Table Of ContentUltrafast switching of photonic entanglement
Matthew A. Hall,1 Joseph B. Altepeter,1 and Prem Kumar1
1Center for Photonic Communication and Computing, EECS Department
Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3118
Todeployandoperateaquantumnetworkwhichutilizes existingtelecommunications infrastruc-
ture, it is necessary to be able to route entangled photons at high speeds, with minimal loss and
signal-band noise, and—most importantly—without disturbing the photons’ quantum state. Here
we present a switch which fulfills these requirementsand characterize its performance at thesingle
photonlevel; itexhibitsa200-psswitching window,a120:1 contrastratio, 1.5dBloss, andinduces
nomeasurabledegradationintheswitchedphotons’entangled-statefidelity(<0.002). Furthermore,
becausethistypeofswitchcouplesthetemporalandspatialdegreesoffreedom,itprovidesanimpor-
tantnewtoolwithwhichtoencodemultiple-qubitstatesinasinglephoton. Asaproof-of-principle
1
1 demonstration of this capability, we demultiplex a single quantum channel from a dual-channel,
0 time-division-multiplexedentangled photon stream, effectivelyperforming acontrolled-bit-flipon a
2 two-qubitsubspace of a five-qubit,two-photon state.
n PACSnumbers:
a
J
6 Switching technologies enable networked rather than Moreover,this switch designis scalable: by its extension
point-to-point communications. Next-generation pho- onecancreatedevicesthatarecapableofcouplingmany
]
h tonic quantum networks will require switches that op- temporal qubits and many spatial qubits. As a proof-of-
p erate with low loss, low signal-band noise, and with- principle demonstration of this capability, we utilize the
- out disturbing the transmitted photons’ spatial, tempo- switchtoperformacontrolled-bit-flipoperationonatwo-
t
n ral, or polarization degrees of freedom [1]. Additionally, qubitsubspaceofatwo-photon,five-qubitsystem,where
a
the switch’s operational wavelength must be compati- a temporally encoded qubit is used as the control and a
u
q ble withalow-loss,non-dispersivetransmissionmedium, spatially encoded qubit is used as the target. This op-
[ such as standard optical fiber’s 1.3-µm zero-dispersion eration is used to demultiplex a single quantum channel
band [2, 3]. Unfortunately, no previously demonstrated fromadual-channel,time-division-multiplexedentangled
2
technology[4]–[15]iscapableofsimultaneously satisfying photon stream encoded into the larger five-qubit space.
v
9 each of the above requirements: waveguide electro-optic
In order to simultaneously achieve low loss and ultra-
7 modulators (EOMs) [16] and resonators [17, 18] can op-
fastswitching,weutilizeanall-optical,fiber-baseddesign
8 erate at very high speeds (10 GHz) but completely de-
4 in which bright 1550-nm pump (C-band) pulses control
stroy any quantum information stored in the polariza-
. the trajectory of 1310-nm (O-band) single-photon sig-
8 tiondegreeoffreedom;micro-electromechanicalswitches
nals (see Fig. 1(b)). Physically, this switch exploits
0
[6, 19] do not degrade the photon’s quantum state, but
0 polarization-insensitive cross-phase modulation (XPM)
operate at very low speeds (<= 250 kHz); polarization-
1 [23] in a nonlinear-optical loop mirror (NOLM) [24], the
independent EOMs [16] can operate at moderate speeds
: reflectivityofwhichisdeterminedbythephasedifference
v
( 100 MHz) but with relatively high loss; and finally,
i ∼ between the clockwise and counter-clockwise propagat-
X traditional 1550-nm devices based on nonlinear-optical
ing paths in a fiber Sagnac interferometer (the “loop”)
r fiber loops [7, 20] generate unacceptably high levels of [25]. To actively control the state of this switch, we
a Raman-induced noise photons (> 1 in-band noise pho-
initially configure an intra-loop fiber polarization con-
ton per 100-ps switching window [21]).
troller such that the loop reflects all incoming pho-
tons. Multiplexingastrong1550-nmpumppulseintothe
Although the requirements for ultrafast entangled-
clockwise or counter-clockwiselooppath then createsan
photonswitchingarecollectivelydaunting,theydescribe
XPM-induced phase shift on the respective clockwise or
adevicethatiscapableofselectivelycouplingthespatial
counter-clockwise signal amplitude, with a π phase shift
and temporal degrees of photonic quantum information.
