Quantum Communications in Space - Esa

QUANTUM COMMUNICATIONS IN SPACE

(“Q**Space**”)

Executive Summary Report

Markus Aspelmeyer, Hannes R. Böhm, Caslav Brukner, Ra**in**er Kaltenbaek, Michael

L**in**denthal, Julia Petsch**in**ka, Thomas Jennewe**in**, Rupert Urs**in**, Philip Walther, Anton

Zeil**in**ger

Mart**in** Pfennigbauer, Walter R. Leeb

INSTITUT

FÜR NACHRICHTENTECHNIK

UND HOCHFREQUENZTECHNIK

Prepared for the European **Space** Agency under

ESTEC/Contract No. 16358/02/NL/SFe

ESTEC Technical Management: J.M.Perdigues (TOS-MMO)

EUROPEAN SPACE AGENCY CONTRACT

REPORT

The work described **in** this report was done under

ESA contract. Responsibility for the contents

resides **in** the author or organization that

prepared it.

May 19, 2003

Abstract

**Quantum** **in**formation science is an **in**trigu**in**g example where purely fundamental and

even philosophical research can lead to a new technology. The developments **in** this young

field experience a worldwide boom. **Quantum** communication provides qualitatively new

concepts, which are much more powerful than their classical counterparts. This report is a

detailed study of the feasibility for adopt**in**g the concepts of fundamental quantum physics and

quantum communications to a space **in**frastructure. It also develops physical and technological

concepts specifically designed for a space environment.

After review**in**g the basics of physics of quantum **in**formation we characterize and compare

quantum communications and classical optical communications. We discuss how to produce,

manipulate, and measure qubits to be employed **in** quantum communication systems. Emphasis

is put on photonic qubits, but we also review the present status of non-photonic realizations

of qubits. We show that the various protocols discussed **in** connection with quantum

**in**formation (like quantum cryptography or quantum teleportation) are feasible with today’s

technology when be**in**g based on photonic entanglement. Several space scenarios are analyzed

**in** detail both with respect to quantum communication applications and to novel fundamental

quantum physics experiments. With the latter one may address, e.g., open questions on

quantum non-locality and on the possible **in**fluence of relativity on quantum coherence.

In the last chapter we establish selection criteria for first proof-of-pr**in**ciple quantum communication

experiments **in** space. Specifically, we propose to realize down-l**in**ks from the

International **Space** Station (ISS) to optical ground stations. For this scenario, we suggest

a series of four experiments, one follow**in**g **in** a logical way from the other, with **in**creas**in**g

complexity and expenditure. We also present a prelim**in**ary design of the experiments. This

**in**cludes block diagrams for space and ground term**in**als, a rough cost estimate, and a description

of the quantum measurements to be performed. For these experiments we propose

the use of entangled photon pairs. They do not only allow the implementation of “standard”

quantum cryptography schemes but also open up new avenues both for fundamental research

and for quantum communication.

i

Contents

1 Introduction 1

1.1 Study objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Economic potential of quantum communication . . . . . . . . . . . . . . . . . 1

2 Pr**in**ciples of physics of quantum **in**formation 1

3 Concepts based on entangled particles 3

3.1 Photons versus particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3.2 Experimental realization and recent results . . . . . . . . . . . . . . . . . . . 3

4 Comparison between quantum communications and classical optical communications

4

5 Technologies for quantum communications **in** space 6

5.1 Preparation of s**in**gle qubits and entangled qubits . . . . . . . . . . . . . . . . 6

5.2 S**in**gle photons vs. entangled photons . . . . . . . . . . . . . . . . . . . . . . . 7

6 Experimental scenarios 8

7 **Quantum** communications applications **in** space 10

7.1 Modules for quantum communication applications . . . . . . . . . . . . . . . 11

7.2 L**in**k attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

8 Fundamental quantum physics experiments 16

9 Selection and design of an experiment 17

9.1 Selected experimental scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 18

9.2 Technological basel**in**e and prelim**in**ary design . . . . . . . . . . . . . . . . . . 18

9.3 Mass, volume, and power consumption of space term**in**al . . . . . . . . . . . . 20

Bibliography 22

ii

1 Introduction

1.1 Study objectives

The study has two ma**in** objectives: The first is to identify and **in**vestigate novel concepts

for space communication systems based on the pr**in**ciples of quantum physics with

emphasis on those areas related to quantum communications. The second objective is to

conceive and def**in**e experiments for the demonstration of fundamental pr**in**ciples

of quantum physics, which make advantageous use of the space **in**frastructure.

The aim of the proposed experiments is to make specific use of the added value of space

**in**frastructure for fundamental quantum experiments and practical quantum communication

based on the generation and manipulation of entangled photon pairs. The purpose of the

proposed experiments is with**in**

• general studies: to establish a free space optical l**in**k suitable as quantum channel for

s**in**gle photons (**in** contrast to exist**in**g optical space technology) and perform a systematic

study on the **in**fluence of l**in**k parameters such as atmospheric birefr**in**gence,

absorption or scatter**in**g on the decoherence of quantum states,

• physical sciences: to perform fundamental quantum physics experiments beyond the

capabilities of earth-bound laboratories by utiliz**in**g the added value of space/ISS environment,

• technology **in**novation and commercial R&D: to perform quantum key distribution between

ISS and a s**in**gle ground station and also between ISS and two different ground

stations to demonstrate the pr**in**ciple feasibility of satellite-based quantum communication.

