The closely related idea of quantum lithography in-

Volves using quantum states of light, such as the NOON

State, to harness the “reduced de-Broglie” wavelength to

lithographically de fi ne λ /2N-sized features4. Signi fi cant

Challenges include achieving arbitrary two-dimensional

Patterns and realizing N-photon resists. For quantum

Metrology, it is important to consider whether the phase

to be measured is fi xed but unkown, or time varying, re-

Quiring a high bandwidth measurement. A recent break-

Through showed that the need for complicated entan-

Gled states could be replaced by increased measurement

Time52, which is useful in the former case. Gravity wave

Detection is an example of the latter case, which can best

Be addressed by the CV approaches described below.

Quantum technologies with bright laser beams

The same non-linear crystal used in SPDC can be used

To deterministically create quantum states of a bright

Laser beam: The variance in the generalized amplitude

Xand phase pof a light beam are bound by the quan-

tum uncertainty relation: ∆x∆p≥~/2. The output of

a laser has ∆x= ∆p; while a ‘squeezed’ state of light

has ∆x6= ∆p. Squeezed states include a beam of only

even numbers of photons (P∞

n=0 cn|2ni, where nis the

Photon number); and entangled two-mode squeezed vac-

uum (p1−q2P∞

n=0 qn|niA|niBwhere q= tanh rand r

Is the squeezing parameter). Such squeezed states can be

Used as an alternative to the (discrete, two-level) qubit

Encoding described above. As with single photons, quan-

Tum entanglement for CV photonic quantum technologies

Can be created in several degrees of freedom of light: the

Most common is amplitude and phase quadratures8; and

Others include polarization53–55 and spatial modes56.

CV quantum communication can be regarded as a

Quantum version of conventional coherent communica-

Tion, where information is encoded in coherent states of

Light—laser light. The essence of CV quantum commu-

nication is an ‘optimum measurement’, which projects

The encoded states onto some entangled basis states, and

Gives us channel capacity beyond the Shannon limit57.

The realisation of this measurement can be regarded as

QIP, and so CV quantum communication and QIP are

Inseparable and since the processing must include co-

Herent states of light it is CV QIP. Quantum metrology

Schemes using adaptive homodyne measurement58 have

been demonstrated59. This type of ‘quantum feedback

And control’ is becoming a powerful tool for quantum

Metrology.

The most fundamental component of CV pho-

Tonic quantum technologies is CV quantum

teleportation10,60,61. The fi delity Fof CV telepor-

Tation is directly determined by the amount of squeezing

(typically quanti fi ed by the reduction in noise level of

The squeezed variable below the unsqueezed shot noise

Value, measured in dB) of the quantum entanglement

resource: F≤(1 + e−2r)−1. Achieving strong squeezing

Is experimentally challenging because losses destroy the

even-photon nature and an in fi nite level of squeezing

is not physically possible—it would require an in fi nite

Amount of energy (number of photons). The long-

Standing record of 6dB of squeezing62 was overcome

Using periodically poled KTiOPO4(PPKTP) as the

Nonlinear medium in a subthreshold optical parametric

Oscillator (OPO) cavity to achieve 7dB of squeezing63,

And 9dB with improvement of phase stability in the

Homodyne measurement64. In 2008 10dB was achieved

With a monolithic MgO:LiNbO3OPO65, which would

correspond to a teleportation fi delity of 0.91. In actual

teleportation experiments a fi delity of 0.83 has been

achieved66, equivalent to 7dB of e ff ective squeezing.

The advantage of QIP with single photon qubits is the

near-unit fi delity of operations; however, the lack of a

Strong optical non-linearity at the single photon level

Means that one has to select success events after the

Processing (as described above) making processing in-

Evitably slow. By contrast, the advantage of QIP with

CVs is the deterministic or unconditional nature of pro-

cessing; while the major disadvantage is non-unit fi delity

Of the processing because of the impossibility to achieve

an in fi nite amount of squeezing (r < ∞). Thus hybridiza-

Tion of qubits and CVs for photonic QIP could be desir-

able for the realization of QIP with unit fi delity and high

Success rate.

Encoding in ‘Schr¨odinger kittens’