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INTRASYSTEM ENTANGLEMENT GENERATOR AND UNAMBIGUOS BELL STATES

DISCRIMINATOR ON CHIP

Fabio Antonio Bovino

Department SBAI, SAPIENZA University of Rome

via Antonio Scarpa 14/16, 00161, Rome

ABSTRACT

Bell measurements, jointly projecting two qubits onto the

so-called Bell basis, constitute a crucial step in many quan-

tum computation and communication protocols, including

dense coding, quantum repeaters, and teleportation-based

quantum computation. A problem is the impossibility of

deterministic unambiguous Bell measurements using passive

linear optics, even when arbitrarily many auxiliary photons,

photon-number-resolving detectors, and dynamical (condi-

tionally changing) networks are available. Current proposals

for going over the 50% upper bound without using experi-

mentally challenging nonlinearities rely on using entangled

photon ancilla states and a sufﬁciently large interferometer to

combine the signal and ancilla modes. We demonstrate that

the novel Multiple Rail architecture, based on the propagation

of a single photon in a complex multipath optical circuit (or

multiwaveguide optical circuit), provides the possibility to

perform deterministic Bell measurements so to unambigu-

ously discrimate all four Bell States.

Index Terms—Quantum communication, Quantum

computing, Entanglement, Quantum gates.

1. INTRODUCTION

Quantum technology is a fundamentally new way of harness-

ing Nature, it has potential for a truly revolutionary innovation

and promise the next generation of products with exciting and

astounding properties that will affect our lives profoundly.

They will have a great inﬂuence in defence, aerospace, energy

and telecommunication sectors. If this process is to continue

in the future, new, quantum technology will replace or sup-

plement what we have now. In particular, Quantum Informa-

tion Technology can support entirely new modes of informa-

tion processing based on so called quantum bits or qubits. Its

eventual impact may be as great as or greater than that of its

classical predecessor. There is almost daily progress in devel-

oping promising technologies for realizing quantum informa-

tion processing with various advantages over their classical

counterparts [1–31].

Thanks to Italian Ministry of Defence, PNRM "Contratto N.168 di Rep.

del 18-03-2016 - Repubblica Italiana "Copernico", for funding.

Photons are unsurpassed as qubits in terms of decoherence

times, mobility and achievability of high-ﬁdelity single-qubit

operations. Thus far, entanglement experiments in optics have

been very clean, and an optical quantum processor would

obviously have an advantage in connecting to a quantum

"network" (no need to convert between stationary and ﬂy-

ing qubits). All-optical quantum processing became feasible

when, in 2001, a breakthrough known as the KLM (Knill-

Laﬂamme-Milburn) scheme showed that scalable quantum

computing is possible using only single-photon sources and

detectors, linear optical circuits and quantum teleportation:

recent approaches based on cluster states or error encod-

ing made an all-optical architecture are serious contenders

for the ultimate goal of a large-scale quantum computer. This

scheme relies on quantum interference with auxiliary photons

at a beam splitter and single-photon detection to induce not-

deterministic interactions. In the past ten years, the KLM

scheme has moved from a mathematical proof of concept,

towards practical realization, with demonstrations of sim-

ple quantum algorithms and theoretical developments that

dramatically reduce the resource overhead [32].

Optical integrated circuits offer great potential for realiz-

ing previously unfeasible large-scale quantum circuits. This

ﬂexible architecture can be used to implement key quantum-

computation elements, including universal Control N OT

(CN OT )gates based on integrated directional couplers and

reconﬁgurable single-qubit operations using phase controllers

based on the thermo-optical effect of silica. However, this

architecture suffers not only of the overhead of multiple

sources, detectors and ancillary qubits, but it needs to main-

tain the complete coherence between single photons in which

qubits are encoded [33–41]. Existing schemes typically

require thousands of single photon sources (photon guns),

temporal (and spectral) coherence between photon sources,

extensive adaptive switching which is experimentally chal-

lenging, noisy large quantum memories for repeat-until-

success strategies. To overcome the KLM limitation, we

introduced a novel architecture and physical representation

of a quantum machine. The idea is based on the exploita-

tion of the so called "Classical Entanglement". “Classical

Entanglement” or “Intra-System Entanglement” should not

be confused with “entanglement simulations in classical op-

7993978-1-5386-4658-8/18/$31.00 ©2019 IEEE ICASSP 2019

Fig. 1. Scheme of a Bell States generator in terms of quantum

gates.

tics”, but it denotes the occurrence of some mathematical

and physical aspects of quantum entanglement in classical

beams of light. We show the possibility to encode the whole

state space in a complex optical circuit based on Multiple

Rail (MR) architecture: any quantum state is encoded and

processed trough different single-mode waveguide [42, 43].