1.2 Economic potential of quantum communication

Although classical **in**formation technology will surly not vanish with the advent of quantum

communication, there is no doubt about the huge economic potential of this new technology.

Gartner-Group analysts predict the advent of quantum communication technologies, both of

quantum computers (with**in** a ten-year time frame) and of quantum cryptography (expected

as early as 2008). “**Quantum** mechanics is now on the speculative edge of comput**in**g research.

(. . . ) Enterprises that show a realistic understand**in**g of advanc**in**g technologies will be more

successful **in** their reassurances” [1]. Also EU politics **in**dicate that quantum communication

research has **in**deed taken the step from the laboratory **in**to the world of **in**dustry. The European

Commission is plann**in**g to move quantum cryptography research from the FET (Future

and Emerg**in**g Technologies) Framework Programme **in**to the ma**in** **in**dustrial applications

programme with**in** the calls of the 6th Framework Programme.

2 Pr**in**ciples of physics of quantum **in**formation

The most fundamental physical concepts of quantum **in**formation science are:

• superposition: The superposition pr**in**ciple allows the description of a physical system

as a superposition of alternatives. This so-called “superposition of states” does not only

1

provide all predictions for the outcome of a physical measurement but has also drastic

consequences for the nature of the physical state which we ascribe to a system.

• qubit: A qubit is the quantum analogue of the classical bit, i.e. **in**formation that is

encoded on an **in**dividual quantum system with separate states |0〉 and |1〉. In contrast to

its classical counterpart, the qubit also entails the possibility of coherent superposition,

i.e. it can be either “0”, “1”, or a superposition of “0” and “1”. It is pr**in**cipally

impossible to obta**in** a perfect copy of a qubit **in** an arbitrary state without destroy**in**g

the **in**formation content of the orig**in**al. This so-called “no-clon**in**g theorem” is the basis

for the security of all quantum cryptographic schemes.

• entanglement: **Quantum** entanglement [2] describes correlations between physical systems

that cannot be modeled by classical means. It is the basis both for fundamental

physical studies and for the design of quantum communication schemes such as quantum

cryptography, quantum dense-cod**in**g or quantum teleportation [3]. The quantum nature

of entanglement is revealed by test**in**g correlations between **in**dependent observers

aga**in**st so-called Bell **in**equalities, which impose a classical limit to such correlations.

Interactions of an entangled system with its environment can result **in** a loss of entanglement,

and **in** an **in**crease of its noise content, i.e. **in** a decrease of its purity. This

so-called “decoherence” is equivalent with a loss of **in**formation.

Communication schemes based on those fundamental quantum physical pr**in**ciples can

be much more efficient (or without any analogue) than their classical counterpart. We have

**in**vestigated the follow**in**g well-known quantum communication protocols:

• **Quantum** key distribution (QKD) us**in**g s**in**gle and entangled qubits: The quantum

cryptographic protocol relies on the transmission and detection of s**in**gle photons. Its

security is based on the impossibility to copy the quantum state of a s**in**gle photon and

is thus guaranteed by the laws of quantum physics [4, 5, 6].

• **Quantum** dense cod**in**g (QDC): This communication protocol utilizes quantum entanglement

as a resource for communication with higher-than-classical channel capacity

[7].

• **Quantum** state teleportation: This means the transfer of a quantum state from one

site to another without the use of a physical carrier but by utiliz**in**g shared entanglement

between transmitter and receiver. The implementation of quantum state teleportation

is also a necessary requirement to establish a quantum repeater, which allows to share

entanglement over arbitrary distances [8].

• Entanglement-enhanced classical communication: This scheme is based on the

transmission of entangled states and is more noise-resistant than any classical communication

[9].

• **Quantum** communication complexity (QCC): This protocol utilizes entanglement

to decrease the amount of **in**formation exchange that is necessary to accomplish computational

tasks between distant parties [10, 11].

2

3 Concepts based on entangled particles

3.1 Photons versus particles

In most state-of-the-art quantum **in**formation experiments, photons are used for several reasons.

They are “easy” to entangle and do not need sophisticated equipment for keep**in**g

the system stable. For photonic qubits, the two bit levels “0” and “1” can for example be

realized by means of polarization, by angular momentum or by energy and time. Treat**in**g

non-photonic systems leads to completely new conditions for creat**in**g and read**in**g out

qubits. Also decoherence effects are more noticeable when work**in**g with particles **in**clud**in**g a

mass or potential. For non-photonic qubits the most common degrees of freedom which are

experimentally accessible to store **in**formation are:

• sp**in**: Every particle conta**in****in**g a sp**in**, like electrons, protons, neutrons or atoms can

be entangled.

• **in**ternal levels: This possibility to realize a qubit exists for atoms, as well as for ions

and molecules, e.g. **in** their modes of vibration

• energy levels: For many schemes this seems to be the easiest way to entangle particles.

The only condition is to own a stable ground- and excited state (|g〉, |e〉).

3.2 Experimental realization and recent results

To build hardware for a quantum **in**formation network or computer, technology will be needed

that enables to manipulate and store qubits. The ma**in** requirements are:

• storage: Qubits have to be stored for a long time, long enough to complete the operation.

• isolation: The quantum system or the collection of qubits must be **in**itialized **in** a welldef**in**ed

state and therefore well-isolated from the environment to m**in**imize decoherence

errors.

• readout: Techniques will be needed to measure the qubits efficiently and reliably.

• gate: For manipulat**in**g a quantum state, arbitrary unitary operators must be available

and controlled to launch the **in**itial state to an arbitrary entangled state.

