Quantum optics, which itself has been a testing ground

For the ideas of quantum information science. For ex-

Ample, quantum entanglement was experimentally tested

Using photons generated from atomic cascades in the

S and early 1980s5,6. In the late 1980s and 1990s,

the non-linear process of ‘spontaneous parametric down-

Conversion’ (SPDC) was shown to be a convenient source

Of pairs7of photons for such fundamental experiments

And to generate quantum states of a bright laser beam—

‘squeezed states’8. SPDC has been used for many fun-

Damental quantum information tasks, including quantum

Teleportation9,10. Similarly, the interaction of single pho-

Tons with single atoms in an optical cavity—cavity quan-

tum electrodynamics (QED)—has been a rich fi eld of

Fundamental science with major applications to photonic

Quantum technologies11

While we don’t yet know exactly what form future

Quantum technologies will take, it seems likely that quan-

Tum information will be transmitted in quantum states of

Light, and that some level of information processing will

Be implemented on these quantum states. It also seems

Clear that if we are to realize these technologies we will

Need to harness and ultimately drive some of the latest

developments in the conventional fi eld of photonics.

Secure communication with photons

Light speed transmission and low noise properties make

Photons indispensable for quantum communication—

Transferring a quantum state from one place to another12.

arXiv:1003.3928v1 [quant-ph] 20 Mar 2010

2

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FIG. 1: a, A qubit can be encoded as the polarization of a

Single photon. bAn arbitrary state of a qubit can be repre-

sented on the Poincare´e or Bloch sphere. cA half wave plate

(λ /2) can be used to rotate this polarisation. dA polarisa-

Tion encoded qubit can be interconverted to a path enecoded

Qubit via a polarising beam splitter.

A quantum bit (or qubit) of information can be encoded

In any of several degrees of freedom—polarization, path,

Time-bin etc. Manipulation at the single photon level is

Usually straightforward—using birefringent waveplates in

The case of polarization for example (Fig. 1).

This ability to transfer quantum states between remote

Locations can be used to greatly enhance communication

Security. We can use the fundamental fact that any mea-

Surement of a quantum system necessarily disturbs that

System to reliably detect the presence of an eavesdrop-

Per. Several commercially available quantum key distri-

Bution (QKD) systems operate on this principle (com-

Prehensive reviews on the topic can be found in Refs.

For example). These QKD systems currently rely on

Attenuated laser pulses rather than single photons—an

approach that has been shown to be su ffi cient for point

To point applications2. However, attenuation of these

weak laser pulses over transmission in fi bre, or free space,

Currently limits the range of such systems to 100’s km

(quantum states of bright light are described below).

The advanced state of our modern communication sys-

tems owes much to the Erbium Doped Fiber Ampli fi er

(EDFA) for it’s ability to amplify optical signals as they

propagate over long distances of optical fi bre. Unfor-

tunately, ampli fi cation of a quantum signal is not so

Straightforward: measurement of the quantum state of

The signal destroys the information (the same disturbance

That enables detection of an eavesdropper). A major

Challenge is to realise a quantum repeater that is able

To store quantum information and implement entangling

Measurements. Ultimately, sophisticated quantum net-

Works will likely require nodes with a small-scale version

Of the quantum information processors described below.

Quantum information processing

The requirements for realizing a quantum computer are

Confounding: scalable physical qubits—two state quan-

Tum systems—that can be well isolated from the en-

Vironment, but also initialised, measured, and control-

Lably interacted to implement a universal set of quantum

Logic gates13. Despite these great challenges, a number

Of physical implementations are being pursued, includ-

Ing nuclear magnetic resonance, ion, atom, cavity quan-

Tum electrodynamics, solid state, and superconducting

Systems. Over the last few years single photons have