causing the switch to transmit all incoming photons. As
In other words, a device that can encode multiple-qubit
XPMis inherentlypolarizationdependent, andpolariza-
quantum states onto a single photon, enabling quantum
tion is often used to encode quantum information, it is
communication protocols that exploit high-dimensional
important that the pump pulse itself be effectively un-
spatio-temporal encodings. In this Letter we describe
polarized. We accomplish this by temporally overlap-
the construction and characterization of an all-optical
ping two orthogonally polarized pump pulses, each with
switch which meets each of the aforementioned require-
a slightly different wavelength [23].
ments, and whose aggregate performance (in terms of
loss, speed, and in-band noise) exceeds that of all avail- Note that the traditional NOLM-based C-band de-
able alternatives [4]–[20] by orders of magnitude [22]. vices are unsuitible for single-photon switching for two
2
damentalproblemsandallowsfornoiseless,highcontrast
switchingoperation. Firstly,thelargeanti-Stokesdetun-
ing ( 35 THz) between the 1550-nm pump pulses and
≈
the 1310-nmsingle-photonpulses reduces contamination
ofthe quantumchannelsbyspontaneousRamanscatter-
ing of the pump ( 2 10 7 background photons per
−
≈ ×
ps of signal pulse). Secondly, in standard single-mode
fiber (the loop medium) this detuning leads to a large
group-velocity difference ( 2 ps/m) between the pump
≈
and signal pulses. This allows the switch to operate in a
regime where the pump pulse walks completely through
thesignal’stemporalmode,providingthetypeofuniform
phase-shift which is essential for high-contrast switching
operation. The effective phase shift is therefore deter-
mined by the total energy in a single pump pulse, re-
gardless of that pulse’s temporal profile. The switching
window,τ,isinturndeterminedbythelengthofthefiber
betweentheWDMs, L,multipliedbythe speedatwhich
the signal sweeps through the pump (i.e., the group ve-
locitydifferencebetweensignalandpump). Forourcase,
τ =L 2ps/m. The turn-ontime ofthis switchingwin-
×
dowissetbythetemporalextentofthepumppulses(i.e.,
the time it takes for the pump to physically enter and
FIG. 1: (a) Entangled photon-pair source and test appara- leave the fiber loop). For 5-nm bandwidth, transform-
tus for ultrafast switching. Nondegenerate entangled photon limited C-band pump pulses, for example, the turn-on
pairs(λsignal =1303.5 nm andλidler=1306.5 nm)aregener- time can be as short as 1 ps.
ated in 500 m of standard single-mode fiber (SMF-28). Sig-
Twokeyexperimentaltechnologiesarerequiredto op-
nalandidlerphotonsareseparatedusingadouble-passgrat-
erate and characterize this type of switch: a short-pulse
ingfilter(DGF).Circ: circulator, FM:Faradaymirror,FPC:
dual-wavelength1550-nmpumpandasourceof1310-nm
fiberpolarizationcontroller,HWP:half-waveplate,PBS:po-
larizing beam-splitter, PDD: polarization dependent delay, entangledphotons. Tocreatethedual-wavelengthpump,
QWP:quarter-waveplate,WDM:wavelength-division multi- two 5-ps duration pulses (1545-nm and 1555-nm wave-
plexer. (b)Thesingle-photonswitch. Thelengthoftheintra- lengths) are spectrally carved with diffraction gratings
loopSMF-28isdirectlyproportionaltotheswitchingwindow. fromtheoutputofa50-MHzrepetitionratemode-locked
An L = 100-m loop (shown) results in a ≈200-ps switching
fiberlaser(IMRAFemtolite Ultra,ModelBX-60),which
window. (c) Reconstructed density matrix of the unswitched
arethenmultiplexedusingapolarizationbeamcombiner
statefor L= 100-m (fidelityto amaximally entangled state,
(PBC). The power necessary to produce a π phase shift
F =99.5%±0.2%). (d)Densitymatrixoftheswitched state
for L = 100 m (F =99.4%±0.4%). Similar reconstructions is obtained by amplifying the multiplexed pulses with a
for the 500-m switch (not shown) yielded F =99.5%±0.2% cascade of erbium-doped fiber amplifiers (EDFAs). A
(unswitched) and F =99.2%±0.2% (switched). long-pass filter with a 1543-nm edge is used after each
EDFA to ensure that the optical gain is confined to the
reasons: Firstly, and most importantly, such switches pumppulsesandthatnocontaminatingO-bandphotons
generate very high levels of Raman-induced background are introduced by the pump preparation process.
