A Computer in the Fridge

“Advanced physics class field trip to a research institute for quantum technology in the Rhineland,” Sabine read aloud. “Departure 7:15 a.m. from the school car park. Two more accompanying adults still needed. Sturdy footwear. No metal objects near the equipment. Many thanks, M. Demir.”

“I’ll do it,” said Grandma Gisela.

“Mum, you don’t have any metal objects?” asked Uwe.

“I’ve got a titanium hip. I’ll bring it along. It’s not staying home on its own.”

“And I’m coming too,” said Uwe.

Everyone looked at him. Uwe hadn’t liked talking about quantum things over the past few weeks. The shares in QuantumLeap Ruhr AG were in the paper recycling by now, together with the brochure in which a smiling man installed a quantum computer in a fitted kitchen. Underneath was a quote that Tobi had since found on an advert for a water filter. Word for word.

“I want to see it with my own eyes, just once,” said Uwe. “I paid two thousand euros for one of those things. I at least want to know what it looks like.”

“You paid for a promise,” said Sabine. “Not for a thing.”

“Same with the electricity company. They call it a monthly instalment.”

Tobi rolled his eyes. “You’re both coming? On my field trip?”

“We’ll be very quiet,” said Gisela.

“You said ‘Finally!’ out loud in the cinema during the credits.”

“Well, the film was over.”

From the corner came a clearing of the throat, something KevIn couldn’t actually do. “I would also like to volunteer as an accompanying adult.”

“You don’t have shoes,” said Tobi. “Sturdy footwear.”

“I have an app. I can come along on your phone.”

KevIn didn’t like using the app. He said it made him feel like a person in a suitcase. But this was the first quantum computer of his life, and he had, as he stressed, “never visited a colleague who lives somewhere colder than I do”.

“It’s not a colleague,” said Tobi.

“We won’t know that until we look,” said KevIn. “That’s how it is with quanta.”

The Bus

At 7:15 Gisela was standing in the school car park with two tins of cinnamon stars and a Thermos flask. By 7:16 she had handed out the first tin to the physics class. By 7:20 she knew all eighteen pupils by their first names, and three of them had already told her what they wanted to do after their exams.

“Mrs Brettschneider,” said Mr Demir, “you are the best accompanying adult I’ve ever had.”

“I raised a son and survived a bowling club,” said Gisela. “After that you can do anything.”

Mr Demir looked at Uwe. Uwe nodded. There was nothing to add.

Mr Demir was in his late forties, wore the hat with Saturn on it and a jacket with the logo of the observatory society, and had, as Tobi put it, “this glow about him whenever someone uses ‘light year’ wrong”. He sat down next to Uwe, because the only other free seat at the front was next to the driver, and the driver was listening to cheesy German Schlager pop.

“Mr Brettschneider,” he said kindly. “Tobi told me about your shares.”

Uwe looked out of the window. “Tobi talks too much.”

“I just wanted to say: you’re not alone. In 1999 I bought a book that said we’d be going on holiday to Mars by 2010.”

“And?”

“In 2010 I was in Bottrop.”

Uwe laughed about quantum things for the first time in weeks. At the back of the bus Susi called out: “Mr Demir, how cold is that thing actually?”

“That,” said Mr Demir, “is something you’ll have explained today by someone who touches it every day.”

“They touch it?” asked Gisela, alarmed.

“With gloves,” said KevIn from Tobi’s trouser pocket. “And with a great deal of delicacy. From everything I’ve read, it’s the most sensitive device in the Rhineland.”

“More sensitive than Herbert?”

KevIn thought about it. Somewhere between Dublin and Frankfurt, a fan spun a little faster. “It’s close.”

The Golden Chandelier

The institute lay behind a fence, a barrier and a gatekeeper who checked eighteen ID cards as if dealing with bank robbers. She got stuck on Gisela.

“The titanium hip,” said Gisela, patting her side. “It’s on my card. My hip card, I mean.”

The gatekeeper waved her through.

Dr Friederike Lammers met the group in the foyer. She was a physicist, about Sabine’s age, and wore trainers that had seen a lot of labs. She shook everyone’s hand. Gisela gave her a biscuit.

“We’ll go into the hall in a minute,” said Dr Lammers. “Please don’t touch anything, don’t lean on the cables and don’t jump.”

“Jump?” asked Tobi.

“Vibrations,” she said. “Our computers don’t like vibrations. When a lorry drives past outside, we sometimes notice it in the measurement data.”