• precision: The quantum gates should be implemented with high fidelity if the device

is to perform reliably.

These str**in**gent hardware requirements unfortunately rule out most known physical systems.

Conventional solid-state architectures such as silicon are ideal for classical **in**formation,

for the same reason they are unsuitable for quantum **in**formation; their stability is given by

the latch**in**g of logic levels, or cont**in**uous monitor**in**g by the environment. The most attractive

candidates for quantum **in**formation processors currently come from the area of atomic physics

and quantum optics. Here, **in**dividual atoms and photons are manipulated **in** a controlled environment

with well understood coupl**in**gs, offer**in**g environmental isolation much better than

**in** other physical systems. An elegant proposal to marry atomic and photonic quantum bits **in**

3

optical cavity quantum electrodynamics (QED) was developed **in** 1997 [12]. In this scheme, a

small number of cold atoms are conf**in**ed to the anti-nodes of a s**in**gle-photon stand**in**g-wave

field **in** a high-f**in**esse Fabry-Pérot optical cavity. Qubit states stored **in** the **in**ternal states

of the atoms can be mapped to the qubit spanned by the number of photons (0 or 1) **in** the

cavity by apply**in**g an appropriate laser pulse from the side. The photon can be made to leak

out of the cavity with**in** a predef**in**ed time w**in**dow, result**in**g **in** an ideal s**in**gle-photon source

for use **in** quantum communication. Moreover, after the photon leaks out of the cavity, it can

be determ**in**istically “caught” **in** a second cavity by the application of another laser pulse,

whose envelope is time reversed with respect to the first pulse.

Several recent experiments from the universities of Harvard and Berkeley have reported

slow**in**g and even “stopp**in**g” of light us**in**g the techniques of electromagnetically **in**duced

transparency **in** an atomic vapor. These experiments offer the potential for mapp**in**g quantum

states of photons onto collective quantum states of the atomic vapor. For example, s**in**glephoton

qubits can be mapped onto a s**in**gle qubit spann**in**g all N-sp**in**s **in** a vapor.

The basic element for the scheme of “atomic ensembles” to entangle atoms or molecules

is a system which has the relevant level structure as shown **in** Fig. 1. All the atoms are **in**itially

prepared **in** the ground state |g〉. Then a sample is illum**in**ated by a short, off-resonant laser

pulse (△|e〉) that **in**duces Raman transition **in**to the state |h〉 by emitt**in**g a Stokes photon.

An emission of only one s**in**gle photon **in** a forward direction results **in** an entangled state of

the two ensembles, where either one atom/molecule of the left or the right box is **in** the state

|h〉 [13].

Although extensive research is be**in**g performed on the utilization of non-photonic systems

for quantum communication, up to date the only practical realizations are based on photons.

Figure 1: Set-up for entanglement generation and relevant structure of the ensemble with |g〉,

the ground state, |h〉 the (long-liv**in**g) metastable state for stor**in**g a qubit and |e〉, the excited

state.

4 Comparison between quantum communications and classical

optical communications

Table 1 summarizes the ma**in** conceptual differences and common properties of the performance

of quantum and classical optical communications. The performance is def**in**ed by

speed, capacity, efficiency, security, technology, complexity and **in**fluence of the atmosphere.

4

It is a common source of misconception that quantum physics does permit communication

at superlum**in**al speeds, although the non-local features of quantum entanglement would

suggest this. The pr**in**cipal limit is still the speed of light!

Classical **Quantum** common prop- differences

Theory Theory erties

speed speed of light,

delays

tronics

**in** elec-

capacity data rate **in**creased capacity via

quantum dense cod**in**g

efficiency basically same possibly higher bit

efficiency rates by quantum

security low beam perfect s**in**-

measurements,

entanglementenhancedcommunication

and computation

security is based on the

divergence gle photon

laws of nature (quan-

makes eaves- sources needed

tum cryptography)

dropp**in**g for perfect se-

more difficult curity

complexity

than for radio

frequency

reasonable, fast progress

there are al- keeps comready

classical plexity at a

systems **in** reasonably low

orbit

level

technology transceivers rapidly evolv- po**in**t**in**g, acqui- there exist no perfect

are state-of**in**g standard sition and track- optical amplifiers for

the-art technologies **in**g is close to quantum **in**formation

state-of-the-art (no-clon**in**g-theorem)

**in**fluence of

wavelength- and

atmosphere

time-dependent

attenuation

Table 1: Comparison between quantum communications and classical optical communications

5

5 Technologies for quantum communications **in** space

5.1 Preparation of s**in**gle qubits and entangled qubits

At present, all quantum communication schemes are based on s**in**gle photons and entangled

photons, s**in**ce this is straight forward to realize and least affected by environmental decoherence

(as compared to massive particles).