Therefore, it is possible to realize quantum devices that show

deterministic and not probabilistic behavior, as quantum en-

tanglers, Bell measurements and teleportation schemes. Bell

measurements, jointly projecting two qubits onto the so-

called Bell basis, constitute a crucial step in many quantum

computation and communication protocols, including dense

coding, quantum repeaters, and teleportation-based quantum

computation. A Multiple Rail (MR) architecture, based on

the propagation of a coherent pulse in a complex multipath

optical circuit, provides unambiguous discrimination of all

four Bell States and deterministic teleportation.

2. BELL STATE SYNTHESIZER

The deep ways that quantum information differs from clas-

sical information involve the properties, implications and

uses of quantum entanglement. Entanglement has always

been a key issue in the ongoing debate about foundation

and interpretation of Quantum Mechanics, since Einstein

and Schroedinger expressed their deep disatisfaction about

this astoneshing part of quantum theory. Untill 1965, when

Bell published the famous inequality by which he demon-

strated that EPR’s (Einnstein, Podolsky and Rosen) local and

realistic vision of world was wrong, all discussions about

entanglement were theoretical, sometimes metatheoretical or

metaphysical. Nowadays entanglement is a physical resource

and a fondamental key for quantum information and in par-

ticular for quantum computing, then it is fundamental task

the engineering of entanglement syntetizer. The logic opera-

tion sequence that provides entanglement syntesis by starting

from two independent qubits is obtained by the circuit shown

in Fig.(1). The circuit represents the product of Hadamard

gate (H) applied to the ﬁrst qubit followed by CN OT , with

the ﬁrst bit as the source and the second bit as the target. It is

straightforward to see that this circuit transforms the standard

basis to the entangled basis

j00i=) j00i=1

p2(j00i+j11i) =

+;

j01i=) j01i=1

p2(j01i+j10i) =

+;

j10i=) j10i=1

p2(j00ij11i) =

;

j11i=) j11 i=1

p2(j01ij10i) =

:(1)

Similarly, we can invert the transformation by running the cir-

cuit backwards (since both CN OT and Hsquare to the iden-

tity); if we apply the inverted circuit to an entangled state,

and then measure both bits, we can learn the value of both

the phase bit and the parity bit. Of course, Hacts on only

one of the qubits; the “nonlocal” part of our circuit is the

CN OT gate – this is the operation that establishes or re-

moves entanglement. We can rewrite the product CN OT

(HI)in CN OT S W AP (IH)S W AP so that we

can implement the Bell Synthesizer by the physical circuit

described in Fig.(2). The Mach-Zehnder (MZ) interferome-

ter is the building block for the engeneering of the quantum

gates. Consider two orthogonal optical modes represented by

the annihilation operators ^aand ^

band the vacuum modes j0ia

and j0ib. We deﬁne our logical qubits as j0i= ^ayj00iand

j1i=^

byj00i.

Fig. 2. Physical implementation of a Bell State Synthesizer.

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Fig. 3. Optical, Electronics and mechanics integration of InP

Entangler. Note the gold contacts for MZ voltage setting and

control, and thermistor to thermalize the integrated structure.

That is, single photon occupation of one mode represents

a logical zero, whilst single photon occupation of the other

represents a logical one. Let us consider a Mach-Zehnder in-

terferometer. The ﬁrst beam splitter prepares a superposition

of possible paths, the phase shifters inside modify quantum

phases in different paths and the second beam-splitter com-

bines all the paths together. By setting the ﬁrst phase-shifter

we are able to control the amplitude probability to obtain the

single photon in the ﬁrst or in the second path. A second phase

shifter can control the phase factor between the two amplitude

probabilities, so that the M Z interferometer is a universal

single qubit gate. The extension of the "dual rail" conﬁgu-

ration to a "multiple rail" one provides two- qubit gates, as

Hadamard,SW AP and C N OT gate.

3. PHYSICAL IMPLEMENTATION OF THE BELL

STATE SYNTHESIZER

We used the InP technology to engineering the Bell States

generator. In Fig.(3) the scheme of the physical chip is

showed. In particular, it is possible to appreciate the smart

input-output system so that a single bundle of eight ﬁber pro-

vides the injection and the extraction of light pulses. The gold

metalized contacts on the top of waveguides provide to set the

voltage for the ﬁne control of each Mach Zehnder interfer-

ometer. To implement the XPauli gates we used symmetric

MZ (i.e without differential path length between upper and

lower arms) which are in cross state by default. To implement

HADAM ARD gate, in order to change the M Z initial state

from cross operation to 50=50 operation, a differential path

length equivalent to a =2phase shift has been introduced.