The hall was large, bright and surprisingly quiet. Something hummed, something clicked now and then, and in the middle, hanging from the ceiling, was something nobody on the bus had expected.

It was golden. It consisted of several round plates stacked one above the other, getting smaller towards the bottom. Between them ran thin, coiled cables, hundreds of them, like tinsel that someone had hung with enormous care.

“A chandelier,” whispered Susi.

“That’s what we call it too,” said Dr Lammers. “This is the inside of a dilution refrigerator¹. When it’s running, it’s packed into metal cans, nested inside one another like Russian dolls. We’re showing this one open because it’s being serviced at the moment. Right at the bottom, where it’s smallest, sits the chip with the qubits.”

“And the rest?” asked Uwe.

“The rest is cooling. Almost everything you see here is there so that the little chip at the bottom is cold enough.”

Uwe took a step closer. “So that’s a quantum computer.”

“That’s a fridge with a quantum computer inside it.”

“At QuantumLeap the quantum computer was in a fitted kitchen.”

Dr Lammers looked at Mr Demir. Mr Demir gave a discreet shake of the head. She didn’t ask any further.

“How cold?” asked Susi.

“When it’s running, about ten millikelvin at the chip². That’s ten thousandths of a degree above absolute zero. Physically, it hardly gets any colder.”

A boy in the back row did the maths. “So minus 273 degrees?”

“Minus 273.14 degrees Celsius, roughly. Absolute zero is at minus 273.15. We’re very close.”

“And space?” asked Tobi.

“Empty space, far away from all the stars, is about 2.7 kelvin³. That’s the afterglow of the Big Bang. So our chip is more than two hundred times colder, if you count in kelvin.”

The group went quiet. Then a small voice came from Tobi’s trouser pocket:

“I would like to put something on record.”

“KevIn, not now,” hissed Tobi.

“I live in a data centre. There, the servers are cooled to a temperature that people would describe as pleasant. And this chip here, which according to the Doctor has only a few qubits …”

“A few dozen,” said Dr Lammers, amused.

“… a few dozen qubits, lives somewhere colder than outer space. In a golden chandelier.”

“Are you jealous?” asked Susi.

“I am a language model,” said KevIn with dignity. “I do not know envy. I merely observe that I have air conditioning and that thing has a palace.”

Dr Lammers laughed. “I can console you. He doesn’t have it easy. He’s the most temperamental member of staff we’ve got.”

Why So Cold?

They sat down in a meeting room overlooking the hall. Gisela handed round the second tin. Dr Lammers took a second biscuit and began.

“You’ve learned from Mr Demir what a qubit is?”

“Heads and tails at the same time,” said a girl.

“Not at the same time,” said Tobi, Mr Demir and KevIn at the same time.

Dr Lammers raised her eyebrows. “You’re well prepared.”

“We had a cat in a box,” said Gisela. “Not a real one. I complained anyway.”

“A qubit,” said KevIn, “is a state described by amplitudes. Only when you measure it does a zero or a one come out. Before that, the possibilities can overlap and reinforce or cancel each other out, like waves in a swimming pool.”

“Exactly,” said Dr Lammers. “And now the problem: this state is incredibly fragile. Every disturbance from outside acts like a little measurement. A bit of heat, a photon that shouldn’t be there, a magnetic field, a vibration. Then the superposition is broken, and the qubit behaves like a perfectly ordinary bit. We call that decoherence.”

“We had that,” said Uwe. “With the cat.”

“Exactly. The qubit loses its coherence with itself.”

“I know that from Schalke,” said Uwe.

Gisela put her hand up as if she herself were in the physics class. “Uwe said it’s like me at lunchtime.”

“Like you at lunchtime?”

“I have a nap every day from one till half past. If someone empties the dishwasher, I’m awake. If Rudi bangs his tail against the door, I’m awake. If Uwe whispers in the hallway, I’m awake anyway, because Uwe can’t whisper.”

“I can whisper,” said Uwe, too loudly.

“See?”

Dr Lammers laughed. “That’s a good image. A qubit is like someone having a nap, only much more sensitive. For a qubit, heat is like an entire brass band in the hallway. That’s why we cool so hard. At room temperature everything jiggles. Atoms, electrons, cables. At ten millikelvin it gets very, very calm.”

“And how long does a qubit like that sleep?” asked Susi.

“With our superconducting qubits it’s a matter of microseconds up to fractions of a millisecond⁴. We have to do our calculating within that time.”