photons at signal wavelenths [21]. These noise pho- The IMRA laser also provides an electrical clock sig-
tons would swamp any single-photon signals, effectively nal for a 1310-nmentangled photon source and an array
“washing out” any two-photon quantum correlations. of four single-photon detectors. The entangled photon
Secondly, traditional NOLM-based devices utilize pump source, described in detail in reference [3] and shown in
pulses which are group-velocity matched to the signals Fig. 1(a),utilizes spontaneousfour-wave-mixinginstan-
beingswitched. Whilethisincreasestheinteractiontime, dard single-mode fiber to produce pairs of polarization-
the nonlinear character of the XPM process limits the entangledphotons from100-pswide, 50-MHzrepetition-
switchingcontrastinthis typeofoperation. Becausethe rate pump pulses at 1305 nm. After switching, the pho-
pump pulse can not be made perfectly square-shaped, ton pairs are measured with a correlated photon detec-
the center of the signal pulse receives a stronger nonlin- tionsystem(NucryptLLC,ModelCPDS-4)consistingof
earphaseshiftthanthepulsewings,makingiteffectively an array of four InGaAs/InP avalanche photodiodes.
impossible to choose a single pump power which maxi- In order to test the switch’s effectiveness for quantum
mizes switching contrast over the entire signal pulse. communications, we measured both active and passive
Our switch designis immune to eachofthese two fun- switching of polarization-entangled photon pairs. Fig.
3
1(b) shows the switch as integrated into the fiber-based
source of entangled photons referenced above. To test
multipleswitchingwindows,looplengthsof500m( 900-
≈
pswindow)and100m( 180-pswindow)wereused. The
≈
insertion loss introducecd by these switches in the O-
band quantum channel was measured to be 1.3 dB (L =
100m,portT),1.7dB(L=100m,portR),1.7dB(L=
500m,portT),and2.1dB(L=500m,portR).Because FIG.2: (a)Single-photonswitchingcontrast. Plotshowsthe
all of these directly measured losses include the 0.4 dB probabilityofroutinganincomingsinglephotonversuspump
energy for L= 500 m and L = 100 m (detector dark counts
or 0.8 dB loss fromone or two passes through an optical
subtracted). (b) Temporal extent of the switching window,
circulator, the raw switching loss for either transmission
as measured using single photons. Plot shows single-photon
through or reflection from the switching loop is 0.9 dB
counts versus relative delay between the single-photon and
(1.3 dB) for the L = 100-m (500-m) loop. thepump pulses (detectordark countssubtracted).
To set a performance benchmark for the switch,
unswitched (no pump) entangled photons from port
photonbackground,from—forexample—Ramanscatter-
R were characterized using coincidence-basis quantum-
ingofthe1550-nmpumppulses. Wemeasuredtheprob-
statetomography[26,27]. Bothsignalandidlerphotons
abilityofgeneratinga1310-nmbackgroundphotoncount
were analyzedusing separate quarter waveplate(QWP),
and found it to be proportionalto L ( 4 10 7m 1).
half waveplate (HWP), polarizing beam splitter (PBS) ≈ × − −
combinations, which together perform arbitrary single In addition to its use as a single-photon router, the
qubit measurements. The measured coincidence rates— switch is a spatio-temporal coupler, enabling the encod-
after subtracting accidental coincidences, a procedure ing and decoding of quantum information into a tem-
which lowers statistical errors [28]—for 36 combinations porally multimode Hilbert space, which is, in principle,
of analyzer settings [29] were subjected to a numeri- boundless. TheextenttowhichthisHilbertspacecanbe
calmaximumlikelihoodoptimization,whichreconstructs effectively accessed, however, is determined by the tem-
thedensitymatrixmostlikelytohaveproducedthemea- poral switching profiles of the devices described above.
