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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
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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’
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