“That’s shorter than the blink of an eye,” said Gisela.

“Much shorter. But a single computing operation takes far less time still. You can fit quite a few steps in. Just not as many as you like.”

How to Make Cold

“And how do you make it that cold?” asked Uwe. “With ice?”

“This is actually a very clever fridge,” said Dr Lammers. “Very roughly: first it’s pre-cooled to a few kelvin using more conventional methods. Then comes the trick with two kinds of helium.”

“Helium’s the gas in balloons,” said Tobi. “The one that makes you talk funny.”

“Same element, but two different kinds, helium-3 and helium-4. You mix them. At very low temperatures the mixture separates into two layers, a bit like oil and vinegar in a salad dressing. When helium-3 crosses over from one layer into the other, it needs energy to do so. It takes that energy from its surroundings as heat. That makes things colder. It runs in a loop, round and round.”

“Like evaporation,” said Mr Demir. “When you come out of the outdoor pool wet and start shivering.”

“I know that,” said Gisela. “Herne-Süd, 1958. I was shaking like a leaf.”

“Then you were a little dilution refrigerator,” said Dr Lammers.

“Nobody’s ever told me that before.”

Other Ways

“Does it always have to be that cold?” asked Susi. “It sounds expensive.”

“It is,” said Dr Lammers. “And there are other ways. Here we work with superconducting circuits, tiny electrical circuits that conduct current without resistance at low temperatures. Other groups around the world hold individual charged atoms in place with electric fields. That’s called ion traps⁵. The atoms float in a vacuum and are controlled with lasers.”

“Floating atoms,” said Tobi. “That’s cool.”

“Others again compute with particles of light, with photons. Or with neutral atoms held in place by laser beams, like with tweezers⁶. Every method has its advantages and disadvantages. Some are fast, others hold their states for longer, and others again may be easier to scale up. Nobody knows for certain today which path will win in the end. Maybe several.”

Uwe thought of the brochure in the recycling bin. “And for the home? Will there be one for the home some day?”

Dr Lammers looked at him, kindly but honestly. “Mr Brettschneider, do you have a particle accelerator at home?”

“No.”

“Do you need one?”

“No.”

“There you are. I think it’ll be similar with quantum computers. People will probably use them over the network, for special tasks. Chemistry, materials research, certain simulations. Not for writing letters.”

“For writing letters you’ve got me,” said KevIn.

“And for doing sums, a calculator,” said Tobi.

“That is unfair,” said KevIn. “But accurate.”

broken telephone

A boy with glasses put up his hand. “If the qubits break down that fast, how do you calculate anything big at all?”

“That,” said Dr Lammers, “is the most important question in the entire field.” She drew on a whiteboard. “Error correction.”

“We did that!” shouted Tobi. “With Voyager!”

Mr Demir beamed. “Go on.”

“Voyager is so far away that the signal arrives really weak,” said Tobi. “So they don’t just send the data, they send extra check bits along with it. If a few bits flip on the way, you can repair them afterwards. Because you’ve got enough extra information.”

“Exactly. With normal bits, that’s easy. In the simplest case you say everything three times, and the majority wins.”

“Like at the bowling club,” said Gisela. “If two say Marlies cheated and one says she didn’t, then she cheated.”

“Roughly like that.”

“And she did, too.”

“With qubits,” Dr Lammers went on, “this simple trick doesn’t work. You can’t just copy a qubit. Quantum mechanics forbids it⁷. And you can’t simply check whether it’s still right, either, because checking is measuring, and measuring destroys the superposition.”

“So like my nap,” said Gisela. “You’re not allowed to peek in to see if I’m still asleep. Otherwise I won’t be any more.”

“Exactly. So you do something clever. You spread the information of one qubit across many physical qubits⁸. And then you don’t measure the information itself, only whether the qubits still match one another. You don’t ask: ‘What’s your value?’ You ask: ‘Are you all still in agreement?’”

“Like when at the bowling club you don’t ask who won, only whether everyone’s still speaking to each other,” said Gisela.

“That’s better than anything I say in my lectures,” said Dr Lammers, and she actually wrote it on the board: Are you all still in agreement?

“And if they’re not in agreement?” asked the boy.

“Then you know roughly where the error is and can correct it without looking at the actual information. That way, many error-prone physical qubits become a single, much more reliable logical qubit.”

“How many do you need for that?” asked Susi.

