### Use of Squeezed Light to perform Distance Measurement below the Standard Quantum Limit

**(from Left to Right) Nicolas Treps, Brahim Lamine and Claude Fabre**

A team of researchers (B. Lamine, N. Treps and C. Fabre) from the Laboratoire Kastler Brossel (LKB) at the University Pierre and Marie Curie (Paris, France) have shown how to use squeezed light to perform distance measurement below the standard quantum limit imposed by the quantum nature of light [1].

Any distance measurement involves the propagation of light between two observers and the measurement of its phase (interferometric measurement, which gives distance within a wavelength) or its amplitude (time of flight measurement, giving absolute measurement). The quantum nature of light introduces fluctuations in the phase and the amplitude of the light used for ranging, therefore leading to a noisy measurement. The scientists have shown how to combine both a time of flight and a phase measurement, using frequency combs and homodyne detection, to minimize the effects of this quantum noise.

When classical light is used, then the sensitivity cannot go below what is called a standard quantum limit, which is smaller than previously existing standard quantum limits based either on interferometric or phase measurement. More interestingly, when squeezed frequency combs are used to perform the measurement, the sensitivity can significantly dive below the previous standard quantum limit. Squeezing light consists in tailoring its quantum fluctuations.

Ranging using frequency combs have already been proposed in the past [2] while it has long ago been realized that quantum resources is a way of improving ranging [3] (in particular entanglement and squeezing). Nevertheless the combination of both technology in an adapted optimal scheme is a major first.

Potential applications could be for future space-based experiments such as DARWIN (to detect Earth-like exoplanets) or LISA (to detect gravitational waves), and even for precise dispersion measurement. Indeed, when dispersion occurs, it does not affect in the same way the phase and the envelope --an effect which can be seen in the detection scheme proposed by the scientists.

**References**

[1] "Quantum Improvement of Time Transfer between Remote Clocks"

B.Lamine, C.Fabre and N. Treps,

Physical Review Letter 101, 123601 (2008). Abstract. [arXiv:0804.1203].

[2] "Absolute measurement of a long, arbitrary distance to less than an optical fringe",

J. Ye, Optics Letters 29, 1153 (2004). Abstract.

[3] "Quantum-enhanced positioning and clock synchronization",

V. Giovannetti, S. Lloyd, and L. Maccone, Nature 412, 417 (2001). Abstract.

Labels: Gravitational Waves, Laser 2, Precision Measurement, Squeezed State

## 2 Comments:

That paper by Lamine, Fabre and Treps, certainly deserves your selection, but I am not sure about your comments, and particularly about the applicability of the technique to LISA :

squeezing techniques are very sensitive to the losses of the optical system, and the losses are very high in LISA (100pW detected, for 1W emitted). So I doubt that squeezing could improve LISA measurements.

Concerning Darwin, the precision requirements of the ranging system is not very high and the SQL is quite sufficient.

The idea has to be implemented, and its applications have to be found

Dear 2physics Team,

The above comment is relevant, here is my response. It is true that squeezing

is very sensitive to losses, as we said in the PRL article. Therefore,

squeezing will not help LISA. BUT remember that our protocol, using

frequency combs instead of continuous wave, even without squeezing,

still represents an improvment in timing. Ok, this is just a factor

square root of two maximum (if Delta omega=omega_0), but for

Gravitational Waves detection, a square root of 2 improvement means a

detection probability multiplied by nearly three (because the volume

seen by the detector increases as a power 3 of the sensitivity).

Anyway,

I am not claiming that frequency combs have to be used in LISA, because

there is so much other difficulties that comes wth the use of frequency

comb, but this is a potentiality for future space missions (and

eventually why not LISA! Indeed, mode locked laser have reached the same

stability than continuous wave, with the advantage of carrying a time

varying envelope giving an absolute range).

The same argument is also valid for DARWIN: using frequency combs could

be a solution to gain in sensitivity (without squeezing). Moreover,

there will be small losses in DARWIN, so that squeezing could have been

used if the instrument was limited by the quantum noise. This is not the

case for DARWIN, but could be the case for future missions (post-Darwin,

post-Grace etc...).

I want to say that I agree with the comment that the proposed technique

is for the moment in advance compare to existing sensitivities (limited

by technical noises except in LIGO/VIRGO, where quantum noise is a

severe limitation above a few hundred Hertz), so that it is true that it

still has to find some "real" applications. Space mission is a good

candidate because the power is limited, so that the standard quantum

limit is bad, and there is low losses, but other ideas are welcome !.

Best regards,

Brahim Lamine

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