On the roof of the Institute of Quantum Communications in Gdynia, the air smelled of salt and ozone. The Baltic breathed dark glass, and sparse yellow points of light burned in the harbour. Silent meteorological drones circled above them, drawing barely visible spirals in the sky. Lena Nowicka sat on the edge of the concrete attic with a mug of tea and counted in her mind how unsuitable she was for someone planning to spend her holidays in a laboratory. From below, through the open hatch, came the hum of fans and the steady crackle of relays. An interferometer, a blue box as thick as a wardrobe, full of fibre optics and crystals, was cooling in B-12. It was it, encased in super-insulation and enveloped in the mint glow of the tubes, that would catch a narrow window of synchronicity with Helicon-2, a probe that had long since passed Neptune and settled on the fringes of cold darkness. Sometimes raw packets of statistics about dust and micrometeorites came from it. Sometimes nothing came. - 'I can't believe you left the tea on the roof,' muttered Nikolai, climbing through the hatch with his rucksack and breath smelling of cinnamon. - 'One descent to get the bars and we're already risking an institute-wide disaster. - 'Relax, it's not cryogenics,' smiled Lena, descending the metal ladder behind him. - When do we have a window? - 'Officially at 00:17,' he replied, throwing his backpack on the table and running his fingers over the touchpad. - But Dr Krawiec wrote that the fluctuations of the ionosphere are nasty today. We've got a margin like an examiner at a high school graduation. The main screen flashed dark purple and a graph slowly rose above the noise line. A thumbnail blinked on the communicator line: Dr Krawiec, across town in a flat full of papers and orchids. - 'Just don't sit at it until dawn,' his voice sounded, dry and matter-of-fact. - The window may be shorter than usual. If you don't see a clear peak by 00:20, you turn off the track and go to bed. No experiments. - 'Sure,' Lena said reflexively. She knew she was lying with her voice. For a fortnight she had been preparing her own script to filter out false correlations. She wanted to test it in battle. The darkness from behind the laboratory glass looked like a silence that could be touched. The laser in the device purred and the cooling clocked in as an even step. Lena felt that familiar mixture of anxiety and excitement, like before the start of a race. Because this was what it was all about: the anticipation of a signal that can tremble inside a person like a string. - 'Okay, we've got the pre-buffer input,' Nikolai announced, tapping on the keyboard. - 'I'm collecting correlation distributions. I can see the phase shift... oh, nice mushroom. Lena, your filters on? - Yes. Autocalibration stable. I'm suppressing rubbish from the arc station transmitter. The graph popped up as if something inside it sighed. A single tooth emerged above the edge of the noise - clean, sharp. - 00:12 - said Nikolai. - It's still too early for a window. It's probably an artefact. The tooth has duplicated itself. Three, four, twelve. They folded into a pulse with a familiar headline signature. Lena leaned over the monitor. The algorithm on the Helicon-2 side always opened the message with the same predictable rhythm. But now, in the log, in the address field, something appeared that had no right to appear: LENA_NOWICKA. - Is this some kind of joke? - Nikolai asked quietly. - Did you paste your name into the simulator? - No - she whispered. Her heart suddenly clutched her chest. - It's coming from the real channel. The quantum signature... wait for it... it fits. The CRC, too. Only the timestamp is... She blinked. The timestamp was based on a very boring mathematics she loved: the rhythm of the reference pulsars from which the clock for the mission flowed. This rhythm was inviolable, because no one could fool a pulsar. Yet the ZT field showed 00:29:08. - This is in seventeen minutes,' Nikolai said slowly. - The signature comes from here, but the time... is from here. And on the front. The lights dimmed slightly in the lab. The UPS beeped warningly, as if someone had moved a heavy cloud over the building. Dr Krawiec on the screen raised his gaze from his notes. - A drop in voltage? - He asked. - Do you have shielding on the tracks? The ionospheric alarm just went up. - 'The shields are there,' Lena replied, her hands cold despite her breathlessness. - 'Doctor... Helikon sent something with an address... my address. And with a time stamp ahead of time. A moment of silence rolled through the communicator. Then: - Don't touch anything. Take a full dump and freeze the buffer. Don't try to respond. Lena nodded and pressed the sequence. The console displayed a text block, still jagged, as if someone had chopped it up in a hurry. Her script glued it together. Letters emerged line by line until she finally read the first sentence: Lena, don't open the door. Leave all the lights on. You are not alone. The skin on the back of her neck tingled. She looked at Nikolai - and saw her own amazement reflected in his eyes. In the bottom corner of the screen someone had added something else, as if in a panic: Set the filter to 31.8. You'll see. Lena twitched. 31.8 was a modulation frequency she only used in tests, never live. It shouldn't work on the Helicon channel. Yet her heart told her to try. Nikolai shook his head, but didn't take the keyboard from her. - Two seconds,' she whispered. - I'm just going to see. Click. The spectrum parted like a curtain. Between the ridges of noise she saw a thin, almost invisible thread. As the filter closed, the thread grew into a wave with coded synchronicity points. And in it - an image. Pixellated but legible: a B-12 glass door, exactly like the one here, ajar by the width of a hand. Across the corridor hung a green mackintosh, her jacket. And then the frame vibrated and showed the lab clock: 00:29. - 'That's not possible,' Nikolai growled out. - 'This is our corridor, but...' - 'Seventeen minutes from now,' finished Lena, unable to take her eyes off the pseudo-picture that was flickering, as if pulled from someone else's memory. For a moment, Dr Krawiec's voice returned, jagged with interference: - Close... buffer... don't open.... The speaker beeped and fell silent. In the same second, a fire door closed somewhere in the building, metal banged against metal, echoes ran down the stairwell. The B-12 lock panel flashed a yellow light, as if examining whether everything was in place. - Do you hear that? - Nikolai straightened up. - Someone is walking in the corridor. Lena froze, listening in. Footsteps. Soft, suspended, unhurried. They stopped just outside the door. Through the glass, she could see nothing but her own reflection and the studio stretched into a black blur. A new line of data appeared on the console screen, as fast as if someone was typing it on the other side of the glass: You have less than a minute. Don't do the spotlight. Don't walk up to the door. Lena swallowed her saliva. She held her hand over the switch of the work lamps, as if this simple movement could suddenly bring daylight here. She didn't flinch. Nikolai pointed to the stairwell's camera preview monitor. The image went noisy and for a split second gave a silhouette - blurry, faceless, more of a shadow than a person. Then the screen darkened again. - 'Probably the camera's dead,' said Nikolai without conviction. - 'Or someone's playing a stupid prank on us. - Someone who can sign with the Helicon key? - Lena shook her head. She felt the intricately constructed certainties crumble into fine sand. The light panels flashed a second time. In the silence there was a sound so distinct that they both turned their heads at the same time: one, a second, a third, a slow, metallic knock on the B-12 door. On the monitor next to it, the countdown flicked and began to run down: 00:59. 00:58. 00:57. And someone, on the other side of the glass, put his hand on the door handle.