Most of the photonic qubit realizations rely on the preparation of coherent s**in**gle-photon

number states of the electromagnetic field (s**in**gle-photon Fock-states). This can be approximated

by a strongly attenuated laser pulse with an ultralow mean photon number. At present,

the best available source for s**in**gle photons is based on spontaneous parametric downconversion

(SPDC). This method relies on the creation of simultaneously created photon pairs

via the (nonl**in**ear) process of parametric down conversion, where one of the two photons is

used as a trigger photon. Other devices are currently under **in**vestigation but have not yet

reached a level of reliable application. Table 2 summarizes the most popular candidates for

s**in**gle-photon generation.

s**in**gle photon max photon rate bandwidth-

source

limiteda operat**in**g conditions comments

SPDC b limited by pump**in**tensity

and repetition

rate (presently

approx. 106 yes room temperature,

ambient pressure

fa**in**t laser

Hz)

limited by repetition yes room temperature,

pulses rate

ambient pressure

color centers limited by state de-

**in** diamond cay time (presently

approx. 106 no room temperature, low collect**in**g

ambient pressure efficiencies

quantum dots

Hz)

limited by repetition no 5 K bandwidth-

rate

limit is approached

with

additional

microcavities

cavity QED limited by state de-

with atoms cay time (presently

approx. 103 mK, UHV

Hz)

Table 2: Summary of present-day technology of s**in**gle photon sources.

a The bandwidth limit is a measure of coherence, given by the ratio between the decay lifetime of a photon

state and its coherence time. Only bandwidth-limited states (ratio = 1) can be used for high-fidelity quantum

communication and quantum computation protocols.

b Spontaneous Parametric Down-Conversion

At present, the most efficient source of entangled photons is spontaneous parametric

down conversion (SPDC), which allows to generate different k**in**ds of entanglement:

• Polarization-entangled qubits: Polarization entanglement can be achieved by di-

6

Figure 2: Source of entangled photons based on spontaneous parametric down-conversion.

The phase match**in**g condition governs the “decay” of the photons such that they emerge

from the crystal along cones. In the degenerate case, i.e. when the two photons have the

same wavelength, the **in**tersection l**in**es of the cones yield entangled photons. This is when

the photons carry no **in**formation about whether they emerged with horizontal or vertical

polarization, yet they will always opposite polarization.

rectly superpos**in**g a spatial signal and idler mode, s**in**ce this erases the specific polarization

**in**formation **in** the modes (see Fig. 2). This type of source is at present the most

effective source for polarization entanglement [14].

• Momentum-entangled qubits: Momentum-entanglement is achieved between different

signal and idler modes such that each s**in**gle photon carries no **in**formation about

the spatial mode it was prepared **in** [15].

• Time-b**in**-entangled qubits: In the case of time-b**in** entanglement, photon pair creation

via SPDC is stimulated by temporally dist**in**guishable pump pulses result**in**g **in**

a state **in** which the s**in**gle photons **in** the signal and idler modes do not carry any

**in**formation about their preparation time (time-b**in** one or two). S**in**ce the pump pulses

are created with**in** an **in**terferometer, a fixed phase relation between subsequent pulses

can be established which **in**tr**in**sically excludes the possibility of ga**in****in**g any tim**in**g

**in**formation and therefore corresponds to maximal entanglement [16].

5.2 S**in**gle photons vs. entangled photons

The mere use of s**in**gle photons (qubits) as the **in**formation carrier of classical bits does not

**in**crease the classical communication capacities. However, this situation changes either if

entangled s**in**gle particles are utilized as carriers of classical **in**formation and sent between

distant users (entanglement enhanced classical communication), or if entangled particles are

be**in**g shared by distant users even before the communication is established (quantum dense

cod**in**g). This means that, **in** order to achieve more efficient communication based

on the pr**in**ciples of quantum communication, the reduction of the signal level to

the s**in**gle-particle regime is not sufficient and the use of entanglement is absolutely

necessary. We therefore suggest **in** the follow**in**g to also consider a source of entangled

photons as a potential source of s**in**gle photons, where one of the photons is used as a trigger

photon.

7

detector

typical

detection

efficiency

typical

dark counts

wavelengths

operat**in**g

temperatures

[Hz] [nm] [ ◦ C]

Si-APD a 40% 200 to 500 700 to 800 -25

SLIK b APD 70% 540 -25

Ge-APD 15% 25000 1000 to 1300 -200

InGaAs 10% to 30% 10 −5 ·gat**in**g frequency 1000 to 1500 -200 to -100

VLPC c 99% ≈ 10 4 to 10 5 500 to 800 -269

Table 3: Present-day technology for s**in**gle photon detection.

a APD: Avalanche-Photodiode

b SLIK: Super Low Ionization k factor

c VLPC: Visible Light Photon Counter

In quantum communication schemes with photons, the s**in**gle photons used must be detected

with a high efficiency, and with a high tim**in**g accuracy. Table 3 gives an overview on

exist**in**g s**in**gle photon detection technologies.

In pr**in**ciple, an identification of an entangled Bell state of two qubits would require

a direct **in**teraction of the two photons. S**in**ce a direct coupl**in**g is too weak to exploit, other

methods with higher efficiencies have to be used. The best method up to now is based on

l**in**ear optics exclusively and can dist**in**guish two out of four Bell states. One of the ma**in**

issues for the utilization of polarization entangled photon pairs is the ability to discrim**in**ate

subsequent pairs, s**in**ce perfect correlations due to entanglement are only established between

s**in**gle pairs. This necessitates the possibility of detect**in**g photon pairs at different observer

positions accurately with respect to a jo**in**t time basis. In practical terms, a tim**in**g accuracy

of the order of some ns is necessary.

6 Experimental scenarios

The scenarios **in**volv**in**g an earth-based transmitter term**in**al allow to share quantum entanglement

between ground and satellite, between two ground stations or between two satellites

and thus to communicate between such term**in**als employ**in**g quantum communication protocols

(see Fig. 3). In the most simple case, a straight up-l**in**k to one satellite-based receiver

can be used to perform secure quantum key distribution between the transmitter station and

the receiver. Here, one of the photons of the entangled pair is be**in**g detected right at the

transmitter site and thus the entangled photon source is used as a triggered source for s**in**gle

photons. If the satellite acts as a relay station, the same protocol can be established between

two distant earth-based communication parties. Shared entanglement between two parties

can be achieved by po**in**t**in**g each of the photons of an entangled pair either towards an earthbased

station and a satellite or towards two separate satellites. Another set of satellite-based

relays can be used to further distribute the entangled photons to two ground stations. Possible

applications for shared entanglement between two parties are quantum key distribution

or entanglement-enhanced communication protocols.