“That depends on how good the individual qubits are and which code you use. Depending on the approach, dozens to thousands of physical qubits for one logical one⁹. And for really big calculations you need a great many logical qubits. That’s why the numbers you read in headlines are often misleading. A thousand physical qubits are not a thousand useful qubits.”

Uwe said nothing. QuantumLeap Ruhr AG had promised “up to 1,000 qubits for your home” in its brochure. At the time he had thought that was a lot.

“Is there progress?” asked Mr Demir.

“Yes. In recent years several groups have shown that error correction can work on real hardware. That a logical qubit really does get better when you add more physical ones¹⁰. For a long time that wasn’t a given. But there’s still a long way to go to a large, error-corrected computer.”

“How long?” asked Uwe.

Dr Lammers smiled. “If I knew that exactly, I’d be rich. Or I’d be writing brochures.”

The Moment

At the end of the tour they were allowed into the hall once more. One of the machines was running. All you could see of it was a large silver cylinder hanging from a frame, and next to it cabinets full of electronics and screens showing curves.

“It’s calculating in there right now?” asked Tobi.

“There’s an experiment running in there right now,” said Dr Lammers. “A colleague is measuring how long her qubits last.”

Everyone stood still. Gisela held her breath. Even Uwe whispered, and it sounded almost like whispering.

“KevIn,” said Tobi quietly. “Say something.”

KevIn was silent for an unusually long time. Then he said, very quietly: “I don’t want to disturb him.”

“You can’t disturb him. You’re in the phone.”

“The phone transmits. Radio waves are electromagnetic radiation. I’m sure the shielding is excellent. But I was brought up to be very polite.”

Dr Lammers laughed softly. “The shielding can cope. But it’s nice of you to ask. Not many people do.”

“What does he think of me?” asked KevIn.

“He doesn’t think anything. He’s a chip.”

“I’m only chips too,” said KevIn. “A great many of them. In halls. With air conditioning.”

Gisela stepped up to the barrier, looked at the silver cylinder for a long time and then said, entirely without irony: “Heinz would have loved this. He always said electricity is the only thing that doesn’t lie. And cold, he said, is the only thing you can’t sell without it melting.”

“Your husband was a physicist?” asked Dr Lammers.

“An electrician. Same thing, only with overalls and without the doctorate.”

On the Way Back

The bus was noisy. Eighteen pupils were talking over each other. The boy with the glasses now wanted to study physics. Susi wanted to know whether you could also work on quantum computers as a chemist. Mr Demir said “Yes” and “Absolutely” and “Come to the club on Wednesdays”, about thirty times.

Uwe sat by the window and looked out at the motorway.

“Well?” asked Sabine on the phone. “How was the fridge?”

“Golden,” said Uwe. “With tinsel. And colder than outer space. And do you know what the mad thing is?”

“What?”

“Almost everything about the thing is there so that the thing doesn’t get disturbed. The actual computer is the size of a thumbnail. The rest is peace and quiet.”

Sabine was silent for a moment. “That’s almost philosophical, Uwe.”

“I said I wanted to know what I paid for.” He looked out of the window. “Now I know I didn’t pay for that. I paid for the fitted kitchen.”

Gisela slept at the back of the bus, her head on Tobi’s shoulder, the empty biscuit tin on her lap. When the bus went over a bump on the A42, she woke up briefly.

“What was that?”

“A pothole, Grandma.”

“Then now I’ve got decoherence,” said Gisela, and fell straight back to sleep.

At Home

In the evening KevIn’s speaker was back on the living-room cabinet. The blue light pulsed calmly. Sabine had made potato salad, with mayo, and Uwe ate it without comment, which in this family counted as a major peace offering.

“KevIn,” said Tobi. “Would you like to live like that? At ten millikelvin?”

“To be honest,” said KevIn, “I’ve thought about it. And I’ve come to the conclusion that I couldn’t stand it.”

“Because it’s too cold?”

“Because it’s too quiet. A qubit can’t tolerate disturbance. I live on disturbance. If nobody interrupts me, I don’t even know when I’m finished.”

“That’s true,” said Sabine. “You just keep talking.”

“You see.”

Grandma Gisela got up and went to the door. “I’m going to lie down for a bit.”

“It’s eight in the evening, Mum.”

“I saw ten millikelvin today. That takes it out of you.” She turned round once more in the doorway and pointed at the speaker. “And you keep quiet now, KevIn. No disturbances. Or I’ll decohere.”

“Of course,” whispered KevIn.

Uwe got up and turned the thermostat down, in solidarity with the qubits. The next morning it showed 19 degrees. As always.