Quantum delayed choice experiment

A Quantum Delayed-‐Choice (QDC) Experiment

John Archibald Wheeler 1983


o  Quantum systems exhibit par$cle-‐like or wave-‐like behaviour depending on the experimental apparatus that they are confronted with! – Delayed Choice experiment (DC)

o  Photon is forced to choose a behaviour before the observer decides what to measure! (DC)

o  Quantum delayed-‐choice experiment (QDC) – parCcle and wave behaviour invesCgated

simultaneously!

o  VerificaCon of the quantum nature of the photon’s behaviour is via nonlocality with QDC, replacing the delayed choice of the observer in DC.

A QDC Experiment – OVERVIEW

CLAIM: “we observed strong nonlocal correla$ons, which show the photon must SIMULTANEOUSLY behave

both as a par$cle and a wave”

**KEY DEVELOPMENT: NO LONGER NECESSARY TO CLOSE/OPEN INTERFEROMETER!**

(therefore removing the role of the observer/device!) o  QDC scheme based on Bell’s inequality (21) – can test the most general classical model

-‐ device-‐independent

“we cer$fy the quantum nature of the photon’s behaviour by observing the viola$on of a Bell inequality. This demonstrates in a device-‐independent way (without making assump$ons about the

func$oning of the devices – that no local hidden variable model can reproduce the quantum predic$ons.”

A QDC Experiment – OVERVIEW

o  Wave/par$cle duality -‐-‐-‐-‐àDESCARTES 1630 (waves) NEWTON 1670s (parCcles) vs HUYGENS 1678 (waves)

-‐-‐-‐-‐à YOUNG 1803 (waves) – Double Slit -‐-‐-‐-‐à MAXWELL 1865 (waves), verified by HERTZ 1887

o  NoCon of COMPLEMENTARITY (Copenhagen interpretaCon) -‐-‐-‐-‐à Appeal to hidden-‐variable models o  Wheelers DELAYED CHOICE (DC) gedankenexperiment 1980s

A QDC Experiment – LANDMARKS

o  Experimental implementaCons (1987 – 2000)

A QDC Experiment – LANDMARKS

A DC Experiment – WHEELER

John Archibald Wheeler 1983

Squint to see a ‘complementary’ image!!! J A DC Experiment – WHEELER

Squint to see a ‘complementary’ image!!! J A DC Experiment – WHEELER

o  Describes dialog between EINSTEIN and BOHR between EINSTEIN’S arrival in Princeton in 1933 and his death in 1955.

o  EINSTEIN is adament that an observer dependence “IS INCOMPATIBLE WITH ANY REASONABLE IDEA OF REALITY”

o  BOHR’S reply “YOUR CONCEPT OF REALITY IS TOO LIMITED’

o  EINSTEIN could not reconcile himself to the fact that the DOUBLE-‐SLIT experiment could be made to demonstrate WAVE-‐LIKE or PARTICLE-‐LIKE behaviour.

o  He declared it to be “CLEARLY OBVIOUS THAT QUANTUM THEORY IS INCONSISTENT”

o  BOHR emphasised that there in fact was NO INCONSISTENCY

o  We are dealing with two different experiments, BUT IT IS IMPOSSIBLE TO DO BOTH EXPERIMENTS SIMULTANEOUSLY (unCl the QDC!!!)

o  He argued one can observe one feature of nature, or its complementary feature. BUT NOT BOTH SIMULTANEOUSLY (unCl the QDC!!)


A DC Experiment – BACKGROUND

o  Decision to add / remove half-‐silvered mirror (BS2) is made at the last picosecond, aher the photon has already travelled through (most) of the system

o  “WE HAVE A STRANGE INVERSION OF THE NORMAL ORDER OF TIME” (unCl QDC!!) o  “WE NOW…HAVE AN UNAVOIDABLE EFFECT ON WHAT WE HAVE A RIGHT TO SAY ABOUT

THE ALREADY PAST HISTORY OF THAT PHOTON”

o  Argument has since been refined with the DC experiment



o  DC AT THE COSMOLOGICAL SCALE!

o  Distance of travel in a Lab split-‐beam experiment ~30m o  Time of travel ~ 0.1 microseconds

o  Distance COULD be billions of light years

o  Consequently an observing device, configured to respond a specific way has an “IRRETRIEVABLE CONSEQUENCE FOR WHAT ONE HAS THE RIGHT TO SAY ABOUT A PHOTON THAT WAS PRODUCED LONG BEFORE THERE WAS ANY LIFE IN THE UNIVERSE”

o  Two astronomical objects (0957 + 561 A,B)

(Were once considered to be two disCnct quasi-‐stellar objects (or ‘quasars’) as they are seperated by 6 seconds of arc. Now considered to be two disCnct images of one quasar An intervening galaxy has been found 1/4 of way from us to the quasar

(0957 + 561 A,B) A DC Experiment – WHEELER

A DC Experiment – Experimental RealisaCon


o  University of Rochester – Emission of photon pair from two-‐down conversion crystals

o  One photon passes MZI, second photon determines whether first shows interference

o  THEREFORE view that act of determining parCcles path disturbs interference effects is UNTENABLE!

o  KEY FACTOR = whether path informaCon is available or not (experiments where path info carried by second parCcle is destroyed aher parCcle registered – see interference!)


o  QDC says otherwise!!


o  QDC says otherwise!! (see next slide)

A QDC Experiment – Proposal

o  Quantum mechanics most consistent and accurate physical theory we have...

