Squeezed Light

Squeezed state

Single photon state

Can light be squeezed? In fact it is the quantum noise of light that can be squeezed. Such squeezed light (a squeezed state of light) is a special form of light that is researched in the field of quantum optics. The light's quantum noise is a direct consequence of the existence of photons, the smallest energy quanta of light. When light is detected with an ideal photo diode, every photon is translated into a photo-electron. For squeezed light the resulting photo-current exhibits surprisingly low noise. The noise is lower than the minimum noise one expects from the existence of independent photons and their statistical arrival times.

The quantum noise of light with independent (uncorrelated) photons is often called shot-noise. The light itself is then in a so-called coherent state, or Glauber state. Roy Glauber was awarded half of the Nobel prize in physics 2005 for his theoretical description of the quantum nature of light. Shot-noise could be expected as the minimum noise possible. And highly stabilized laser sources almost achieve this low noise level. However, squeezed light can even show les noise than Glauber states.

Squeezed states of light belong to the class of nonclassical states of light. Other nonclassical states are single photon states and, more general, number states (Fock states).

What are the applications of squeezed light?

Entanglement generation

Entanglement generation

Having less noise, squeezed light has applications in optical communication [1] and optical measurements [2]. Using squeezed light, weaker signals can be transmitted with the same signal to noise ratio and the same light power. Squeezed light can be used to Squeezed light can be used to distribute secret keys to two distant parties to perform quantum cryptography.

Squeezed light composes an attractive field of research in fundamental quantum physics. In particular it allows the production of entangled states of light and the experimental demonstration of the so-called Einstein-Podolsky-Rosen (EPR) paradox. This paradox constitutes the essence of Albert Einstein's objections against quantum theory being a complete theory [3]. Through the observation of the EPR paradox in several experiments [4] local hidden variable theories could be excluded. Quantum theory indeed seems to be a complete theory and Einstein's objections not justified. Squeezed light has been used to demonstrate quantum teleportation [5,6]

Squeezed light for gravitational wave detectors

GEO600 site


Highly topical is the planned application of squeezed light in gravitational wave detectors. This proposal was made 25 years ago, but only now the appropriate squeezed light is indeed available [7,8]. Gravitational wave detectors are high-power laser interferometers with km-scale arm lengths. Their goal is the astronomical observation of oscillations of space and time caused by black holes, vibrating neutron stars, super nova explosions, and the big bang.

Squeezed light is now belonging to the selected key technologies for future generations of such detectors. Once realized, it will be the first "real" application of nonclassical states of light. Squeezed light will be used to circumvent the demands for even higher laser powers in future gravitational wave detectors. Existing detectors are being operated with several kilowatts of laser power in the interferometer. Unfortunately the suspended optics of the interferometer are heated up and are deformed by such high powers. Squeezed light can be used to increase the detector sensitivity without increasing the laser power. The squeezed light technique might even enable the cryogenic cooling of the interferometer mirrors. Cryogenic cooling will reduce the thermally excited proper motion of the mirrors and is another key technology for future gravitational wave detectors.

How can squeezed light be generated?

Pumped crystal

Squeezed light source

Photon number statistic

Squeezed light can be generated in nonlinear optical crystals. Ultraviolett or visible light, either pulsed or continuous wave, is focussed into a highly transparent but birefringent crystal, for example of magnesium-oxide doped lithium niobate. The light polarizes the crystal material which leads to an effect called parametric down-conversion (PDC) in which one pump photon produces two daughter photons of about twice the wavelength. Depending on the experimental setup this process is also called optical parametric oscillation (OPO) or optical parametric amplification (OPA). In all settings photon pairs are randomly produced, however the photons of each pair show quantum correlations and are not independent fromeach other.

In typical PDC experiments a rather small rate of well-separated single photon pairs are produced. In typical OPO or OPA experiments heaps of photon pairs and even higher order pairs consisting of 4, 6 or greater even numbers of photons are produced. This is the regime of squeezed light.

Squeezed light was first demonstrated in 1985 [9]. Recently, a squeezing factor of 10 (10 dB) was realized at the Albert-Einstein-Institute in Hannover [10].

How can squeezed light be detected?

Homodyne Detector

Projective Measurement

The number of photon pairs in squeezed light is often too high to be detected with state of the art single photon detectors. In most cases squeezed light is therefore detected using a different detector type, a so-called homodyne detector. Such detectors are also based on the internal photo-electric effect and still every photon is precisely converted into one photo electron. However, homodyne detectors do not aim for a photon number measurement. They measure the strengths of amplitude and phase fluctuations, I.e. the modulations of the light field, which are entailed by the photon statistics.

A homodyne detector is a very useful device since it can handle much higher photon rates. Consequently, electronic dark currents is less of a problem. One should note that homodyne detection finds it direct analogy in many classical applications of light, fort example in laser interferometry and in optical communication, being based on amplitude and phase modulations.


AEI Hannover

DFG ScienceTV: The Wave Hunters

Institut für Optik, Universität Erlangen-Nürnberg

Department of Physics, ANU, Canberra

Tokyo University

LIGO Laboratory, MIT, Cambridge

Institut d'Optique, Orsay


[1] Yamamoto, Haus, Rev. Mod. Phys. 58, 1001 (1986),
Preparation. Measurement and information capacity of optical quantum states.

[2] C. M. Caves, Phys. Rev. D 23, 1693 (1981),
Quantum mechanical noise in an interferometer.

[3] A. Einstein, B. Podolsky, and N. Rosen, Phys. Rev. 47, 777 (1935),
Can quantum mechanical description of physical reality be considered complete?

[4] A. Aspect, P. Grangier and G. Roger, Phys. Rev. Lett. 49, 91 (1982),
Experimental Realization of Einstein-Podolsky-Rosen-Bohm Gedankenexperiment: A new violation of Bell's Inequalities.

[5] A. Furusawa, J. L. Sørensen, S. L. Braunstein, C. A. Fuchs, H. J. Kimble und E. S. Polzik, Science 282, 706 (1998),
Unconditional quantum teleportation.

[6] W. P. Bowen, Nicolas Treps, B. C. Buchler, R. Schnabel, T. C. Ralph, H.-A. Bachor,T..Symul, and P. K. Lam, Phys. Rev. A 67, 032302 (2003),
Experimental investigation of continuous-variable quantum teleportation.

[7] H. Vahlbruch, S. Chelkowski, B. Hage, A. Franzen, K. Danzmann, and R. Schnabel, Phys. Rev. Lett. 95, 211102 (2005),
Demonstration of a squeezed light enhanced power- and signal-recycled Michelson Interferometer.

[8] H. Vahlbruch, S. Chelkowski, B. Hage, A. Franzen, K. Danzmann, and R. Schnabel, Phys. Rev. Lett. 97, 011101 (2006),
Coherent control of vacuum squeezing in the gravitational-wave detection band.

[9] R.E. Slusher, L.W. Hollberg, B. Yurke, J.C. Mertz, and J. F. Valley, Phys. Rev. Lett. 55, 2409 (1985),
Observation of Squeezed States Generated by Four-Wave Mixing in an Optical Cavity

[10] H. Vahlbruch, M. Mehmet, N. Lastzka, B. Hage, S. Chelkowski, A. Franzen, S. Gossler, K. Danzmann, and R. Schnabel, submitted (2007), arXiv: 0706.1431,
Observation of squeezed light with 10dB quantum noise reduction