In a second scenario, a transmitter with an entangled photon source is placed

8

Relay

Receiver

Receiver

Receiver

Transmitter

Transmitter

Receiver

Receiver

Relay

Transmitter

Relay

Receiver

(a) (b)

Receiver

Transmitter

Receiver

(c) (d)

(e)

Figure 3: Scenarios for satellite-aided quantum communication with an Earth-based transmitter

term**in**al. The transmitter term**in**al distributes entangled photon pairs to the receivers

which can perform an entanglement-based quantum communication protocol. As **in**dicated,

a relay module redirects and/or manipulates qubit states without actually detect**in**g them.

9

on a space-based platform. This allows not only longer l**in**k distances because of reduced

**in**fluence of atmospheric turbulence; It will also be the preferred configuration for global quantum

communication, s**in**ce only one down-l**in**k per photon of the entangled pair is necessary

to share entanglement between two earth-based receivers. Aga**in**, already a simple downl**in**k

allows to establish a s**in**gle-photon l**in**k, e.g., for quantum cryptography. In this configuration,

a key exchange between two ground stations is also possible. To this end each of the two

ground stations has to establish a quantum key with the satellite. S**in**ce the space term**in**al has

access to both keys, it can transmit a logical comb**in**ation of the keys, which can then be used

by either ground station or both ground stations such that they arrive at the same key. This

logical comb**in**ation can easily be chosen such that it cannot reveal any **in**formation about the

key. Note that the key does not have to be generated simultaneously at both receiver stations.

In pr**in**ciple, a quantum key exchange can be performed between arbitrarily located ground

stations. However, **in** all such scenarios based on s**in**gle photons the security requirement for

the transmitter term**in**al is as high as it is for the ground station. Only the use of entangled

states sent to two separate ground stations allows **in**stantaneous key exchange between these

two communicat**in**g earth-bound parties and also relaxes the security requirement for the

transmitter module. Furthermore, more advanced quantum communication schemes will be

feasible. The required shared entanglement can be established either by two direct downl**in**ks

or by us**in**g additional satellite relay stations. **Quantum** entanglement can also be distributed

between a ground station and a satellite or between two satellites. In an even more elaborate

scheme a third party might be **in**volved, which is capable of perform**in**g a Bell-state analysis

on two **in**dependent photons. This allows quantum state teleportation and even entanglement

swapp**in**g and could thus resemble a large-scale quantum repeater for a truly global quantum

communication network. Applied to quantum cryptography, this third party might be used

to control the communication between the two other parties **in** a “Third-Man” cryptography

protocol. For example, **in** a polarization-based experiment, depend**in**g on whether he performs

a simple polarization analysis of the **in**dependent photons or a Bell-state measurement,

the “Third-Man” can communicate secretly with either of the two parties (or with both) or

he can control whether the two can communicate secretly or not without know**in**g the content

of the communication. The presented application scenarios for quantum communication

applications are summarized **in** Fig. 4.

7 **Quantum** communications applications **in** space

We identify the follow**in**g fields of potential space applications for quantum communication

schemes:

• Secure communication with quantum cryptography: Two ma**in** applications

have to be considered: Secure communication to and from satellites and secure communication

between arbitrary distant earth-based parties. As an example, remotecontroll**in**g

satellite-based systems necessitates truly secure communication l**in**ks between

a ground-station and a satellite. Another example **in** which security is mandatory

**in** a space environment would be the Galileo system, where protection aga**in**st **in**tentional

manipulation is of highest importance. Clearly, the technique of quantum key

distribution can also be used for secure **in**ter-satellite communication.

• Efficient communication and computation us**in**g quantum dense cod**in**g, quan-

10

Figure 4: **Quantum** communication **in** space: experimental scenarios

tum teleportation and quantum communication complexity: A large economic

potential of quantum communication lies **in** its superior **in**formation process**in**g capabilities,

which exceed those of classical methods by far.

• Deep space communication us**in**g quantum telecomputation and communication

complexity: Deep space communication is a specific example where resources

for data transmission are limited. **Quantum** communication complexity could provide a

unique means of **in**formation transport with very little consumption of communication

resources.

7.1 Modules for quantum communication applications

A transmitter module consists of a photon source for s**in**gle photons or entangled photon

pairs (**in**clud**in**g passive or active manipulation of s**in**gle qubit-states), a module for tim**in**g

synchronization with the receiver station, a classical channel to the receiver station and a

connection to an up-l**in**k or down-l**in**k (see Fig. 5).

A receiver module comprises one ore more **in**put channels, **in** each of which manipulation

of qubits can be performed **in**dependently. Depend**in**g on whether active (remote) control

of optical elements for qubit manipulation (such as polarizer or a polarization retarder) is

possible or not, we dist**in**guish active and passive receiver modules. Additionally, each receiver

is equipped with s**in**gle photon detectors at each **in**put port, a receiver module for timesynchronization

and a classical channel for communication with the transmitter (see Fig. 6).

In most cases, both receiv**in**g and transmitt**in**g classical **in**formation is required.

