magnet wire. These windings carry a higher voltage but a lower current. They’re protected by several layers of tape, a copper-foil electrostatic shield, and more tape.

The outer “low impedance” windings are made with thicker insulated wire and fewer turns. They handle a lower voltage but a higher current.

All of the windings are wrapped around a black plastic bobbin. Two pieces of ferrite ceramic are bonded together to form the magnetic core at the heart of the transformer.

This article appears in the February 2023 print issue.

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