sured results. Figs. 1(c) and 1(d), respectively, show re- To characterize the shape and width of the switching
constructeddensitymatricesforpassivelyswitched(port window,weintroducearelativedelaybetweenthesignal
R) and actively switched (port T) entangled photons, and the pump pulses. Sweeping this delay while mea-
after reflection or transmission through the L = 100-m suringtheswitchedsinglephotonswemapthe switching
loop. SimilarreconstructionswereperformedfortheL= window(showninFig. 2(b)). Notethatthetemporalex-
500-mloop;inallfourcases,the fidelity ofthe measured tentofthe photonsbeingswitchedblurs thetrue switch-
state toa maximally-entangledstate exceeded99.0%. In ingwindow. Toquantitativelyestimatetheextentofthis
addition,nomeasureablestatedegradationresultedfrom blurring, we directly measure the temporal shape of the
active versus passive switching. 1305-nm pump pulses used to create the test photons.
We perform this measurement by constructing a third L
Another important metric for both quantum andclas-
sical routers is the switching contrast—or the ratio of = 2-m switch with a switching window of 10-ps (full
≈
width at half maximum). Using the now-characterized
power directed to the desired output port divided by
1305-nmclassicalpumppulses,were-measuretheswitch-
the power directed to the complementary output port.
ingwindowswhileapplyinganumerical-fitdeconvolution
Fig. 2(a) shows the single-photon switching contrast as
to the results. In this way we obtain the instantaneous
afunction ofthe pump-pulse energy. Acontrastof120:1
temporal widths of the 100-m and 500-m switching win-
is achieved at a pump energy of 2.5 nJ for L = 500 m.
dows to be 180 ps and 900 ps, respectively.
Although the switching contrast is expected to be inde-
pendentofL,the 100-mdatadoesnotappearto achieve The switch’s ability to manipulate spatial and tem-
full contrast—only 43:1. This artifact is due to a long, poral quantum information has the potential to en-
low-power tail ( 370-ps total pulse width) in the 1305- able new quantum communication protocols. As an
≈
nm pumppulses that drivethe entangledphotonsource. example of this functionality, we use the switch to
Although the entangled photon production rate is pro- demultiplex a single quantum channel from a dual-
portionalto the pump-power squared, this still results in channel entangled photon stream, on which we en-
a longer than 200-ps pulse width for the entangled pho- code a five-qubit space (see Fig. 3(a)) defined by sig-
tons used to test the switch. As a result, the switching nal and idler polarization qubits (Hs,i , Vs,i ), sig-
window is too short to completely envelop the photon nal and idler temporal qubits (ts0|,i , ts1i,i|), aind an
| i | i
to be switched, resulting in an artifically lower switch- idler spatial qubit (Ti , Ri —see Fig. 1(b)). Using
| i | i
ing contrastfor the L = 100-mloop. We expect the true this encoding, we create the five-qubit hyper-entangled
switchingcontrasttobethesameinbothcases( 120:1). state Φ = c1 ψ1 ts0 ti0 Ti + c2 ψ2 ts1 ti1 Ti ,
Closely related to contrast is the generate≥d single- where | ψi1 |1 (cid:0)i|His| Hi|i +i Vs V|i (cid:1)i,| i|ψ2i| i
| i ≡ √2 | i| i | i| i | i ≡
4
1 (cid:0)Hs Hi Vs Vi (cid:1), and c1 and c2 are arbitrary
√2 | i| i−| i| i
coefficients. Measuring Φ using polarization-basis to-
| i
mography while tracing out temporal degrees of free-
dom and projecting into the idler spatial mode Ti will
| i
yieldahighlymixedstate,exactlytheresultoneexpects
from a simultaneous measurement of multiple entangled
quantum channels. A switch capable of implementing
a controlled-NOT operation which couples the spatial
and temporal qubits (see Fig. 3(a)), however, would
transform Φ into the state: Φ′ = c1 ψ1 ts0 ti0 Ti +