11

Synchronization

Input

S**in**gle Photon /

Entangled Photon

Source

Data

Aquisition

Transmitter Module

Qubit

Manipulation

Photonic

Qubits

Synchronization

Control Data Classical Channel

Up/Down

L**in**k

Figure 5: Transmitter module for quantum communication. (The data acquisition module

allows the storage of data. It might well be that, e.g. **in** case of quantum key distribution

systems, the key is not generated immediately.)

L**in**k

Down/Up

L**in**k

Photonic Qubits

Synchronization

Classical Channel

Passive/Active

Qubit-Manipulation

Data

Aquisition

Receiver Module

Detection

Unit

Figure 6: Receiver module for quantum communication.

12

Measurement

Output

Control Data

In/Out

L**in**k

7.2 L**in**k attenuation

The maximal acceptable l**in**k attenuation for a quantum communication system based on

entangled photons is determ**in**ed by the tim**in**g resolution and the dark count rates of the detectors

used, as well as by the net production rate of the source. As the m**in**imum signal-tonoise

ratio we assume that necessary for the violation of a Bell **in**equality. Roughly speak**in**g, a

total l**in**k efficiency of ηl**in**k = ηl**in**k1ηl**in**k2 ≈ 10 −6 (−60 dB) is necessary. The l**in**k attenuation

is also important for determ**in****in**g the number of photon pairs that can be received **in** a certa**in**

time w**in**dow. This could be crucial **in** scenarios where the l**in**ks are only available for short

times as is for example the case **in** up-l**in**ks to low orbit**in**g satellites. We **in**vestigated the l**in**k

attenuation for optical free-space l**in**ks **in**volv**in**g space **in**frastructure. The attenuation factor

calculated **in**cludes the effect of beam diffraction, attenuation and turbulence-**in**duced beam

spread**in**g caused by the atmosphere, receive aperture diameter, losses with**in** the telescopes

act**in**g as antennas, as well as antenna po**in**t**in**g loss. The calculations have been performed

for wavelengths of 800 nm and 1 550 nm. The first wavelength is reasonable because the best

photon source exists for 800 nm, while the second wavelength is ma**in**ly used **in** telecom systems.

Figure 7 summarizes the scenarios considered based on satellites **in** geostationary orbit

(GEO) and **in** low-earth orbit (LEO). Such satellites may serve as a platform for transmitters

or receivers. We presently do not envision the use of passive relays, e.g. retro-reflectors or

mirrors, because of the high l**in**k loss they would **in**troduce and because of the difficulty to

implement a po**in**t-ahead angle.

For the case of a LEO-based transmitter or receiver (l**in**k 1 **in** Fig. 7), l**in**k attenuation

poses no problems. Even for quite small telescopes onboard the LEO satellite, the attenuation

factor is well below 60 dB for all cases. Figure 8 is a contour plot of the l**in**k attenuation as

a function of transmitter and receiver aperture diameter (DT , DR) for the ground-to-LEO

upl**in**ks operated at a wavelength of λ = 800 nm. Two additional vertical scales give the l**in**k

distance L for 30 cm receive telescope aperture as well as for the receive telescope aperture

for a l**in**k distance of L = 500 km. The correspond**in**g plot for LEO-to-ground downl**in**ks is

shown **in** Fig. 9. The attenuation is much larger for the upl**in**k than for the downl**in**k due

to the pronounced **in**fluence of atmospheric turbulence for the upl**in**k, where the turbulent

layers are close to the transmitter. Another consequence of turbulence is that **in**creas**in**g the

transmitter aperture for the upl**in**k beyond 60 cm hardly decreases the l**in**k attenuation.

13

Figure 7: Summary of l**in**ks **in**volv**in**g Earth-based ground stations and LEO satellites or GEO

satellites

14

D R /L [cm/km]

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0

L [km] for D R =30cm

400 −

500 −

600 −

800 −

1000 −

2000 −

5000 −

D R [cm] for L=500km

40

35

30

25

20

15

10

l**in**k attenuation A [dB] for λ=800nm, T R =T T =0.8, L p =0.2, A atm =1dB, r 0 =9cm.

55 55

60 45

60

65 70 65

5

75

50

85

90

50

40

45

35

40

80

30

35

55

60

70

50

30

40

45

25

35

0

0 20 40 60

D [cm]

T

80 100 120

30

40

45

45

50

50

55

55

60

60

65 70 65 70 65

75 75 75

Figure 8: Contour plot of l**in**k attenuation A (**in** dB) as a function of transmitter and receiver

aperture diameter (DT , DR) and l**in**k distance L for ground-to-LEO upl**in**ks at λ = 800 nm

D R /L [cm/km]

0.2

0.15

0.1

0.05

0

L [km] for D R =100cm

450 −

500 −

600 −

800 −

1000 −

2000 −

5000 −

D R [cm] for L=500km

120

100

80

60

40

20

35

45

40

50

55

45

60

50 55

65

40

30

35

60

70 75

85 80

90

l**in**k attenuation A [dB] for λ=800nm, T R =T T =0.8, L p =0.2, A atm =1dB.

25

30

45

20

25

50

55

40

35

15

20

30

15

65 7060

25

45

0

0 5 10 15 20

D [cm]

T

25 30 35 40

20

10

40

55 50

Figure 9: As Fig. 8 but for LEO-to-ground downl**in**ks.