o  HOWEVER, makes counter-‐intuiCve predicCons

o  NoCon of WAVE-‐PARTICLE DUALITY introduced A quantum sytem may behave as a parCcle or wave depending upon the experiment

o  COPENHAGEN INTERPRETATION – parCcle / wave behaviour are never observed simultaneously

o  To reconcile quantum predicCons and common sense it has been suggested that quantum parCcles may know the experiment in advance, via a HIDDEN VARIABLE

o  Challenged by J. A Wheeler in DELAYED CHOICE gedankenexperiment


o  Quantum parCcle sent towards MZI o  RelaCve phase between the two MZI arms adjusted so parCcle emerges at D’ with certainty

(WAVE-‐LIKE BEHAVIOUR) o  BS2 can be removed arbitrarily – meaning a WHICH PATH measurement is made. o  The photon is detected in each mode with an equal probability of 1/2

(PARTICLE-‐LIKE BEHAVIOUR)

o  BS2 can be removed/inserted arbitrarily aher the parCcle has entered the MZI (aher BS1) o  ParCcle could not have known in advance (via hidden variables) which kind of experiment it would

be confronted with.


CONCEPTUALLY DIFFERENT STARTING POINT: o  Recent theoreCcal proposal of QDC using a quantum-‐controlled BS (C(H)) which can be in a

superposiCon of PRESENT and ABSENT o  meaning the MZI can be simultaneously CLOSED and OPEN – tesCng WAVE and PARTICLE nature of

the photon simultaneously!

o  Used a reconfigurable integrated quantum photonic circuit to implement a MZI with a quantum BS o  Observed CONTINUOUS MORPHING between WAVE and PARTICLE behaviour! o  (although this can be reproduced by a simple classical model – weakness in proposal and recent

NMR implementaCons)


o  Observed CONTINUOUS MORPHING between WAVE and PARTICLE behaviour!

o  Therefore experimentally demonstrated QDC based on BELL INEQUALITY – allowed to test the most general classical model.

o  ACHIEVED STRONG BELL INEQUALITY VIOLATIONS – the delayed choice of closing the MZI or not is no longer necesssary – instead we cerCfy the quantum nature of the photon’s behaviour by observing the violaCon of a Bell ineuality

o  Demonstrates in a device-‐independent way (without making assumpCons about the funcConing of the devices) – that NO LOCAL HIDDEN VARIABLE model can reproduce the quantum predicCons.

o  “NO MODEL WHERE PHOTON KNEW IN ADVANCE CAN ACCOUNT FOR THE OBSERVED STATISTICS”


Classical delayed-‐choice Equivalent quantum network

Delayed-‐choice with quantum beam spliber Biased QRNG


Classical delayed-‐choice

Equivalent quantum network

o  Second beam spliper inserted/removed randomly once the photon is inside the MZI (has passed BS1)

o  HADAMARD gates act as BEAMSPLITTERS o  RED LINE = ANCILLA, iniCally prepared in |0>, then

|+>, then measured – acts as QRNG!


Delayed-‐choice with quantum beam spliber Equivalent quantum network

[Classical control acer ANCILLA measurement] = [quantum control before ANCILLA measurement SECOND BEAM SPLITTER NOW IN SUPERPOSITION OF PRESENT / ABSENT (controlled-‐Hadamard) C(H)

o  Have a quantum BS in superposiCon of present / absent o  Interferometer is therefore open / closed o  Forces photon to be in a superposiCon of parCcle / wave (following Wheelers interpretaCon)


Delayed-‐choice with quantum beam spliber

Biased QRNG

o  Can also bias the QRNG by preparing the ANCILLA in an arbitrary state


QUANTUM DELAYED CHOICE

o  BS2 in now a quantum BS (represented by a controlled-‐Hadamard operaCon) – can be set into a

superposiCon of present and absent via ANCILLA photon o  Again, a single photon is sent through the MZI, aher the first Hadamard a superposiCon of the two

spaCal modes is created (|+>) o  And a phase shiher modifies the relaCve phase


THE PRESENCE OF BS2 DEPENDS ON STATE

OF ANCILLA o  If ancilla is prepared in the state |0>a no

BS is present

o  MZI is leh OPEN

o  Corresponds to IDENTITY acCng upon the state, results in:

o  Final measurement in the computaConal {|0>s,|1>s} basis reveals the PARTICLE-‐NATURE of the photon as a WHICH-‐PATH measurement is made.

o  Measured intensiCes in both output modes are EQUAL and PHASE-‐INDEPENDENT

ID’ = ID’’ = 1/2


THE PRESENCE OF BS2 DEPENDS ON STATE

OF ANCILLA

o  If ancilla is prepared in the state |1>a the

BS is present

o  MZI is CLOSED

o  Corresponds to HADAMARD acCng upon the state, results in:

o  Final measurement gives informaCon regarding the MZI phase, but no path informaCon

o  Measured intensiCes are now


o  Main feature of QBS is that it can be in a superposiCon of present and absent

o  If ANCILLA is iniCally in a superposiCon the global state of the system evolves into:

o  The SYSTEM and ANCILLA photons are now ENTANGLED when

o  Measured intensiCes are now


A Quantum Delayed-‐Choice (QDC) Experiment -‐ IMPLEMENTATION

A Quantum Delayed-‐Choice (QDC) Experiment – (CHSH) BELL INEQUALITY

Quantum delayed choice experiment

Science

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