The length increment is 118nm assuming an effective refrac-

tive index neff = 3:28@1:55m wavelength. This length

increment is in the order of the process resolution (about

200nm) making the initial state uncertain. The InP Chip

is integrated in a custom packaging which provides the ex-

ternal electrical connectorization for each quantum gate and

thermalization thanks to a thermistor. The electrical control

of each Mach Zender is performed by a Custom Polarizing

Board. A mechanical structure provides the assembly of all

single components. The metallic holed base also gives a

strong support to protect the bundle of Polarizing Maintain-

ing ﬁbers. After the packaging and the integration the Mach

Zehnders of each quantum gate were characterized in terms

of Voltage-Current curve and in terms of optical response.

4. EXPERIMENTAL BELL STATE GENERATION

AND UNAMBIGUOS DISCRIMINATION WITHOUT

ANCILLA

In the experiment, an attenuated coherent state with an

avarage number of photon = 0:1was injected in the ﬁrst

waveguide of the Bell State generator. The ﬁrst waveguide

corresponds to the state j00i, so that at the output of the

chip we have the excitation of the Bell state j+i, that is

the quantum superposition of the state j00iand j11iwith the

same probability amplitude. Now we introduce the operation

represented in Fig.(4): the Bell state generator creates an

entangled state, which is subjected to a total swap gate with

a supplementary phase shifting. After the operation a Bell

measurement is applied on the new state. In the physical

implementation the Polarization Maintaining output ﬁbers

are short-cutted and an external phase modulator is inserted

to control the relative phase of the states j00iand j11i. The

phase shifter provides the state j00i+ei' j11i=p2that is

re-injected in the chip and measured by four single photon

detectors (ID Quantique), one detector for each input (output)

ﬁber. In particular, only the ﬁrst and the third waveguides,

corresponding to the states j00iand j10i;will be excited, and

the following phase dependent response will be obtained:

out = cos2'

2j00i+isin2'

2j10i:

Fig. 4. The Bell state generator create an entangled state,

which experiments a total swap gate and a supplemetary

phase shifting. Then a Bell measurement provides the dis-

crimination of the new state..

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Fig. 5. Discrimination of entangled states by a Bell measure-

ment. By changing the voltage applied to the external phase-

shifter the state +is transformed in the state . The chip

provides unambiguos Bell measurements with a visibility bet-

ter than 99%. The error bar is smaller of the dot dimension.

The experimental results are shown in Fig.(5). The visi-

bility of the curves, deﬁned as (Max Min)=(M ax +M in),

is better than 99%, and provides a clear discrimination of the

states. The outputs corresponding to the state j01iand j11i

are negligeble and not reported in Fig.(5). A Conventional

Bell measurement, based on a 50=50 Beam Splitter and two

Polarizing Beam Splitters, can not discrimante between +

and state [20]:

5. CONCLUSIONS

We demonstrated the operation of Bell state synthesizer on

chip based on InP technology. With the same chip an un-

ambiguos Bell measurement was performed with very high

visibility. The high level experiment and the results show that

more complex circuits can be built with the same architec-

ture and technology to perform more complicated logic opera-

tions. Finally, the introduction of the multiple rail architecture

could make a practical quantum computer a reality. The in-

vestigation has started from the designing of building blocks,

and it follows up with their engeneering and ﬁnal integration

in a ﬁrst pototype of reconﬁgurable quantum processor able

to perform complex operations.

The architecture is efﬁcient and can be extended to high

number of qubits. The important difference with other

schemes of quantum computing is that we do not need to

repeat the calculation a lot of times with “single photon

states”, but we just perform the operation in a single shot by

using a pulse with a lot of photons. In fact, with this archi-

tecture each photon of the pulse will interact only with itself,

and it will contribute to the total result of the computation in a

single shot. This feature will provide the use of off-the-shelf

integrated laser systems and classical detectors (not single

photon detectors): then the engineering, and the realization

of large circuits able to perform very complex calculations is

possible with the current technology. As an example, to com-

plete an elaboration with a “conventional” optical quantum

computer we need to repeat calculation for 1 sec with a clock

of 1GHz; with this new architecture we can perform one cal-

culation per clock’s cycle: we can perform 1 Giga operation

per second. An other amazing property of this architecture is

the possibility of reconﬁguration of the gate: it realizes a sort

of quantum FPGA. This demonstrates that, in principle, the

same device can solve different kind of computations.

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