15

35

15

30

65

25

35

60

25

10

20

5

4555

5040

30

40

35

The long distance **in** l**in**ks between GEO and ground (l**in**k 2 **in** Fig. 7) results **in**

a relatively high attenuation. With a ground station aperture of D = 100 cm and a GEO

term**in**al aperture of D = 30 cm one will meet the 60 dB requirement **in** a downl**in**k, but not

**in** an upl**in**k (compare Tab. 4).

While from a technological po**in**t of view a satellite-to-satellite l**in**k is the most demand**in**g

configuration it offers highly attractive scientific possibilities. It allows to cover, **in**

pr**in**ciple, arbitrarily large distances and might thus also be a possibility for further novel fundamental

tests on quantum entanglement. We calculated the attenuation factor as a function

of transmitter and receiver aperture diameter for a LEO-LEO l**in**k (l**in**k 3 **in** Fig. 7). The

60 dB-limit poses no problem for LEO-LEO l**in**ks with reasonable l**in**k distance. For GEO-

GEO l**in**ks (l**in**k 4 **in** Fig. 7) with a distance of L = 45 000 km, an attenuation of A = 55 dB

would result for DT = DR = 30 cm.

Numerical results for the attenuation A are summarized **in** Tab. 4. The cited values apply

for the follow**in**g default parameters: ground based telescope diameter = 100 cm, space based

telescope diameter = 30 cm; l**in**k distances: 500 km for ground-LEO, 36 000 km for ground-

GEO, 2 000 km for LEO -LEO, 40 000 km for GEO-GEO, 35 500 km for LEO-GEO.

800 nm

1 550 nm

ground

based

transmitter

LEO

transmitter

GEO

transmitter

ground

based

receiver

6 dB

12 dB

44 dB

49 dB

LEO

receiver

27 dB

26 dB

28 dB

34 dB

53 dB

59 dB

GEO

receiver

64 dB

63 dB

53 dB

59 dB

54 dB

60 dB

LEO

retroreflector

31 dB

36 dB

GEO

retroreflector

105 dB

110 dB

Table 4: Comparison of the attenuation factors result**in**g for default parameters. The values

with**in** the boxes correspond to a wavelength of 1 550 nm, while the others stand for 800 nm

8 Fundamental quantum physics experiments

For a demonstration of fundamental quantum physics experiments, we focus on the use of entangled

two-particle states to test for non-classical correlations violat**in**g a Bell-type **in**equality.

Test**in**g for the violation of a Bell **in**equality, a so-called Bell experiment, is probably the most

straight forward fundamental experiment **in** quantum physics [17]. Mak**in**g use of the space

environment and the present technological **in**frastructure **in** space, a class of novel experiments

devoted to tests of a Bell **in**equality can be designed:

• Bell experiments over long distances beyond the capabilities of earth-bound laboratories,

address**in**g the question of persistence of quantum correlations over distances from

several hundred kilometers (**in** the case of LEO satellites) up to solar distances.

16

• An ultimate Bell experiment, concerned with ultimately clos**in**g the f**in**al **in**terpretation

loophole **in** such experiments.

• Experiments test**in**g different models for the physical collapse of the wave-function.

With a long-term vision **in** m**in**d, we are also propos**in**g a set of possible future fundamental

experiments mak**in**g use of quantum entanglement **in** space:

• Experiments concern**in**g special and general relativistic effects on quantum entanglement.

• Wheeler’s delayed choice experiment [18].

• A novel test of the Lens-Thirr**in**g effect, mak**in**g use of entanglement-enhanced **in**terferometry.

• An experimental test of Gödel’s cosmological model of a rotat**in**g universe.

9 Selection and design of an experiment

For a trade-off between possible experiments and their various manifestations we used the

follow**in**g evaluation criteria:

• state-of-the-art of present-day technology,

• technical complexity (l**in**k attenuation, PAT requirements, status of space qualification,

. . . ),

• detrimental **in**fluence of atmosphere,

• compatibility with available ground and space systems,

• novelty and scientific impact of experiment,

• added value with respect to space.

Concern**in**g the last po**in**t one has to consider that **in** free-space transmission systems, the

lack of atmosphere is a major advantage for bridg**in**g large distances beyond those presently

bridgeable between earth-bound term**in**als. Further, space l**in**ks do not encounter the problem

of obscured l**in**e-of-sight by unwanted objects or due to the curvature of the Earth. The long

distances that are manageable **in** pr**in**ciple when go**in**g **in**to space are essential with respect to

both fundamental and application aspects. Us**in**g space is presently the only method which

allows to establish quantum key distribution between arbitrary users on Earth. Earth-based

photonic propagation **in** quantum experiments us**in**g glass fibres is limited to some 100 km

with present-day technology. In the long run, the **in**fluence of gravitation on quantum physics,

albeit m**in**or, might be accessible **in** a space-based large-scale experiment us**in**g quantum

entanglement for further fundamental tests.

17

9.1 Selected experimental scenarios

Our f**in**al aim is to have available one transmitter module and two receiver modules. In order

to cover large distances it would be best to have all modules **in** space. However, we presently

discard this scheme due to the huge technological (and f**in**ancial) effort. We propose the

transmitter to be placed **in** space, preferably on the ISS, as the term**in**al can be modified

and upgraded more easily than on a satellite. We suggest to place the receiver modules on

Earth, as this allows maximum flexibility for the manipulation and detection of the photons.

Specifically, we propose to follow a stepwise approach. The upgrade from experiment 1 to 2

(see below) requires the **in**volvement of a second ground station (with a receiver term**in**al),

that from experiment 2 to 3 the modification of the transmitter at the ISS **in** the form of

replac**in**g the – **in**ternal – receiver by a second telescope to be po**in**ted to the second ground

station. This modification can be performed more easily if two telescopes are present from

the beg**in**n**in**g. Before go**in**g **in**to space, transmitter and receiver term**in**als are to be tested

along a horizontal terrestrial l**in**k.

