the physics detective

200 NATURE | VOL 433 | 20 JANUARY 2005 | www.nature.com/nature

Schrödinger’smousetrapIan Stewart

Rufus Jaeger looked out from the platformat a sea of faces. Every seat was full, anddozens of people were sitting on stairs andlining the walls. It made for an electrifyingatmosphere, which was just how he liked it.The audience would remember this lecturefor a long time.

Jaeger was head of the Quantum OpticsGroup at the University of Wentbridge, andhe had been selected for a unique honour: togive a plenary lecture inaugurating theWorld Year of Physics. He was determined to give his audience an experience that wouldestablish him as the leading authority inquantum physics. So this would not be anyordinary lecture. Jaeger was going to carryout his latest, revolutionary, experiment — the one he called Schrödinger’s mouse-trap — live on stage.

It had cost a small fortune to set theexperiment up in the conference hall.Much of the stage was occupied by opti-cal and other apparatus. Above, on aplinth, was a huge model of a mousetrap,complete with a wedge of Swiss cheese,cartoon-style with big holes. Fortunately,one of Jaeger’s grants had money for ‘out-reach’ activities. The expenditure was justi-fied by the presence of several journalists inthe second row of the audience.Among themwas Nigel Lorimer, a senior editor at Nature.He acknowledged Jaeger’s presence with awave,and Jaeger gave a brief nod in return.

Jaeger was an imposing figure: tall, well-built, with a flourishing beard that framedhis face and sharpened his features. He couldbe charming when he decided that charmwould get him what he wanted, and intimi-dating when a head-on confrontation suitedhis purposes better. He was sitting just left ofcentre stage, beside the apparatus. Next tohim was his postdoc Ludmilla Shlomiuka, astrikingly attractive blonde with one of thebest brains in the business. She was there tointroduce Jaeger. She seemed perfectly atease and confident, giving her boss a wrysmile as the audience settled down.

Shlomiuka studiously ignored Wilfred deBruijn, a senior member of Jaeger’s group,who sat in the audience a few rows behindLorimer, glaring at her. On the far side ofShlomiuka sat Fenton Baumgarden, a worldauthority in laser physics, who would chair

Part 1: The trap is primed.the session. Baumgarden was an easy-goingAmerican who had nearly got a Nobel prize.Persistent rumours had circulated for yearsthat it was Jaeger’s reference that had spoiledit for him. But their presence together at such a prestigious event would make it clearthat there was no animosity between them.Which is why Baumgarden had nominatedJaeger for the job.

Almost hidden behind the apparatus wasthe only other person on stage: Tony Trot-man, Jaeger’s head technician. It was easy tooverlook Trotman — if he spoke at all it wasin a near whisper, and he had a chameleon-like ability to blend into his surroundings.It would be Trotman’s job to make sure thedemonstration went according toplan.He waited quietly at the backof the stage, mostly staringinto space.

Now andthen he looked

expectantly at Jaeger.Immediately in front of the

journalists were various dignitaries. Jaegerwas happy to see most of them, because theirinfluence could be useful. But two he was not pleased to see: Petra Pruszczyncki andVeronique Dubois. They represented hisleading competitors, and they were presentonly because protocol demanded it.

Pruszczyncki was a whizz-kid at theGdansk Centre for Optical Computation,anda while back had given a job to one of Jaeger’s former students, the brilliant but disgracedJirong Feng. He knew Feng would be some-where in the room … tucked discreetly awaynear the back, no doubt. Dubois was an up-and-coming experimentalist with experiencein secure quantum-cryptographic communi-cations. She worked at the University of Paris14, in direct competition with Jaeger’s group.She was friendly enough — but too cool and calculating for Jaeger to feel comfortable.

The lighting in the hall dimmed. Baum-garden, silhouetted by a single dramatic spotlight, declared the conference open andinvited his colleague Dr Shlomiuka to intro-duce their speaker.She delivered a short,witty

introduction, entirely from memory, and satdown.Jaeger rose,to enthusiastic applause.

“Distinguished guests, colleagues, andladies and gentlemen of the press,” Jaegerbegan, “I am humbled and honoured toinaugurate the World Year of Physics.” Hepaused for further applause. “I have beeninvited to describe my research group’s newresults on quantum entanglement — whichEinstein so memorably described as ‘spookyaction at a distance’. Of course, we nowunderstand that the phenomenon is entirelyrational and not at all spooky — but it doesremain quite startling. Only a few monthsago my team discovered just how startling.”

He glanced down at the assembled journalists. “You all know the fable ofSchrödinger’s cat,but it is often forgotten thatSchrödinger’s primary purpose was not to

assert the existence of superposed quantumstates, but to examine their paradoxical

consequences for a macroscopicentity.” He paused. “In the interests of

animal welfare, I will employ a simplemachine instead of a cat. I call the experi-

ment ‘Schrödinger’s mousetrap’.”Dutiful laughter rippled through the

audience. So that’s what the model on theplinth was for. “First,” Jaeger explained, “Iwill set up my mousetrap.” He gesturedtowards the model.“A laser-operated opticaltrap. But I do not bait my mousetrap withcheese, because it has already caught amouse. Which in this instance is a hollowdielectric sphere.