| i | i | i| i| i| i
c2 ψ2 ts1 ti1 Ri . This demultiplexed state should ex-
| i| i| i| i
hibitmaximalentanglementwhenprojected intothespa- FIG. 3: (a) Diagram of the five degrees of freedom in
tial mode Ti , because even after tracing over the tem- a multiplexed entangled photon stream, which can be de-
poral degr|eesiof freedom only the maximally entangled multiplexed by applying the controlled switch operation
(shown). (b) Arrangement of detected quantum information
poTlaoriziamtipolnemsteantte |ψth1iiswopuroldofb-oef-pprreisnecnipt.le test of the channels for the five-qubit state |Φi = c1|ψ1i|ts0i|ti0i|Tii+
switch’s ability to couple spatial and temporal degrees c2|ψ2i|ts1i|ti1i|Tii, where |ψ1i ≡ √12(cid:0)|Hsi|Hii+|Vsi|Vii(cid:1)
of freedom, we modify our O-band entangled-photon and |ψ2i≡ √12(cid:0)|Hsi|Hii−|Vsi|Vii(cid:1). Tracing overthe tem-
source [3] by pumping it with a pair of pulses separated poral qubit and projecting into spatial mode |Tii, we recon-
struct a highly mixed density matrix with F = 58.9% ±
by ∆t t1 t0 300ps. The polarizations of the
≡ − ≈ 0.5%. (c) Arrangement of quantum information channels
leading and trailing pump pulses are chosen such that
after active switching to output T (i.e., demultiplexing),
t√hce2u(|nHnpoirm+ail|iVzepdi)ptu1mwphiscthatuepoisn√ScF1W(|HMpgii+ve|sVtphie)to0u+t- cw2h|ψic2hi|tss1hio|uti1ldi|Rpirio.dBuyceptrhoejecsttiantge:in|tΦo′isp=atica1l|ψm1oid|tes0i|T|ti0iii|wTeiire+-
put signal-idler state Φ . For the demultiplexing test, coverthemaximallyentangledstate|ψ1i(F =98.6%±0.7%).
| i
we choose c1/c2 1.25 and ∆t 300 ps.
≈ ≈
Figure 3(b) shows the experimentally measured den-
nelfromatime-division-multiplexedstreamofentangled
sity matrix for the multiplexed quantum channels. As
photons. Very few fundamental limitations apply to this
expected, the state is highly mixed; its fidelity to the
typeofswitchdesign. Withcarefullydesignedfibercom-
nearest maximally entangled state is only 58.9%. Uti-
ponents,onehasthepotentialtodramaticallyreducethe
lizing the 100-m switch we then demultiplex (i.e., ac-
switch’s loss. In principle the only unavoidable switch-
tively switch) only the first quantum channel (t = t0), ing losses are fiber transmissionlosses (0.15–0.2dB/km)
creating the state Φ . As shown in Fig. 3(d), after
| ′i and circulator insertion losses (waveguide-based circula-
demultiplexing we are able to recover the high fidelity
tors with a 0.05 dB insertion loss have been designed
(98.6%) of the target state to a maximally entangled
and simulated [30]). Additionally, decreasing L to a few
state. Because the cross-Kerr phase shift has previously
meters will reduce the switch’s speed to 10 ps while
been shown to maintain spatial and temporal coherence ≈
simultaneously decreasing the background by an order
inNOLMswitches[7,20],weanticipatethatthisswitch’s
of magnitude. Even without these improvements, how-
cross-Kerr-based demultiplexing operation is in fact co-
ever, this switch represents an important new tool for
herent and equivalent to the controlled-NOT operation
manipulating spatianlly- and temporally-encoded quan-
depicted in Fig. 3(a). Moreover, unlike LOQC-based
tum information.
controlled-NOT gates, this switch is completely deter-
This research was supported in part by the DARPA
ministic and easily extensible, capable of independently
ZOE program (Grant No. W31P4Q-09-1-0014) and the
tunable couplings (e.g., controlled-π/4) to many tempo-
NSF IGERT Fellowship (Grant No. DGE-0801685).
ral qubits encoded onto the same photon (by changing
the control-pulse’s intensity as a function of time). By
cascadingseveralswitches,itisalsopossibletocoupleto
multiple spatial qubits.
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