Experiment 1 : **Space**-based transmitter of entangled photons and one ground-based receiver.

One photon is sent to the ground station, and the other is detected directly at

the transmitter. Experimental achievements: (a) the first demonstration of quantum

key distribution us**in**g s**in**gle photons (via BB84) between space and ground and (b)

**in**vestigation of the **in**fluence of long-distance propagation on s**in**gle photons beyond

distances achievable on Earth.

Experiment 2 : **Space**-based transmitter of entangled photons and two **in**dependent groundbased

receivers, with only one down-l**in**k at a time, i.e. one photon is sent to a ground

station, and the other is detected directly at the transmitter. No modification of the

modules is required. Experimental achievement: **Quantum** key exchange between two

**in**dependent ground stations over distances not feasible with earth-bound technology.

Experiment 3 : **Space**-based transmitter and simultaneous operation to two **in**dependent

ground-based receivers. Experimental achievements: (a) test**in**g Bell’s **in**equality over

distances only achievable with space technology, (b) quantum key distribution based on

entanglement.

Experiment 4 : Establishment of an upl**in**k: **Space**-based receiver, ground-based transmitter

module and an additional **in**dependent ground-based qubit source. Also, a groundbased

receiver module capable of a Bell-state measurement is necessary. Experimental

achievements: (a) quantum key distribution **in** upl**in**k-configuration; (b) quantum state

teleportation between a ground station and a satellite.

9.2 Technological basel**in**e and prelim**in**ary design

The transmitter term**in**al comprises the entangled photon source, modules for polarization

sensitive manipulation and detection of s**in**gle photons, a classical optical PAT system and

a telescope for the establishment of the downl**in**k (see Fig. 10). Each of the photons of the

entangled pairs is coupled **in**to optical fibers which are subject to polarization control via

(piezomechanical) bend**in**g of the fibers. Depend**in**g on the stage of the experiment, the two

photons of the entangled pair are either both sent through separate telescopes or only one is

sent while the other one is immediately detected. The reference laser of the PAT subsystem

18

Figure 10: Block diagram of the transmitter term**in**al.

is l**in**early polarized and optionally pulsed to provide both an orientational and a tim**in**g

reference frame between transmitter and receiver site.

The receiver term**in**al comprises a s**in**gle-photon analysis and detection subsystem (analogous

to the unit used **in** the transmitter term**in**al) and a classical optical subsystem consist**in**g

of a telescope and PAT equipment (see Fig. 11). Another polarization analysis subsystem

monitors the polarization of the transmitter reference laser, which is used to compensate for

any orientational misalignment between transmitter and receiver polarization. The signal

from this analysis is used to properly orientate the polarization **in** the s**in**gle photon beam

path. The reference laser of the receiver station(s) are operat**in**g at a wavelength differ**in**g

from the transmitter reference laser **in** order to clearly separate the two dist**in**ct signals for a

more accurate polarization analysis of the transmitter reference laser.

For the quantum communication experiments **in** m**in**d, we take **in**to account the follow**in**g

four European ground stations:

• Observatorio Del Teide, Tenerife (ES), Instituto de Astrof´ysica de Canarias

• Centro di Geodesia Spaziale, Matera (IT), Italian **Space** Agency (ASI)

• Estacion de Observacion de Calar Alto (ES), Instituto Geografico Nacional

• Observatorio de Sierra Nevada (ES), Instituto de Astrof´ysica de Andaluc´ya

19

Figure 11: Block diagram of the receiver term**in**al.

9.3 Mass, volume, and power consumption of space term**in**al

The basis for - necessarily very crudely - estimat**in**g mass, volume, power consumption, and

cost (MVPC) were various free-space optical term**in**als designed (and **in** a few cases flown)

for conventional laser communication. Specifically, we used data of the follow**in**g term**in**als:

OPALE and PASTEL (ESA´s SILEX system), SOTT (Matra Marconi), CDT (Bosch Telecom),

OPTEL-25 (Contraves **Space**), LUCE (Japan), LCDE (JEM/Japan). All this **in**formation

together with quite some amount of educated guess led to Tab. 5, which presents the

results of our MVPC estimates, both for the term**in**al with a s**in**gle telescope (as needed for

experiment 1 and 2) and one with two telescopes (experiments 3 and 4). In the table we

dist**in**guish between four subunits:

• the source of entangled photons (**in**clud**in**g polarization control, polarization analysis,

and reference detection),

• the optical head consist**in**g of the optical bench, the telescope (**in**clud**in**g the coarse

po**in**t**in**g mechanism),

• the devices needed for PAT (**in**clud**in**g the reference laser, the po**in**t ahead mechanism,

and the acquisition and track**in**g sensor),

• electronics for driv**in**g and controll**in**g the opto-electronic and opto-mechanic devices,

as well as the harness.

20

**Quantum**

source

Optical

head

PAT

Electronics

Total for

s**in**gletelescope

term**in**al

Total for

dualtelescope

term**in**al

Mass [kg] 35 40 20 20 115 185

Volume [cm 3 ] 50x30x30 50x30x30 30x30x30 50x20x20 50x60x60 50x90x60

Power [W] 80 15 40 50 185 265

Table 5: Estimate for mass, volume, and power of term**in**als with a s**in**gle telescope and with

a dual telescope.

21

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23