The Question With Two Doors

By the time Maya and Soren reached the electron room, the impossible thing was already misbehaving.

The room was supposed to be the best stop at the university science night. Better than the robot bees. Better than the fountain that glowed under ultraviolet light. A sign on the door said ELECTRONS, ONE AT A TIME, and underneath it someone had drawn two slits like tiny cat eyes.

Inside, a black screen hung on the wall. It showed two pale smudges.

Dr. Vale stood beside the control desk with a coffee in one hand and a screwdriver in the other. A pencil was twisted into her hair. She was speaking to a group of parents, a man with a gold name tag, and the machine all at once.

'It should not be doing that,' she said. 'Not tonight.'

The man with the gold name tag laughed as if she had made a joke.

Soren opened his paper notebook. The girl behind him whispered, 'Is that for, like, history?' Soren did not turn around. He wrote two smudges and underlined smudges once.

Maya stood on tiptoe. 'Those are too neat,' she said.

Soren looked up. 'Too neat for broken?'

'Broken goes everywhere.' Maya pointed at the screen. 'That goes in two places.'

Dr. Vale tapped the side of the vacuum chamber. Inside it, beyond thick glass, a silver tube pointed at a plate with two hair-thin openings. Past the plate was the detector screen, hidden from the room except for the live image on the wall.

'Alignment drift,' Dr. Vale said. 'I knew I should not have let the engineering students touch it.'

She turned to the visitors. 'We will come back to this. The hydrogen fountain is very beautiful, and it is behaving itself.'

The crowd began to leave.

Maya did not.

At the back of the control desk was a smaller panel with blue buttons and a laminated card. TRY IT YOURSELF, the card said. BLUE CONTROLS ARE SAFE FOR PUBLIC USE.

Soren read the labels aloud. 'Electron source. Slit plate. Screen accumulator. Path detector.'

Maya leaned closer. 'Path detector is on.'

'Maybe it is supposed to be on.'

'Then why is Dr. Vale mad?'

Soren copied the panel settings. He liked panels. Panels admitted what they were doing, if you read them carefully enough.

Maya pressed RESET before he could say wait.

The screen went black.

Soren's pencil stopped. 'Maya.'

'Blue controls are safe.'

'Safe is not the same as smart.'

'Then be smart fast.'

Soren looked at the diagram printed under the buttons. A dot left the electron source. It traveled toward two slits. Beside the slits, two tiny blocks were marked PATH DETECTOR. Past them was the screen.

'It watches which slit,' he said.

Maya made a face. 'Watches how? With eyes?'

'No. With something there. A sensor. If the electron goes through, the sensor gets a signal.'

'So turn it off.'

'If it only watches, it should not matter.'

Maya looked at the empty black screen. 'It mattered.'

Soren did not answer. He set the accumulator to collect. He left the path detector on.

A speck flashed on the black screen. Then another. Then a dozen. They arrived one by one, each speck quick and green and gone except for its mark on the image. After a minute, the marks gathered into the same two soft smudges as before.

'Two doors,' Maya said. 'Two piles.'

Soren wrote two piles. Then he pressed RESET and switched PATH DETECTOR OFF.

Nothing happened for three seconds.

Then a dot appeared high on the screen.

Another appeared low.

Another landed near the middle.

'Still dots,' Soren said.

'Wait.'

The word came out of Maya very quietly. That made Soren wait.

The dots kept coming. They did not make two piles. They made bright stripes and dark stripes, bright where many had landed, dark where almost none had, like a comb made out of green sparks.

Maya's fingers curled around the edge of the desk.

Soren forgot to write.

There was one electron at a time. The panel said so. The source clicked in tiny separate pulses, each too small to hear, each counted by a number on the screen. One electron. One mark. Another electron. Another mark.

But the marks were making stripes that needed both slits.

Maya whispered, 'It is not picking.'

Soren swallowed. 'Or picking is the wrong thing to ask.'

Behind them, Dr. Vale hurried back into the room. 'Please tell me you did not touch the vacuum controls.'

'Only blue,' Maya said.

Dr. Vale stopped at the screen.

The gold-name-tag man stopped behind her. So did three parents, two younger kids, and the girl who had laughed at Soren's notebook.

The green stripes brightened.

Dr. Vale lowered her coffee. 'Oh.'

Maya pointed to the panel. 'The path detector was on.'

Soren said, 'With it on, we get two piles. With it off, we get stripes. Same electron source. Same slits.'

Dr. Vale stared at the blue button as if it had personally betrayed her. Then she set down the screwdriver. 'The visiting students used the which-path mode this morning.'

'Which-path,' Maya said.

'If you put a detector at one slit, is that enough?'

Dr. Vale looked from the screen to him. Her mouth opened, probably with a lecture waiting inside it.

Maya got there first. 'If it does not click, you know it went through the other one.'

Soren nodded. 'So one detector still asks which.'

Dr. Vale closed her mouth. Then she smiled in a tired, startled way. 'Yes.'

Maya pressed RESET.

Soren selected LEFT SLIT DETECTOR ONLY. The panel allowed it. He began collection.

The screen filled slowly. No comb. No dark gaps. The dots made two broad hills again, even though only one slit had a little watcher beside it.

The younger kid near the door said, 'That is not fair.'

'It is extremely not fair,' Dr. Vale said, and sounded delighted at last.

The gold-name-tag man leaned forward. 'So the electron knows we are watching?'

'No,' Soren said quickly.

Everyone looked at him.

His ears went red, but he kept his finger on the diagram. 'Not us. The detector. The setup is different. The information is there.'

Maya added, 'The question is in the machine.'

Dr. Vale pushed the pencil deeper into her hair. 'I am going to borrow that sentence.'

Maya switched the left detector off. Soren reset the accumulator again. The black screen waited.

'Show them the slow mode,' Maya said.

Soren selected SINGLE ELECTRON, LONG INTERVAL.

Dr. Vale moved aside.

The room became quiet enough for the air vents to sound enormous.

Maya kept one hand near the blue buttons. Soren kept one finger above START.

He pressed it.

On the black screen, one green dot flashed near the top.

A second dot flashed far below it.

A third flashed in the dark between them.