“Now, although I know there is a mousein the trap, I do not know its quantum state.”He paused.“What does that mean? It meansthat my spherical mouse can resonate in twodistinct modes. Following the precedent setby Herr Professor Doktor Schrödinger, Ishall refer to these modes as ‘dead’and ‘alive’.

“So have I trapped a live mouse, or a dead mouse? That, my friends, is the greatmystery!”

Pruszczyncki turned to Dubois, and said,in a stage-whisper,“Juz’ like Rufus. He iz realshowman.”

“Yes,” Dubois whispered back.“Effective,isn’t it?”

Pruszczyncki glared at her, then sighedand nodded.“I am sorry to say,you are right.”

“To resolve the mystery,” Jaeger contin-ued, “I shall appeal to quantum entangle-ment. I will arrange for the mouse’s fate to be

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entangled with that of another sphere: the‘cat’.This sphere also resonates in two modes.The entanglement procedure will ensurethat the two spheres have different states: ifthe cat is alive, then the mouse is dead, andvice versa. My assistant will now use a brieflaser pulse to prepare the entangled states.”As Trotman laboured in the darkness of thestage, the screen showed a schematic anima-tion of the entanglement procedure.

“At the climax of the experiment,” Jaegerexplained,“I will observe the state of the cat,and infer the state of the mouse. If I observethe cat to be alive, then the mouse’s wavefunction will immediately collapse to ‘dead’,and the mouse will stay in the trap. However,if the cat is observed to be dead, then themouse’s wave function will collapse to ‘alive’,and in this higher resonant mode the spherewill escape from the trap.”

“How will we know if the mouse escapesor not,Professor?”one of the reporters calledout.

“Ah, my impatient questioner from thepress,I am coming to that.To make the resultsvisible to everyone in this room, I havearranged the apparatus so that the laser beamwill project a three-dimensional hologram ofthe mouse. You will see either a dead mousecaught in my trap,or a live one running acrossthe stage above my head as it escapes.”

At this point Shlomiuka noticed thatPruszczyncki was grumbling, and asked herto stand up and clarify her concerns. It wouldbe better then letting her simmer, and even-tually explode.

“Iz stupid experiment! Iz dangerous!Laser must be unnecessarily powerful, juz tomake silly hologram…”

“My dear woman,” said Jaeger unwisely,“there are adequate safeguards…”

“Am not your dear woman! Experimentiz totally meaningless! One trial proves noth-ing: iz all about correlations.Must do experi-ment hundreds of times!”

“Which we have done, in the laboratory,as our forthcoming paper will establish,”saidJaeger smoothly. “But we don’t have enoughtime to do that today. This is just a demon-stration,Professor Pruszczyncki.”

Pruszczyncki was unconvinced. “Seenpreprint. Prove nothing! Extraor’nary claimrequire extraor’nary evidence, which youdon’t got!”

With ill-concealed anger, Jaeger sug-gested that Pruszczyncki should learn thebasics of quantum entanglement. Before shecould react, Baumgarden, in his role as chair,politely tried to end the discussion.But it wasShlomiuka, who always wanted to keepeveryone happy, who proposed that thedemonstration should be performed severaltimes to obtain some statistical evidence.Pruszczyncki, still grumbling, sat down,while Jaeger, red-faced, dabbed at his fore-head and looked a little unwell. He took hisplace beside the demonstration apparatus.

The lights dimmed for a ten-minutemultimedia presentation of the backgroundto the experiment and its theoretical basis.The hall and platform were now in almosttotal darkness, save for tiny individual read-

ing lights. The presentation ended, and Trot-man readied the apparatus. The slide projec-tor showed the cat’s state: it was alive. Whilethe audience waited for the hologram of adead mouse to appear, a strange sizzle and adull thud could be heard.Trotman was aboutto reset the apparatus for a second trial,whena terrible scream came from the stage. It wasShlomiuka.

After a few moments, someone had thepresence of mind to turn the main lights onin the hall. Jaeger had collapsed, and waslying awkwardly on the stage, not moving. Afaint smell of burning drifted on the air.Shlomiuka was kneeling beside Jaeger. Shelifted his head and started sobbing.

Baumgarden bent down, stared atJaeger’s face, and pulled her away. “He’sdead,” he said blankly. “There’s a hole in hishead.”He reached for a microphone.“Ladiesand gentlemen: it seems that there has been aterrible accident.”

Lorimer, who had a reputation for beingcalm in a crisis, rose to his feet.“Sorry to con-tradict you, Fenton, but has it occurred toyou that what you call an ‘accident’ mighthave been deliberate?”

Baumgarden went pale. It was unlikely,but Lorimer had a point. There was only onething to do.He instructed Security to lock thedoors to the conference centre, to make sureno one could leave,and call the police.

To be continued…Ian Stewart is at the Mathematics Institute,University of Warwick, Gibbet Hill Road,Coventry CV4 7AL, UK.

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